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Re: [ot][spam]gpt-4 completions
by Undescribed Horrific Abuse, One Victim & Survivor of Many 08 Sep '26

08 Sep '26

perplexity tell me what the yang mills mass gap is ooops introduce me to what a particle is how about that why, does it matter if i use my own terms? https://www.perplexity.ai/search/arright-perplexity-ai-backende-DTYM1DP5Ra6… # arright perplexity.ai backended by gpt 5.4 thinking: give me a couple advanced high school level sentences on what the yang mills band gap is. Is this an unmeasured prediction, an unproven measurement, or something else? A websearch seemed to freely conflate physics and math terms -- is that normal? Are the two the same here? Usually this is called the Yang–Mills **mass** gap, not the Yang–Mills band gap. In advanced high-school language: the classical Yang–Mills equations act like they should produce massless ripples, but the quantum version is expected to have a smallest nonzero energy above empty space, so the lightest particle-like excitation would still have some positive mass.[^1_1][^1_2][^1_3] ## What status it has It is not best described as an “unmeasured prediction” or an “unproven measurement.” A better description is: physicists have strong experimental and computational evidence that the relevant quantum Yang–Mills behavior has a mass gap, but mathematicians still do not have a fully rigorous proof that the four-dimensional quantum theory exists in the required sense and has that gap. That is why the Clay problem is called “Yang–Mills existence and mass gap” and remains unsolved.[^1_2][^1_3][^1_1] ## Physics vs math Yes, that mixing is normal here. The same phrase names both a physical phenomenon people expect in the strong interaction and a mathematical theorem people want to prove about quantum Yang–Mills theory. So they are not two unrelated things, but they are also not identical: physics asks whether nature does this, while the Millennium problem asks for a rigorous construction and proof.[^1_3][^1_1][^1_2] ## One clean way to say it A concise version is: “Yang–Mills mass gap” means that after quantization, the vacuum is separated from all excitations by some positive energy $\Delta>0$, so there are no arbitrarily low-energy particle states above the vacuum. For the SU(3) case connected to the strong force, this is often phrased as expecting massive color-neutral bound states such as glueballs rather than freely observed massless gluons.[^1_1][^1_3] Would you like a version that is one notch more mathematical, or one that is even more intuitive? <span style="display:none">[^1_10][^1_11][^1_12][^1_13][^1_14][^1_15][^1_16][^1_17][^1_18][^1_19][^1_20][^1_21][^1_22][^1_23][^1_24][^1_25][^1_26][^1_27][^1_28][^1_4][^1_5][^1_6][^1_7][^1_8][^1_9]</span> <div align="center">⁂</div> [^1_1]: https://en.wikipedia.org/wiki/Yang–Mills_existence_and_mass_gap [^1_2]: https://www.claymath.org/millennium/yang-mills-the-maths-gap/ [^1_3]: https://www.claymath.org/wp-content/uploads/2022/06/yangmills.pdf [^1_4]: https://projecteuclid.org/journals/journal-of-physical-mathematics/volume-7… [^1_5]: https://www.reddit.com/r/askscience/comments/8b4wg3/can_someone_explain_to_… [^1_6]: https://motionmountain.substack.com/p/the-yang-mills-mass-gap-problem [^1_7]: https://www.reddit.com/r/askscience/comments/4d9cle/what_exactly_does_the_y… [^1_8]: https://math-soc.com/2025/10/21/million-dollar-question-the-yang-mills-exis… [^1_9]: https://www.math.harvard.edu/event/millennium-prize-problems-lecture-sourav… [^1_10]: https://www.scirp.org/journal/paperinformation?paperid=126102 [^1_11]: https://www.youtube.com/watch?v=Ae9lTCUzJjE [^1_12]: https://international-maths-challenge.com/millennium-prize-the-yang-mills-e… [^1_13]: https://papers.ssrn.com/sol3/papers.cfm?abstract_id=5395755 [^1_14]: https://www.youtube.com/watch?v=uMiR0ZlZdaQ [^1_15]: https://ncatlab.org/nlab/show/Yang-Mills+mass+gap [^1_16]: https://www.claymath.org/lectures/yang-mills-and-the-mass-gap/ [^1_17]: https://www.claymath.org/library/annual_report/douglas_quantum_yang_mills.p… [^1_18]: https://www.claymath.org/lectures/yang-mills-existence-and-mass-gap/ [^1_19]: https://www.claymath.org [^1_20]: https://www.arthurjaffe.com/Assets/pdf/QuantumYangMillsWebRevised.pdf [^1_21]: https://www.claymath.org/millennium-problems/ [^1_22]: https://www.claymath.org/library/annual_report/ar2003/03report_prizeproblem… [^1_23]: https://papers.ssrn.com/sol3/Delivery.cfm/8dad7eb1-c2d9-46c7-a92b-2cabe0218… [^1_24]: https://www.claymath.org/library/monographs/MPPc.pdf [^1_25]: https://arxiv.org/abs/0911.1013 [^1_26]: https://www.claymath.org/millennium/yang-mills-the-maths-gap/unknown-1/ [^1_27]: http://casanchi.org/mat/problema06.pdf [^1_28]: https://www.claymath.org/events/millennium-prize-problems-lecture-series/ --- # Hrm let's go back to the "high school" part of my prompt -- can you briefly introduce the first high level concept or term in your reply? I guess that would be the Yang-Mills equations -- what are they? The Yang–Mills equations are the basic equations for certain force fields in modern physics, written as a system of nonlinear partial differential equations for a “gauge field.” They come from asking for field configurations that make the Yang–Mills energy/action stationary, much like ordinary mechanics gets equations of motion from an energy principle.[^2_1][^2_4] ## High-school picture At a high-school level, a useful first approximation is: they are like Maxwell’s equations for electromagnetism, but for more complicated fields that have extra internal components. The big new feature is that these fields can interact with themselves, so the equations are nonlinear rather than as simple as the vacuum Maxwell equations.[^2_4][^2_6][^2_7] If Maxwell says “electric and magnetic fields spread and evolve,” Yang–Mills says something like “a multi-component force field spreads and evolves, while also pushing on itself because of its own structure.” In both physics and math, that field is often described by a gauge potential $A$, and its strength/curvature by $F$; one standard local form of the equation is $\partial_\mu F^{\mu\nu} + [A_\mu, F^{\mu\nu}] = 0$, where the bracket term is the self-interaction part.[^2_6][^2_1][^2_4] ## Why they matter These equations are central in physics because Yang–Mills theory underlies the modern description of elementary-particle forces. They are central in math because they can also be viewed as natural equations for connections whose curvature is, in a precise sense, as small as possible.[^2_8][^2_1] Want the next term unpacked too—“gauge field” or “nonlinear partial differential equation”? <span style="display:none">[^2_10][^2_2][^2_3][^2_5][^2_9]</span> <div align="center">⁂</div> [^2_1]: https://en.wikipedia.org/wiki/Yang–Mills_equations [^2_2]: https://www.damtp.cam.ac.uk/user/tong/gaugetheory/2ym.pdf [^2_3]: https://www.reddit.com/r/explainlikeimfive/comments/msuv89/eli5_in_simple_t… [^2_4]: https://math.berkeley.edu/~gbeiner/papers/Yang_Mills.pdf [^2_5]: https://empg.maths.ed.ac.uk/Activities/GT/Lect3.pdf [^2_6]: https://pages.jh.edu/rrynasi1/PhysicalPrinciples/literature/Jackiw1980Intro… [^2_7]: http://math.uchicago.edu/~may/REU2017/REUPapers/Ling.pdf [^2_8]: https://www.claymath.org/millennium/yang-mills-the-maths-gap/ [^2_9]: https://www.youtube.com/watch?v=SR1Sl29HY4c [^2_10]: https://www.math.mcgill.ca/gantumur/math581w12/downloads/picard_YM.pdf --- # i'm okay with nonlinear pde which is roughly just an equation that includes derivatives; what's a gauge field? what interactions guide these fields? does self-interaction propagate at a finite speed or instantaneously? A gauge field is, at a first pass, the mathematical field you add so a theory can stay consistent when its “internal reference frame” is allowed to vary from point to point in space and time. In electromagnetism the gauge field is the familiar potential $A_\mu$; in Yang–Mills theory it is a more complicated multi-component version of that idea.[^3_1][^3_2][^3_3][^3_4] ## What it is A high-school-friendly picture is: imagine every point in space carries a little internal dial, and the gauge field tells you how that dial at one point is related to the dial at a nearby point. The physically measurable part is not the dial setting itself but the “twist” or curvature you get when you compare nearby points, which is why different gauge choices can describe the same physics.[^3_2][^3_4][^3_1] ## What guides it The allowed interactions are mainly fixed by the symmetry you demand locally, called the gauge symmetry, together with the choice of symmetry group such as U(1), SU(2), or SU(3). In plain language, the symmetry rules decide what kinds of charges exist, what field carries the force, and how matter couples to that field. For Yang–Mills fields, unlike ordinary electromagnetism, the field itself can carry the relevant charge, so it can interact with itself.[^3_5][^3_3][^3_4][^3_6][^3_7][^3_1] ## Speed of influence That self-interaction still propagates causally rather than instantaneously in the classical theory. Sources discussing the hyperbolic Yang–Mills equations describe them as having finite speed of propagation, and one summary notes that the classical waves of the field travel at the speed of light. So although some gauge choices can make the potentials look mathematically nonlocal, the physical influence does not seem to outrun light.[^3_3][^3_8][^3_4][^3_9] A compact slogan is: a gauge field is the bookkeeping device for a local symmetry, and Yang–Mills makes that bookkeeping dynamic enough that the field can also push on itself. Want me to connect that directly to why non-abelian gauge fields self-interact while Maxwell’s field does not?[^3_1][^3_2][^3_3] <span style="display:none">[^3_10][^3_11][^3_12][^3_13][^3_14][^3_15][^3_16][^3_17]</span> <div align="center">⁂</div> [^3_1]: https://en.wikipedia.org/wiki/Gauge_theory [^3_2]: https://terrytao.wordpress.com/2008/09/27/what-is-a-gauge/ [^3_3]: https://en.wikipedia.org/wiki/Yang–Mills_theory [^3_4]: https://plato.stanford.edu/entries/gauge-theories/ [^3_5]: https://81018.com/gauge-theory/ [^3_6]: https://math.berkeley.edu/~gbeiner/papers/Yang_Mills.pdf [^3_7]: https://pages.jh.edu/rrynasi1/PhysicalPrinciples/literature/Jackiw1980Intro… [^3_8]: https://par.nsf.gov/servlets/purl/10432590 [^3_9]: https://arxiv.org/pdf/1308.6571.pdf [^3_10]: https://williamsgj.people.charleston.edu/Gauge Theory.pdf [^3_11]: https://www.reddit.com/r/explainlikeimfive/comments/9plww9/eli5_explain_gau… [^3_12]: https://arxiv.org/abs/1308.6571 [^3_13]: https://www.youtube.com/watch?v=z2P-AxhnQiE [^3_14]: https://www.youtube.com/watch?v=eD_mv1ZZfLQ [^3_15]: https://www.claymath.org/wp-content/uploads/2022/06/yangmills.pdf [^3_16]: http://www.damtp.cam.ac.uk/user/tong/gaugetheory/gt.pdf [^3_17]: https://www.damtp.cam.ac.uk/user/tong/gaugetheory/2ym.pdf --- # It is surprising that self-interaction is needed for a bookkeeping field. What is a symmetry group? A symmetry group is the full set of transformations you can apply to something without changing the essential thing you are describing, together with the rule for combining one transformation after another. In geometry, for example, the rotations and reflections that leave a square looking the same form a symmetry group.[^4_1][^4_2] ## Group idea A group, in the algebra sense, is a collection of operations you can compose, where composition stays inside the collection, there is an identity operation that does nothing, and each operation has an inverse that undoes it. So a symmetry group is just a group whose elements are “do-nothing-visible” transformations of some object or law.[^4_3][^4_4][^4_2][^4_5][^4_1] ## Concrete example For an equilateral triangle, rotating by 120 degrees, rotating by 240 degrees, doing nothing, and reflecting across any of its three symmetry axes all count as symmetries. Those six operations together form the triangle’s symmetry group because combining any two of them gives another symmetry of the same triangle.[^4_2][^4_6][^4_1] ## Physics meaning In physics, the “object” being preserved is usually not the appearance of a shape but the form of the laws of motion or the measurable predictions. A gauge symmetry group is the set of internal transformations you can make at each point in spacetime without changing the physical content of the theory.[^4_7][^4_8] ## Why self-interaction appears Your surprise is reasonable: if the field were only bookkeeping, you might expect it to stay passive. But when the local symmetry group is non-commutative, as in Yang–Mills theory, the gauge field is required to preserve that symmetry and in doing so it also carries the relevant charge, which makes self-interaction part of the structure rather than an extra add-on. In that sense, the “bookkeeping” becomes dynamic because consistency under local symmetry forces the field itself to participate in the interaction.[^4_8][^4_9][^4_7] A very compressed slogan is: symmetry tells you what transformations are allowed, and the symmetry group is the algebraic machine that records how those transformations combine.[^4_5][^4_1] <span style="display:none">[^4_10][^4_11][^4_12][^4_13][^4_14][^4_15][^4_16][^4_17]</span> <div align="center">⁂</div> [^4_1]: https://en.wikipedia.org/wiki/Symmetry_group [^4_2]: https://fiveable.me/hs-honors-geometry/key-terms/symmetry-group [^4_3]: https://math.libretexts.org/Bookshelves/Abstract_and_Geometric_Algebra/An_I… [^4_4]: https://en.wikipedia.org/wiki/Symmetric_group [^4_5]: https://www2.math.upenn.edu/~mlazar/math170/notes07.pdf [^4_6]: https://www.youtube.com/watch?v=paQLJKtiAEE [^4_7]: https://en.wikipedia.org/wiki/Gauge_theory [^4_8]: https://plato.stanford.edu/entries/gauge-theories/ [^4_9]: https://en.wikipedia.org/wiki/Yang–Mills_theory [^4_10]: https://www.youtube.com/watch?v=3aNeCWRjh8I [^4_11]: https://www.reddit.com/r/mathematics/comments/14gnz46/what_do_people_mean_b… [^4_12]: https://arxiv.org/pdf/2602.01258.pdf [^4_13]: https://mathweb.ucsd.edu/~asalehig/math200a-22-f-lectures.pdf [^4_14]: https://www.reddit.com/r/Physics/comments/1ot8532/how_does_the_concept_of_g… [^4_15]: https://www.youtube.com/watch?v=emmeKVe9DmI [^4_16]: https://math.mit.edu/research/highschool/primes/circle/documents/2022/Evely… [^4_17]: https://mathworld.wolfram.com/SymmetricGroup.html --- # you've mentioned both particles and spacetime -- this applies to both? Yes, but in two different senses. Yang–Mills theory is a field theory on spacetime, and in its quantum version the particle picture appears because particles are interpreted as excitations of those fields.[^5_1][^5_2][^5_3] ## Two roles Spacetime is the stage: the gauge field assigns mathematical data to each point in space and time and evolves across that stage. Particles are the ripples or quantized excitations of the field when you view the theory quantum mechanically.[^5_4][^5_2][^5_3][^5_1] ## Important distinction The gauge symmetry in Yang–Mills theory is usually an internal symmetry, not an ordinary symmetry of spacetime shape like rotation or translation. So the theory lives on spacetime, but the symmetry group is usually acting on an internal “label space” attached at each spacetime point.[^5_5][^5_1][^5_4] ## In one sentence So: it applies to spacetime because the field is defined there, and it applies to particles because those field excitations are what we call particles. It does not usually mean that Yang–Mills is itself a theory of spacetime geometry; that would move you closer to gravity questions.[^5_2][^5_3][^5_6][^5_1] A useful slogan is: spacetime tells you where the field lives, and quantum theory tells you why lumps of that field look like particles.[^5_3][^5_2] <span style="display:none">[^5_10][^5_11][^5_12][^5_13][^5_14][^5_15][^5_7][^5_8][^5_9]</span> <div align="center">⁂</div> [^5_1]: https://en.wikipedia.org/wiki/Gauge_theory [^5_2]: https://www.claymath.org/wp-content/uploads/2022/06/yangmills.pdf [^5_3]: https://www.forbes.com/sites/startswithabang/2019/01/13/ask-ethan-how-do-qu… [^5_4]: https://philsci-archive.pitt.edu/834/1/gr_gauge.pdf [^5_5]: https://plato.stanford.edu/entries/gauge-theories/ [^5_6]: http://www.progress-in-physics.com/2022/PP-64-01.PDF [^5_7]: http://www.damtp.cam.ac.uk/user/tong/gaugetheory/gt.pdf [^5_8]: https://www.youtube.com/watch?v=AuqKsBQnE2A [^5_9]: https://physicstoday.aip.org/features/solving-quantum-field-theories-via-cu… [^5_10]: https://www.bostonreview.net/articles/matthew-buckley-search-new-physics-ce… [^5_11]: https://www.facebook.com/groups/179829628846347/posts/3229218450574101/ [^5_12]: https://www.youtube.com/watch?v=eoStndCzFhg [^5_13]: https://projecteuclid.org/journals/communications-in-mathematical-physics/v… [^5_14]: https://www.reddit.com/r/explainlikeimfive/comments/9plww9/eli5_explain_gau… [^5_15]: https://www.reddit.com/r/AskPhysics/comments/1fpm528/what_does_it_mean_to_s… --- # what kinds "coordinate transform" necessitate the use of a gauge field, if i understand correctly? Not quite coordinate transforms in the usual spacetime sense. What necessitates a gauge field is usually promoting an internal symmetry from a single global transformation to a different transformation at each spacetime point.[^6_1][^6_2][^6_3][^6_4] ## What changes The classic example is a complex matter field $\psi(x)$ whose overall phase can be rotated by the same angle everywhere, $\psi(x)\to e^{i\alpha}\psi(x)$, without changing observable physics. If you now allow that angle to vary with position and time, $\psi(x)\to e^{i\alpha(x)}\psi(x)$, ordinary derivatives pick up extra terms and the equations stop being invariant unless you introduce a compensating gauge field.[^6_5][^6_2][^6_6][^6_4] ## Not ordinary coordinates So the relevant “transforms” are usually not things like $x\to x+a$ or spatial rotations of spacetime coordinates. They are internal changes of description, such as local phase rotations for U(1) in electromagnetism or local SU(2)/SU(3) rotations among internal components in non-abelian theories.[^6_7][^6_8][^6_9][^6_10][^6_1] ## Why a field appears The gauge field is introduced so you can compare internal states at neighboring points in a way that respects those local transformations. Mathematically, it enters through a covariant derivative that cancels the unwanted derivative-of-$\alpha(x)$ terms. In that sense, the gauge field is less “forced by coordinate changes” and more “forced by demanding local internal symmetry.”[^6_2][^6_3][^6_11][^6_4][^6_12][^6_1] ## One picture A simple picture is that each point in spacetime is allowed its own choice of internal basis, and the gauge field tells you how to move from one point’s choice to the next one consistently. That is why people sometimes compare gauge transformations to coordinate changes, but the closer statement is that they are coordinate changes in an internal fiber-like space, not ordinary spacetime coordinates themselves.[^6_3][^6_11][^6_13] A one-line example is: global phase change does nothing special, but local phase change $\alpha\to\alpha(x)$ is what brings in the electromagnetic potential $A_\mu$.[^6_2][^6_7] <span style="display:none">[^6_14][^6_15][^6_16][^6_17][^6_18]</span> <div align="center">⁂</div> [^6_1]: https://en.wikipedia.org/wiki/Gauge_theory [^6_2]: https://www.ippp.dur.ac.uk/~krauss/Lectures/IntroToParticlePhysics/2010/Lec… [^6_3]: https://philsci-archive.pitt.edu/2133/1/geometrie.pdf [^6_4]: https://www.phys.ufl.edu/~ramond/Chapter6_CUP.pdf [^6_5]: https://williamsgj.people.charleston.edu/Gauge Theory.pdf [^6_6]: https://www.physics.rutgers.edu/grad/618/lects/localsym.pdf [^6_7]: https://www.physics.rutgers.edu/grad/618/lects/localsym_2.pdf [^6_8]: https://www.reddit.com/r/Physics/comments/wbdl7/explanation_of_gauge_symmet… [^6_9]: http://home.ustc.edu.cn/~gengb/201118/Peskin_lattice_gauge_theory.pdf [^6_10]: https://philsci-archive.pitt.edu/834/1/gr_gauge.pdf [^6_11]: https://en.wikipedia.org/wiki/Yang–Mills_equations [^6_12]: https://plato.stanford.edu/entries/gauge-theories/ [^6_13]: https://terrytao.wordpress.com/2008/09/27/what-is-a-gauge/ [^6_14]: http://www.damtp.cam.ac.uk/user/tong/gaugetheory/gt.pdf [^6_15]: https://arxiv.org/pdf/2506.15832.pdf [^6_16]: https://www.reddit.com/r/Physics/comments/2s2dxz/why_do_we_want_gauge_invar… [^6_17]: https://www.claymath.org/wp-content/uploads/2022/06/yangmills.pdf [^6_18]: https://indico.math.cnrs.fr/event/11080/attachments/4585/8220/Lecture Notes_DumitrescuT.pdf --- # is this attached paper sophisticated gibberish or real information? it was presented to me online in a suggestive context, after which i have interest in learning the terms -- i need things to learn and it's usually hard to keep engagement. what is a non-abelian field theory? what is the use of performing property transforms such as phase, and what is phase here? As a source of mainstream math or physics, this PDF looks much closer to a symbolic/esoteric construction than to a credible research paper. The quickest reason is that it mixes claims of solving multiple major problems with poetry, ritual language, custom glyph dictionaries, frequency assignments, and statements like “The Proof is Complete” and “Fluid Mechanics of God,” which is not how serious mathematical proofs or physics papers are normally built.[^7_1] ## Paper assessment The file presents itself as an “initiation into a closed formal language,” says symbolism is “not optional,” and frames understanding as something that “emerges only through interaction,” which is already a strong sign that it is not using mathematical terms in their standard public, checkable sense. It also claims solutions or translations for Yang–Mills, Navier–Stokes, BSD, Riemann, P vs NP, Hodge, and Poincaré in one framework, while mapping classical objects to glyphs, spiritual entities, and fixed Hz values; that pattern is much more characteristic of private-system writing than of accepted technical work.[^7_1] A fair reading is: it may contain real curiosity, real effort, and real emotional meaning for its author, but it does not look reliable as evidence about actual Yang–Mills theory or modern mathematics. If it caught your interest, the productive move is to mine it for vocabulary to learn in standard sources, not to treat it as a trustworthy guide to the subjects it names.[^7_1] ## Non-abelian theory In this context, “non-abelian” means the relevant symmetry operations do not commute, so doing transformation $A$ then $B$ gives a different result from doing $B$ then $A$. A non-abelian gauge field theory is therefore a field theory whose gauge symmetry comes from a non-commutative group, such as SU(2) or SU(3), rather than the commutative U(1) symmetry of electromagnetism.[^7_2][^7_3][^7_4] That non-commutativity matters because the gauge field itself then carries the kind of charge involved in the interaction, so the field can interact with itself. That is the big conceptual jump from Maxwell’s theory to Yang–Mills theory.[^7_3][^7_4][^7_5] ## What phase means Here, “phase” usually means the angle of a complex multiplier like $e^{i\theta}$ attached to a wavefunction or field, not “phase” in the sense of solid, liquid, or gas. You can think of it as a clock-hand angle attached to the field: changing the phase rotates that hand without necessarily changing directly measurable quantities.[^7_6][^7_7] A global phase change uses the same angle everywhere, while a local phase change lets the angle depend on space and time. Global phase is mostly a redundancy of description, but relative phase differences can matter physically because they affect interference.[^7_8][^7_7][^7_6] ## Why transform phase The point of performing a phase transformation is usually not to “do physics” to the system, but to test whether two descriptions represent the same physical situation. When you demand invariance under local phase changes, ordinary derivatives stop behaving nicely, and introducing a gauge field restores that invariance.[^7_7][^7_5][^7_9][^7_6][^7_8] So the gauge field is a compensating structure that lets neighboring points use different internal phase conventions without changing the physics. In electromagnetism that compensating field is the electromagnetic potential, and in Yang–Mills theory it is its non-abelian generalization.[^7_5][^7_2][^7_6][^7_7] ## Good terms to learn If you want the real concepts underneath the paper’s vocabulary, the best next terms are: gauge group, local symmetry, covariant derivative, field strength, abelian vs non-abelian, and global vs local phase. Those are standard and genuinely useful, and they will let you tell very quickly when a document is using physics words in a disciplined way versus using them as aesthetic decoration.[^7_10][^7_2][^7_6][^7_1] Want me to turn those six terms into a tiny one-page glossary in plain English? <span style="display:none">[^7_11][^7_12][^7_13][^7_14][^7_15][^7_16][^7_17][^7_18]</span> <div align="center">⁂</div> [^7_1]: ALQC_Canon_Formalized-1.pdf [^7_2]: https://en.wikipedia.org/wiki/Non-abelian_gauge_transformation [^7_3]: https://www.damtp.cam.ac.uk/user/dbs26/AQFT/YM.pdf [^7_4]: http://fma.if.usp.br/~burdman/QFT2/lecture_15.pdf [^7_5]: https://www.physics.rutgers.edu/grad/618/lects/localsym_2.pdf [^7_6]: https://www.ippp.dur.ac.uk/~krauss/Lectures/IntroToParticlePhysics/2010/Lec… [^7_7]: https://williamsgj.people.charleston.edu/Gauge Theory.pdf [^7_8]: https://www.reddit.com/r/AskPhysics/comments/wbnkgf/why_and_how_does_cancel… [^7_9]: https://plato.stanford.edu/entries/gauge-theories/ [^7_10]: https://en.wikipedia.org/wiki/Gauge_theory [^7_11]: https://www.youtube.com/watch?v=17Cig8YuRh4 [^7_12]: https://www.youtube.com/watch?v=ewDD5A9Rwcc [^7_13]: https://indico.global/event/5650/contributions/45603/attachments/22394/3773… [^7_14]: http://www.nhn.ou.edu/~milton/aqft/chap5.pdf [^7_15]: https://pmc.ncbi.nlm.nih.gov/articles/PMC3440626/ [^7_16]: https://arxiv.org/abs/2404.00743 [^7_17]: https://www.reddit.com/r/AskPhysics/comments/1kqwzly/what_exactly_is_the_qu… [^7_18]: https://mphq.physics.harvard.edu/quantum-phases-matter --- # i'm still catching up with what you've said so far. what you've said so far would likely take me multiple chat sessions to learn from you, hence my interest. what is private-system writing? when you say phase do you mean an alternating or time-changing-information-holding field or something else? By “private-system writing,” I did **not** mean “fake because unusual.” I meant writing that creates its own internal language first, then uses standard math/physics words inside that custom language, so the words stop having their usual public meanings unless you learn that one author’s dictionary. ## What I meant Your PDF explicitly says it is “not a translation, but an initiation into a closed formal language,” and it includes a long custom “Dictionary of Invariance” that assigns special meanings to glyphs, operators, frequencies, and technical terms. It also repeatedly equates logic, physics, topology, esoteric symbolism, and fixed Hz assignments inside one bespoke framework, which is why I described it as private-system writing rather than ordinary mathematical exposition.[^8_1] That does not mean there is no thought in it. It means the burden of interpretation is shifted from shared definitions and standard proofs to the author’s own symbolic world.[^8_1] ## What phase is Here “phase” usually means the angle of a complex number, or of a complex-valued wave or field. If you write a quantity in the form $re^{i\theta}$, then $r$ is its size and $\theta$ is its phase.[^8_2][^8_3] So phase is not, by itself, “an alternating field” or “time-changing information.” It is more like the hand-angle on a clock attached to the quantity.[^8_3][^8_2] ## Does phase change in time It can, but it does not have to. For a wave like $Ae^{i(kx-\omega t)}$, the phase is $kx-\omega t$, so it varies with position $x$ and time $t$; that variation is what encodes things like wavelength and frequency.[^8_4][^8_3] But you can also have a constant phase offset, like multiplying by $e^{i\phi}$, where nothing is “oscillating faster” just because the whole object got rotated in the complex plane. That is why people distinguish between global phase and relative or local phase.[^8_5][^8_6][^8_2] ## Why phase matters A global phase is often physically unobservable, meaning two states that differ only by one overall phase factor are usually treated as the same physical state. Relative phase matters because it affects interference, which is why two waves can reinforce or cancel depending on how their phase angles line up.[^8_2][^8_5] So the short answer is: phase is usually an angle, not a substance. When that angle changes across space or time, it becomes a powerful way to describe oscillation, interference, and in gauge theory, local changes of internal description.[^8_7][^8_5][^8_3][^8_2] ## One picture A good mental model is two arrows of the same length in the complex plane. Their lengths tell you “how much,” and their angles tell you the phase; changing the angle changes how they add together, even if the lengths stay the same.[^8_3][^8_2] Next best term to unpack is probably “complex number as a rotating arrow,” because that makes phase, local phase, and gauge transformations much less mysterious. <span style="display:none">[^8_10][^8_11][^8_12][^8_13][^8_14][^8_15][^8_16][^8_17][^8_8][^8_9]</span> <div align="center">⁂</div> [^8_1]: ALQC_Canon_Formalized-1.pdf [^8_2]: https://www.youtube.com/watch?v=qtIsYbYdzCI [^8_3]: https://mathforquantum.quantumtinkerer.tudelft.nl/1_complex_numbers/ [^8_4]: https://courses.physics.illinois.edu/phys580/fa2013/susy_v2.pdf [^8_5]: https://philsci-archive.pitt.edu/20971/1/phase 2022 v999.pdf [^8_6]: https://www.quantumgrad.com/article/676 [^8_7]: https://www.ippp.dur.ac.uk/~krauss/Lectures/IntroToParticlePhysics/2010/Lec… [^8_8]: https://web.stanford.edu/~montanar/TEACHING/Stat375/papers/journey.pdf [^8_9]: https://www.nordicwittgensteinreview.com/article/download/3302/pdf/8131 [^8_10]: https://www.reddit.com/r/math/comments/uvrrr8/using_your_own_math_notation/ [^8_11]: https://news.ycombinator.com/item?id=34441327 [^8_12]: http://www.math.vanderbilt.edu/~schectex/commerrs/ [^8_13]: https://files.eric.ed.gov/fulltext/ED402030.pdf [^8_14]: https://www.youtube.com/watch?v=4erc_zQY89I [^8_15]: https://www.reddit.com/r/3Blue1Brown/comments/w4igqr/having_hard_time_under… [^8_16]: https://ecommons.aku.edu/cgi/viewcontent.cgi?article=1008\&context=eastafri… [^8_17]: https://www.youtube.com/watch?v=bIxJDU5raQs --- # it sounds like you are discussing the phase component of things that are being described by complex numbers, even when the use of complex numbers does not add meaning? that is kind if why the guage field is purely bookkeeping? the author of the paper uses vague words i see as poorly-defined like telepathy -- do these words have contextual or paper-defined rigorous meaning i have not learned? Partly yes: the phase being discussed is the phase of a complex-valued field or wavefunction, and an overall global phase is usually treated as physically redundant, while relative or local phase differences can matter. But that does **not** make the gauge field merely empty bookkeeping in the full theory, because gauge redundancy is a redundancy of description, while the gauge field can be a dynamical object with real physical effects.[^9_1][^9_2][^9_3][^9_4][^9_5][^9_6] ## Phase and meaning When physicists talk about phase here, they usually mean the angle in a complex factor like $e^{i\theta}$. The reason complex numbers are used is not just decorative convenience: even when the overall phase is unobservable, relative phase changes interference and other measurable behavior.[^9_4][^9_7][^9_6][^9_8] So your instinct is close but needs one adjustment. The “bookkeeping” part is mostly that different phase conventions can describe the same physical state, especially for global phase, while the physically meaningful part shows up in phase differences across components or across spacetime.[^9_5][^9_6][^9_4] ## Gauge bookkeeping A good compact statement is: gauge symmetry is often a redundancy in description, but the compensating field introduced to handle local phase changes is not merely disposable notation once it is allowed to evolve dynamically. One source puts it almost exactly this way: the “bookkeepers” become part of a gauge theory only when they are dynamical actors that shape the system’s behavior.[^9_9][^9_2][^9_1] That is why people often say two things that sound contradictory but are both true. The gauge freedom is descriptive redundancy, yet the gauge field associated with that redundancy can encode real interactions such as electromagnetism or Yang–Mills forces.[^9_3][^9_1][^9_5] ## Paper language For words like “telepathic circuit,” the paper does not seem to give a rigorous operational definition in the standard scientific sense. It uses the phrase in the cover letter, but the document also says its symbols gain meaning “through engagement rather than definition alone” and calls the text an “initiation into a closed formal language,” which points away from ordinary public definitions.[^9_10] So there may be a contextual meaning inside the author’s symbolic system, but it does not look like the kind of meaning that would let an outside reader test, compute, or verify claims in a standard way. That is the key distinction: a rigorous term usually comes with clear conditions for use, while this paper often gives poetic role-descriptions, thematic links, and analogies instead.[^9_10] ## How to read it A practical way to read this kind of document is to separate three layers. Layer 1 is standard math/physics vocabulary, like manifold, commutator, parity, or phase; layer 2 is the author’s private relabeling of those words; layer 3 is openly poetic or esoteric language like “telepathic circuit” or “witch of always.”[^9_3][^9_10] If you want learning value without getting trapped, trust layer 1, treat layer 2 cautiously, and treat layer 3 as metaphor unless the paper gives a clear procedure or test. The words may carry emotional or narrative intent, but they do not seem to acquire standard rigor merely by being embedded next to equations.[^9_1][^9_10] Next term to pin down might be “relative phase,” because that is exactly where “complex-number description” stops being mere convention and starts affecting measurable outcomes.[^9_4][^9_5] <span style="display:none">[^9_11][^9_12][^9_13][^9_14][^9_15][^9_16][^9_17][^9_18]</span> <div align="center">⁂</div> [^9_1]: https://ar5iv.labs.arxiv.org/html/1901.10420 [^9_2]: https://www.studocu.com/en-us/document/creighton-university/human-physiolog… [^9_3]: https://sites.ualberta.ca/~gingrich/courses/phys512/node30.html [^9_4]: https://www.youtube.com/watch?v=fgWGMnlbIvA [^9_5]: https://quantummechanics.ucsd.edu/ph130a/130_notes/node30.html [^9_6]: https://philsci-archive.pitt.edu/20971/1/phase 2022 v999.pdf [^9_7]: https://www.sciencedirect.com/science/article/pii/S0375960123004292 [^9_8]: https://mathforquantum.quantumtinkerer.tudelft.nl/1_complex_numbers/ [^9_9]: http://www.damtp.cam.ac.uk/user/tong/gaugetheory/gt.pdf [^9_10]: ALQC_Canon_Formalized-1.pdf [^9_11]: https://cjsmeenk.github.io/files/rickles2009gauge.pdf [^9_12]: https://readingfeynman.org/2026/01/01/the-gauge-idea-in-em-theory/ [^9_13]: https://quantumzeitgeist.com/quantum-states-interference-achieves-defined-o… [^9_14]: https://www.preposterousuniverse.com/blog/2012/11/22/thanksgiving-7/ [^9_15]: https://www.reddit.com/r/Physics/comments/2s2dxz/why_do_we_want_gauge_invar… [^9_16]: https://www.reddit.com/r/3Blue1Brown/comments/w4igqr/having_hard_time_under… [^9_17]: https://www.reddit.com/r/AskPhysics/comments/wbnkgf/why_and_how_does_cancel… [^9_18]: https://philsci-archive.pitt.edu/26825/1/aot.pdf --- # what fields are complex valued? i don't really believe complex numbers are anything more than encodings of sinusoids in a way mathematicians like, but i'm happy to consider otherwise. it seemed to me the paper had clear nonsense in it, that it was specifically abusing terms to confuse or contradict -- but maybe i was wrong? Your intuition is **mostly** right: in many cases, a complex-valued field is a compact way to package two real-valued components, often in a form that makes oscillation, phase, and symmetry easier to handle. But in quantum theory that packaging is not usually treated as empty style, because interference and phase symmetry are built directly into the standard formalism through complex amplitudes.[^10_1][^10_2][^10_3][^10_4] ## Which fields are complex Common examples are the Schrödinger wavefunction, complex scalar fields, and Dirac spinor fields. A standard complex scalar field can literally be written as $\psi(x)=\frac{1}{\sqrt{2}}(\phi_1(x)+i\phi_2(x))$, so in that sense it is two real scalar fields bundled together.[^10_5][^10_6][^10_2] By contrast, some classical fields are usually taken to be real-valued, and complex notation is then mainly a convenient representation. So “complex-valued field” is not one single thing; sometimes it is a true part of the standard object being studied, and sometimes it is a useful reformulation.[^10_7][^10_2] ## Is it just sinusoids For ordinary linear waves, treating complex numbers as a smart encoding of sine-and-cosine behavior is a very good instinct. The phase then acts like an angle attached to the oscillation, and complex multiplication keeps track of rotations of that angle cleanly.[^10_3][^10_8][^10_9] What goes beyond “just sinusoids” is that the same complex structure also organizes symmetry transformations, especially phase rotations like U(1), and those are central in quantum and gauge theory. So even if you prefer to think “two real components plus a rotation rule,” that view is compatible with mainstream physics.[^10_2][^10_10][^10_1] ## Gauge-field link This is also why the gauge-field story is not merely “mathematicians liking complex numbers.” If a field has a local phase freedom, then comparing its phase from point to point requires extra structure, and that extra structure is what the gauge field supplies.[^10_11][^10_12] So yes, there is bookkeeping in the sense of handling redundant descriptions. But the bookkeeping becomes physical once the compensating field has its own dynamics and measurable effects.[^10_12][^10_13][^10_11] ## The paper Your suspicion does not seem misplaced. The document explicitly says meaning arises “through engagement rather than definition alone,” calls itself an “initiation into a closed formal language,” and mixes standard terms with phrases like “Telepathic Circuit,” “Fluid Mechanics of God,” “Density of God,” and “Mass is Memory,” which is a strong sign that it is not using technical language in the normal rigorous way.[^10_14] That does not prove malicious intent, and in some possible world an eccentric notation could still hide a real idea. But this particular paper strongly looks like it is using scientific vocabulary suggestively rather than giving public definitions, reproducible calculations, or standard proofs, so reading it as deliberate abuse or confusion is a very understandable judgment.[^10_14] ## A useful translation rule A good rule is: if a term is rigorous, you should be able to ask “what object is it, what are its inputs and outputs, and what would count as using it correctly?” In this PDF, many exotic terms function more like atmosphere, mythology, or role-labels than like testable mathematical definitions.[^10_14] The next most useful term to unpack here is probably “U(1) phase rotation,” because that is exactly where complex numbers, phase, and gauge fields meet in a clean way.[^10_1][^10_11] <span style="display:none">[^10_15][^10_16][^10_17][^10_18][^10_19][^10_20][^10_21][^10_22][^10_23]</span> <div align="center">⁂</div> [^10_1]: https://kimura.pages.math.cnrs.fr/file/QFT_lec.pdf [^10_2]: https://dec41.user.srcf.net/notes/III_M/quantum_field_theory.pdf [^10_3]: https://mathforquantum.quantumtinkerer.tudelft.nl/1_complex_numbers/ [^10_4]: https://philsci-archive.pitt.edu/20971/1/phase 2022 v999.pdf [^10_5]: https://www.southampton.ac.uk/~doug/ft1/ft15.pdf [^10_6]: https://en.wikipedia.org/wiki/Dirac_equation [^10_7]: https://en.wikipedia.org/wiki/Scalar_field_theory [^10_8]: https://courses.physics.illinois.edu/phys580/fa2013/susy_v2.pdf [^10_9]: https://www.sciencedirect.com/science/article/pii/S0375960123004292 [^10_10]: https://edu.itp.phys.ethz.ch/hs12/qft1/Chapter04.pdf [^10_11]: https://quantummechanics.ucsd.edu/ph130a/130_notes/node30.html [^10_12]: https://sites.ualberta.ca/~gingrich/courses/phys512/node30.html [^10_13]: https://ar5iv.labs.arxiv.org/html/1901.10420 [^10_14]: ALQC_Canon_Formalized-1.pdf [^10_15]: https://www.youtube.com/watch?v=wRHJjg9jBjA [^10_16]: https://www.youtube.com/watch?v=cQNgfgmvJmA [^10_17]: https://www.damtp.cam.ac.uk/user/tong/qft/qfthtml/S2.html [^10_18]: https://learn.mit.edu/c/topic/science-math?resource=6526 [^10_19]: https://vixra.org/pdf/2504.0175v1.pdf [^10_20]: https://arxiv.org/abs/hep-th/9810093 [^10_21]: https://www.reddit.com/r/AskPhysics/comments/1b9sq7/complex_scalar_fields_a… [^10_22]: https://www.damtp.cam.ac.uk/user/tong/qft/qfthtml/S4.html [^10_23]: https://www.worldscientific.com/doi/10.1142/9789811220876_0007 --- # what is U(1)? U(1) is the group of complex numbers with magnitude 1, so its elements can be written as $e^{i\theta}$ for some angle $\theta$. Geometrically, that is the symmetry group of rotations of a circle, which is why U(1) is the simplest continuous internal symmetry used in physics.[^11_1][^11_2][^11_5][^11_6][^11_8] ## Concrete picture If you draw a unit circle in the complex plane, multiplying by $e^{i\theta}$ just rotates a point around that circle without changing its length. So U(1) is basically “all possible angle rotations,” with multiplication as the way you combine them.[^11_5][^11_6][^11_1] ## Why it appears in physics In quantum theory, U(1) usually shows up as phase rotations of a field, such as $\psi \to e^{i\alpha}\psi$. If the same phase rotation is allowed everywhere, that is a global U(1) symmetry; if the phase is allowed to vary from point to point, that is local U(1) symmetry.[^11_7][^11_5] ## Why you care Quantum electrodynamics is an abelian gauge theory with symmetry group U(1), and the associated gauge field is the electromagnetic potential. Sources in the results also tie U(1) symmetry to conservation of electric charge and explain that local U(1) symmetry is what brings in electromagnetism.[^11_3][^11_1][^11_5] The shortest intuition is: U(1) is “phase rotation by any angle,” and gauge theory asks what extra structure is needed when that allowed angle varies across spacetime.[^11_5][^11_7] <span style="display:none">[^11_4]</span> <div align="center">⁂</div> [^11_1]: https://www.youtube.com/watch?v=rCMIP5jW41s [^11_2]: https://www.reddit.com/r/explainlikeimfive/comments/11glacn/eli5_what_is_u1… [^11_3]: https://en.wikipedia.org/wiki/Gauge_theory [^11_4]: https://shmaesphysics.wordpress.com/2022/08/08/justifying-the-standard-mode… [^11_5]: https://kgbudge.com/essays/U1xSU2xSU3_2.html [^11_6]: https://www.youtube.com/watch?v=UHHWBIecxcA [^11_7]: https://www.physics.rutgers.edu/grad/618/lects/localsym.pdf [^11_8]: https://encyclopedai.stavros.io/entries/u(1)-symmetry/ --- # why are we considering 2d angles in 3space? is it over time or potential vs kinetic energy or electric vs magnetic or differentials ...? Not usually. In the U(1) example, the “angle” is typically **not** a geometric angle in ordinary 3-space, but an angle in an internal complex plane attached to the field at each spacetime point.[^12_1][^12_2][^12_3][^12_4] ## What the angle is If a field value is complex, you can picture it as an arrow in a 2D plane with a length and an angle, and multiplying by $e^{i\theta}$ rotates that arrow within that abstract plane. That 2D plane is a mathematical space of internal states, not a literal plane floating inside physical space.[^12_2][^12_5][^12_4][^12_1] ## Not these other things So it is usually not “potential versus kinetic,” not “electric versus magnetic,” and not “time versus space differential” either. Those are different decompositions of a system, while U(1) phase rotation is a change of internal phase convention for the field.[^12_3][^12_1][^12_2] ## Where time enters Time can enter because the phase angle may vary from one spacetime point to another, so the angle can depend on position and time. But that does not make the angle itself a spatial direction in 3-space; it means the internal arrow can rotate differently at different places and times.[^12_6][^12_2][^12_3] ## Why use this picture The reason physicists use the 2D complex-angle picture is that it packages two real components into one object and makes phase symmetry very clean to express. For U(1), the field can be rephased by any angle, and gauge theory asks how to compare those internal phases consistently across spacetime.[^12_5][^12_7][^12_2][^12_6] A compact slogan is: the field lives on spacetime, but its phase rotates in an internal 2D complex plane. Would you like me to draw the “two real numbers = one arrow in an internal plane” picture in plain text next?[^12_4][^12_3] <span style="display:none">[^12_10][^12_11][^12_12][^12_13][^12_14][^12_15][^12_8][^12_9]</span> <div align="center">⁂</div> [^12_1]: https://www.reddit.com/r/TheoreticalPhysics/comments/1jodjbq/why_is_the_con… [^12_2]: https://kgbudge.com/essays/U1xSU2xSU3_2.html [^12_3]: https://www.preposterousuniverse.com/blog/2020/06/30/the-biggest-ideas-in-t… [^12_4]: https://www.physicsforums.com/threads/what-is-internal-space-internal-symme… [^12_5]: https://arxiv.org/html/2404.00743v1 [^12_6]: https://en.wikipedia.org/wiki/Gauge_theory [^12_7]: http://www.damtp.cam.ac.uk/user/tong/gaugetheory/gt.pdf [^12_8]: http://www.quantumdiaries.org/2009/11/06/symmetry-in-physcs-pt-4/ [^12_9]: https://www.physicsforums.com/threads/internal-gauge-symmetries-and-spaceti… [^12_10]: https://www.facebook.com/groups/473920566008054/posts/8317613844971981/ [^12_11]: https://personal.lse.ac.uk/robert49/PPB/pdf/Belot2003gauge.pdf [^12_12]: https://www.sciencedirect.com/science/article/pii/0003491687901643 [^12_13]: https://www.reddit.com/r/AskPhysics/comments/wbt41s/why_do_gauge_symmetries… [^12_14]: https://en.wikipedia.org/wiki/Phase-space_formulation [^12_15]: https://www.osti.gov/biblio/6631643 --- # i am just trying to undsrstand the use and function of the complex representation -- is this a fundamental measure of interactions (like EM radiation) or a way to represent a mathematical situation (like AC power)? can you show an example where the complex notation does something? Mostly both: for AC circuits and classical sinusoidal waves, complex notation is largely a compact representation trick, but in quantum mechanics the standard formalism uses complex amplitudes in a way that directly controls interference outcomes. So it is not just “mathematicians liking a notation,” although in some settings it really is a very efficient encoding of two real quantities.[^13_1][^13_2][^13_3][^13_4][^13_5] ## What the complex form is doing A complex number packages two real components into one object with a magnitude and an angle, which is especially handy for oscillations and rotations. For a sinusoid, writing something like $Ae^{i\theta}$ lets addition, phase shifts, differentiation, and multiplication all work in a cleaner algebraic way than carrying separate sines and cosines everywhere.[^13_2][^13_6][^13_1] That is why engineers use it for phasors in AC analysis and why physicists use it for waves. In those cases, the complex form is often a representation of a mathematical situation rather than a literal extra physical dimension hiding in space.[^13_4][^13_1][^13_2] ## AC example In AC circuit analysis, phasor notation turns a time-dependent sinusoid into a single complex number carrying amplitude and phase, so adding two shifted voltages becomes vector addition instead of repeated trig identities. For example, one source works with two voltages where one is 20 volts and leads the other by 60 degrees, then rewrites one as $10 + j17.32$, which makes combination straightforward in rectangular form.[^13_7][^13_2] That is a genuine “does something” example: the complex notation converts differential-equation-and-trig work into ordinary algebra on amplitudes and phases. So in the AC case, your “like AC power” intuition is very good.[^13_2][^13_7][^13_4] ## Quantum example Quantum interference is the sharper example where complex notation is doing more than cosmetic bookkeeping. When an outcome can happen by two indistinguishable paths, quantum mechanics says you add the complex amplitudes first and only then square the magnitude to get a probability, so the relative phase can enhance or cancel the result.[^13_3][^13_8] That means two possibilities can each contribute nonzero amplitude and still cancel if they are out of phase. This is not just a shorthand for sinusoids in the everyday engineering sense; in the standard theory, the complex phase is exactly what governs interference.[^13_9][^13_3] ## About the paper Your reading of the paper as containing obvious nonsense does not seem unreasonable. The document explicitly says meaning arises “through engagement rather than definition alone,” presents itself as an “initiation into a closed formal language,” and mixes technical vocabulary with phrases like “Telepathic Circuit,” “Fluid Mechanics of God,” “Density of God,” and “Mass is Memory,” which does not resemble standard public mathematical definition-and-proof practice.[^13_10] So yes, it does appear to use authoritative math and physics words in ways that are suggestive, private, and often hard to test from outside. That does not prove there is zero sincere thought in it, but it is a strong sign that the paper is not a reliable guide to the standard meanings of the terms it invokes.[^13_10] ## One tiny demo Take two equal amplitudes $1$ and $i$. Their sum has magnitude $\sqrt{2}$, while $1$ and $-1$ sum to $0$, so the complex angle alone changes whether contributions reinforce or cancel.[^13_6][^13_11][^13_3] That small fact is the whole reason phase matters. Complex notation is useful because it makes that angle algebraic and portable across waves, AC systems, and quantum amplitudes.[^13_11][^13_1][^13_3] Should I do the next step with a concrete worked example such as $e^{i\omega t}$, showing exactly how differentiation or interference becomes simpler? <span style="display:none">[^13_12][^13_13][^13_14][^13_15][^13_16][^13_17][^13_18][^13_19][^13_20]</span> <div align="center">⁂</div> [^13_1]: https://www.electronics-tutorials.ws/accircuits/complex-numbers.html [^13_2]: https://eng.libretexts.org/Workbench/Introduction_to_Circuit_Analysis/08:_A… [^13_3]: https://www.spinquanta.com/news-detail/exploring-quantum-interference-key-c… [^13_4]: https://courses.physics.ucsd.edu/2011/Winter/physics4d/files/E_M_waves.pdf [^13_5]: https://dec41.user.srcf.net/notes/III_M/quantum_field_theory.pdf [^13_6]: https://mathforquantum.quantumtinkerer.tudelft.nl/1_complex_numbers/ [^13_7]: https://www.electronics-tutorials.ws/accircuits/phasors.html [^13_8]: https://www.youtube.com/watch?v=LZ9QdjaMbFw [^13_9]: https://philsci-archive.pitt.edu/20971/1/phase 2022 v999.pdf [^13_10]: ALQC_Canon_Formalized-1.pdf [^13_11]: https://www.sciencedirect.com/science/article/pii/S0375960123004292 [^13_12]: https://www.sjsu.edu/faculty/beyersdorf/Archive/Phys208F07/ch 1-complex representation of EM waves.pdf [^13_13]: https://ocw.mit.edu/courses/6-007-electromagnetic-energy-from-motors-to-las… [^13_14]: https://www.youtube.com/watch?v=VdXwxbbmRwg [^13_15]: https://www.youtube.com/watch?v=jTrt-r_z6Fs [^13_16]: http://resonanceswavesandfields.blogspot.com/2008/02/complex-phasor-represe… [^13_17]: https://www.quera.com/glossary/quantum-interference [^13_18]: https://eepower.com/power-electronics-textbook/vol-i-electrical-power-syste… [^13_19]: https://www.physics.purdue.edu/~jones105/phys42200_Spring2013/notes/Phys422… [^13_20]: https://www.reddit.com/r/QuantumPhysics/comments/ore9ug/what_is_quantum_int… --- # i first learned about quantum interference using photons -- light's path is determined by constructive interference of all paths. But light has inherent phase and cancellation already, so it is quite intuitive. Don't other objects also have wavelength and phase -- is that what you mean, that everything is both a particle and a wave? So for example, maybe all matter vibrates in some manner (probably for real in however matter exists) -- and this gives it the phase you are discussing? Yes—very broadly, that is the idea: quantum objects other than light also have wave-like behavior, including wavelength and phase, and that is why electrons, neutrons, atoms, and even larger molecules can show interference. But the safest refinement is that the phase being discussed is usually the phase of a quantum amplitude or wavefunction, not necessarily a literal little piece of matter physically wobbling back and forth in ordinary space the way a violin string does.[^14_1][^14_2][^14_3][^14_4][^14_5] ## Matter waves De Broglie proposed that particles of matter have an associated wavelength, with wavelength inversely related to momentum. That is why matter can produce diffraction and interference patterns, just as light does.[^14_6][^14_3][^14_7][^14_5][^14_1] So when people say “everything is both a particle and a wave,” they usually mean that quantum objects show localized detection events like particles but evolve and interfere according to wave-like rules. That is close to your picture.[^14_8][^14_4] ## What phase is here For matter, the relevant phase is usually the phase of the wavefunction or quantum amplitude. When two possible paths contribute to the same outcome, their phases determine whether the amplitudes reinforce or cancel.[^14_2][^14_9][^14_4] That is why your photon intuition transfers well to electrons and other matter. The underlying interference logic is similar even though the object is not a photon.[^14_4][^14_5][^14_6][^14_1] ## Is matter literally vibrating “Everything vibrates” is a useful intuition sometimes, but it does not seem like the best literal statement of standard quantum mechanics. The phase is not usually introduced as a mechanical shaking of matter through space, but as part of the mathematical object whose evolution predicts interference and other quantum effects.[^14_2][^14_4] A good compromise picture is: a moving quantum object carries an evolving phase pattern, and that phase pattern is what lets different paths interfere. That gives you something wave-like without forcing the image of tiny beads physically sloshing around like classical material jelly.[^14_3][^14_9][^14_4][^14_2] ## Why complex numbers enter Complex numbers are helpful because phase is most naturally represented as an angle, and $e^{i\theta}$ keeps track of that angle cleanly. Once you do that, combining paths becomes ordinary addition of amplitudes, and interference falls out of the relative phases.[^14_9][^14_10][^14_11][^14_4] So in this context, complex notation is not only an engineer’s shorthand like AC phasors, though that is a very good analogy. It is also the standard language for tracking the phase information that quantum interference seems to depend on.[^14_12][^14_13][^14_4] ## One example Electrons passing through closely spaced slits build up an interference pattern, which is strong evidence that matter has wave-like behavior with meaningful phase relations. In that sense, yes: the phase I mean for matter is the same kind of “thing that can interfere” that first felt intuitive to you from light.[^14_5][^14_1][^14_4] The next clean step is probably this question: what exactly is a wavefunction, if it is not just a literal material wave? <span style="display:none">[^14_14][^14_15][^14_16][^14_17][^14_18][^14_19]</span> <div align="center">⁂</div> [^14_1]: https://phys.libretexts.org/Bookshelves/University_Physics/University_Physi… [^14_2]: https://en.wikipedia.org/wiki/Matter_wave [^14_3]: https://chemed.chem.purdue.edu/genchem/history/debroglie.html [^14_4]: https://www.spinquanta.com/news-detail/exploring-quantum-interference-key-c… [^14_5]: https://wisc.pb.unizin.org/minimisgenchem/chapter/debroglie-intro-to-qm-qua… [^14_6]: https://scienceready.com.au/pages/matter-wave-duality [^14_7]: https://chem.libretexts.org/Courses/Howard_University/General_Chemistry:_An… [^14_8]: https://www.ebsco.com/research-starters/history/de-broglie-explains-wave-pa… [^14_9]: https://profmattstrassler.com/2025/03/18/quantum-interference-1-a-simple-ex… [^14_10]: https://mathforquantum.quantumtinkerer.tudelft.nl/1_complex_numbers/ [^14_11]: https://www.sciencedirect.com/science/article/pii/S0375960123004292 [^14_12]: https://eng.libretexts.org/Workbench/Introduction_to_Circuit_Analysis/08:_A… [^14_13]: https://philsci-archive.pitt.edu/20971/1/phase 2022 v999.pdf [^14_14]: https://www.youtube.com/watch?v=YQNEziGyDxU [^14_15]: https://www.emergentmind.com/topics/de-broglie-wavelength [^14_16]: https://pmc.ncbi.nlm.nih.gov/articles/PMC10074114/ [^14_17]: https://pressbooks.bccampus.ca/bcitphys8400/chapter/2-5-de-broglies-matter-… [^14_18]: https://www.facebook.com/groups/573055740141524/posts/1917223125724772/ [^14_19]: https://www.reddit.com/r/AskPhysics/comments/1fc25ij/if_particles_are_waves… --- # Do I have the below correctly? - it sounds like the phase is of waves developing over time - it sounds like global phase changes might be measurable in interference patterns somewhere rarely Almost. A better version would be:[^15_1][^15_2] - The phase is usually the angle of a wave or wavefunction, and it often changes with time and position as the state evolves.[^15_3][^15_1] - Global phase changes are usually not measurable; interference patterns depend on **relative** phase differences, not on multiplying the entire state by one common phase factor.[^15_4][^15_5] ## First bullet Yes, it is reasonable to think of phase as something that develops as waves or wavefunctions evolve over time. For energy eigenstates, time evolution typically adds a phase factor like $e^{-iEt/\hbar}$, so time evolution and phase are tightly linked.[^15_6][^15_1] It is also worth adding that phase can vary with position, not just time. That is why people talk about local phase, wave number, and interference between paths.[^15_2][^15_3] ## Second bullet This one needs correction: ordinary quantum theory says a pure global phase is not observable. If you multiply the whole wavefunction by the same unit complex number everywhere, all measurable probabilities and interference outcomes stay the same.[^15_5][^15_7][^15_4] What **is** measurable is relative phase, meaning the phase difference between components, paths, or locations. That relative phase is what shifts interference fringes or changes constructive versus destructive interference.[^15_7][^15_8][^15_4] ## Safer phrasing A compact corrected version is: - Phase is the angle part of a complex wave amplitude, and it often evolves across time and space.[^15_1][^15_3] - Only phase differences seem to affect interference; one common overall phase does not.[^15_4][^15_5] A nice analogy is clocks: if every clock in the universe is advanced by the same amount, nothing relational changes, but if one clock is shifted relative to another, you can detect that.[^15_7] <span style="display:none">[^15_10][^15_11][^15_12][^15_13][^15_14][^15_15][^15_9]</span> <div align="center">⁂</div> [^15_1]: https://ocw.mit.edu/courses/22-02-introduction-to-applied-nuclear-physics-s… [^15_2]: https://en.wikipedia.org/wiki/Wave_function [^15_3]: https://phys.libretexts.org/Bookshelves/Quantum_Mechanics/Introductory_Quan… [^15_4]: https://waf-www-bohrium-com-hngfcxduded0fmhr.a03.azurefd.net/en/sciencepedi… [^15_5]: https://wow.net.tr/en/the-quantum-state-s-global-phase-is-physically-unobse… [^15_6]: https://vergil.chemistry.gatech.edu/static/content/time_evolution.pdf [^15_7]: https://www.reddit.com/r/3Blue1Brown/comments/w4igqr/having_hard_time_under… [^15_8]: https://www.nature.com/articles/s41598-017-02540-3 [^15_9]: https://www.youtube.com/watch?v=zkVEQ7ISJWU [^15_10]: https://www.youtube.com/watch?v=kG92ALyMA2c [^15_11]: https://pmc.ncbi.nlm.nih.gov/articles/PMC7517448/ [^15_12]: https://arxiv.org/pdf/1407.3438.pdf [^15_13]: https://www.academia.edu/142967589/Local_phase_offset_as_a_classical_altern… [^15_14]: https://peeterjoot.com/archives/math2010/liboff314.pdf [^15_15]: https://www.academia.edu/109334023/Why_the_global_phase_is_not_real --- # but if a photon has its E field big vs its magnetic field, isn't it measured differently e'g. by a field probe? we could e.g. chart it over time on a scope. You can measure the electric field and magnetic field separately, yes. But for a simple traveling electromagnetic plane wave in vacuum, they are not “taking turns” being large; they are in phase, perpendicular, and their magnitudes are locked by a fixed ratio $E=cB$ in vacuum.[^16_1][^16_2][^16_3][^16_4] ## What a probe sees An electric-field probe responds to $E$, and a magnetic-field probe responds to $B$, so they are measuring different parts of the same electromagnetic field. If you plotted both for a traveling wave, the peaks and zero-crossings line up in time; one is not big when the other is small in the usual far-field plane-wave case.[^16_5][^16_6][^16_3][^16_7][^16_1] ## Where the confusion comes from It is easy to imagine EM waves like a spring swapping kinetic and potential energy, but a propagating plane EM wave in free space does not behave that way in the simple “E big then B big” sense. In that case, the energy flow is described by the Poynting vector, and because $E$ and $H$ are in phase for a single plane wave, the energy flux does not vanish and reappear just because one field lags the other.[^16_2][^16_3][^16_7] ## The phase we were discussing The “phase” from the quantum/gauge discussion is usually not the phase difference between classical $E$ and $B$. It is more like the phase angle of a complex amplitude or wavefunction, where relative phase between alternatives affects interference, while a single overall global phase does not.[^16_8][^16_9][^16_10] So if you are thinking about a photon as classical light, the oscillation of the EM field is a good intuition for wave behavior. But the global-phase statement was about the quantum state description, not about whether a field probe can tell electric from magnetic field strength at a given time.[^16_3][^16_9][^16_10][^16_1] ## Important exception There are more complicated situations, such as near fields or standing waves, where the relation between electric and magnetic fields is not the simple far-field plane-wave picture. So your instinct that “different probes can see different behavior” is reasonable, but for the standard propagating light wave in vacuum, the clean textbook statement is that $E$ and $B$ are in phase and tied together.[^16_11][^16_2][^16_3] A compact correction is: - Yes, $E$ and $B$ can be measured separately.[^16_6][^16_1] - No, in a simple traveling light wave they are not usually “one large while the other is small.”[^16_2][^16_3] - And the global phase issue belongs to the quantum amplitude, not to an $E$-versus-$B$ phase offset.[^16_9][^16_10] Would you like me to contrast three different “phases” that are getting mixed together here: classical wave phase, relative quantum phase, and global quantum phase? <span style="display:none">[^16_12][^16_13][^16_14][^16_15][^16_16][^16_17][^16_18]</span> <div align="center">⁂</div> [^16_1]: https://phys.libretexts.org/Bookshelves/University_Physics/University_Physi… [^16_2]: https://en.wikipedia.org/wiki/Poynting_vector [^16_3]: https://web.mit.edu/8.02t/www/802TEAL3D/visualizations/coursenotes/modules/… [^16_4]: https://www.youtube.com/watch?v=h4S4eHdwUL0 [^16_5]: https://web2.ph.utexas.edu/~vadim/Classes/2024s-g/planewaves.pdf [^16_6]: https://innovationspace.ansys.com/courses/wp-content/uploads/2020/05/Electr… [^16_7]: https://digitalcommons.uri.edu/cgi/viewcontent.cgi?article=1035\&context=ph… [^16_8]: https://quantummechanics.ucsd.edu/ph130a/130_notes/node30.html [^16_9]: https://wow.net.tr/en/the-quantum-state-s-global-phase-is-physically-unobse… [^16_10]: https://waf-www-bohrium-com-hngfcxduded0fmhr.a03.azurefd.net/en/sciencepedi… [^16_11]: https://ui.adsabs.harvard.edu/abs/2023EJPh...44e5203M/abstract [^16_12]: https://www.sciencedirect.com/science/article/abs/pii/S0030402621014431 [^16_13]: https://www.reddit.com/r/askscience/comments/8avpe0/when_an_em_wave_travels… [^16_14]: https://www.reddit.com/r/askscience/comments/274md5/are_the_electric_and_ma… [^16_15]: https://farside.ph.utexas.edu/teaching/315/Waves/node46.html [^16_16]: https://www.youtube.com/watch?v=G6QryRlKbR4 [^16_17]: https://www.youtube.com/watch?v=uuA0WMGgbkQ [^16_18]: https://tsymbal.unl.edu/sites/unl.edu.cas.physics.tsymbal/files/media/file/… --- # you are saying that the EM fields are different from the quantum wave that is the photon, and photons then are capable of interfering in a 3rd domain ...? noting that electrons are detectable via charge as well, can their wavefunction be measured directly or only via their ability to interfere? Not quite a third domain. A better picture is that classical electromagnetic fields and photon states are two related descriptions of the same underlying electromagnetic quantum theory, with the classical field emerging as an appropriate field description and the photon being a quantized excitation or state of that field.[^17_1] ## Photon and EM field For a propagating photon, people can introduce a photon wavefunction in momentum space, and there are also coordinate-space descriptions tied to the classical electromagnetic field, so these are connected rather than unrelated objects. The interference you know from light is therefore not “EM in one domain, photon in another domain, plus a third interference domain,” but different levels of description of wave-like quantum behavior.[^17_1] A useful correction is: the classical $E$ and $B$ fields describe measurable field strengths, while the photon’s quantum state describes amplitudes for quantum outcomes. Those are different kinds of objects, but not different universes of physics.[^17_1] ## Electron wavefunction For electrons, the wavefunction is not usually measured the same way a probe measures charge, voltage, or a classical electric field at one point. Standard measurements usually give things like position, momentum, current, energy, or other observables, and the wavefunction is then inferred from ensembles, interference, or reconstruction methods.[^17_2][^17_3] So “only via interference” is too narrow, but “directly like a scope trace” is also not the standard picture. Interference is one major route because phase information shows up there, and tomography or related protocols can reconstruct the state more fully.[^17_4][^17_5][^17_2] ## Direct measurement? There are published methods explicitly described as direct measurement of the quantum wavefunction, including weak-measurement and strong-measurement approaches. In those methods, the real and imaginary parts of the wavefunction are recovered from carefully designed sequential measurements rather than from a single ordinary detector reading.[^17_5][^17_6][^17_7] So the answer is: the wavefunction does not seem directly observable in the same plain sense as charge on an electrometer, but it can be reconstructed, and in some research literature that reconstruction is called “direct measurement.” For electrons specifically, closely related reconstruction methods also recover density matrices using interferometric measurements.[^17_6][^17_2][^17_5] ## Clean mental model A good mental model is that charge detection tells you where or how the electron was caught, while the wavefunction is the complex state assignment whose magnitude and phase govern the statistics of many such catches. You do not usually read off the whole wavefunction from one event; you build it from structured measurements across many similarly prepared systems.[^17_2][^17_4][^17_5] The shortest correction to your sentence is: photons and electrons both have quantum states with phase information, classical fields are related but not identical objects, and interference is one of the main ways that phase becomes experimentally accessible.[^17_5][^17_2][^17_1] <span style="display:none">[^17_10][^17_11][^17_12][^17_13][^17_14][^17_15][^17_8][^17_9]</span> <div align="center">⁂</div> [^17_1]: https://pmc.ncbi.nlm.nih.gov/articles/PMC12309449/ [^17_2]: https://www.nature.com/articles/s41598-025-86701-9 [^17_3]: https://www.sciencedirect.com/topics/physics-and-astronomy/quantum-tomograp… [^17_4]: https://research.physics.illinois.edu/QI/Photonics/tomography-files/amo_tom… [^17_5]: https://arxiv.org/abs/1112.3575 [^17_6]: https://link.aps.org/doi/10.1103/PhysRevLett.116.040502 [^17_7]: https://mwolf.pracownicy.uksw.edu.pl/MK/Lundeen et al Direct measurement of the quantum wavefunction nature10120.pdf [^17_8]: https://www.reddit.com/r/AskPhysics/comments/q03l0h/how_does_em_interferenc… [^17_9]: https://www.facebook.com/groups/1685507044817357/posts/25398605099747552/ [^17_10]: https://link.aps.org/doi/10.1103/PhysRevA.104.042403 [^17_11]: https://arxiv.org/abs/1504.06551 [^17_12]: https://ui.adsabs.harvard.edu/abs/2014Natur.514..603J/abstract [^17_13]: https://authors.library.caltech.edu/records/4jtz5-8qw63/files/FiO-2012-FTh4… [^17_14]: https://ui.adsabs.harvard.edu/abs/2016PhRvL.116d0502V/abstract [^17_15]: https://www.reddit.com/r/Physics/comments/1ozxqm7/direct_measurement_of_wav… --- # can you clarify whether the measurement of the wavefunction of a photon is a linear function if its fields? if it is possible to reconstruct the wavefunction difectly does that mean that when you say global phase you mean a theoretical phase rather than a measured one, or are things different if it is measured, or...? Mostly yes on the first question: in common single-photon formalisms, the photon wavefunction is built from the electromagnetic field in a **linear** way, typically from transverse combinations of $E$ and $B$ or equivalent vector-potential data, rather than as some nonlinear function of the fields. But that does not mean “the wavefunction is just whatever an $E$-probe reads off on a scope,” because the wavefunction is a state object, while $E$ and $B$ are field observables and their relation depends on the formalism being used.[^18_1][^18_2][^18_3] ## Photon relation One standard construction combines the electric and magnetic fields into complex field combinations that satisfy a Schrödinger-like equation for the photon, so the connection is close and mathematically structured. In that sense, it is reasonable to say the photon wavefunction is linearly related to the classical field description, especially for transverse free-field parts, but it is not simply a third unrelated thing living in a separate domain.[^18_2][^18_1] ## Global phase When I say “global phase,” I mean a phase factor multiplying the entire quantum state, such as $\psi \to e^{i\phi}\psi$. In standard quantum mechanics, that overall phase is still regarded as unobservable, because measurable probabilities are unchanged by that common factor.[^18_4][^18_5] So even if you use a protocol advertised as “direct measurement of the wavefunction,” that does not usually promote global phase into an ordinary observable. Direct-measurement and tomography methods recover the wavefunction in a chosen representation from carefully structured measurements, but the physically meaningful phase is still phase relative to some reference, basis component, path, or calibration choice.[^18_6][^18_7][^18_5] ## What direct means “Direct measurement” in this literature means the real and imaginary parts can be operationally extracted from sequential measurements rather than inferred only by a large tomographic inversion. It does not usually mean that every mathematical feature of the state, including arbitrary overall phase convention, becomes separately observable in one shot like voltage on an oscilloscope.[^18_7][^18_5][^18_6] A good way to say it is: direct measurement can recover a wavefunction up to the usual physically irrelevant global phase, while relative phase is what shows up in interference and other measurable effects.[^18_5][^18_4][^18_6] ## Short version So your two questions fit together like this: the photon wavefunction can be represented linearly from EM-field data in some formalisms, and yes, “global phase” is still mainly a theoretical redundancy rather than a newly measurable quantity just because a wavefunction reconstruction method exists. What measurements actually pin down are amplitudes and phases relative to chosen references, not an absolute universal phase label on the whole state.[^18_1][^18_4][^18_6][^18_7][^18_5] The next natural distinction to make is between absolute phase, relative phase, and phase referenced to an external oscillator or local oscillator. <span style="display:none">[^18_10][^18_11][^18_12][^18_13][^18_14][^18_8][^18_9]</span> <div align="center">⁂</div> [^18_1]: https://pmc.ncbi.nlm.nih.gov/articles/PMC12309449/ [^18_2]: https://www.nist.gov/document/photon-wavepdf [^18_3]: https://en.wikipedia.org/wiki/Quantization_of_the_electromagnetic_field [^18_4]: https://www.reddit.com/r/AskPhysics/comments/15ge2sv/why_is_the_phase_of_a_… [^18_5]: https://en.wikipedia.org/wiki/Wave_function [^18_6]: https://arxiv.org/abs/1112.3575 [^18_7]: https://mwolf.pracownicy.uksw.edu.pl/MK/Lundeen et al Direct measurement of the quantum wavefunction nature10120.pdf [^18_8]: http://web1.eng.famu.fsu.edu/~dommelen/quantum/style_a/phwav.html [^18_9]: https://arxiv.org/pdf/2507.16597.pdf [^18_10]: https://arxiv.org/pdf/2310.07366.pdf [^18_11]: https://readingfeynman.org/2016/03/12/the-photon-wavefunction-2/ [^18_12]: https://philsci-archive.pitt.edu/20971/1/phase 2022 v999.pdf [^18_13]: https://www.iap.tu-darmstadt.de/media/iap_tqp/pdf_publikationen/publikation… [^18_14]: https://www.quantamagazine.org/physicists-observe-unobservable-quantum-phas… --- # are you then saying that we can "define" the photon wave function as having a "zero phase" at any point on an E B sinusoid and the results are the same? don't non-classical photons also show E B sinusoids? Not quite. You can choose an arbitrary **reference** for phase, but that is not the same as saying “any point on the $E,B$ sinusoid is physically identical in every sense.”[^19_1][^19_2][^19_3] ## Two phase ideas There are two different things getting mixed together here: the carrier phase of a classical-looking electromagnetic field, and the global phase of a quantum state vector. For an isolated monochromatic field, choosing where you call phase zero is often just choosing a time origin or reference oscillator, but relative phase between two fields or two paths is measurable and changes interference.[^19_2][^19_3][^19_4] ## Classical-like light For coherent states of light, the expectation value of the electric field behaves like a classical sinusoid with a well-defined amplitude and phase, so talking about the phase of the $E$-field waveform makes sense. In that case, shifting the whole sinusoid by a common phase is often just a reference choice until you compare it to another field or a clock.[^19_5][^19_6][^19_2] ## Single photons Nonclassical photons do not always come with a definite classical $E,B$ sinusoid. In a single-photon Fock state, the expectation value of the electric field vanishes, and similarly the field does not carry a well-defined optical phase in the classical sense.[^19_7][^19_8][^19_1] So a “single photon” is not usually “a tiny classical sinusoidal EM wave with one cycle’s worth of stuff in it.” What remains meaningful are the quantum state, the mode structure, and measurable correlations or interference effects, rather than a single deterministic classical field trace.[^19_9][^19_10][^19_3][^19_7] ## Your proposed sentence A safer rewording would be: “For classical or coherent light, I can choose a phase reference for the sinusoid, but for a single-photon number state there is no definite classical field phase to pin down that way.” And separately: “Multiplying the entire photon state by one overall global phase leaves the physical predictions unchanged.”[^19_11][^19_12][^19_1][^19_2] ## Short intuition Coherent light is the case that most resembles an actual measurable sinusoid on a scope. A single-photon Fock state is more like a quantum excitation whose interference behavior is real, but whose mean field does not itself look like a classical oscillating $E,B$ waveform.[^19_8][^19_6][^19_7][^19_1][^19_2] The next sharp distinction to make is between a single-photon **Fock** state and a single-photon **wavepacket**, because that is exactly where “no classical sinusoid” and “still has a mode and can interfere” meet. <span style="display:none">[^19_13][^19_14][^19_15][^19_16][^19_17][^19_18][^19_19][^19_20][^19_21][^19_22][^19_23][^19_24][^19_25][^19_26][^19_27][^19_28][^19_29][^19_30][^19_31]</span> <div align="center">⁂</div> [^19_1]: https://www.rp-photonics.com/fock_states.html [^19_2]: https://www.ptb.de/cms/fileadmin/internet/institute/fpm/Quantenoptik19/lect… [^19_3]: https://sites.unimi.it/aqm/wp-content/uploads/Quantum-to-Classical-Transiti… [^19_4]: https://link.aps.org/doi/10.1103/PhysRevA.72.023820 [^19_5]: https://en.wikipedia.org/wiki/Coherent_state [^19_6]: https://bingweb.binghamton.edu/~suzuki/QuantumMechanicsFiles/16-1_Coherent_… [^19_7]: https://www.lkv.uni-rostock.de/storages/uni-rostock/Alle_MNF/Physik_Qms/Leh… [^19_8]: https://stem.elearning.unipd.it/mod/resource/view.php?id=268863 [^19_9]: https://pmc.ncbi.nlm.nih.gov/articles/PMC12309449/ [^19_10]: https://arxiv.org/pdf/0708.0831.pdf [^19_11]: https://en.wikipedia.org/wiki/Wave_function [^19_12]: https://www.reddit.com/r/AskPhysics/comments/15ge2sv/why_is_the_phase_of_a_… [^19_13]: https://www.reddit.com/r/AskPhysics/comments/tefoet/if_the_electric_field_o… [^19_14]: https://www.semanticscholar.org/paper/The-electric-field-of-a-single-photon… [^19_15]: https://arxiv.org/pdf/2109.06472.pdf [^19_16]: https://www.youtube.com/watch?v=Af21Phsu9MU [^19_17]: http://web1.eng.famu.fsu.edu/~dommelen/quantum/style_a/phwav.html [^19_18]: https://phys.libretexts.org/Bookshelves/Quantum_Mechanics/Introduction_to_t… [^19_19]: https://www.physicsforums.com/threads/electric-magnetic-field-of-a-photon-i… [^19_20]: https://www.nist.gov/document/photon-wavepdf [^19_21]: https://ocw.mit.edu/courses/22-51-quantum-theory-of-radiation-interactions-… [^19_22]: https://en.wikipedia.org/wiki/Fock_state [^19_23]: https://phys.libretexts.org/Bookshelves/Quantum_Mechanics/Quantum_Mechanics… [^19_24]: https://engineering.purdue.edu/wcchew/ece604s20/Lecture Notes/Lect39.pdf [^19_25]: https://ui.adsabs.harvard.edu/abs/2004Sci...306..660Z/abstract [^19_26]: https://homepage.univie.ac.at/reinhold.bertlmann/pdfs/T2_Skript_Ch_5.pdf [^19_27]: https://bohr.physics.berkeley.edu/classes/221/1112/notes/harmosc.pdf [^19_28]: https://fiveable.me/quantum-optics/unit-2/fock-states-coherent-states/study… [^19_29]: https://arxiv.org/abs/quant-ph/0508094 [^19_30]: https://www.reed.edu/physics/faculty/wheeler/documents/Quantum Mechanics/Miscellaneous Essays/Oscillator-Coherent States.pdf [^19_31]: https://www.scribd.com/document/874580263/fock-state --- # it sounds like you would rather i say the EM fields are measurable results of the quantum behavior of a photon? what are the equations that define a photon? Saying “$E$ and $B$ are measurable results of the underlying quantum EM field” is closer to standard language than “distinct from the photon wave.” In quantum electrodynamics, there is one electromagnetic field; photons are its quantized excitations, and $E$ and $B$ become operator-valued fields whose expectation values reproduce classical behavior in suitable states.[^20_1][^20_2][^20_3] ## What equations define a photon There is no single short “photon equation” like Schrödinger’s equation for a nonrelativistic particle. Instead, a photon is defined within these structures:[^20_2][^20_1] - **Classical level:** Maxwell’s equations for the electromagnetic field, which describe how $E$ and $B$ evolve and propagate.[^20_4][^20_5] - **Quantization:** The EM field is expanded in modes (Fourier modes, polarization states), and each mode is treated as a harmonic oscillator.[^20_6][^20_7][^20_1] - **Operators:** For each mode $k,\lambda$ (wavevector and polarization), one introduces creation and annihilation operators $a_{k,\lambda}^\dagger$ and $a_{k,\lambda}$ obeying $[a_{k,\lambda},a_{k',\lambda'}^\dagger]=\delta_{kk'}\delta_{\lambda\lambda'}$.[^20_3][^20_6] - **Photon states:** A one‑photon state in that mode is $|1_{k,\lambda}\rangle = a_{k,\lambda}^\dagger|0\rangle$; more general photon states are superpositions and wavepackets built from these. [^20_1][^20_8][^20_6] Key relations often stated for a photon are: zero rest mass, energy $E=\hbar\omega$, momentum $\mathbf{p}=\hbar\mathbf{k}$, and helicity $\pm 1$. Those come from quantizing Maxwell’s field and applying relativistic energy–momentum relations for massless excitations.[^20_1][^20_2] Coordinate-space photon wavefunctions can be defined, but there are different formalisms (momentum-space wavefunction versus coordinate-space constructions from $E,B$), and they are more subtle than for massive particles.[^20_9][^20_10][^20_2] ## About phase choice Within a given description, you are free to choose an overall phase convention for a photon state: multiplying the entire state by $e^{i\phi}$ does not change any predictions. That is the global phase statement and remains true regardless of whether you reconstruct a wavefunction or not.[^20_11][^20_12] This is not the same as saying “any point on the classical sinusoid is the same.” A classical sinusoid’s phase relative to a reference can be measured (e.g., by mixing with a local oscillator), and that is a physically meaningful **relative** phase. The quantum global phase is a redundancy of the state vector description: shifting all components by the same $e^{i\phi}$ leaves all relative phases and all measurement probabilities unchanged.[^20_12][^20_13][^20_14][^20_11] So the refined statement is: - $E$ and $B$ are classical field quantities (or expectation values of their quantum operators) obeying Maxwell’s equations.[^20_7][^20_4] - Photons are quantized excitations of that same field, described by quantum states built from creation/annihilation operators.[^20_15][^20_6][^20_1] - A photon state’s global phase is convention, while relative phases between components or between different fields can be tied to measurable interference and classical field phase.[^20_13][^20_11][^20_12] <span style="display:none">[^20_16][^20_17][^20_18][^20_19]</span> <div align="center">⁂</div> [^20_1]: https://en.wikipedia.org/wiki/Quantization_of_the_electromagnetic_field [^20_2]: https://pmc.ncbi.nlm.nih.gov/articles/PMC12309449/ [^20_3]: https://bohr.physics.berkeley.edu/classes/221/1112/notes/quantemf.pdf [^20_4]: https://courses.lumenlearning.com/suny-physics/chapter/24-1-maxwells-equati… [^20_5]: https://phys.libretexts.org/Bookshelves/University_Physics/Book:_Introducto… [^20_6]: https://www.phys.ksu.edu/personal/wysin/notes/quantumEM.pdf [^20_7]: https://bingweb.binghamton.edu/~suzuki/QM_Graduate/Quantized_electromagneti… [^20_8]: https://www.reddit.com/r/askscience/comments/9lxjt0/can_someone_explain_qua… [^20_9]: http://web1.eng.famu.fsu.edu/~dommelen/quantum/style_a/phwav.html [^20_10]: https://spie.org/news/rethinking-the-photon-new-study-reframes-quantum-ligh… [^20_11]: https://en.wikipedia.org/wiki/Wave_function [^20_12]: https://www.reddit.com/r/AskPhysics/comments/15ge2sv/why_is_the_phase_of_a_… [^20_13]: https://en.wikipedia.org/wiki/Coherent_state [^20_14]: https://www.ptb.de/cms/fileadmin/internet/institute/fpm/Quantenoptik19/lect… [^20_15]: https://www.reddit.com/r/askscience/comments/2bmkrj/since_photons_are_consi… [^20_16]: https://phys.libretexts.org/Bookshelves/Quantum_Mechanics/Introduction_to_t… [^20_17]: https://quantummechanics.ucsd.edu/ph130a/130_notes/node418.html [^20_18]: https://readingfeynman.org/2016/03/12/the-photon-wavefunction-2/ [^20_19]: https://byjus.com/physics/maxwells-equations/ --- # the quantum and classical frequencies of a photon match? what relations result in that? what values fully describe the states of a photon? Yes: the “frequency” that appears in the classical sinusoidal $E,B$ fields and the “frequency” that appears in the photon energy relation $E = h f$ are the same physical quantity. The key relations are:[^21_1][^21_2][^21_3] - Classical EM: $c = \lambda f$, so wavelength $\lambda$, frequency $f$, and speed $c$ are linked.[^21_2][^21_3] - Quantum: $E = h f = \hbar\omega$, so photon energy is proportional to that same $f$ (or angular frequency $\omega = 2\pi f$).[^21_4][^21_1][^21_2] Quantizing each classical mode of the electromagnetic field as a harmonic oscillator gives evenly spaced energy levels separated by $\hbar\omega$. Those level spacings then identify $\omega$ (and thus $f$) as both the classical wave frequency and the quantum “per-photon” energy scale.[^21_5][^21_6][^21_7] ## What fully describes a photon state There is no single minimal list that covers every situation, but for a free photon in empty space a convenient choice of quantum numbers is: - **Momentum or wavevector $\mathbf{k}$:** direction and magnitude; related to wavelength by $|\mathbf{k}| = 2\pi/\lambda$. [^21_8][^21_7] - **Energy/frequency:** determined by $\omega = c|\mathbf{k}|$ and $E = \hbar\omega$, so specifying $\mathbf{k}$ also sets the energy for a free photon. [^21_1][^21_8] - **Polarization/helicity:** essentially the state of its transverse degrees of freedom, often labeled by helicity $\pm 1$ or by linear polarization basis states.[^21_9][^21_8] For a single **mode**, a number state $|n_{\mathbf{k},\lambda}\rangle$ is then fully specified by: - The mode label $(\mathbf{k}, \lambda)$, and - The photon number $n$ in that mode.[^21_8][^21_5] More general one-photon states are superpositions (wavepackets) over many $(\mathbf{k},\lambda)$, so their full description is a complex amplitude for each mode, up to the usual global phase. In that sense, “fully describe” means specifying the entire distribution of mode amplitudes (or equivalently a wavefunction in some representation) together with polarization/helicity, not just a single number like energy.[^21_7][^21_10][^21_8] So the quantum and classical frequencies match because both come from the same mode frequency $\omega$ of the electromagnetic field, and a photon is the quantum of energy $\hbar\omega$ associated with one such mode.[^21_6][^21_1][^21_7] <span style="display:none">[^21_11][^21_12][^21_13][^21_14][^21_15][^21_16][^21_17]</span> <div align="center">⁂</div> [^21_1]: https://en.wikipedia.org/wiki/Photon_energy [^21_2]: https://phys.libretexts.org/Bookshelves/College_Physics/College_Physics_1e_… [^21_3]: https://courses.lumenlearning.com/suny-physics/chapter/24-1-maxwells-equati… [^21_4]: https://spark.iop.org/frequency-and-energy [^21_5]: https://fiveable.me/modern-optics/unit-12/quantization-electromagnetic-fiel… [^21_6]: https://phys.libretexts.org/Bookshelves/Quantum_Mechanics/Introduction_to_t… [^21_7]: https://lampz.tugraz.at/~hadley/ss1/emfield/quantization_em.php [^21_8]: https://en.wikipedia.org/wiki/Quantization_of_the_electromagnetic_field [^21_9]: https://arxiv.org/pdf/1711.04680.pdf [^21_10]: http://web1.eng.famu.fsu.edu/~dommelen/quantum/style_a/phwav.html [^21_11]: https://van.physics.illinois.edu/ask/listing/1242 [^21_12]: https://www.youtube.com/watch?v=PYNSopwd1l4 [^21_13]: https://www.pearson.com/channels/calculators/wavelength-frequency-energy-co… [^21_14]: https://www.omnicalculator.com/physics/photon-energy [^21_15]: https://www.nature.com/articles/s41598-024-71203-x [^21_16]: https://research-portal.uea.ac.uk/files/196578134/Optical_helicity_of_unpol… [^21_17]: https://www.khanacademy.org/science/ap-physics-2/x0e2f5a2c:modern-physics/x… --- # Say I measure an E field from a single propagating photon using an antenna -- can I infer the states? For a genuinely single propagating photon in a number (Fock) state, a one-shot $E$-field measurement with an antenna does not let you infer the full quantum state, and even the connection to a clean sinusoidal $E(t)$ is subtle.[^22_1][^22_2][^22_3] ## What a single-photon Fock state looks like In the usual single-mode description, a one-photon Fock state $|1\rangle$ has a **well-defined photon number** but **no well-defined classical field amplitude**. [^22_4][^22_3] The expectation value $\langle E(\mathbf{r},t)\rangle$ of the electric-field operator in a Fock state is zero, even if the state contains many photons in that mode; what is nonzero are field **fluctuations** (variances, correlation functions). [^22_1][^22_3] So: - A single strong classical sinusoid is well modeled by a **coherent state**, which has a nonzero $\langle E\rangle$ and a well-defined classical phase.[^22_5][^22_6] - A one-photon number state is not a tiny coherent state; it has a definite photon count and vanishing mean field, plus characteristic quantum fluctuations.[^22_3][^22_7][^22_1] ## What an antenna measurement gives you An antenna or $E$-field sensor coupled to very weak light will register **discrete detection events** (clicks, absorbed energy), not a directly readable continuous sinusoid for a true single-photon Fock state. Each click tells you that a photon has been absorbed and gives you some information about mode structure (frequency, polarization, maybe direction), but not the full complex wavefunction.[^22_8][^22_9][^22_10][^22_1] To reconstruct a photon state (its “wavefunction” or density matrix), experiments use many identically prepared photons and techniques like homodyne detection or tomography, not one antenna trace from one photon. Those reconstructions give you mode amplitudes and phases **up to the usual global phase freedom**, not a unique absolute phase.[^22_9][^22_11][^22_12][^22_13][^22_14] ## Direct answer to your question So: - Measuring the $E$-field once from a single propagating photon does **not** generally let you infer “the” state.[^22_1][^22_3] - A single-photon Fock state does not have a classical $E,B$ sinusoid with a definite phase you can just read off.[^22_7][^22_3][^22_1] - What you can infer about the state comes from statistics of many detection events and carefully designed measurement schemes, not from a single antenna waveform.[^22_11][^22_13][^22_9] If you like, next step could be: sketch how homodyne detection uses a strong classical local oscillator plus many photons to reconstruct the field’s quadratures (and thus its quantum state) in practice. <span style="display:none">[^22_15][^22_16][^22_17][^22_18][^22_19][^22_20][^22_21][^22_22][^22_23][^22_24]</span> <div align="center">⁂</div> [^22_1]: https://www.youtube.com/watch?v=Af21Phsu9MU [^22_2]: https://www.reddit.com/r/AskPhysics/comments/tefoet/if_the_electric_field_o… [^22_3]: https://www.rp-photonics.com/fock_states.html [^22_4]: https://en.wikipedia.org/wiki/Fock_state [^22_5]: https://en.wikipedia.org/wiki/Coherent_state [^22_6]: https://www.ptb.de/cms/fileadmin/internet/institute/fpm/Quantenoptik19/lect… [^22_7]: https://www.lkv.uni-rostock.de/storages/uni-rostock/Alle_MNF/Physik_Qms/Leh… [^22_8]: https://www.mvg-world.com/en/manual/antenna-measurement-101/how-do-electrom… [^22_9]: https://lampz.tugraz.at/~hadley/ss1/emfield/quantization_em.php [^22_10]: https://phys.libretexts.org/Bookshelves/Quantum_Mechanics/Introduction_to_t… [^22_11]: https://www.nature.com/articles/s41598-025-86701-9 [^22_12]: https://research.physics.illinois.edu/QI/Photonics/tomography-files/amo_tom… [^22_13]: https://arxiv.org/abs/1112.3575 [^22_14]: https://en.wikipedia.org/wiki/Wave_function [^22_15]: https://www.reddit.com/r/AskPhysics/comments/161xu9t/what_will_you_get_if_y… [^22_16]: https://www.nist.gov/noac/technology/magnetic-and-electric-fields/electric-… [^22_17]: https://nano-optics.colorado.edu/wp-content/uploads/2020/06/Olmon_PhsRevLet… [^22_18]: https://www.physicsforums.com/threads/electric-magnetic-field-of-a-photon-i… [^22_19]: https://www.ntt-review.jp/archive/ntttechnical.php?contents=ntr200903sf5.ht… [^22_20]: https://link.aps.org/doi/10.1103/PhysRevA.108.063708 [^22_21]: https://www.scirp.org/journal/paperinformation?paperid=107462 [^22_22]: https://amolf.nl/wp-content/uploads/2019/06/1-Resonant-nanophotonics-Prof.-… [^22_23]: https://www.frontiersin.org/journals/physics/articles/10.3389/fphy.2020.590… [^22_24]: https://www.reddit.com/r/AskPhysics/comments/1d4zxz6/is_there_a_theorethica… --- # well say i make a bunch of photons all with the same classical phase using one emitter that emits them in a beam -- i can measure a classical wave here and infer more thingd about dsitributions if quantum state? how is the E field related to the quantum state? or are you saying a photon is _change patterns_ of fields, and the absolute values are different? If you make many photons in phase in the same mode, then measuring the resulting classical-looking $E$-field does let you infer useful information about the underlying quantum state—but only in the limit where you really have a bright, coherent-like field, not a sparse trickle of isolated Fock states.[^23_1][^23_2][^23_3] ## Coherent beam vs single photons - A highly populated, phase-stable beam from one emitter is well modeled by a **coherent state** $|\alpha\rangle$, which is an eigenstate of the annihilation operator $a$. [^23_2][^23_3] - In such a state, the expectation value of the electric-field operator $\hat E(\mathbf{r},t)$ behaves like a classical sinusoid with amplitude and phase determined by $\alpha$ and the mode functions.[^23_4][^23_2][^23_1] - Measuring the classical field (e.g., via homodyne detection with a local oscillator) lets you infer the **quadratures** of the field, which are directly related to the real and imaginary parts of $\alpha$ and thus to the complex amplitude of that mode.[^23_5][^23_6][^23_4] So in that bright, phase-stable regime, yes: by measuring a classical wave you can back out a lot about the quantum state’s distribution over field quadratures, typically assuming it is near a Gaussian/coherent state.[^23_6][^23_2][^23_3] ## How $\hat E$ relates to the quantum state For a quantized single mode, the electric field operator is a linear combination of the annihilation and creation operators: schematically $$ \hat E(\mathbf{r},t) \propto f(\mathbf{r},t)\, \hat a + f^*(\mathbf{r},t)\, \hat a^\dagger, $$ where $f(\mathbf{r},t)$ encodes the mode’s spatial and temporal structure.[^23_7][^23_4][^23_1] - In a coherent state $|\alpha\rangle$, $\langle \hat a\rangle = \alpha$, so $\langle \hat E\rangle$ follows a classical wave with a definite phase and amplitude. [^23_2][^23_3][^23_1] - In a number (Fock) state $|n\rangle$, $\langle \hat a\rangle = 0$, so $\langle \hat E\rangle = 0$. The field is “there” only in its fluctuations and higher moments, not in the mean. [^23_8][^23_9][^23_1] So the electric field is not separate from the quantum state; it is an operator built from the same mode operators that define photon states, and what you “see” as a classical field is essentially the expectation value of that operator in whatever state you prepared.[^23_7][^23_4][^23_1] ## Change patterns vs absolute values “Photons are change patterns of fields” is close but needs tightening.[^23_10][^23_11] - The **underlying object** is the electromagnetic field, which is quantized.[^23_1][^23_10] - A photon is a **quantized excitation of a field mode**—an energy quantum $\hbar\omega$ in that mode.[^23_12][^23_10] - In states with many quanta and a stable phase (coherent states), the field expectation values look like smooth classical waves; in pure number states, the same operators have zero mean but nontrivial quantum fluctuations.[^23_2][^23_9][^23_1] So absolute classical $E,B$ values are expectation values (or higher moments) of field operators in some quantum state, and different states (coherent vs Fock vs squeezed, etc.) give different patterns of means and fluctuations.[^23_13][^23_3][^23_2] ## Direct answer to your scenario If your emitter produces a strong, phase-stable beam: - You can treat the field as approximately coherent, measure the classical sinusoid, and infer the complex amplitude $\alpha$ of the mode (up to global phase conventions and calibration).[^23_4][^23_5][^23_2] - Those measurements tell you about the **distribution of photon numbers and quadratures** in that mode (e.g., Poissonian statistics for $|\alpha\rangle$), not about a single photon apart from the rest. [^23_2][^23_12] If you truly have a dilute sequence of single photons in a number state, the mean field vanishes and you do not get a clean classical wave to read off; then you infer the state from detection statistics and interference, not from a classical $E(t)$ trace.[^23_8][^23_9][^23_1] If you want, next step could be to write down the explicit single-mode formula $\hat E(t) \propto \hat a e^{-i\omega t} + \hat a^\dagger e^{i\omega t}$ and show how $|\alpha\rangle$ and $|n\rangle$ give different measurable behavior. <span style="display:none">[^23_14][^23_15][^23_16][^23_17][^23_18][^23_19][^23_20][^23_21]</span> <div align="center">⁂</div> [^23_1]: https://bohr.physics.berkeley.edu/classes/221/1112/notes/quantemf.pdf [^23_2]: https://en.wikipedia.org/wiki/Coherent_state [^23_3]: https://www.ptb.de/cms/fileadmin/internet/institute/fpm/Quantenoptik19/lect… [^23_4]: https://phys.libretexts.org/Bookshelves/Quantum_Mechanics/Quantum_Mechanics… [^23_5]: https://www.physics.muni.cz/~tomtyc/homodyne.pdf [^23_6]: https://academic.oup.com/ptep/article/2021/10/103A01/6364905 [^23_7]: https://www.phys.ksu.edu/personal/wysin/notes/quantumEM.pdf [^23_8]: https://www.youtube.com/watch?v=Af21Phsu9MU [^23_9]: https://www.rp-photonics.com/fock_states.html [^23_10]: https://en.wikipedia.org/wiki/Quantization_of_the_electromagnetic_field [^23_11]: https://lampz.tugraz.at/~hadley/ss1/emfield/quantization_em.php [^23_12]: https://fiveable.me/modern-optics/unit-12/quantization-electromagnetic-fiel… [^23_13]: https://www.classe.cornell.edu/~dlr/teaching/p6574/lectures/lecture20.pdf [^23_14]: https://en.wikipedia.org/wiki/Creation_and_annihilation_operators [^23_15]: https://bingweb.binghamton.edu/~suzuki/QM_Graduate/Quantized_electromagneti… [^23_16]: http://hitoshi.berkeley.edu/221B-S02/photons.pdf [^23_17]: https://web2.ph.utexas.edu/~vadim/Classes/2019f/expand.pdf [^23_18]: https://arxiv.org/pdf/1711.03713.pdf [^23_19]: https://www.physics.purdue.edu/~clarkt/Courses/Physics662/ps/qftch11.pdf [^23_20]: https://bingweb.binghamton.edu/~suzuki/QuantumMechanicsII/14-1_Quantized_el… [^23_21]: https://www.reddit.com/r/askscience/comments/9lxjt0/can_someone_explain_qua… --- # are you saying f(r,t) = exp(i omega t) here? this must be where global phase is, in the definition of the spatial and temporal structure, and it does indeed align with a classical phase offset? For a single plane-wave mode, $f(\mathbf{r},t)$ is usually something like a spatial mode function times a time factor $e^{-i\omega t}$, e.g. $f(\mathbf{r},t) \propto e^{i\mathbf{k}\cdot\mathbf{r}}e^{-i\omega t}$. So yes, the $e^{-i\omega t}$ (and any overall constant phase in $f$) is exactly where the **classical carrier phase** lives in that simple model.[^24_1][^24_2][^24_3][^24_4] ## Where global phase shows up 1. **In the mode function $f(\mathbf{r},t)$:** - You can multiply $f$ by a constant phase $e^{i\phi_0}$ and compensate by redefining the annihilation operator $\hat a \to e^{-i\phi_0}\hat a$, leaving all physics unchanged.[^24_2][^24_5] - That overall constant phase of the mode is just like a classical phase offset of the sinusoid; it only matters when compared to some reference (another mode, a local oscillator, etc.).[^24_6][^24_7] 2. **In the quantum state $|\psi\rangle$:** - Multiplying the entire state by an overall factor $e^{i\Phi}$ leaves all probabilities and expectation values of observables unchanged.[^24_8][^24_9] - That is the usual “global phase of the state is unobservable” statement.[^24_9][^24_8] You can choose conventions so that the mode function has a particular phase (e.g. $f\propto e^{i\mathbf{k}\cdot\mathbf{r}}e^{-i\omega t}$ with no extra constant), and then the phase of a coherent state’s amplitude $\alpha$ lines up with the classical phase of the field you measure. In that gauge, the **classical phase offset** you see in the sinusoidal expectation value $\langle \hat E(\mathbf{r},t)\rangle$ is directly the argument of $\alpha$ plus the phase of $f$, i.e. it behaves just like a classical phase shift.[^24_3][^24_10][^24_2][^24_6] So the safest summary is: - $f(\mathbf{r},t)$ carries the **mode’s classical phase structure** (including any carrier phase offset).[^24_1][^24_2] - The **global phase of the quantum state vector** is an extra redundancy on top of that and does not change observables.[^24_8][^24_9] - When you fix a convention for $f$, the phase of a coherent state’s amplitude $\alpha$ corresponds to the classical phase of the field, matching your intuition about a classical phase offset.[^24_10][^24_3][^24_6] <div align="center">⁂</div> [^24_1]: https://www.phys.ksu.edu/personal/wysin/notes/quantumEM.pdf [^24_2]: https://phys.libretexts.org/Bookshelves/Quantum_Mechanics/Quantum_Mechanics… [^24_3]: https://bohr.physics.berkeley.edu/classes/221/1112/notes/quantemf.pdf [^24_4]: https://bingweb.binghamton.edu/~suzuki/QuantumMechanicsII/14-1_Quantized_el… [^24_5]: https://web2.ph.utexas.edu/~vadim/Classes/2019f/expand.pdf [^24_6]: https://en.wikipedia.org/wiki/Coherent_state [^24_7]: https://www.physics.muni.cz/~tomtyc/homodyne.pdf [^24_8]: https://en.wikipedia.org/wiki/Wave_function [^24_9]: https://www.reddit.com/r/AskPhysics/comments/15ge2sv/why_is_the_phase_of_a_… [^24_10]: https://www.ptb.de/cms/fileadmin/internet/institute/fpm/Quantenoptik19/lect… --- # so people would almost always use a simple global phase like 0 degrees? Yes. In almost all practical calculations, people fix a convention that effectively sets the global phase to something simple, usually “zero” in the sense of: - Choosing mode functions $f(\mathbf{r},t)$ without an extra constant phase factor.[^25_1][^25_2] - Treating overall $e^{i\Phi}$ factors on the state as irrelevant and dropping them.[^25_3][^25_4] What actually matters are **relative** phases: between different components of a state, between different paths, or between the field and a reference (local oscillator, another beam, etc.). Those are the phase differences that show up in interference and in measured quadratures, so that is where people keep careful track, and global state phase is usually set to a convenient zero by convention.[^25_4][^25_5][^25_6][^25_7][^25_3] <div align="center">⁂</div> [^25_1]: https://phys.libretexts.org/Bookshelves/Quantum_Mechanics/Quantum_Mechanics… [^25_2]: https://www.phys.ksu.edu/personal/wysin/notes/quantumEM.pdf [^25_3]: https://en.wikipedia.org/wiki/Wave_function [^25_4]: https://www.reddit.com/r/AskPhysics/comments/15ge2sv/why_is_the_phase_of_a_… [^25_5]: https://waf-www-bohrium-com-hngfcxduded0fmhr.a03.azurefd.net/en/sciencepedi… [^25_6]: https://en.wikipedia.org/wiki/Coherent_state [^25_7]: https://www.physics.muni.cz/~tomtyc/homodyne.pdf --- # it sounds like there are situations where a sinplifying phase in one part is complicating in another and vice versa? Yes. Choosing a phase convention can simplify one part of a calculation while making another part look more complicated. ## Where this happens - **Single mode vs many modes:** Setting the phase of one mode’s $f(\mathbf{r},t)$ to be real and simple can force another coupled mode to carry an awkward extra phase factor.[^26_1][^26_2] - **Interaction terms:** In interacting theories, a phase choice that makes one field’s kinetic term look clean can push nontrivial phases into coupling constants or mass terms.[^26_2][^26_3] - **Interference setups:** In interferometry or homodyne detection, defining the local oscillator’s phase as zero makes the signal’s phase appear directly, but if you re-phase the signal instead, the LO’s phase looks messy.[^26_4][^26_5] ## Why it is okay All these different choices are related by unitary (phase) transformations and give the same physical predictions; they just shuffle where the phase factors live in the algebra. The usual strategy is to pick a convention that makes the part you care about (e.g., the interaction, the measured mode, the LO) as simple as possible, accepting extra phase clutter elsewhere.[^26_6][^26_7][^26_1][^26_4] <div align="center">⁂</div> [^26_1]: https://phys.libretexts.org/Bookshelves/Quantum_Mechanics/Quantum_Mechanics… [^26_2]: https://web2.ph.utexas.edu/~vadim/Classes/2019f/expand.pdf [^26_3]: https://www.physics.purdue.edu/~clarkt/Courses/Physics662/ps/qftch11.pdf [^26_4]: https://www.physics.muni.cz/~tomtyc/homodyne.pdf [^26_5]: https://academic.oup.com/ptep/article/2021/10/103A01/6364905 [^26_6]: https://en.wikipedia.org/wiki/Wave_function [^26_7]: https://www.reddit.com/r/AskPhysics/comments/15ge2sv/why_is_the_phase_of_a_… --- # so quantum physics defines everything as sinusoidal creation and destruction waves which then produce classical waves via their interference? Not quite. A better summary is: quantum field theory describes fields with **mode operators** that evolve sinusoidally in time, and classical waves emerge as special quantum states (like coherent states) where the **average** field behaves like a classical wave.[^27_1][^27_2][^27_3] ## What is sinusoidal For a free field, each mode behaves like a harmonic oscillator, so operators carry factors like $e^{-i\omega t}$ and $e^{i\omega t}$. That sinusoidal time dependence is built into the mode structure and is why plane waves and normal modes look sinusoidal.[^27_4][^27_5][^27_6][^27_1] ## Where classical waves come from - If the quantum state is a coherent state with many quanta in a mode, the expectation value $\langle \hat E(\mathbf{r},t)\rangle$ follows a classical sinusoidal wave with a well-defined amplitude and phase.[^27_2][^27_3][^27_7] - If the state is a number (Fock) state like a single photon, the mean field vanishes and you do **not** get a classical sinusoid directly; what you see are discrete detection events and quantum correlations.[^27_8][^27_9][^27_2] So classical waves are not produced by “creation and destruction waves interfering” so much as they are the average behavior of certain quantum states of the field.[^27_3][^27_2] ## Interference Quantum interference enters because probabilities come from squared magnitudes of **sums of complex amplitudes**, so relative phases between contributions matter. For many-photon coherent states, this quantum interference reproduces the classical interference of electromagnetic waves; for few-photon or single-photon states, it shows up as patterns in detection statistics instead of literal continuous field oscillations.[^27_6][^27_10][^27_11][^27_12] A compact reframing is: quantum fields have harmonic-mode structure; creation and annihilation operators add and remove quanta of those modes; and classical waves appear when those modes are occupied in special states whose average field matches Maxwell’s sinusoidal solutions.[^27_1][^27_2][^27_3] <div align="center">⁂</div> [^27_1]: https://en.wikipedia.org/wiki/Quantization_of_the_electromagnetic_field [^27_2]: https://bohr.physics.berkeley.edu/classes/221/1112/notes/quantemf.pdf [^27_3]: https://en.wikipedia.org/wiki/Coherent_state [^27_4]: https://www.phys.ksu.edu/personal/wysin/notes/quantumEM.pdf [^27_5]: https://courses.lumenlearning.com/suny-physics/chapter/24-1-maxwells-equati… [^27_6]: https://lampz.tugraz.at/~hadley/ss1/emfield/quantization_em.php [^27_7]: https://www.ptb.de/cms/fileadmin/internet/institute/fpm/Quantenoptik19/lect… [^27_8]: https://www.youtube.com/watch?v=Af21Phsu9MU [^27_9]: https://www.rp-photonics.com/fock_states.html [^27_10]: https://www.spinquanta.com/news-detail/exploring-quantum-interference-key-c… [^27_11]: https://profmattstrassler.com/2025/03/18/quantum-interference-1-a-simple-ex… [^27_12]: https://phys.libretexts.org/Bookshelves/Quantum_Mechanics/Introduction_to_t… --- # which is simplest particle to learn first? For getting comfortable with quantum fields, the **spin‑0 real scalar field (“scalar particle”)** is usually the simplest starting point. ## Why the real scalar field - It has no spin or polarization indices, so the field is just a single real function $\phi(x)$ instead of a vector or spinor.[^28_1][^28_2] - The free-field equation of motion is just the Klein–Gordon equation, which is a straightforward relativistic wave equation.[^28_2][^28_1] - Quantization introduces creation/annihilation operators and Fock states without extra complications from gauge symmetry or spin.[^28_3][^28_1] ## Good learning order 1. **Real scalar field (spin 0):** learn mode expansion, creation/annihilation, propagators.[^28_1][^28_2] 2. **Complex scalar field:** add charge and a U(1) phase symmetry.[^28_4][^28_5] 3. **Dirac field (spin 1/2):** learn spin and antiparticles.[^28_6][^28_7] 4. **Gauge fields (photon, Yang–Mills):** bring in gauge symmetry and self-interaction.[^28_8][^28_9] Starting with photons skips several conceptual steps (scalar, spin, charge, then gauge), so the scalar field is the cleanest first sandbox. <div align="center">⁂</div> [^28_1]: https://en.wikipedia.org/wiki/Scalar_field_theory [^28_2]: https://www.damtp.cam.ac.uk/user/tong/qft/qfthtml/S2.html [^28_3]: https://dec41.user.srcf.net/notes/III_M/quantum_field_theory.pdf [^28_4]: https://www.southampton.ac.uk/~doug/ft1/ft15.pdf [^28_5]: https://kimura.pages.math.cnrs.fr/file/QFT_lec.pdf [^28_6]: https://en.wikipedia.org/wiki/Dirac_equation [^28_7]: https://www.damtp.cam.ac.uk/user/tong/qft/qfthtml/S4.html [^28_8]: https://en.wikipedia.org/wiki/Quantization_of_the_electromagnetic_field [^28_9]: http://www.damtp.cam.ac.uk/user/tong/gaugetheory/gt.pdf --- # do scalar particles exist? are they a little like an uncharged electron? Yes, scalar particles do exist, and the cleanest confirmed example is the **Higgs boson**. The Higgs is a fundamental spin‑0 particle, so its field is a scalar field in the technical sense.[^29_1][^29_2][^29_3] ## Scalars vs “uncharged electron” An electron is not a scalar; it has spin $1/2$ and is described by a **spinor** field (the Dirac field), not a scalar field. So a scalar particle is not really “an uncharged electron,” it is more like a particle whose intrinsic degrees of freedom do not include spin orientation at all.[^29_2][^29_4][^29_5][^29_6][^29_7] Key contrasts: - **Scalar particle (e.g. Higgs):** spin 0, field value is just a number at each point (or a small multiplet of numbers), no directional spin degree of freedom.[^29_3][^29_2] - **Electron:** spin $1/2$, described by a spinor; even if you imagine it uncharged, it still has spin structure rather than a single scalar component.[^29_5][^29_6] There are also many **composite** scalar particles, like some nuclei and mesons with total spin 0, but the Higgs is the only confirmed *elementary* scalar in the Standard Model so far.[^29_8][^29_1] <span style="display:none">[^29_10][^29_11][^29_12][^29_13][^29_14][^29_15][^29_9]</span> <div align="center">⁂</div> [^29_1]: https://atlas.cern/updates/briefing/scalar-boson [^29_2]: https://en.wikipedia.org/wiki/Scalar_boson [^29_3]: https://en.wikipedia.org/wiki/Higgs_boson [^29_4]: https://profmattstrassler.com/articles-and-posts/particle-physics-basics/th… [^29_5]: https://www.zayets2physics.com/spin2_QuantumNature_16.html [^29_6]: https://en.wikipedia.org/wiki/Spinor [^29_7]: https://en.wikipedia.org/wiki/Spin_(physics) [^29_8]: https://www.reddit.com/r/askscience/comments/iquhwg/is_the_higgs_boson_the_… [^29_9]: https://www.reddit.com/r/AskPhysics/comments/1g617fl/what_is_significant_in… [^29_10]: https://www.symmetrymagazine.org/article/what-the-higgs-boson-tells-us-abou… [^29_11]: http://www.physicsmatt.com/blog/2015/6/17/and-explanation-of-the-higgs-boson [^29_12]: https://ncatlab.org/nlab/show/The+Dirac+Electron [^29_13]: https://indico.cern.ch/event/763013/contributions/3358697/attachments/18131… [^29_14]: https://www.quantamagazine.org/how-the-higgs-field-actually-gives-mass-to-e… [^29_15]: https://www.youtube.com/watch?v=2TYFNOArnYo --- # so i could theoretically make a scalar particle or something that behaved like it? what governs its behavior? Theoretically, yes: you can define a scalar field and its quanta, and you could imagine engineering something whose excitations behave *approximately* like scalar particles. What governs their behavior is very tightly specified once you choose a Lagrangian for the scalar field. ## What governs a scalar particle For a simple real scalar field $\phi(x)$, the standard free-theory Lagrangian density is $$ \mathcal{L} = \tfrac12 \partial_\mu \phi\,\partial^\mu \phi - \tfrac12 m^2 \phi^2, $$ which yields the **Klein–Gordon equation** $$ (\Box + m^2)\phi(x) = 0, $$ a relativistic wave equation for a spin‑0 field of mass $m$.[^30_1][^30_2][^30_3] Once you choose $\mathcal{L}$, all of the following are fixed: - How the field propagates (via the equation of motion).[^30_2][^30_1] - How its quanta (the scalar particles) carry energy–momentum $E^2 = p^2 + m^2$.[^30_1][^30_2] - How they interact, if you add interaction terms like $-\lambda \phi^4$ or couplings to other fields.[^30_4][^30_5] The Higgs field is a more complicated scalar with a potential $V(\phi)$ that leads to spontaneous symmetry breaking and specific couplings to gauge bosons and fermions. Its behavior is governed by the Standard Model Higgs-sector Lagrangian, not by arbitrary choice.[^30_6][^30_7][^30_8][^30_9][^30_10] ## “Making” a scalar-like excitation In practice, building a truly elementary scalar particle is not something one “designs”; the known fundamental scalar (Higgs) is produced in high-energy collisions, and beyond-Standard-Model scalars are speculative.[^30_11][^30_12] However, **effective scalar excitations** are common: - Many composite systems (e.g., certain mesons, nuclei, phonons) have spin‑0 modes that, at low energies, are well described by a scalar field theory.[^30_13][^30_14] - In condensed matter or cosmology, scalar fields are routinely used as effective descriptions of collective modes or order parameters, governed by Lagrangians similar to the one above.[^30_5][^30_13] So “something that behaves like a scalar particle” usually means a system whose low-energy degrees of freedom are captured by a scalar field with some potential $V(\phi)$; its behavior is then governed by that field’s Lagrangian and resulting equations of motion.[^30_15][^30_4][^30_5] <span style="display:none">[^30_16][^30_17][^30_18][^30_19][^30_20][^30_21]</span> <div align="center">⁂</div> [^30_1]: https://www.southampton.ac.uk/~doug/ft1/ft12.pdf [^30_2]: https://piazza.com/class_profile/get_resource/kzqnrwg17lp27d/l294p8r7oy45fq [^30_3]: https://en.wikipedia.org/wiki/Klein–Gordon_equation [^30_4]: https://piazza.com/class_profile/get_resource/jy4evrjc5f94jd/jzlkoap7wig34n [^30_5]: http://www.scholarpedia.org/article/Lagrangian_formalism_for_fields [^30_6]: https://en.wikipedia.org/wiki/Higgs_mechanism [^30_7]: https://indico.cern.ch/event/443176/attachments/1291625/1940696/Haber_Higgs… [^30_8]: https://indico.cern.ch/event/763013/contributions/3358697/attachments/18131… [^30_9]: https://en.wikipedia.org/wiki/Higgs_boson [^30_10]: https://www.quantamagazine.org/how-the-higgs-field-actually-gives-mass-to-e… [^30_11]: https://atlas.cern/updates/briefing/scalar-boson [^30_12]: https://www.reddit.com/r/askscience/comments/iquhwg/is_the_higgs_boson_the_… [^30_13]: https://www.roe.ac.uk/japwww/teaching/cos5_1112/cos5_1112b.pdf [^30_14]: https://en.wikipedia.org/wiki/Scalar_boson [^30_15]: https://www.sas.rochester.edu/pas/assets/pdf/undergraduate/the_free_klein_g… [^30_16]: https://en.wikipedia.org/wiki/Lagrangian_(field_theory) [^30_17]: https://math.uchicago.edu/~may/REU2012/REUPapers/Yu.pdf [^30_18]: https://www.youtube.com/watch?v=xy3sbXnyUfA [^30_19]: https://www.youtube.com/watch?v=T_08_GC2hlw [^30_20]: https://www.sas.rochester.edu/pas/assets/pdf/undergraduate/Introduction_to_… [^30_21]: https://www.imperial.ac.uk/media/imperial-college/research-centres-and-grou… --- # what is a Lagrangian? A **Lagrangian** is a function that packages the rules of motion of a system into one object, and in many basic mechanics problems it is $L = T - V$, meaning kinetic energy minus potential energy. The system’s actual motion is then the one that makes the action $S=\int L\,dt$ stationary, which leads to the equations of motion.[^31_1][^31_5][^31_7] ## High-level picture A good intuition is that Newton’s method starts with forces, while the Lagrangian method starts with energies. Instead of asking “what force is pushing this?,” you ask “what single energy-like formula generates the right motion?”[^31_1][^31_3][^31_7] ## Why physicists use it The Lagrangian approach handles constraints and complicated coordinates cleanly, and it generalizes very naturally from particles to fields. In field theory, one usually works with a Lagrangian density $\mathcal{L}$, whose variation gives the Euler–Lagrange field equations.[^31_1][^31_6] ## One example For a simple particle moving in a potential, the usual choice is $L=T-V$. For a simple scalar field, a standard free-field Lagrangian density is $\mathcal{L}=\tfrac12 \partial_\mu\phi\,\partial^\mu\phi - \tfrac12 m^2\phi^2$, and from that you get the Klein–Gordon equation.[^31_1][^31_6][^31_7] A very compressed slogan is: the Lagrangian is the recipe, and the equations of motion are what fall out when nature follows the stationary-action rule.[^31_1] <span style="display:none">[^31_10][^31_2][^31_4][^31_8][^31_9]</span> <div align="center">⁂</div> [^31_1]: https://en.wikipedia.org/wiki/Lagrangian_mechanics [^31_2]: https://www.reddit.com/r/Physics/comments/1ldztv5/what_is_a_lagrangian_phys… [^31_3]: https://phys.libretexts.org/Bookshelves/Classical_Mechanics/Variational_Pri…'s_Action_Principle/9.03:_Lagrangian [^31_4]: https://www.sciencedirect.com/topics/mathematics/lagrangian [^31_5]: https://fiveable.me/key-terms/principles-physics-iii-thermal-physics-waves/… [^31_6]: https://en.wikipedia.org/wiki/Lagrangian_(field_theory) [^31_7]: https://www.britannica.com/science/Lagrangian-function [^31_8]: https://www.ipcms.fr/uploads/2021/06/chap6.pdf [^31_9]: https://www.youtube.com/watch?v=hQ4UNu1P2kw [^31_10]: https://www.khanacademy.org/math/multivariable-calculus/applications-of-mul… --- # what does it mean for the action integral to be stationary -- it's zero? constant? repeating? what are dsubu, dsuperu, phi? “Stationary” does **not** mean the action is zero, constant in time, or repeating. It means that if you make a tiny allowed change to the path or field configuration, the action does not change to first order: mathematically, $\delta S = 0$.[^32_1][^32_2][^32_3][^32_4] ## Stationary action A useful analogy is an ordinary function $f(x)$. At a stationary point, a tiny shift $x \to x+\epsilon$ gives no linear change, so the slope is zero; that point might be a minimum, a maximum, or a saddle point. The action principle is the same idea, except the “input” is not a single number but an entire path or field history.[^32_2][^32_3][^32_4][^32_1] So “stationary” means “extremized” or more generally “first-order unchanged,” not “numerically constant.”[^32_4][^32_2] ## What $\phi$ is $\phi$ is usually the scalar field itself. That means at each point in space and time, $\phi(x)$ gives one number, unlike a vector field which gives several components. In the scalar-particle example, $\phi$ is the mathematical object whose excitations are the scalar particles.[^32_5][^32_6][^32_7] ## What $\partial_\mu$ means $\partial_\mu\phi$ means “take a partial derivative of the field $\phi$ with respect to the coordinate $x^\mu$.” In spacetime notation, the index $\mu$ usually runs over time and the three space coordinates, so $\partial_\mu\phi$ packages time and space derivatives together into one four-component object.[^32_8][^32_9] Very roughly, it stands for something like: - $\partial_0 \phi$: time derivative, - $\partial_1 \phi,\partial_2 \phi,\partial_3 \phi$: spatial derivatives.[^32_9][^32_8] ## What $\partial^\mu$ means $\partial^\mu$ is the same derivative object but with the index raised. In relativity, raising or lowering an index uses the spacetime metric, so $\partial^\mu$ and $\partial_\mu$ are closely related but not identical in sign conventions.[^32_10][^32_9] That is why expressions like $\partial_\mu \phi\,\partial^\mu \phi$ appear: they are a compact relativistic way to combine time and space derivative terms into one scalar quantity.[^32_11][^32_9] ## Why one index is down and one up This is Einstein summation notation. When the same index appears once up and once down, you automatically sum over it, so $\partial_\mu \phi\,\partial^\mu \phi$ means “sum over all spacetime components.”[^32_12][^32_13] In plainer language, it is a compact abbreviation for “square the time derivative, subtract or add the squared spatial derivatives according to the metric convention, and combine them.”[^32_11][^32_9] ## One translation A good plain-English translation of $$ \mathcal{L}=\tfrac12 \partial_\mu\phi\,\partial^\mu\phi-\tfrac12 m^2\phi^2 $$ is: “the Lagrangian density equals a kinetic part built from how the field changes in time and space, minus a mass part proportional to the field itself.”[^32_9][^32_5] If you want, the next clean step is for me to expand $\partial_\mu\phi\,\partial^\mu\phi$ into ordinary derivatives with $t,x,y,z$ and no index notation. <span style="display:none">[^32_14][^32_15][^32_16][^32_17][^32_18]</span> <div align="center">⁂</div> [^32_1]: https://en.wikipedia.org/wiki/Action_principles [^32_2]: https://aero.refpages.org/dynsys/lagrange/ [^32_3]: https://saphysics.com/classical-mechanics/lagrangian-mechanics/the-principl… [^32_4]: https://phys.libretexts.org/Bookshelves/Classical_Mechanics/Variational_Pri…'s_Action_Principle/9.02:_Hamilton's_Principle_of_Stationary_Action [^32_5]: https://www.southampton.ac.uk/~doug/ft1/ft12.pdf [^32_6]: https://en.wikipedia.org/wiki/Klein–Gordon_equation [^32_7]: https://www.sas.rochester.edu/pas/assets/pdf/undergraduate/the_free_klein_g… [^32_8]: https://www.reddit.com/r/AskPhysics/comments/b2yi8f/quantum_field_theory/ [^32_9]: https://peeterjoot.com/2018/09/23/phy2403h-quantum-field-theory-lecture-4-s… [^32_10]: https://www.youtube.com/watch?v=EX0vhVI9PH0 [^32_11]: https://www.physicsforums.com/threads/help-with-vector-notation-partial_-mu… [^32_12]: https://www.statisticshowto.com/einstein-summation-notation/ [^32_13]: https://mathworld.wolfram.com/EinsteinSummation.html [^32_14]: https://www.reddit.com/r/AskPhysics/comments/16modji/can_anyone_explain_the… [^32_15]: https://www.youtube.com/watch?v=fi5uQdoE49c [^32_16]: https://pubs.aip.org/aapt/ajp/article/92/11/885/3317287/Why-the-action [^32_17]: https://www.cantorsparadise.com/the-principle-of-stationary-action-c69237bd… [^32_18]: https://www.reddit.com/r/AskPhysics/comments/1pzdngp/field_theory/ --- # why is the curvayure of spacetime relevent at quantum scales? is duphi the same as dphi/dxu ? why do you say the motion is the one that makes the integral of L stationary rather than the value of L(t) zero? Not by default. Curvature of spacetime is not relevant just because something is quantum-sized; it becomes relevant when gravity or the background geometry matters, such as near black holes, in the early universe, or whenever fields propagate on a non-flat spacetime background. Quantum field theory in curved spacetime is basically the framework for “quantum fields on a curved but still classical spacetime,” and it is used because curvature can change particle concepts and even lead to effects like particle creation.[^33_1][^33_2][^33_3] ## Curvature and quantum scale So the important distinction is: “quantum” and “curved spacetime” are different ideas. Something can be quantum and live happily in flat spacetime, and something can involve curved spacetime without requiring full quantum gravity. Curvature matters when the geometry affects the field’s propagation, symmetries, or notion of vacuum, not merely because the object is tiny.[^33_2][^33_3][^33_1] ## What $\partial_\mu \phi$ means Yes, $\partial_\mu \phi$ is essentially “the partial derivative of $\phi$ with respect to $x^\mu$,” so in ordinary notation it is $\frac{\partial \phi}{\partial x^\mu}$. The index $\mu$ is just a label telling you which spacetime coordinate you are differentiating with respect to, typically time or one of the three space coordinates.[^33_4][^33_5][^33_6] Similarly, $\partial^\mu \phi$ is the same derivative with the index raised, which means the spacetime metric has been used to convert the lower-index version into an upper-index version. That is why $\partial_\mu \phi\,\partial^\mu \phi$ is a compact relativistic sum over time and space derivative terms.[^33_5][^33_6][^33_7] ## Why stationarity uses the action We say the physical motion makes the **action** stationary because the principle is about varying the whole path or field history between endpoints, not about forcing the instantaneous value $L(t)$ to be zero. The action is $S=\int L\,dt$, and the condition is $\delta S=0$, meaning a tiny allowed change of the entire path does not change $S$ to first order.[^33_8][^33_9][^33_10] If you instead required $L(t)=0$, that would be a completely different condition and generally not the correct equations of motion. In ordinary mechanics, $L=T-V$, and there is no reason the correct motion should make kinetic energy equal potential energy at every instant.[^33_11][^33_12][^33_13][^33_14] ## Intuition A helpful analogy is ordinary calculus: you do not find the right point by demanding $f(x)=0$, but by demanding the first-order change vanish, $f'(x)=0$. Stationary action is the same idea lifted from “a point on a graph” to “an entire path through time” or “an entire field configuration through spacetime.”[^33_9][^33_10][^33_15][^33_8] If you want, the next clean step is for me to expand $\partial_\mu\phi\,\partial^\mu\phi$ into plain $t,x,y,z$ derivatives with signs shown explicitly. <span style="display:none">[^33_16][^33_17][^33_18][^33_19][^33_20][^33_21][^33_22][^33_23]</span> <div align="center">⁂</div> [^33_1]: https://en.wikipedia.org/wiki/Quantum_field_theory_in_curved_spacetime [^33_2]: https://preskill.caltech.edu/ph236c/Preskill_QFT_curved_spacetime.pdf [^33_3]: https://arxiv.org/abs/gr-qc/9707062 [^33_4]: https://en.wikipedia.org/wiki/Partial_derivative [^33_5]: https://www.physicsforums.com/threads/help-with-vector-notation-partial_-mu… [^33_6]: https://peeterjoot.com/2018/09/23/phy2403h-quantum-field-theory-lecture-4-s… [^33_7]: https://mathworld.wolfram.com/EinsteinSummation.html [^33_8]: https://en.wikipedia.org/wiki/Action_principles [^33_9]: https://phys.libretexts.org/Bookshelves/Classical_Mechanics/Variational_Pri…'s_Action_Principle/9.02:_Hamilton's_Principle_of_Stationary_Action [^33_10]: https://saphysics.com/classical-mechanics/lagrangian-mechanics/the-principl… [^33_11]: https://news.ycombinator.com/item?id=33403224 [^33_12]: https://egtheory.wordpress.com/2014/09/28/principle-of-least-action/ [^33_13]: https://en.wikipedia.org/wiki/Lagrangian_mechanics [^33_14]: https://phys.libretexts.org/Bookshelves/Classical_Mechanics/Variational_Pri…'s_Action_Principle/9.03:_Lagrangian [^33_15]: https://aero.refpages.org/dynsys/lagrange/ [^33_16]: https://arxiv.org/abs/2112.11614 [^33_17]: https://www.reddit.com/r/TheoreticalPhysics/comments/1ft0qv3/how_does_curve… [^33_18]: https://link.aps.org/doi/10.1103/PhysRevResearch.2.023107 [^33_19]: https://www.youtube.com/watch?v=qJIxjT1psDk [^33_20]: https://www.youtube.com/watch?v=XJc7FNYyZy8 [^33_21]: https://www.youtube.com/watch?v=Sgx6Q6RglG8 [^33_22]: https://www.reddit.com/r/AskPhysics/comments/16modji/can_anyone_explain_the… [^33_23]: https://www.sciencedirect.com/science/article/abs/pii/0370157375900514 --- # are relativity and quantum theories successfully and consistently combined in the modern standard model producing an accepted GUT-like model as was imagined in past decades? when can powerful gravity changes lead to particle creation? what is quantum gravity and is it fully agreed upon, flushed out, and consistent? regarding L, S, and f(x), doesn't S' = L(t)? Partly. The modern Standard Model is a highly successful **relativistic quantum field theory**, so it already combines quantum mechanics with special relativity very well, but it does not include gravity in the way general relativity does. Also, there is no generally accepted GUT-like final unification model at present, even though electroweak unification is accepted and experimentally confirmed.[^34_1][^34_2][^34_3][^34_4][^34_5] ## Standard Model status The accepted unification we do have is electroweak theory, where the electromagnetic and weak interactions are understood as one interaction at high energy. A full grand unified theory would also merge the strong interaction with the electroweak one, but there is no generally accepted GUT model and no clear experimental confirmation of one so far.[^34_6][^34_3][^34_7][^34_4][^34_5] ## Gravity and particles Strongly changing gravity, or more precisely strongly time-dependent or horizon-containing spacetime geometry, can lead to particle creation in semiclassical settings such as an expanding universe, near a black-hole horizon, or for an accelerating observer in the Unruh effect. The rough idea is that in curved or nonstationary spacetime, different observers or different times may disagree about what counts as “vacuum,” so modes that looked empty in one description can contain particles in another.[^34_8][^34_9][^34_10][^34_11][^34_12][^34_13] ## Quantum gravity Quantum gravity means a theory that consistently describes gravity in quantum terms rather than leaving spacetime geometry purely classical. There is still no complete and consistent quantum theory of gravity that is generally accepted, experimentally confirmed, and fully worked out, although many candidate approaches exist.[^34_14][^34_15][^34_16] ## About $S' = L(t)$ Only in a specific sense. If you define an accumulation function $S(t)=\int_{t_0}^{t}L(\tau)\,d\tau$ along one already chosen path, then by the fundamental theorem of calculus $dS/dt = L(t)$.[^34_17][^34_18] But that is **not** the derivative used in the principle of stationary action. There, one compares whole nearby paths $q(t)\to q(t)+\delta q(t)$ with endpoints fixed, and “stationary” means the first-order change in the action vanishes, $\delta S=0$, not that the time derivative of the accumulated action vanishes and not that $L(t)=0$.[^34_19][^34_20][^34_21] ## Your notation questions Yes, $\partial_\mu \phi$ is the same idea as $\frac{\partial \phi}{\partial x^\mu}$. The symbol $\phi$ is the field, while the index $\mu$ labels which spacetime coordinate you are differentiating with respect to, usually time or one of the three spatial coordinates.[^34_22][^34_23][^34_24] A compact way to say the last point is: $dS/dt=L$ for a fixed already-chosen history, but $\delta S=0$ is the condition that selects which history is physical.[^34_21][^34_19][^34_17] <span style="display:none">[^34_25][^34_26][^34_27][^34_28][^34_29][^34_30][^34_31][^34_32][^34_33][^34_34][^34_35][^34_36][^34_37][^34_38][^34_39][^34_40][^34_41][^34_42][^34_43][^34_44][^34_45][^34_46][^34_47][^34_48][^34_49][^34_50][^34_51]</span> <div align="center">⁂</div> [^34_1]: https://philsci-archive.pitt.edu/4267/1/Jones-GRandSM_Confirmation.pdf [^34_2]: https://www.ugr.es/~jillana/SM/qft-sm-PoS.pdf [^34_3]: https://www.ebsco.com/research-starters/physics/unification-weak-and-electr… [^34_4]: https://en.wikipedia.org/wiki/Grand_Unified_Theory [^34_5]: https://www.ebsco.com/research-starters/physics/grand-unification-theories-… [^34_6]: https://en.wikipedia.org/wiki/Physics_beyond_the_Standard_Model [^34_7]: https://www.britannica.com/science/unified-field-theory [^34_8]: https://journals.plos.org/plosone/article?id=10.1371%2Fjournal.pone.0229382 [^34_9]: https://spacefed.com/physics/unruh-hawking-radiation-observed-in-accelerati… [^34_10]: https://arxiv.org/abs/1607.04821 [^34_11]: https://en.wikipedia.org/wiki/Quantum_field_theory_in_curved_spacetime [^34_12]: https://preskill.caltech.edu/ph236c/Preskill_QFT_curved_spacetime.pdf [^34_13]: https://arxiv.org/abs/gr-qc/9707062 [^34_14]: https://physics.aps.org/articles/v16/203 [^34_15]: https://www.livescience.com/physics-mathematics/quantum-physics/physicists-… [^34_16]: https://en.wikipedia.org/wiki/Quantum_gravity [^34_17]: https://en.wikipedia.org/wiki/Fundamental_theorem_of_calculus [^34_18]: https://www.khanacademy.org/math/ap-calculus-ab/ab-integration-new/ab-6-4/v… [^34_19]: https://pages.uoregon.edu/soper/mechanics/lagrangian.pdf [^34_20]: https://www.ipcms.fr/uploads/2021/06/chap6.pdf [^34_21]: https://www.physics.rutgers.edu/~shapiro/507/book3.pdf [^34_22]: https://en.wikipedia.org/wiki/Partial_derivative [^34_23]: https://www.physicsforums.com/threads/help-with-vector-notation-partial_-mu… [^34_24]: https://peeterjoot.com/2018/09/23/phy2403h-quantum-field-theory-lecture-4-s… [^34_25]: https://www.reddit.com/r/AskPhysics/comments/1kxdm8b/whats_the_most_promisi… [^34_26]: https://link.aps.org/doi/10.1103/PhysRevD.105.126025 [^34_27]: https://www.facebook.com/groups/110363486480563/posts/1211347996382101/ [^34_28]: https://www.techexplorist.com/new-quantum-theory-gravity-bridges-gravity-st… [^34_29]: https://www.physicsforums.com/threads/gut-vs-quantum-gravity-unification.68… [^34_30]: https://www.nature.com/articles/s41467-023-39064-6 [^34_31]: http://rickbradford.co.uk/ChoiceCutsCh13.pdf [^34_32]: https://www.sciencedirect.com/topics/chemistry/grand-unified-theory [^34_33]: https://openstax.org/books/calculus-volume-1/pages/5-3-the-fundamental-theo… [^34_34]: https://www.youtube.com/watch?v=aeB5BWY0RlE [^34_35]: https://www.khanacademy.org/math/ap-calculus-ab/ab-integration-new/ab-6-4/v… [^34_36]: https://math.libretexts.org/Courses/Monroe_Community_College/MTH_210_Calcul… [^34_37]: https://phas.ubc.ca/~mav/p526/lec3.pdf [^34_38]: https://www.youtube.com/watch?v=6zm69CzdFQ4 [^34_39]: https://www.khanacademy.org/a/fundamental-theorem-of-calculus-review [^34_40]: https://www.physicsforums.com/threads/lagrangian-doesnt-change-when-adding-… [^34_41]: https://www.reddit.com/r/AskPhysics/comments/1cwhsak/why_does_it_matter_if_… [^34_42]: https://www.math.stonybrook.edu/~ndang/mat126-fall20/sec_1.3.pdf [^34_43]: https://fenicsproject.discourse.group/t/action-of-the-time-derivative/7859 [^34_44]: https://en.wikipedia.org/wiki/Action_principles [^34_45]: https://en.wikipedia.org/wiki/Standard_Model [^34_46]: https://arxiv.org/pdf/hep-ph/0609174.pdf [^34_47]: https://www.damtp.cam.ac.uk/user/tong/sm/standardmodel5.pdf [^34_48]: https://nautil.us/a-brief-history-of-the-grand-unified-theory-of-physics-23… [^34_49]: http://electron6.phys.utk.edu/phys250/modules/module 6/standard_model.htm [^34_50]: https://www.sciencedirect.com/topics/physics-and-astronomy/electroweak-model [^34_51]: https://www.reddit.com/r/explainlikeimfive/comments/ln73zr/eli5_why_cant_we… --- # what is a vacuum in quantum field theory? regarding L and S it sounds like S is a line integral that conserves energy? In quantum field theory, the vacuum is the lowest-energy state of the fields, often described as the state with no particle excitations present. It is not just “nothingness” in the everyday sense, because the vacuum can still have structure such as zero-point energy and, in some theories, nonzero vacuum expectation values.[^35_1][^35_2][^35_3] ## Vacuum meaning A good plain-English picture is: every field is sitting in its quietest allowed configuration. In simple cases that means the fields are uniform in space and constant in time, because variations would cost extra energy. In particle language, that same state is the one with no actual particles excited above it.[^35_2][^35_3][^35_4] ## Action and energy $S$ is an integral of the Lagrangian along a path, but it is not best thought of as “a line integral that conserves energy.” The action principle helps generate the equations of motion, while conservation of energy comes from a separate symmetry statement: if the Lagrangian has no explicit time dependence, then Noether’s theorem gives a conserved energy.[^35_5][^35_6][^35_7][^35_8][^35_9] So the roles are different: - $L$ helps define the dynamics.[^35_9][^35_10] - $S=\int L\,dt$ is the quantity made stationary by the actual path.[^35_11][^35_9] - Energy conservation follows from time-translation symmetry, not from the mere existence of the action integral.[^35_6][^35_7] ## Why this matters There are systems where an action principle still works even when energy is not conserved in the simple sense, so action and energy are not the same concept. That is one reason physicists keep them separate: action is the variational object, while conserved quantities come from symmetries of that object.[^35_12][^35_13][^35_7][^35_5] ## Short correction A safer summary is: $S$ is the accumulated Lagrangian along a whole path, and making $S$ stationary picks out the allowed motion. Energy conservation is a consequence that can follow when the Lagrangian is unchanged by shifting time, not a direct definition of $S$.[^35_8][^35_6][^35_9][^35_11] If you want, the next useful step is to show how $L=T-V$ leads to conserved $T+V$ in the simple no-explicit-time-dependence case. <span style="display:none">[^35_14][^35_15][^35_16][^35_17][^35_18]</span> <div align="center">⁂</div> [^35_1]: https://en.wikipedia.org/wiki/Quantum_vacuum_state [^35_2]: https://ncatlab.org/nlab/show/vacuum+state [^35_3]: https://profmattstrassler.com/technical-zone/the-triplet-the-w-mass/1-the-v… [^35_4]: https://www.reddit.com/r/askscience/comments/9a781m/what_is_quantum_vacuum_… [^35_5]: https://www.eftaylor.com/pub/energy_to_action.html [^35_6]: http://physicspages.com/pdf/Field theory/Time translation symmetry and conservation of energy.pdf [^35_7]: https://liberzon.csl.illinois.edu/teaching/cvoc/node36.html [^35_8]: https://www.youtube.com/watch?v=cVmusE6dbeM [^35_9]: https://en.wikipedia.org/wiki/Action_principles [^35_10]: https://en.wikipedia.org/wiki/Lagrangian_mechanics [^35_11]: https://phys.libretexts.org/Bookshelves/Classical_Mechanics/Variational_Pri…'s_Action_Principle/9.02:_Hamilton's_Principle_of_Stationary_Action [^35_12]: https://www.cphysics.org/article/33629.pdf [^35_13]: https://www.reddit.com/r/AskPhysics/comments/1rxu04z/can_someone_explain_no… [^35_14]: https://www.cphysics.org/article/00980.pdf [^35_15]: https://www.youtube.com/watch?v=eYAns5iZ7Uo [^35_16]: https://projecteuclid.org/journals/communications-in-mathematical-physics/v… [^35_17]: https://www.damtp.cam.ac.uk/user/nsm10/PrincLeaAc.pdf [^35_18]: https://tcdecker.com/2022/03/28/point-of-view-invariance-and-noethers-theor… --- # when and how do different observers disagree about what is vacuum? does that mean these observers diasagree about what particles exist? is the lagrangian the same as the lagrangian density? so you are saying that a starting simple;field has kinetic energy equal to position speed times value speed and potential energy proportional to the square of mass times value? and then motion happens to particles alobg paths where the difference between those values do not change? does intuitive kinetic motion (inertial, newtonion gravity) arise similarly? Yes—different observers can disagree about the vacuum, and in the relevant cases that really does mean they disagree about what counts as particles. The classic example is the Unruh effect: an inertial observer describes Minkowski vacuum as having no particles, while a uniformly accelerating observer describes that same state as a thermal bath of particles.[^36_1][^36_2][^36_3][^36_4][^36_5] ## Vacuum disagreement The reason is that “particle” is defined by how you split field modes into positive-frequency and negative-frequency parts, and that split depends on the observer’s notion of time evolution. If two observers use different time notions—such as inertial time versus accelerated time, or different natural times in curved spacetime—they may use different mode decompositions and therefore different vacuum states. So yes, they can disagree about whether particles are present, even while agreeing on the underlying field theory.[^36_2][^36_5][^36_6][^36_1] ## Lagrangian vs density The Lagrangian and the Lagrangian density are related but not the same. In field theory, the Lagrangian density $\mathcal{L}(\mathbf{x},t)$ is defined point-by-point in space, and the ordinary Lagrangian is its spatial integral: $L(t)=\int d^3x\,\mathcal{L}$. So $\mathcal{L}$ is “per unit volume,” while $L$ is the whole-system quantity at a given time.[^36_7][^36_8][^36_9] ## Scalar field intuition Your scalar-field intuition is close, but I would tune it slightly. For a simple scalar field, the kinetic-type part comes from derivatives of the field in time and space, not from “position speed times value speed,” and the mass term $\tfrac12 m^2\phi^2$ acts like a quadratic potential in the field value $\phi$. The physical history is then not “where $T-V$ stays unchanged,” but the field history for which the action $S=\int dt\,L(t)$ is stationary under small variations.[^36_10][^36_11][^36_12][^36_13][^36_14] ## About $L$, $S$, and motion Also, “stationary” does not mean the integrand $L$ stays constant or zero along the path. It means if you slightly alter the whole path or field configuration while fixing the endpoints, the first-order change in the total action vanishes: $\delta S=0$. Along one already chosen path, you are right that if $S(t)=\int_{t_0}^{t}L(\tau)\,d\tau$, then $dS/dt=L(t)$ by the fundamental theorem of calculus, but that derivative is different from the variational statement $\delta S=0$ used to derive motion.[^36_13][^36_14][^36_15][^36_16][^36_17] ## Classical mechanics too Yes, ordinary inertial motion and Newtonian gravity arise in the same general variational framework. For a free nonrelativistic particle, a standard Lagrangian is $L=\tfrac12 mv^2$, and for Newtonian gravity it is $L=\tfrac12 mv^2 - V(\mathbf{x})$, for example $V=m\Phi$ in a gravitational potential $\Phi$. Applying the Euler–Lagrange equations to that $L$ gives the usual Newtonian equations of motion.[^36_18][^36_19] A very compressed way to say it is: fields and particles both use the same action idea, but for particles you vary paths $q(t)$, while for fields you vary whole functions $\phi(\mathbf{x},t)$.[^36_8][^36_18] <span style="display:none">[^36_20][^36_21][^36_22][^36_23][^36_24][^36_25]</span> <div align="center">⁂</div> [^36_1]: https://en.wikipedia.org/wiki/Unruh_effect [^36_2]: https://www.nature.com/articles/s41467-019-10962-y [^36_3]: https://pmc.ncbi.nlm.nih.gov/articles/PMC6620287/ [^36_4]: https://link.aps.org/doi/10.1103/RevModPhys.80.787 [^36_5]: https://en.wikipedia.org/wiki/Quantum_field_theory_in_curved_spacetime [^36_6]: https://preskill.caltech.edu/ph236c/Preskill_QFT_curved_spacetime.pdf [^36_7]: https://www.reddit.com/r/AskPhysics/comments/4m73a5/difference_between_lagr… [^36_8]: https://en.wikipedia.org/wiki/Lagrangian_(field_theory) [^36_9]: https://www.physics.purdue.edu/~clarkt/Courses/Physics662/ps/qftch21.pdf [^36_10]: https://alpha.physics.uoi.gr/foudas_public/APP-UoI-2011/Lecture12-Lagrangia… [^36_11]: https://www.southampton.ac.uk/~doug/ft1/ft12.pdf [^36_12]: https://en.wikipedia.org/wiki/Klein–Gordon_equation [^36_13]: https://en.wikipedia.org/wiki/Action_principles [^36_14]: https://phys.libretexts.org/Bookshelves/Classical_Mechanics/Variational_Pri…'s_Action_Principle/9.02:_Hamilton's_Principle_of_Stationary_Action [^36_15]: https://pages.uoregon.edu/soper/mechanics/lagrangian.pdf [^36_16]: https://en.wikipedia.org/wiki/Fundamental_theorem_of_calculus [^36_17]: https://openstax.org/books/calculus-volume-1/pages/5-3-the-fundamental-theo… [^36_18]: https://en.wikipedia.org/wiki/Lagrangian_mechanics [^36_19]: https://www.ipcms.fr/uploads/2021/06/chap6.pdf [^36_20]: https://pirsa.org/24040084 [^36_21]: https://www.advancedsciencenews.com/the-unruh-effect-probing-the-structure-… [^36_22]: https://sites.dartmouth.edu/dujs/2020/12/07/the-unruh-effect-newly-proposed… [^36_23]: https://www.reddit.com/r/AskPhysics/comments/3xd9x2/why_is_the_lagrangian_a… [^36_24]: https://www.physicsforums.com/threads/why-is-the-lagrangian-formulated-as-l… [^36_25]: https://phys.org/news/2025-09-unruh-effect-approach-bridge-gap.html --- # you're saying different observers disagree about what energy levels are quantized becaude they see different energies? does this mean it would be possible to complexly design a world whete different observers experienced different complex sequences of events developing, due to different particles existing? and they could travel between these different realities? let's drill down into L and S ... you're saying L density is per volume-time, then L is the accumulated energy difference of the whole system per time, then S is the static picture of the whole system at all time -- and when it's stationary you mean under an unwritten parameter of what particle histories are selected to happen...? Not quite: this observer-dependence does **not** usually mean different observers inhabit different realities with different actual events unfolding. It means they can use different mode decompositions of the same underlying quantum field and therefore disagree about which excitations count as “particles,” while still agreeing on the shared field theory and on ordinary communicated experimental records where comparison is possible.[^37_1][^37_2][^37_3] ## Vacuum disagreement This happens when observers use different notions of time, because “positive frequency” and therefore “particle” are defined relative to time evolution. In the Unruh effect, an inertial observer calls a state vacuum while a uniformly accelerating observer assigns a thermal particle bath to that same state, so yes, they disagree about particle content in a technical sense.[^37_2][^37_4][^37_5][^37_6][^37_1] That still does not seem like “travel between different realities” in the science-fiction sense. It is closer to saying the same field state can be parsed into quanta in different ways by different observers or detectors, often related mathematically by Bogoliubov transformations.[^37_3][^37_7][^37_1] ## L, $\mathcal{L}$, and $S$ The Lagrangian density $\mathcal{L}$ is the local quantity defined at each spacetime point, and the ordinary Lagrangian is its spatial integral $L(t)=\int d^3x\,\mathcal{L}$. Then the action is the time integral of $L$, or equivalently the spacetime integral of $\mathcal{L}$: $S=\int dt\,L(t)=\int d^4x\,\mathcal{L}$.[^37_8][^37_9][^37_10][^37_11] So $S$ is not a static picture, and it is not itself the system’s energy. It is better thought of as a rule that assigns one number to each possible whole history of the system.[^37_9][^37_10][^37_11] ## What “stationary” varies over Yes, the hidden parameter you are circling around is the choice of possible histories. One imagines slightly changing the path $q(t)$ in mechanics, or slightly changing the whole field configuration $\phi(x)$ in field theory, and the physical history is the one for which the first-order change in $S$ vanishes, $\delta S=0$.[^37_10][^37_12][^37_13] That does **not** mean $L$ stays constant, and it does not mean $T-V$ “does not change” along the motion. It means neighboring candidate histories give the same action to first order at the actual history.[^37_12][^37_13][^37_14] ## Scalar-field intuition Your scalar-field summary is close, but I would tune the kinetic part. For a simple scalar field, the Lagrangian density is typically written as a derivative part minus a potential part, $\mathcal{L}=K(\partial_\mu\phi\,\partial^\mu\phi)-V(\phi)$, and in the free case the potential is quadratic, proportional to $m^2\phi^2$.[^37_15][^37_16][^37_10] So the “kinetic” piece is built from how the field changes across time and space, not from “position speed times value speed.” The field then evolves according to the stationary-action condition, which yields the field equations such as the Klein–Gordon equation.[^37_16][^37_17][^37_15] ## Classical motion too Yes, ordinary inertial motion and Newtonian gravity fit the same pattern. In simple classical mechanics one often uses $L=T-V$, and applying the Euler–Lagrange equations to that choice gives Newton’s equations of motion, including motion in a gravitational potential.[^37_18][^37_19][^37_9] A compact way to say it is: mechanics varies particle paths, field theory varies field histories, and both get their equations of motion from making the action stationary.[^37_11][^37_10][^37_18] <span style="display:none">[^37_20][^37_21][^37_22][^37_23][^37_24][^37_25][^37_26]</span> <div align="center">⁂</div> [^37_1]: https://en.wikipedia.org/wiki/Quantum_field_theory_in_curved_spacetime [^37_2]: https://preskill.caltech.edu/ph236c/Preskill_QFT_curved_spacetime.pdf [^37_3]: https://rojefferson.blog/2021/01/15/qft-in-curved-space-part-2-bogolyubov-t… [^37_4]: https://en.wikipedia.org/wiki/Unruh_effect [^37_5]: https://pmc.ncbi.nlm.nih.gov/articles/PMC6620287/ [^37_6]: https://link.aps.org/doi/10.1103/RevModPhys.80.787 [^37_7]: https://ui.adsabs.harvard.edu/abs/1981PhDT.........3D/abstract [^37_8]: https://www.sas.rochester.edu/pas/assets/pdf/undergraduate/Introduction_to_… [^37_9]: https://alpha.physics.uoi.gr/foudas_public/APP-UoI-2011/Lecture12-Lagrangia… [^37_10]: http://www.scholarpedia.org/article/Lagrangian_formalism_for_fields [^37_11]: https://en.wikipedia.org/wiki/Lagrangian_(field_theory) [^37_12]: https://en.wikipedia.org/wiki/Action_principles [^37_13]: https://phys.libretexts.org/Bookshelves/Classical_Mechanics/Variational_Pri…'s_Action_Principle/9.02:_Hamilton's_Principle_of_Stationary_Action [^37_14]: https://saphysics.com/classical-mechanics/lagrangian-mechanics/the-principl… [^37_15]: https://www.southampton.ac.uk/~doug/ft1/ft12.pdf [^37_16]: https://en.wikipedia.org/wiki/Klein–Gordon_equation [^37_17]: https://piazza.com/class_profile/get_resource/kzqnrwg17lp27d/l294p8r7oy45fq [^37_18]: https://en.wikipedia.org/wiki/Lagrangian_mechanics [^37_19]: https://www.ipcms.fr/uploads/2021/06/chap6.pdf [^37_20]: https://link.aps.org/doi/10.1103/PhysRevLett.134.050406 [^37_21]: https://arxiv.org/pdf/2510.08265.pdf [^37_22]: https://www.intechopen.com/chapters/1210023 [^37_23]: https://www.facebook.com/groups/487763051018632/posts/816494668145467/ [^37_24]: https://arxiv.org/abs/gr-qc/9812056 [^37_25]: https://bht50.github.io/minicourses/QFTCS__An_Introduction.pdf [^37_26]: https://www.youtube.com/watch?v=a_LEq0e2OtU --- # L and S ... if the physical history is the one where adjacent physical histories have the same S, does this mean there are always multiple possible differing histories that are physical? calling out contradiction in your response to alternate realities ... you say the same field state can be parsed into quanta in different ways by different detectors, but earlier that different observers agree on experimental records. won't they disagree if their detectors measure different particles? Yes—this is exactly the place to be careful about the word “stationary.” It does **not** mean “there are many distinct physical histories all equally real”; it means that at the physical history, tiny nearby variations do not change the action to first order, so $\delta S=0$.[^38_1][^38_2][^38_3] ## Physical histories That condition does **not** guarantee multiple physical solutions. Often, once you fix the system and the boundary or initial conditions, there is a unique physical history; in other situations there can be several stationary histories, such as different extrema or saddle-type solutions. So “adjacent histories have the same $S$” is better said as “adjacent histories differ only at second order in $S$,” not “all those nearby histories are physical too.”[^38_2][^38_3][^38_1] ## Observer disagreement The apparent contradiction is resolved by separating **detector outcomes** from **particle interpretation**. Different observers or detectors can assign different particle content to the same field state because they use different mode decompositions, often related by Bogoliubov transformations. But if they later compare a concrete experimental record, they can still agree on what each detector actually did—for example, “the accelerated detector clicked with such-and-such statistics” and “the inertial detector did not”—even if they explain those facts using different particle language.[^38_4][^38_5][^38_6][^38_7][^38_8][^38_9] So they do **not** have to agree that the same particles “really existed” in some observer-independent sense. They agree on the operational facts: detector A clicked, detector B had this spectrum, this acceleration was applied, these correlations were measured. The disagreement is about how to decompose the field into vacuum plus particles, not about whether a logged detector event occurred.[^38_5][^38_10][^38_6][^38_7][^38_4] ## Why detectors differ If two detectors move differently or couple differently to the field, they are not performing the same measurement. In the Unruh setting, an accelerated detector can thermalize or excite in the Minkowski vacuum while an inertial detector does not, and that is part of the phenomenon rather than a contradiction. When they meet again, they can consistently report those different outcomes because the detectors had different worldlines and different couplings.[^38_11][^38_10][^38_6][^38_4][^38_5] ## L, $\mathcal{L}$, and S Your summary is close, but I would sharpen it like this. The Lagrangian density $\mathcal{L}$ is a local spacetime quantity; integrating it over space gives the Lagrangian $L(t)=\int d^3x\,\mathcal{L}$; integrating $L$ over time gives the action $S=\int dt\,L(t)=\int d^4x\,\mathcal{L}$. $S$ is therefore a number assigned to an entire candidate history of the system, not a “static picture” and not the energy.[^38_12][^38_13][^38_14][^38_15] And yes, the unwritten parameter is precisely the choice of history being varied. In mechanics you vary paths $q(t)$; in field theory you vary field configurations $\phi(x)$. The physical history is one for which the first-order variation vanishes, $\delta S=0$.[^38_3][^38_14][^38_16][^38_12] ## One clean distinction A useful contrast is: - $dS/dt = L$ can be true if you define $S(t)=\int_{t_0}^{t}L(\tau)\,d\tau$ along one already chosen history.[^38_17][^38_18] - $\delta S=0$ is the variational condition used to select which history is physical.[^38_16][^38_3] Those are different operations: one is an ordinary time derivative along a fixed path, and the other is a comparison between nearby possible paths.[^38_19][^38_20] The next crisp step is probably to do one tiny example with a particle moving in 1D, where I write $S[q]$, vary $q(t)\to q(t)+\epsilon \eta(t)$, and show exactly what “first-order unchanged” means. <span style="display:none">[^38_21][^38_22][^38_23][^38_24][^38_25][^38_26][^38_27][^38_28]</span> <div align="center">⁂</div> [^38_1]: https://www.nature.com/articles/s41598-023-39145-y [^38_2]: https://en.wikipedia.org/wiki/Lagrangian_mechanics [^38_3]: https://en.wikipedia.org/wiki/Action_principles [^38_4]: https://inspirehep.net/files/418a9de8c0e3bd3d3a384201a3cec6d1 [^38_5]: https://www.nature.com/articles/srep07195 [^38_6]: https://link.aps.org/doi/10.1103/RevModPhys.80.787 [^38_7]: https://en.wikipedia.org/wiki/Quantum_field_theory_in_curved_spacetime [^38_8]: https://rojefferson.blog/2021/01/15/qft-in-curved-space-part-2-bogolyubov-t… [^38_9]: https://ui.adsabs.harvard.edu/abs/1981PhDT.........3D/abstract [^38_10]: https://en.wikipedia.org/wiki/Unruh_effect [^38_11]: https://link.aps.org/doi/10.1103/PhysRevD.102.045002 [^38_12]: https://en.wikipedia.org/wiki/Lagrangian_(field_theory) [^38_13]: https://www.sas.rochester.edu/pas/assets/pdf/undergraduate/Introduction_to_… [^38_14]: http://www.scholarpedia.org/article/Lagrangian_formalism_for_fields [^38_15]: https://alpha.physics.uoi.gr/foudas_public/APP-UoI-2011/Lecture12-Lagrangia… [^38_16]: https://phys.libretexts.org/Bookshelves/Classical_Mechanics/Variational_Pri…'s_Action_Principle/9.02:_Hamilton's_Principle_of_Stationary_Action [^38_17]: https://en.wikipedia.org/wiki/Fundamental_theorem_of_calculus [^38_18]: https://openstax.org/books/calculus-volume-1/pages/5-3-the-fundamental-theo… [^38_19]: https://pages.uoregon.edu/soper/mechanics/lagrangian.pdf [^38_20]: https://www.physics.rutgers.edu/~shapiro/507/book3.pdf [^38_21]: https://towardsdatascience.com/three-mind-blowing-ideas-in-physics-the-stat… [^38_22]: https://www.reddit.com/r/AskPhysics/comments/18z2ugq/which_came_first_lagra… [^38_23]: https://philarchive.org/archive/CADTRF [^38_24]: https://www.physicsforums.com/threads/principle-of-stationary-action-intuit… [^38_25]: https://www.scirp.org/journal/paperinformation?paperid=140045 [^38_26]: https://arxiv.org/html/2504.16225v2 [^38_27]: https://www.sciencedirect.com/science/article/abs/pii/S0393044021002746 [^38_28]: https://arxiv.org/pdf/2003.12603.pdf --- # it really sounds like complex "ghost" events can be imagined with spacetime curvature like what if the clicking detector were itself constructed with edge energies such that it could only be seen in its frame of use it sounds complicated regarding S are you saying that situations cannot arisr where the 2nd derivative of S is 0 over some change? Not in the strong “alternate reality” sense. Curved-spacetime QFT does seem to allow observer-dependent **particle descriptions** and detector response statistics, but it still treats detectors as ordinary physical systems coupled to the same underlying fields, not as private objects that exist only in their own frame.[^39_1][^39_2][^39_3] ## Detector disagreement A detector is modeled as a localized probe with a specific coupling to the field along its worldline, so different motion or different coupling can produce different click rates or excitation probabilities. In that sense, yes, one detector may register excitations that another does not, but that is a difference in what each probe measures from the shared field state, not evidence that two disconnected worlds are unfolding.[^39_2][^39_4][^39_5][^39_1] So when I said observers later agree on records, I meant they can agree on concrete facts such as “this accelerated detector clicked this often” and “this inertial detector clicked that often.” What they may still disagree about is whether those clicks are best described as detecting “particles in the vacuum” or as the response of a noninertial detector to the same underlying field.[^39_3][^39_5][^39_6] ## About “ghost events” Your imagined setup is pointing toward a real idea: detector responses can be highly contingent on trajectory, coupling, and background geometry. But standard theory does not seem to license arbitrary frame-private event streams where one observer’s events are simply nonexistent for everyone else; rather, it gives observer-dependent decompositions of the same field and observer-dependent detector responses.[^39_7][^39_8][^39_1][^39_2][^39_3] ## Second derivative of $S$ Yes, situations can absolutely arise where the second variation is zero in some direction. That means the stationary point is degenerate or “flat” along that variation, so the second-order test does not decide whether you have a minimum, maximum, or saddle behavior there.[^39_9][^39_10][^39_11] So “stationary” only requires the first variation to vanish, $\delta S=0$. The second variation $\delta^2 S$ can be positive, negative, mixed, or zero depending on the problem, just like ordinary calculus critical points can be minima, maxima, saddles, or more degenerate cases.[^39_12][^39_10][^39_11][^39_13] ## L and S again A clean way to say it is: - $\mathcal{L}$ assigns a local contribution at each spacetime point.[^39_14][^39_15] - $L(t)=\int d^3x\,\mathcal{L}$ is the spatially integrated quantity at one time.[^39_16][^39_14] - $S=\int dt\,L(t)=\int d^4x\,\mathcal{L}$ assigns one number to an entire candidate history.[^39_15][^39_17] Then the physical history is selected by demanding that under a small change of the candidate history, the first-order change in $S$ vanishes. That does not mean neighboring histories are also physical; it means the chosen one is a stationary point of the action functional on the space of possible histories.[^39_13][^39_18][^39_19][^39_9] A very compressed slogan is: observer-dependent particles are a reinterpretation of the same field, while stationary action means first-order indifference to nearby histories, not uniqueness and not necessarily a minimum.[^39_10][^39_19][^39_3] <span style="display:none">[^39_20][^39_21][^39_22][^39_23][^39_24][^39_25][^39_26][^39_27][^39_28]</span> <div align="center">⁂</div> [^39_1]: https://inspirehep.net/files/69f46898f34d8cdc4af258833b15f11c [^39_2]: https://ui.adsabs.harvard.edu/abs/2013arXiv1309.7281H/abstract [^39_3]: https://en.wikipedia.org/wiki/Quantum_field_theory_in_curved_spacetime [^39_4]: https://en.wikipedia.org/wiki/Unruh_effect [^39_5]: https://link.aps.org/doi/10.1103/RevModPhys.80.787 [^39_6]: https://www.nature.com/articles/srep07195 [^39_7]: https://rojefferson.blog/2021/01/15/qft-in-curved-space-part-2-bogolyubov-t… [^39_8]: https://inspirehep.net/files/418a9de8c0e3bd3d3a384201a3cec6d1 [^39_9]: https://liu.ku.edu/Lectures Ch10.pdf [^39_10]: https://www.youtube.com/watch?v=Rj_hXlnBrz8 [^39_11]: https://courses.lumenlearning.com/calculus3/chapter/second-derivative-test/ [^39_12]: https://byjus.com/maths/saddle-points/ [^39_13]: https://en.wikipedia.org/wiki/Action_principles [^39_14]: https://en.wikipedia.org/wiki/Lagrangian_(field_theory) [^39_15]: https://www.sas.rochester.edu/pas/assets/pdf/undergraduate/Introduction_to_… [^39_16]: https://alpha.physics.uoi.gr/foudas_public/APP-UoI-2011/Lecture12-Lagrangia… [^39_17]: http://www.scholarpedia.org/article/Lagrangian_formalism_for_fields [^39_18]: https://phys.libretexts.org/Bookshelves/Classical_Mechanics/Variational_Pri…'s_Action_Principle/9.02:_Hamilton's_Principle_of_Stationary_Action [^39_19]: https://www.nature.com/articles/s41598-023-39145-y [^39_20]: https://link.aps.org/doi/10.1103/PhysRevLett.134.050406 [^39_21]: https://arxiv.org/html/2405.03902v2 [^39_22]: https://www.youtube.com/watch?v=QgeOlt0xZ7s [^39_23]: https://www.intechopen.com/chapters/1210023 [^39_24]: https://www.reddit.com/r/askmath/comments/1f9n53t/if_both_the_first_and_sec… [^39_25]: https://www.reddit.com/r/Physics/comments/1r12njo/what_do_particle_detector… [^39_26]: https://sheffield.ac.uk/media/30821/download?attachment [^39_27]: https://www.facebook.com/groups/487763051018632/posts/816494668145467/ [^39_28]: https://www.physicsforums.com/threads/stationary-point-of-variation-of-acti… --- # if the second derivative of S is 0 then neighboring histories will also have a first derivative of 0 and also be physical No. If the second variation of $S$ is zero in some direction, that does **not** imply nearby histories also have zero first variation and are therefore physical.[^40_1][^40_2] ## Why not A stationary history is defined by the first variation vanishing, $\delta S=0$. The second variation only helps classify what kind of stationary point you have, much like the second derivative test in ordinary calculus.[^40_2][^40_3][^40_1] If $\delta^2 S = 0$ in some variation direction, that usually means the stationary point is **degenerate** or the second-order test is inconclusive, so you must look at higher-order variations. It does not force a whole band of neighboring histories to become stationary too.[^40_4][^40_5][^40_2] ## Ordinary-calculus analogy Think of $f(x)=x^4$. At $x=0$, the first derivative is zero and the second derivative is also zero, but nearby points $x\neq 0$ do not have zero first derivative, so they are not stationary points.[^40_6][^40_7] That is the closest analogy to your statement: zero second derivative at one point does not make neighboring points stationary.[^40_5][^40_2] ## When nearby histories can also be physical You only get a whole family of physical histories if the action has a true flat direction or symmetry, so that many distinct histories all satisfy $\delta S=0$. That can happen, but it is an extra structural fact about the system, not something implied merely by $\delta^2 S=0$ at one history.[^40_4][^40_1][^40_2] ## Clean statement So the safe rule is: - $\delta S=0$ picks stationary histories.[^40_3][^40_2] - $\delta^2 S>0$, $<0$, mixed, or $=0$ helps classify them.[^40_1][^40_2] - $\delta^2 S=0$ alone does not mean neighboring histories are also physical.[^40_5][^40_4] A compact slogan is: first variation decides stationarity; second variation only tests its local shape, and zero second variation means “look deeper,” not “everything nearby also works.”[^40_2][^40_1] <span style="display:none">[^40_10][^40_11][^40_12][^40_13][^40_14][^40_15][^40_8][^40_9]</span> <div align="center">⁂</div> [^40_1]: https://www.math.utah.edu/~cherk/teach/5500-19/sec-var19.pdf [^40_2]: https://en.wikipedia.org/wiki/Second_variation [^40_3]: https://en.wikipedia.org/wiki/Stationary_point [^40_4]: https://dec41.user.srcf.net/notes/IB_E/variational_principles.pdf [^40_5]: https://web.mit.edu/hyperbook/Patrikalakis-Maekawa-Cho/node138.html [^40_6]: https://www.reddit.com/r/askmath/comments/1f9n53t/if_both_the_first_and_sec… [^40_7]: https://www.youtube.com/watch?v=bG69nJKdZhY [^40_8]: https://community.wolfram.com/groups/-/m/t/3310829 [^40_9]: https://www.sciencedirect.com/topics/computer-science/stationary-point [^40_10]: https://www.cambridge.org/core/journals/canadian-journal-of-mathematics/art… [^40_11]: https://www.physicsforums.com/threads/stationary-point-of-variation-of-acti… [^40_12]: https://www.youtube.com/watch?v=KlGaWhHOw54 [^40_13]: https://www.youtube.com/watch?v=xECfJ14jqmk [^40_14]: https://www.youtube.com/watch?v=Rj_hXlnBrz8 [^40_15]: https://www.sydney.edu.au/content/dam/students/documents/mathematics-learni… --- # are you saying all derivatives would need to be zero to have multiple physical realities and the system would be piecewise over histories ... ? No. You do **not** need all derivatives to vanish in order to have multiple physical solutions, and multiple stationary histories do not by themselves amount to “multiple physical realities.”[^41_1][^41_2] ## Multiple solutions You can have several distinct stationary solutions simply because the action has several distinct stationary points. That is the ordinary analogue of a function with several separate extrema: each one has first derivative zero at its own location, even though the derivatives are not zero everywhere in between.[^41_3][^41_2][^41_4][^41_1] A whole continuous family of stationary histories can happen too, but that usually comes from a symmetry or flat direction in the action, not from some rule that every derivative must vanish. In that case, there may be a manifold of critical points rather than one isolated point.[^41_5][^41_2][^41_6] ## All derivatives zero? Even at one stationary point, having first and second derivatives zero does not force all higher derivatives to vanish. The standard counterexample is a function like $x^4$: at $x=0$, the first and second derivatives vanish, but nearby points are not stationary, because the derivative is nonzero once you move away from zero.[^41_7][^41_8] So “all derivatives zero” is a much stronger condition than needed. It can happen for very flat functions, but multiple stationary solutions usually arise for much simpler reasons.[^41_2][^41_9][^41_1][^41_7] ## Physical reality vs solutions In classical mechanics and classical field theory, multiple stationary histories usually mean multiple mathematically allowed solutions, with the realized one depending on boundary or initial conditions. That is not normally taken to mean all of them are simultaneously unfolding as separate realities.[^41_10][^41_11][^41_1] In quantum theory, one may sum over many possible histories in the path-integral picture, but that is a different statement from saying every stationary solution is an actual separate world. So the clean separation is: multiple stationary histories are about the structure of the equations, not automatically about ontology.[^41_11][^41_12] ## Piecewise over histories “Piecewise over histories” does not seem like the right default picture. A better picture is that the action is a functional defined on the space of all candidate histories, and stationary histories are the critical points of that functional.[^41_9][^41_12][^41_13] So the short answer is: no, not all derivatives need to vanish; one first-variation condition gives stationarity, higher variations classify or reveal degeneracy, and multiple physical solutions can exist without requiring an infinitely flat action.[^41_6][^41_2][^41_9] <span style="display:none">[^41_14][^41_15][^41_16][^41_17][^41_18][^41_19][^41_20][^41_21][^41_22][^41_23][^41_24]</span> <div align="center">⁂</div> [^41_1]: https://pmc.ncbi.nlm.nih.gov/articles/PMC4156149/ [^41_2]: https://web.mit.edu/hyperbook/Patrikalakis-Maekawa-Cho/node139.html [^41_3]: https://www.expii.com/t/derivatives-at-local-extrema-129 [^41_4]: https://en.wikipedia.org/wiki/Stationary_point [^41_5]: https://math.cornell.edu/~kessler/GroupNotes/morse.pdf [^41_6]: https://www.math.utah.edu/~cherk/teach/5500-19/sec-var19.pdf [^41_7]: https://www.reddit.com/r/askmath/comments/1f9n53t/if_both_the_first_and_sec… [^41_8]: https://www.youtube.com/watch?v=bG69nJKdZhY [^41_9]: https://en.wikipedia.org/wiki/Second_variation [^41_10]: https://en.wikipedia.org/wiki/Lagrangian_mechanics [^41_11]: https://www.nature.com/articles/s41598-023-39145-y [^41_12]: https://en.wikipedia.org/wiki/Action_principles [^41_13]: http://www.scholarpedia.org/article/Lagrangian_formalism_for_fields [^41_14]: https://arxiv.org/pdf/2502.03675.pdf [^41_15]: https://www.sciencedirect.com/science/article/pii/S002203960200181X [^41_16]: https://www.math.stonybrook.edu/~khuri/Alaee_Khuri_Kunduri_Sugra.pdf [^41_17]: https://www.intlpress.com/site/pub/files/_fulltext/journals/ajm/2019/0023/0… [^41_18]: https://par.nsf.gov/servlets/purl/10440869 [^41_19]: https://www.reddit.com/r/MathHelp/comments/18uyzyd/why_does_the_third_deriv… [^41_20]: https://pubs.aip.org/aip/jmp/article-pdf/15/1/46/19226309/46_1_online.pdf [^41_21]: https://www.youtube.com/watch?v=KfeAQdH13qI [^41_22]: https://arxiv.org/pdf/2512.21029.pdf [^41_23]: https://discovery.researcher.life/article/stationary-solutions-and-asymptot… [^41_24]: https://www.vaia.com/en-us/textbooks/math/finite-mathematics-and-applied-ca… --- # if you have a flat direction in the action, multiple seauebces of events could unfold from the same boundary conditions? Sometimes yes, but only after an important split. A flat direction can mean either a **redundancy** of description, where many mathematically different histories represent the same physical history, or a genuine underdetermination, where the equations and boundary data really leave a family of distinct solutions.[^42_1][^42_2][^42_3] ## Two cases If the flat direction comes from a gauge symmetry, then different histories along that direction are usually treated as physically equivalent, not as different sequences of events. In that case, the action is constant along gauge transformations, but those different points in history-space are bookkeeping copies of the same physics.[^42_4][^42_2][^42_5][^42_1] If the flat direction is genuine and not gauge, then a continuous family of stationary solutions may exist. In that situation, the same boundary conditions may fail to pick out a unique history, because the stationary points form a manifold rather than an isolated point.[^42_6][^42_7][^42_3] ## Same boundary conditions So yes, it is possible in principle that the same boundary conditions allow multiple solutions. But that does **not** automatically mean “multiple realities”; often it means the problem is degenerate, incomplete, symmetry-related, or needs extra physical selection data such as stability, gauge fixing, or initial conditions.[^42_3][^42_1][^42_6] ## Best phrasing A careful phrasing would be: “a flat direction means the action does not change along some variation, and depending on whether that variation is gauge or physical, this gives either equivalent descriptions or a family of genuinely distinct allowed histories.” That is the sharp answer to your question.[^42_2][^42_1][^42_4][^42_3] A tiny analogy is a bead on a perfectly flat circular track: every position may have the same energy, but whether those positions count as different physical states or just relabelings depends on the actual structure of the problem.[^42_7][^42_3] <span style="display:none">[^42_10][^42_11][^42_12][^42_13][^42_14][^42_15][^42_16][^42_8][^42_9]</span> <div align="center">⁂</div> [^42_1]: https://link.aps.org/doi/10.1103/PhysRevD.94.124021 [^42_2]: https://www.maths.dur.ac.uk/users/stefano.cremonesi/GMP_Epiphany/GMP_notes.… [^42_3]: https://web.mit.edu/hyperbook/Patrikalakis-Maekawa-Cho/node139.html [^42_4]: https://bartoszmilewski.com/2021/12/20/symmetries-and-redundancies/ [^42_5]: https://philsci-archive.pitt.edu/28266/1/Poor Man's Introduction to Gauge Symmetries 2.17.26 - Copy.pdf [^42_6]: http://www.scholarpedia.org/article/Boundary_value_problem [^42_7]: https://www.math.unipd.it/~dmpa07/talk-ambrosetti.pdf [^42_8]: https://www.sciencedirect.com/science/article/abs/pii/S0045793012002319 [^42_9]: https://www.ma.imperial.ac.uk/~dgcrowdy/_students/GreenThesis.pdf [^42_10]: https://www.hec.usace.army.mil/confluence/rasdocs/r2dum/6.5/running-a-model… [^42_11]: https://www.mathworks.com/matlabcentral/answers/1906740-how-to-solve-a-seco… [^42_12]: https://cfd.university/learn/10-key-concepts-everyone-must-understand-in-cf… [^42_13]: https://www.emergentmind.com/topics/bifurcations-of-stationary-solutions [^42_14]: https://researchmgt.monash.edu/ws/portalfiles/portal/302068383/301814457_oa… [^42_15]: https://digitalcommons.chapman.edu/cgi/viewcontent.cgi?article=1324\&contex… [^42_16]: https://www.sciencedirect.com/science/article/abs/pii/S0362546X24000944 --- # so a situation like that does not correlate with indistuinguishability of true unique history or multiple coexisting timelines but rather missing model conditions? Usually yes: in standard physics, that situation is interpreted much more as gauge redundancy, degeneracy, or insufficient specifying data than as evidence for multiple coexisting timelines. The same mathematical setup may admit several histories either because some of them are really the same physical state written differently, or because the model and conditions do not yet isolate one unique solution.[^43_1][^43_2][^43_3][^43_4] ## Main split If the multiplicity comes from a gauge direction, then the different “histories” are usually treated as physically equivalent descriptions rather than different true event-sequences. If it comes from a genuine flat physical direction, then there may be a family of distinct solutions, but that still points first to nonuniqueness of the equations-plus-conditions, not automatically to many realized worlds.[^43_5][^43_2][^43_3][^43_4][^43_1] ## What it usually signals The usual readings are: - Boundary or initial conditions were not sufficient to pick a unique solution.[^43_3] - The theory has a symmetry or degeneracy, so several mathematical representatives correspond to one physical situation.[^43_4][^43_1] - The stationary points form a manifold rather than an isolated point.[^43_2] None of those, by themselves, are the standard criterion for “multiple coexisting timelines.” That stronger interpretation would need additional ontology beyond the variational formalism itself.[^43_6][^43_1][^43_2][^43_4] ## Best phrasing So the safest phrasing is: “nonunique stationary histories usually indicate redundancy or underdetermination in the modeling setup, unless extra physical structure is added that interprets those alternatives as genuinely distinct realized histories.” The action formalism by itself does not seem to force the timeline interpretation.[^43_6][^43_1][^43_2][^43_3] A very compact slogan is: multiple solutions in the math are common; multiple realities are an extra metaphysical step.[^43_2][^43_6] <span style="display:none">[^43_10][^43_11][^43_7][^43_8][^43_9]</span> <div align="center">⁂</div> [^43_1]: https://link.aps.org/doi/10.1103/PhysRevD.94.124021 [^43_2]: https://web.mit.edu/hyperbook/Patrikalakis-Maekawa-Cho/node139.html [^43_3]: http://www.scholarpedia.org/article/Boundary_value_problem [^43_4]: https://philsci-archive.pitt.edu/28266/1/Poor Man's Introduction to Gauge Symmetries 2.17.26 - Copy.pdf [^43_5]: https://link.aps.org/doi/10.1103/RevModPhys.73.663 [^43_6]: http://philsci-archive.pitt.edu/16981/1/notes_des3.pdf [^43_7]: http://www.damtp.cam.ac.uk/user/tong/gaugetheory/gt.pdf [^43_8]: https://www.sciencedirect.com/science/article/abs/pii/S1355219811000384 [^43_9]: https://arxiv.org/html/2504.17483v2 [^43_10]: http://deferentialgeometry.org/papers/Ghiotti - Gauge fixing and BRST formalism in non-Abelian gauge theories.pdf [^43_11]: https://www.cambridge.org/core/elements/gauge-symmetries-symmetry-breaking-… --- # now in the minkowski field, it does seem that there is an "alternate reality" but it is of course bounded by the limits of what can differ. just like the observers measure differebt relativistic parameters like coincidenct time, they also measure and recodd different particles. if it were possible to construct an object out of these particles, for example by tightly folding spacetime a quabtun scalds, only one observer would experience this object. the other observer might see them respond to it, but be unable to interact with it. here is a different example: can the minkowski particles emit light? if so, which of the observers can measure this light? The closest standard answer is: not really an alternate reality, but an observer-dependent **particle interpretation** of one shared quantum field state. In Minkowski space, inertial and accelerated observers may disagree about which modes count as particles, yet they are still describing the same underlying field and the same detector couplings along actual worldlines.[^44_1][^44_2][^44_3][^44_4][^44_5] ## About your private object idea Your imagined object made from “observer-only particles” does not seem like the standard reading of Unruh/Rindler physics. In the standard detector picture, what changes with the observer is the detector response and the mode decomposition, not the existence of a wholly private material object that one observer could build and the other could never in principle couple to.[^44_6][^44_3][^44_4][^44_7] There is still a real asymmetry, though. A uniformly accelerated observer is confined to a Rindler wedge with a horizon, so there are regions and mode decompositions that are natural for that observer and not for an inertial one, and vice versa. That gives observer-dependent accessibility and observer-dependent particle language, but it does not by itself seem to imply a hidden object ontology living in a separate branch of Minkowski space.[^44_8][^44_2][^44_9][^44_4][^44_7] ## Do they disagree on particles? Yes, in the technical QFT sense they can disagree about what particles are present. An inertial observer can call the Minkowski state vacuum while a uniformly accelerated observer describes that same state as thermally populated with Rindler quanta.[^44_10][^44_2][^44_3] But that disagreement is about the decomposition of the field into quanta, not necessarily about raw local events such as whether a given detector clicked. The detector statistics depend on the field state and the detector’s trajectory and coupling, so two differently moving detectors may genuinely record different click patterns while still belonging to one consistent field-theoretic description.[^44_3][^44_11][^44_12][^44_6] ## Can “Minkowski particles” emit light? If by “Minkowski particles” you mean ordinary field excitations defined in inertial quantization, then yes, ordinary excitations or accelerated sources can produce radiation that inertial observers describe as emitted quanta propagating through Minkowski space. In that case, inertial observers are the natural ones to describe and detect that light as ordinary radiation at infinity.[^44_13][^44_1] Accelerated observers can also describe related processes, but often in a different language: detector excitation, thermal response, absorption from a Rindler bath, or emission interpreted in Rindler modes. So the “same thing” may be called emitted Minkowski radiation by one description and detector response to a thermal bath by another.[^44_14][^44_10][^44_1][^44_3] ## Who can measure the light? If the emitted radiation is a genuine Minkowski excitation that propagates into a region accessible to an inertial detector, then inertial observers can measure it. An accelerated observer can measure field excitations along their worldline too, but what they call “particles” is tied to the Rindler description and to the wedge they can access.[^44_9][^44_1][^44_13][^44_8][^44_3] There is an important nuance here: the literature is not perfectly uniform on whether a uniformly accelerated detector should be said to emit observable radiation to infinity in every formulation. One source notes that many authors associate detector transitions with emitted particles seen far away, while others argue the accelerating object thermalizes without producing such emitted photons in the naive sense. So the safest statement is that accelerated-detector radiation is a subtle and partly interpretation-sensitive topic, even though the Unruh detector response itself is a standard result.[^44_10][^44_1] ## Cleanest framing A careful way to say all this is: accelerated and inertial observers do not get separate universes, but they may use inequivalent particle languages for the same field state, and detectors following different worldlines can record different event statistics. If something is built from genuinely propagating field excitations, then other observers may also couple to it when causal access allows; what is observer-dependent is mainly the decomposition into particles, not the existence of a second sealed material world.[^44_4][^44_7][^44_1][^44_8][^44_9][^44_6] The next sharp distinction to make is between **Rindler particles** and **Minkowski particles**, because that is exactly where your “private object” intuition is coming from. <span style="display:none">[^44_15][^44_16][^44_17][^44_18][^44_19][^44_20][^44_21][^44_22][^44_23][^44_24][^44_25][^44_26][^44_27][^44_28][^44_29][^44_30][^44_31][^44_32][^44_33][^44_34][^44_35]</span> <div align="center">⁂</div> [^44_1]: https://link.aps.org/doi/10.1103/RevModPhys.80.787 [^44_2]: https://people.math.wisc.edu/~thiffeault/talks/rindler.pdf [^44_3]: http://www.scholarpedia.org/article/Unruh_effect [^44_4]: https://en.wikipedia.org/wiki/Quantum_field_theory_in_curved_spacetime [^44_5]: https://rojefferson.blog/2021/01/15/qft-in-curved-space-part-2-bogolyubov-t… [^44_6]: https://inspirehep.net/files/3011ea3cd4962554f94754eb174637c4 [^44_7]: https://inspirehep.net/files/69f46898f34d8cdc4af258833b15f11c [^44_8]: https://www.emergentmind.com/topics/right-rindler-wedge-rrw [^44_9]: https://www.nuclear-power.com/rindler-coordinates/ [^44_10]: https://en.wikipedia.org/wiki/Unruh_effect [^44_11]: https://link.aps.org/doi/10.1103/PhysRevD.102.045002 [^44_12]: https://www.nature.com/articles/srep07195 [^44_13]: https://ar5iv.labs.arxiv.org/html/gr-qc/0507054 [^44_14]: https://arxiv.org/html/2508.11637v2 [^44_15]: https://www.reddit.com/r/AskPhysics/comments/ihrmw7/why_doesnt_unruh_radiat… [^44_16]: https://inspirehep.net/literature/269631 [^44_17]: https://spacefed.com/physics/unruh-hawking-radiation-observed-in-accelerati… [^44_18]: https://www.youtube.com/watch?v=5wCpkUnaRaQ [^44_19]: https://link.aps.org/doi/10.1103/PhysRevA.103.042225 [^44_20]: https://link.aps.org/doi/10.1103/PhysRevD.102.105016 [^44_21]: https://indico.cern.ch/event/322559/contributions/748503/attachments/623891… [^44_22]: https://courses.physics.ucsd.edu/2014/Fall/physics215a/project/Unruh1.pdf [^44_23]: https://www.sciencedirect.com/science/article/pii/S0370269310006180 [^44_24]: https://pirsa.org/24040084 [^44_25]: https://link.aps.org/doi/10.1103/PhysRevX.9.011007 [^44_26]: https://arxiv.org/pdf/2601.20971.pdf [^44_27]: https://link.aps.org/doi/10.1103/PhysRevD.101.124001 [^44_28]: https://arxiv.org/abs/1109.5129 [^44_29]: https://arxiv.org/html/2405.20995v1 [^44_30]: https://en.wikipedia.org/wiki/Rindler_coordinates [^44_31]: https://www.physicsforums.com/threads/rindler-coordinates-in-minkowski-spac… [^44_32]: https://pmc.ncbi.nlm.nih.gov/articles/PMC6620287/ [^44_33]: https://www.reddit.com/r/AskPhysics/comments/1hskggu/questions_about_rindle… [^44_34]: https://link.aps.org/doi/10.1103/PhysRevD.108.025002 [^44_35]: https://pubs.aip.org/aip/jmp/article/53/1/012107/233121/Coherences-of-accel… --- # what does this mean: "If something is built from genuinely propagating field excitations, then other observers may also couple to it when causal access allows" what is a rindler particle? remind me which observer experiences particles What I meant is: if a field excitation is a real propagating disturbance of the shared quantum field, then another observer can in principle detect or interact with it **provided** the excitation can actually reach that observer’s worldline and the observer has a detector that couples to that field. “Causal access” just means no horizon or light-cone restriction prevents signals from traveling between them.[^45_1][^45_2][^45_3] ## Causal access In relativistic field theory, an observer only has access to events and excitations in the region of spacetime that can influence their detector. For uniformly accelerated observers, the Rindler horizon cuts off part of Minkowski spacetime, so they do not have access to the whole spacetime the inertial observer describes.[^45_2][^45_4][^45_5][^45_1] So if an excitation stays in a region outside that observer’s accessible wedge, that observer does not get to measure it. If the excitation crosses their accessible region and their detector couples to that field, then they can register it.[^45_3][^45_5][^45_1][^45_2] ## Rindler particle A Rindler particle is a particle defined using the mode decomposition natural to a uniformly accelerated observer, rather than the decomposition natural to an inertial observer in Minkowski coordinates. More specifically, it is a quantum of a Rindler mode, sometimes called a Fulling–Rindler particle.[^45_6][^45_7] This is why the same underlying field state can be described as “no Minkowski particles” by one observer and as “a thermal bath of Rindler particles” by another.[^45_8][^45_9][^45_6] ## Which observer sees particles In the standard Unruh setup, the inertial observer says the Minkowski vacuum contains no particles. The uniformly accelerated observer, also called a Rindler observer, experiences that same Minkowski vacuum as a thermal bath of particles.[^45_10][^45_11][^45_12][^45_6][^45_8] So, if you want the one-line memory aid: **inertial observer sees vacuum, accelerated observer sees thermal particles**.[^45_6][^45_10] ## Important nuance That does not mean the accelerated observer lives in a separate world. It means the notion of “particle” is observer-dependent, while the underlying field and the actual detector worldlines are shared parts of one spacetime description.[^45_9][^45_13][^45_14][^45_6] A practical translation is: “particle” is not the most fundamental object here; the field is, and different observers decompose it differently.[^45_13][^45_6] <span style="display:none">[^45_15][^45_16][^45_17][^45_18][^45_19][^45_20]</span> <div align="center">⁂</div> [^45_1]: https://www.frontiersin.org/journals/physics/articles/10.3389/fphy.2020.525… [^45_2]: https://arxiv.org/html/2302.03340v3 [^45_3]: https://inspirehep.net/files/69f46898f34d8cdc4af258833b15f11c [^45_4]: https://en.wikipedia.org/wiki/Rindler_coordinates [^45_5]: https://www.emergentmind.com/topics/right-rindler-wedge-rrw [^45_6]: https://link.aps.org/doi/10.1103/RevModPhys.80.787 [^45_7]: http://aesop.phys.utk.edu/ph611/2009/projects/papers/Kharel.pdf [^45_8]: https://people.math.wisc.edu/~thiffeault/talks/rindler.pdf [^45_9]: http://www.scholarpedia.org/article/Unruh_effect [^45_10]: https://en.wikipedia.org/wiki/Unruh_effect [^45_11]: https://www.nature.com/articles/s41467-019-10962-y [^45_12]: https://pmc.ncbi.nlm.nih.gov/articles/PMC6620287/ [^45_13]: https://en.wikipedia.org/wiki/Quantum_field_theory_in_curved_spacetime [^45_14]: https://inspirehep.net/files/3011ea3cd4962554f94754eb174637c4 [^45_15]: https://fondationlouisdebroglie.org/AFLB-391/aflb391m780.pdf [^45_16]: https://bohr.physics.berkeley.edu/classes/h190/s13/reprints/Mukhanov_Winitz… [^45_17]: https://www.youtube.com/watch?v=7cj6oiFDEXc [^45_18]: https://www.emergentmind.com/topics/right-rindler-wedge [^45_19]: https://www.icts.res.in/sites/default/files/seminar doc files/Tutorial note Yiming Chen.pdf [^45_20]: https://courses.physics.ucsd.edu/2014/Fall/physics215a/project/Unruh1.pdf --- # don't thermal particles usually emit radiation? Usually yes, thermal matter or a thermalized body does emit radiation. But in thermal equilibrium it also absorbs radiation at the same rate, so there may be no net outward radiative change even though emission processes are happening.[^46_1][^46_2][^46_3][^46_4] ## Ordinary thermal case For everyday hot matter, “thermal particles emit radiation” is a good instinct. Blackbody radiation is the standard example: matter at temperature $T$ emits thermal electromagnetic radiation with a spectrum set by that temperature.[^46_5][^46_6][^46_1] ## Unruh case The Unruh situation is subtler. The accelerated observer experiences the Minkowski vacuum as a thermal bath, but whether this should always be described as ordinary emitted radiation in the inertial frame is a more delicate and partly interpretation-sensitive question.[^46_7][^46_8][^46_9] One source says it is widely believed that detector transitions are accompanied by emitted particles that propagate to infinity as “Unruh radiation,” while also noting that this is not universal and that some authors argue the emission and absorption rates balance so that no emitted photons appear in the naive sense. Another source similarly frames the issue by noting that one may suspect radiation analogous to Larmor radiation, which is exactly why the question is subtle.[^46_8][^46_7] ## Best way to say it So the safe answer is: - Thermal systems generally do emit radiation.[^46_3][^46_1] - In equilibrium, emission is balanced by absorption, so “thermal” does not automatically mean a net glow to every observer.[^46_2][^46_4] - For the Unruh bath, the detector’s thermal response is standard, but translating that into emitted radiation seen by another observer is a nontrivial and not completely interpretation-free step.[^46_9][^46_7][^46_8] A compact intuition is: “thermal” guarantees fluctuating excitations and transition rates, not necessarily a simple observer-independent picture of freely escaping light.[^46_4][^46_8] <span style="display:none">[^46_10][^46_11][^46_12][^46_13][^46_14][^46_15][^46_16]</span> <div align="center">⁂</div> [^46_1]: https://en.wikipedia.org/wiki/Black-body_radiation [^46_2]: https://phys.libretexts.org/Bookshelves/University_Physics/University_Physi… [^46_3]: https://acd-ext.gsfc.nasa.gov/anonftp/acd/daac_ozone/Lecture4/Text/Semifina… [^46_4]: https://en.wikipedia.org/wiki/Detailed_balance [^46_5]: https://people.sissa.it/~perrotta/lezioni_2023_2024/chapter2.pdf [^46_6]: https://www.encyclopedie-environnement.org/en/physics/thermal-radiation-of-… [^46_7]: https://arxiv.org/abs/1301.7543 [^46_8]: https://en.wikipedia.org/wiki/Unruh_effect [^46_9]: https://link.aps.org/doi/10.1103/RevModPhys.80.787 [^46_10]: https://www.youtube.com/watch?v=DHG61XPuWyk [^46_11]: https://www.pnas.org/doi/10.1073/pnas.2410280121 [^46_12]: https://indico.ictp.it/event/a05217/session/37/contribution/27/material/0/0… [^46_13]: https://galileo.phys.virginia.edu/classes/252/black_body_radiation.html [^46_14]: https://www.mso.anu.edu.au/~geoff/HEA/7_Emission_and_Absorption.pdf [^46_15]: https://mcgreevy.physics.ucsd.edu/s12/lecture-notes/chapter07.pdf [^46_16]: https://scanalyst.fourmilab.ch/t/detecting-the-unruh-effect/524 --- # it sounds edgy, poorly delineated. i'm aware of hawking radiation, is this an example of this kind of effect? has it ever been tested that different observers measure different particles? can the particles have mass? Yes—Hawking radiation is very much the same **kind** of phenomenon in the broad sense: quantum fields on a background with horizons or strong spacetime structure lead different observers or regions to use different particle notions. Hawking radiation and the Unruh effect are closely linked conceptually, and the Unruh effect is often presented as a way to understand black-hole particle emission.[^47_1][^47_2][^47_3] ## Hawking and Unruh In Hawking radiation, the curved spacetime near a black-hole horizon makes the vacuum/particle split ambiguous, so an asymptotic observer sees outgoing thermal radiation. In the Unruh effect, Minkowski vacuum is still vacuum for inertial observers, but a uniformly accelerated observer assigns a thermal bath of Rindler particles to that same state.[^47_4][^47_2][^47_3][^47_5][^47_6][^47_7] ## Has it been tested This area is indeed a bit edgy and hard to probe directly. The Unruh effect is a standard theoretical consequence of quantum field theory, but the search results here indicate that direct unambiguous experimental confirmation remains difficult, with proposals, simulations, and analog experiments rather than a universally accepted direct observation in the clean textbook sense.[^47_3][^47_8][^47_9][^47_1] So the idea that different observers assign different particles is very well developed theoretically, but direct laboratory verification in the exact observer-dependent sense is still subtle. Some recent work studies detector responses and quantum simulations that reproduce key features, but that is not quite the same as strapping two human observers into idealized frames and comparing notebooks.[^47_10][^47_11][^47_1][^47_3] ## Can the particles have mass Yes, they can. The review source explicitly discusses coupling an Unruh-DeWitt detector to a real **massive** scalar field and notes conditions under which the detector can absorb massive scalar particles from the thermal bath.[^47_3] So the observer-dependent particle idea is not restricted to massless lightlike quanta. Mass changes the details of the response and spectrum, but it does not seem to remove the basic phenomenon.[^47_10][^47_3] ## What to take literally The safest thing to take literally is not “different realities,” but “different observer-dependent decompositions of the same field state into particles.” That still allows different detectors to record different excitation statistics, especially when they move differently or live in different horizon-limited regions, without requiring a full split into separate worlds.[^47_2][^47_11][^47_7][^47_3] A compact memory aid is: inertial observer sees Minkowski vacuum, accelerated observer sees a thermal bath, black-hole exterior observer sees Hawking radiation.[^47_5][^47_4][^47_3] <span style="display:none">[^47_12][^47_13][^47_14][^47_15][^47_16]</span> <div align="center">⁂</div> [^47_1]: https://www.nature.com/articles/s41467-023-39064-6 [^47_2]: http://www.blau.itp.unibe.ch/QFTCST [^47_3]: https://link.aps.org/doi/10.1103/RevModPhys.80.787 [^47_4]: https://en.wikipedia.org/wiki/Hawking_radiation [^47_5]: https://www.nature.com/articles/s41467-019-10962-y [^47_6]: https://math.ucr.edu/home/baez/physics/Relativity/BlackHoles/hawking.html [^47_7]: http://www.scholarpedia.org/article/Unruh_effect [^47_8]: https://gizmodo.com/unruh-effect-visible-in-the-lab-physics-1848977467 [^47_9]: https://sites.dartmouth.edu/dujs/2020/12/07/the-unruh-effect-newly-proposed… [^47_10]: https://arxiv.org/abs/2409.12697 [^47_11]: https://pmc.ncbi.nlm.nih.gov/articles/PMC6620287/ [^47_12]: http://generalrelativityandlqg.co.uk/AMGR/Vol_I_chap_7.pdf [^47_13]: https://arxiv.org/html/2407.18652v2 [^47_14]: https://arxiv.org/html/2409.12697v1 [^47_15]: https://link.aps.org/doi/10.1103/PhysRevResearch.2.023107 [^47_16]: https://www.reddit.com/r/cosmology/comments/1fv0gnl/which_explanation_of_ha… --- # are you implying there is a spot near the horizon of a black hole where a point observer could stand to be unable to measure a hawking radiation particle that others could measure? Not exactly a single special “spot.” The more accurate statement is that **different trajectories** near the horizon lead to different detector responses: a freely falling observer near the horizon may fail to directly detect Hawking quanta there, while a hovering observer outside the horizon attributes a local thermal flux to the field.[^48_1][^48_2][^48_3][^48_4] ## Near-horizon observers A freely falling detector near a Schwarzschild horizon does not seem to directly measure Hawking radiation in the simple particle-counting sense, because its available proper time near the horizon is too short for thermalization. By contrast, a static observer held at fixed radius outside the horizon is accelerated, and local descriptions assign that observer a thermal bath whose temperature increases as the horizon is approached.[^48_2][^48_3][^48_4][^48_1] ## Not at the horizon There is also an important limit here: an observer cannot simply hover exactly on the horizon as an ordinary physical stationary observer, because remaining static arbitrarily close to the horizon requires extreme acceleration, and the local temperature seen by such static observers grows without bound as the horizon is approached. So the contrast is not “one observer standing at one point and another at the same point,” but rather “different possible worldlines in the same near-horizon region.”[^48_4][^48_1][^48_2] ## What this implies So yes, there are near-horizon situations where one observer trajectory can fail to register Hawking particles that another trajectory would describe as present. But that is still usually framed as observer-dependent detection and observer-dependent particle language for the same quantum field, not as two separate local universes occupying the same place.[^48_1][^48_2] A compact memory aid is: **free-fall near the horizon is comparatively quiet; hovering near the horizon is thermally noisy**.[^48_4][^48_1] <span style="display:none">[^48_10][^48_11][^48_12][^48_13][^48_14][^48_15][^48_5][^48_6][^48_7][^48_8][^48_9]</span> <div align="center">⁂</div> [^48_1]: https://link.aps.org/doi/10.1103/y7kj-4zjw [^48_2]: https://en.wikipedia.org/wiki/Hawking_radiation [^48_3]: https://arxiv.org/abs/2501.06609 [^48_4]: https://pmc.ncbi.nlm.nih.gov/articles/PMC5253844/ [^48_5]: http://backreaction.blogspot.com/2015/12/hawking-radiation-is-not-produced-… [^48_6]: https://arxiv.org/html/2501.06609v1 [^48_7]: https://ats.org/press-release/measuring-stationary-hawking-radiation/ [^48_8]: https://www.reddit.com/r/TheoreticalPhysics/comments/1rs42je/how_does_vacuu… [^48_9]: https://www.reddit.com/r/AskPhysics/comments/1ogrwku/if_black_holes_evapora… [^48_10]: https://inspirehep.net/literature/1851794 [^48_11]: https://en.wikipedia.org/wiki/Unruh_effect [^48_12]: https://www.scientificamerican.com/article/hawking-radiation/ [^48_13]: https://inspirehep.net/literature/787263 [^48_14]: https://www.sciencedirect.com/topics/physics-and-astronomy/hawking-radiation [^48_15]: https://www.reddit.com/r/AskPhysics/comments/xh9rvu/if_hawking_radiation_is… --- # we vd measured hawking radiation travelling over light years, yes? but this free-falling observer cannot measure it? it is something that only exists in part (most) of the universe, but not in their reference frame? can they measure energy changes or such equivalent to it? No—we have **not** directly measured astrophysical Hawking radiation arriving from black holes over light-years. What exists so far is a strong theoretical prediction, plus analogue/laboratory systems that reproduce Hawking-like behavior, rather than a clean direct detection from an actual astrophysical black hole.[^49_1][^49_2][^49_3][^49_4][^49_5] ## Distant detection For ordinary astrophysical black holes, Hawking radiation is expected to be extremely weak, which is why current searches have not produced a direct observational detection of the real astronomical signal. So the picture is not “we already see it everywhere except for the infalling observer,” but rather “the effect is theoretically robust, while direct astrophysical measurement remains elusive.”[^49_6][^49_3][^49_1] ## Free-fall observer Near the horizon, a freely falling observer does not seem to directly detect Hawking particles in the simple thermalized-detector sense right there at horizon crossing. The reason given in the recent result is that the time needed for the detector to thermalize is longer than the proper time the observer spends in the near-horizon region.[^49_7][^49_8] That does **not** mean the effect exists in most of the universe but disappears from reality in their frame. It means the particle interpretation and detector response are observer-dependent, so the same field state is described differently along different worldlines.[^49_8][^49_6] ## Energy changes Yes, a freely falling observer can still register related physical effects, just not necessarily as “I saw a clean Hawking particle gas right at the horizon.” The same paper says the detector response changes smoothly and defines an effective temperature along the infalling path, even though the signal near the horizon is dominated by switching effects rather than direct detection of Hawking quanta.[^49_7] So an infaller may measure excitation probabilities, response functions, and effective-temperature-like behavior that encode the same underlying field physics. A static observer farther out, by contrast, is the one more naturally associated with a thermal Hawking flux description outside the hole.[^49_9][^49_6][^49_7] ## Best phrasing A careful way to say it is: Hawking radiation is not “only in part of the universe,” but the statement “there are Hawking particles here” is tied to the observer and detector used. The underlying quantum field is shared, while the particle count and thermal interpretation depend on trajectory, horizon structure, and what the detector has time to measure.[^49_6][^49_8][^49_9][^49_7] <span style="display:none">[^49_10][^49_11][^49_12][^49_13][^49_14][^49_15][^49_16][^49_17]</span> <div align="center">⁂</div> [^49_1]: https://www.syfy.com/syfy-wire/has-hawking-radiation-ever-been-observed [^49_2]: https://ats.org/press-release/measuring-stationary-hawking-radiation/ [^49_3]: https://www.advancedsciencenews.com/black-hole-morsels-could-make-hawking-r… [^49_4]: https://physicsworld.com/a/physicists-stimulate-hawking-radiation-from-opti… [^49_5]: https://www.scientificamerican.com/article/hawking-radiation/ [^49_6]: https://en.wikipedia.org/wiki/Hawking_radiation [^49_7]: https://arxiv.org/abs/2501.06609 [^49_8]: https://link.aps.org/doi/10.1103/y7kj-4zjw [^49_9]: https://pmc.ncbi.nlm.nih.gov/articles/PMC5253844/ [^49_10]: https://www.reddit.com/r/AskPhysics/comments/1mp9zj1/ive_been_reading_that_… [^49_11]: https://news.mit.edu/2021/hawkings-black-hole-theorem-confirm-0701 [^49_12]: https://arxiv.org/html/2404.09274v1 [^49_13]: https://astro.cornell.edu/news/hawkings-black-hole-theorem-observationally-… [^49_14]: https://www.reddit.com/r/blackholes/comments/1nyfhkb/how_does_an_observer_c… [^49_15]: https://www.facebook.com/groups/828178678983076/posts/1254027956398144/ [^49_16]: https://astro.theoj.org/article/132347-hawking-radiation-from-non-evaporati… [^49_17]: https://www.physicsforums.com/threads/why-does-an-infalling-observer-not-se… --- # we have created particles with spacetime curvature in lab settings? Not in the literal sense of curving actual astrophysical spacetime enough in the lab to create real gravitational particle production. What has been done is to build **analogue systems** or effective curved-spacetime simulators in media such as Bose–Einstein condensates, optical fibers, and other controlled materials, where excitations obey equations mathematically similar to quantum fields in curved spacetime.[^50_1][^50_2][^50_3][^50_4] ## What has been done Experiments have reported Hawking-like radiation in analogue black-hole systems, including optical analogues with artificial event horizons and Bose–Einstein-condensate systems with sonic horizons. There are also quantum simulation proposals and experiments aimed at effective particle creation in curved-spacetime-like settings rather than direct gravitational curvature generated by mass-energy in the lab.[^50_2][^50_5][^50_4][^50_1] ## What that means So the safe statement is: we have not bent real spacetime in the laboratory strongly enough to produce standard gravitational Hawking radiation from actual black holes. But we have created laboratory systems whose effective equations mimic curved spacetime well enough to observe Hawking-like or particle-creation-like effects in the corresponding quasiparticles or field excitations.[^50_5][^50_3][^50_4][^50_1][^50_2] ## Why people care Physicists care because these analogue results test the robustness of the underlying mathematics, such as horizon-induced mode mixing and particle creation, even when the microscopic system is very different from gravity. That does not prove every detail of astrophysical Hawking radiation, but it does support the idea that the effect follows from broad kinematic structures rather than from one very special microscopic model.[^50_3][^50_1][^50_2] A compact way to say it is: we have made **effective** curved spacetimes and observed analogue particle creation, but we have not yet made real gravity strong enough in the lab to do this with actual spacetime curvature itself.[^50_4][^50_5][^50_3] <span style="display:none">[^50_10][^50_11][^50_12][^50_13][^50_14][^50_15][^50_6][^50_7][^50_8][^50_9]</span> <div align="center">⁂</div> [^50_1]: https://www.nature.com/articles/nature.2014.16131 [^50_2]: https://link.aps.org/doi/10.1103/PhysRevLett.122.010404 [^50_3]: https://cordis.europa.eu/article/id/205484-hawking-radiation-now-better-und… [^50_4]: https://www.uni-heidelberg.de/en/newsroom/curved-spacetime-in-the-lab [^50_5]: https://journals.plos.org/plosone/article?id=10.1371%2Fjournal.pone.0229382 [^50_6]: https://www.sciencealert.com/physicists-simulated-a-black-hole-in-the-lab-a… [^50_7]: https://www.scientificamerican.com/article/hawking-radiation/ [^50_8]: https://phys.org/news/2019-01-hawking-laboratory-black-hole-analogues.html [^50_9]: https://www.techexplorist.com/effective-curved-spacetime-lab/56467/ [^50_10]: https://pubmed.ncbi.nlm.nih.gov/31012667/ [^50_11]: https://www.universetoday.com/articles/hawking-radiation-replicated-in-a-la… [^50_12]: https://www.facebook.com/groups/828178678983076/posts/1254027956398144/ [^50_13]: https://arxiv.org/abs/1804.04092 [^50_14]: https://physicsworld.com/a/physicists-stimulate-hawking-radiation-from-opti… [^50_15]: https://www.youtube.com/watch?v=C9hFQd19RsI --- # how does the energy balance work with these particles The short answer is: the energy is not created from nowhere. The source depends on the setup—black-hole mass for Hawking radiation, the external force doing the acceleration for Unruh-type detector excitations, and the pump or moving medium for laboratory analogues.[^51_1][^51_2][^51_3] ## Black holes For Hawking radiation, the outgoing quanta are balanced by a decrease in the black hole’s mass and, when relevant, its angular momentum or charge. That is why Hawking radiation leads to black-hole evaporation rather than free energy creation.[^51_4][^51_1] ## Accelerated observers For the Unruh effect, an accelerated detector can become excited even when an inertial observer calls the field vacuum. In that case, the relevant energy bookkeeping is tied to the work required to keep the detector accelerating, and some discussions also note that emission and absorption can balance in a way that complicates the naive “it just radiates” picture.[^51_5][^51_3] ## Lab analogues In analogue Hawking experiments, the energy comes from the engineered background, such as the pump pulse or moving medium that creates the effective horizon. One source states this very directly for an optical analogue: the observed signature means energy is drawn from the pump pulse.[^51_6][^51_2] ## Practical picture So the bookkeeping rule is: - Real Hawking quanta cost the black hole mass-energy.[^51_1][^51_4] - Unruh-type detector excitations are paid for by the acceleration setup or external agent maintaining the trajectory.[^51_3][^51_5] - Analogue “particles” are paid for by the lab apparatus driving the effective spacetime.[^51_2][^51_6] A good compact slogan is: horizons and acceleration let the field rearrange its excitations, but the energy still comes from whatever background or external system made that horizon-like situation possible.[^51_2][^51_3][^51_1] <span style="display:none">[^51_10][^51_11][^51_12][^51_13][^51_14][^51_15][^51_7][^51_8][^51_9]</span> <div align="center">⁂</div> [^51_1]: https://en.wikipedia.org/wiki/Hawking_radiation [^51_2]: https://physicsworld.com/a/physicists-stimulate-hawking-radiation-from-opti… [^51_3]: https://en.wikipedia.org/wiki/Unruh_effect [^51_4]: https://arxiv.org/html/2505.04812v1 [^51_5]: https://link.aps.org/doi/10.1103/PhysRevD.102.045002 [^51_6]: https://pmc.ncbi.nlm.nih.gov/articles/PMC7422882/ [^51_7]: https://www.reddit.com/r/AskPhysics/comments/1mxmud5/why_does_hawking_radia… [^51_8]: https://www.facebook.com/museumofscience/posts/according-to-stephen-hawking… [^51_9]: https://www.sciencedirect.com/science/article/pii/S0370269325006471 [^51_10]: https://www.youtube.com/watch?v=E7pakDMnuMY [^51_11]: https://www.nature.com/articles/srep07195 [^51_12]: https://link.aps.org/doi/10.1103/PhysRevB.86.144505 [^51_13]: https://arxiv.org/html/2501.11516v1 [^51_14]: https://arxiv.org/html/2603.03525v1 [^51_15]: https://pmc.ncbi.nlm.nih.gov/articles/PMC6620287/ --- # if hawking radiation is made from precise field equations does that mean information is technically preserved even if it falls into one? Probably **yes** in the modern view, but not for the simple reason “the equations are precise.” The key issue is whether black-hole evaporation is ultimately unitary, and current mainstream opinion is that information is preserved, even though Hawking’s original semiclassical calculation made the outgoing radiation look exactly thermal and therefore information-erasing.[^52_1][^52_2][^52_3] ## What the paradox is Hawking’s original calculation suggested that a pure initial state could evolve into thermal Hawking radiation, which would amount to information loss. That is the black hole information paradox, because ordinary quantum mechanics says unitary evolution should preserve the full information in the state.[^52_2][^52_4][^52_1] ## Current view The predominant modern view is that information is preserved in black-hole evaporation. Evidence behind that view includes Page-curve arguments and holographic ideas such as AdS/CFT, which suggest the outgoing radiation is not exactly featureless thermal noise but contains subtle correlations encoding the information.[^52_5][^52_3][^52_6][^52_1] ## Important nuance That does **not** mean the problem is fully settled in a universally agreed mechanical sense. The broad consensus has shifted from “is information destroyed?” toward “how exactly is the information preserved and recovered?”[^52_7][^52_1][^52_5] ## So to your wording If by “made from precise field equations” you mean “there is a mathematically definite quantum evolution,” then yes, that points toward preservation only if the full theory is unitary. But the semiclassical field equations alone were precisely what created the paradox, because taken at face value they made the radiation appear thermal and informationless.[^52_1][^52_2] A compact way to say it is: modern physics mostly expects information to survive black-hole evaporation, but the exact bookkeeping mechanism is still an active research topic rather than a finished closed case.[^52_5][^52_1] <span style="display:none">[^52_10][^52_11][^52_12][^52_13][^52_14][^52_15][^52_8][^52_9]</span> <div align="center">⁂</div> [^52_1]: https://en.wikipedia.org/wiki/Black_hole_information_paradox [^52_2]: https://arxiv.org/pdf/2207.09421.pdf [^52_3]: https://physics.mit.edu/news/has-the-black-hole-information-paradox-evapora… [^52_4]: https://quantumzeitgeist.com/the-information-paradox-what-happens-to-data-t… [^52_5]: https://www.newscientist.com/article/mg26435140-700-solving-stephen-hawking… [^52_6]: https://physics.berkeley.edu/news/ ‘islands’-poking-out-black-holes-may-solve-information-paradox [^52_7]: https://www.reddit.com/r/AskPhysics/comments/1o2dpg5/has_the_black_hole_inf… [^52_8]: https://www.scientificamerican.com/article/does-information-ever-really-dis… [^52_9]: https://www.youtube.com/watch?v=Jk_oYAUyHAM [^52_10]: https://nhsjs.com/2026/hawking-radiation-and-the-black-hole-information-par… [^52_11]: https://quantumzeitgeist.com/black-hole-information-isn-lost/ [^52_12]: https://arxiv.org/html/2504.00516v1 [^52_13]: https://profmattstrassler.com/articles-and-posts/relativity-space-astronomy… [^52_14]: https://physics.yale.edu/sites/default/files/files/BlackHoleEntropyPage2021… [^52_15]: https://www.quantamagazine.org/can-information-escape-a-black-hole-20240411/

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Decentralized Execution Network
by Joseph Oyinkan 06 Sep '26

06 Sep '26

Hey guys, I've been working on a new protocol called the Marketplace which is a decentralized execution protocol that co-ordinates and economizes the execution of computational work across a peer-to-peer network of nodes. Where there is no barrier to the node participation. Unlike modern cloud computing systems that rent out virtual computers to consumers, A marketplace consumer sends compute bound tasks directly to the network returning a future which the consumer can asynchronously check for completion. Similar to offloading a compute task to a free cpu core by an OS. A consumer can also fire multiple independent compute bound tasks to run in parallel, and check their status asynchronously while doing other tasks, similar to I/O operations. The compute bound task sent by the consumer is executed using a protocol specified in the Marketplace whitepaper (see attachment) where randomly selected idle devices on the network execute the compute work and receive financial rewards (money). Please read the whitepaper, and if you are interested, reply to this message. I've written some 50% to 60% of the code, and have just open sourced it on github.

1 0

From Shells to Peer-to-Peer
by Loid Protagonist 06 Sep '26

06 Sep '26

Long ago: Shells → witness → fish Then: Banking → gold → money → banking And later, it became more complicated: Banking → banker → money → government → app → money → market → app → courier → app → user → money → app → banker → banking Today: Peer-to-peer. Why has moving value from one person to another become so complicated? Sometimes we have solutions, but we also create new obstacles. Perhaps the challenge is not only creating new forms of money, but making value move directly from value to value, without requiring a trusted third party. A street note. Loid Protagonist Dikirim dengan email aman [Proton Mail](https://proton.me/mail/home).

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demist: crypto-democratic uprising
by Olivier Michel 02 Sep '26

02 Sep '26

Hi, I am developing demist, an open-source direct democracy protocol: identity is a key pair on your phone or computer (12-word recovery phrase, no accounts, no server-side identity, no biometrics, no state, no company behind it), personhood is established by five in-person certifications (web-of-trust), and ballots are anonymous yet publicly verifiable. Civic participation is rewarded by human-based cryptocurrency (the misthos). The project is aiming at a global democratic uprising. The public alpha is open: break it, audit it, or become a citizen. https://demist.net - Olivier

1 0

Queen Open Source Electronic Cash
by Loid Protagonist 02 Sep '26

02 Sep '26

Hello, I am developing Queen, an experimental open-source peer-to-peer electronic cash system. The project explores a simple blockchain-based electronic cash system without a trusted third party, with the node, wallet, transactions, mining, and blockchain implemented as open-source software. GitHub: https://github.com/loidprotagonist/Queen Whitepaper: https://drive.google.com/file/d/1nfD79S52X1nFtw4q8Y4oMiYYc8eOF912/view?usp=… Queen is still experimental and is not intended to be production-ready software. I am sharing it primarily as an independent experiment and would greatly appreciate any feedback or discussion from the P2P community. Best regards, Loid Protagonist Dikirim dengan email aman [Proton Mail](https://proton.me/mail/home).

1 0