In a sci.physics.foundations post, Jay Yablon has brought to light an obscure article by Hans C. Ohanian on the nature of the intrinsic spin of quantum objects and kindly loaded it onto the web: What is Spin? Am J. Phys. 54 (6) June 1986. The abstract is:
According to the prevailing belief, the spin of the electron or some other particle is a mysterious internal angular momentum for which no concrete physical picture is available, and for which there is no classical analog. However, on the basis of an old calculation by Belinfante [Physica 6 887 (1939)], it can be shown that the spin may be regarded as an angular momentum generated by a circulating flow of energy in the wave field of the electron. Likewise, the magnetic moment may be regarded as generated by a circulating flow of charge in the wave field. This provides an intuitivelyl appealing picture and establishes that neither the spin nor the magnetic moment are “internal” — they are not associated with the internal structure of the electron, but rather with the structure of the field. Furthermore, a comparison between calculations of angular momentum in the Dirac and electromagnetic fields shows that the spin of the electrons is entirely analogous to the angular momentum carried by a classical circularly polarized wave.
If you’re interested in the foundations of physics, the above is well worth reading. My efforts on quantum mechanics has been to look at things at a qubit level, where one reduces the number of degrees of freedom down to an absolute minimum. The calculations in the above are of momentum density, and energy density and the like. It’s nice to see them done explicitly.