What is spin?
arXiv:physics/0308027
Abstract
This is a late answer to question #79 by R.I. Khrapko, "Does plane wave not carry a spin?," Am. J. Phys. /69/, 405 (2001), and a complement (on gauge invariance, massive spin 1 and 1/2, and massless spin 2 fields) to the paper by H.C. Ohanian, "What is spin?," Am. J. Phys. /54/, 500--505 (1985). In particular, it is confirmed that "spin" is a classical quantity which can be calculated for any field using its definition, namely that it is just the non-local part of the conserved angular momentum. This leads to explicit expressions which are bilinear in the fields and which agree with their standard "quantum" counterparts.
Submitted to Am. J. Phys., 6 pages