The fundamental subatomic particles of physics come in two varieties: bosons and fermions. All particles have spin, intrinsic angular momentum. Bosons have spin in whole-number amounts: 0, 1, 2, etc. — for instance, particles of light, photons, have spin 1 and are thus bosons. Fermions have half-integral spin: 1/2, 3/2, 5/2, .... Examples of fermions include electrons, protons, and neutrons.
The difference between bosons and fermions is important to all of nature. For deep reasons of quantum mechanics, bosons "like" to clump together, statistically, and they can form coherent systems of big waves that move in phase, as happens in lasers. Fermions, on the other hand, are absolutely forbidden to be in the same state as each other. Hence, electrons, being good fermions, can't all drop down to their lowest possible energy levels near the nuclei of atoms. That's extraordinarily fortunate, since otherwise all the chemical elements would be pretty much like hydrogen — and it might be tough to get an interesting universe out of such crude building blocks!
The magic of the world is that putting together two such simple sets of rules — one for bosons, another for fermions — can yield the amazing complexity of large-scale systems we observe in our lives every day.
Thursday, June 03, 1999 at 21:33:25 (EDT) = 1999-06-03
(correlates: InfiniteInAllDirections, 2007-09-01 - Half Marathon Practice, Standard Model, ...)