phssra@mathcs.emory.edu (Scott R. Anderson) (07/20/89)
In article <Added.IYfxRrG00Ui38Gzk9=@andrew.cmu.edu> GILL@QUCDNAST.BITNET writes: >Will Martin asked several questions, which got various responses, some >of which were rather dubious attempts at physics. > >>2) Regarding elementary particles -- one of their characteristics is called >>"spin". Is this REALLY "spin" the way a top or gyroscope spins -- that is, > >Here there was some misinformation. Spin is NOT angular momentum!! >Rather, it is a purely quantum mechanical phenomenum (like the quantites >of colour you mention above). People mistakenly think that it is an >angular momentum because of its mathematical properties. In the >mathematics of quantum mechanics, spin transforms and has operators that >are identical to the normal angular momentum operator, though with its >own unique eigenvalues (values that it can take on in particular >physical situations)....In fact, spin is an intrinsic feature of any >particle, just like mass and electrical charge.... >(Another example of using macroscopic terms for unrelated microscopic >processes.) It depends on your definition of angular momentum, I guess. If you insist that angular momentum is something that is defined by L = r x p, then, yes, spin is not angular momentum. But this is a little like saying that energy is something that is defined by K = (1/2)mv^2, and potential energy or mass energy are not really energy. However, if something acts like angular momentum and is interchangeable with angular momentum, then physicists generally prefer to generalize the concept of angular momentum. Different sources of angular momentum are then qualified, when necessary. So, physicists now speak about orbital angular momentum, spin angular momentum, and total angular momentum (the sum of the two). The latter is a conserved quantity, so, for example, when an electron in an atom changes its orbital state and loses one unit of orbital angular momentum, a photon is emitted which carries away one unit of spin angular momentum. The necessary qualification here is that the name "spin" is a misnomer; as pointed out above it is an intrinsic property of elementary particles, and has nothing to do with them spinning like a top. That seemed like a possibility in 1925, and though it was disproven by Dirac in 1928, the name has stuck. * * ** Scott Robert Anderson gatech!emoryu1!phssra * * * ** phssra@unix.cc.emory.edu phssra@emoryu1.bitnet * * * * * ** * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *