- Category: Science & Technology
(Damon English/UC Berkeley) By Bob Sanders, Media Relations BERKELEY — Of all the assumptions underlying quantum mechanics and the theory that describes how particles interact at the most elementary level, perhaps the most basic is that particles are either bosons or fermions. Bosons, such as the particles of light called photons, play by one set of rules; fermions, including electrons, play by another.
Seven years ago, University of California, Berkeley, physicists asked a fundamental and potentially disturbing question: Do bosons sometimes play by fermion rules? Specifically, do photons act like bosons all the time, or could they sometimes act like fermions?
Based on the results of their experiment to test this possibility, published June 25 in the journal Physical Review Letters, the answer is a solid "no."
The theories of physics – including the most comprehensive theory of elementary particles, Quantum Field Theory, which explains nature's electromagnetic, weak and strong nuclear forces (but not gravity) — rest on fundamental assumptions, said Dmitry Budker, UC Berkeley professor of physics. These assumptions are based on how the real world works, and often produce amazingly precise predictions. But some physicists would like to see them more rigorously tested.
"Tests of (these assumptions) are very important," said Budker. "Our experiment is distinguished from most other experimental searches for new physics in that others can usually be incorporated into the existing framework of the standard model of particles and forces. What we are testing are some of the fundamental assumptions on which the whole standard model is based."
A few of the important quantum numbers of particles are:
Electric charge. Quarks may have 2/3 or 1/3 electron charges, but they only form composite particles with integer electric charge. All particles other than quarks have integer multiples of the electron's charge.
Color charge. A quark carries one of three color charges and a gluon carries one of eight color-anticolor charges. All other particles are color neutral.
Flavor. Flavor distinguishes quarks (and leptons) from one another.
Spin. Spin is a bizarre but important physical quantity. Large objects like planets or marbles may have angular momentum and a magnetic field because they spin. Since particles also to appear to have their own angular momentum and tiny magnetic moments, physicists called this particle property spin. This is a misleading term since particles are not actually "spinning."
Spin is quantized to units of 0, 1/2, 1, 3/2 (times Planck's Constant, ) and so on....MORE...
