From: Georgi Guninski <guninski@guninski.com>
>Disclaimer: I am lamer in physics.
>I think the analogy shows that FTL (superluminal) speed exist and is
>experimentally observed, this doesn't contradict relativity.
>What contradicts relativity is _communication_ or clock synchronization
>faster than light.
>IIRC if you point powerful projector at the Moon and move it slightly,
>this light on Moon will move FTL at least from your point of view.
>Jim's horse was something like this.
Yes, this is what I was alluding to when I used the term, "phase velocity".
that the phase can travel far faster than 'c', nevertheless the information the wave transmits
is limited to its "group velocity", which cannot travel faster than 'c'.
The moon's diameter is about 2169 miles. If you aimed a laser beam to one edge
of the moon, and then quickly (0.01 second) moved the beam to the other edge,
the spot would look like it's travelling at 216,900 miles per second, well beyond 'c'.
(ignoring the fact that the moon is a spheroid, not a flat surface, of course.)
There is no actual communication from those two points on opposite sides of the moon,
nor are massive particles moving from one point to the other.
I learned my physics from course 8.03 at MIT, with the coursebook by Bekefi and
Barrett, titled "Electromagnetic Waves and Vibrations".
https://mitpress.mit.edu/books/electromagnetic-vibrations-waves-and-radiation
Unfortunately, that was the last advanced physics course I took, having to focus on my
major, chemistry. The next Physics course, that I didn't take, was 8.04, "Quantum
Not that I didn't learn quantum at all, but I learned it from the direction of chemistry.
That class was 5.61, and 5.62, Physical Chemistry.