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Date: Fri, 14 Dec 2001 09:33:49 -0500 (EST)
From: AIP listserver
To: physnews-mailing@aip.org
Subject: update.568
PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 568 December 7, 2001 by Phillip F. Schewe, Ben Stein,
and James Riordon
ULTRASOUND SCANS ARE AUDIBLE TO A FETUS,
[SSZ: text deleted]
TRACKING DNA MOTION WITH PICOMETER ACCURACY.
[SSZ: text deleted]
BREAKING A QUANTUM SYMMETRY ON THE TABLETOP.
A recurrent theme in art and science, the concept of symmetry has
become a powerful scientific tool for the analysis of physical
systems. However, under special circumstances, a "quantum
anomaly" occurs: the laws of quantum physics break a system's
apparent symmetry. After a long search, a research group (Horacio
Camblong, University of San Francisco, camblong@usfca.edu, and
collaborators at Universidad Nacional de La Plata, Argentina) has
found a relatively simple example of a quantum anomaly: the
interaction of a polar molecule with an electron. A polar molecule,
despite being neutral, has a permanent separation of electric
charge--a dipole. This dipole produces an electric field, which can
capture electrons if it is strong enough. Can such an arrangement
exist as a stable ion, with its "extra" electron? The researchers
formulated the answer to this question in the language of
symmetry. In physics, symmetry means that a system, such as the
molecule-electron arrangement, behaves the same after you
perform a change to it, such as stretching the molecule to larger
scales and making appropriate adjustments to other variables in the
system. At first glance, the electron-molecule interaction exhibits
a remarkable scale invariance: the system "looks" the same when
viewed from different scales in space and time--at least in a
classical physics description which treats the molecule as a dipole
and the electron as a point of charge. But this tidy picture breaks
down with a proper treatment of the system, as prescribed by
quantum field theory. A quantum field theory treatment requires
the process of renormalization, which removes certain
mathematical infinities and inconsistencies from the quantum
approach. This process also makes the molecule's energy levels
discrete or quantized rather than continuous. Examining the
system this way, the researchers found that the scale invariance
broke down. In fact, a large body of existing evidence, both
experimental and numerical, supports their conclusion. While all
other known quantum anomalies occur at high energies (an
example is chiral symmetry in nuclear physics), the work suggests
that quantum symmetry breaking can occur at much lower
energies, in the domain of interacting electrons and molecules.
(Camblong et al., Physical Review Letters, 26 November 2001.)
