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What is density of spin?

09.11.11 12:06

See: https://groups.google.com/group/sci.physics.electromag/browse_thread/thread/e612f171d04c7592?hl

André Gsponer [1], following Humblet, affirms that the density of spin is ds/dV=ExA and the total angular momentum density is dM/dV=Rx(ExB). But these quantities are spatially separated! I asked Grosner about the real angular momentum distribution. The point is:

Allen et al. wrote [2]: “in a beam, the ratio [spin to energy] changes from place to place” (p. 300)

Simmonds and Guttmann wrote [3]: “the electric and magnetic fields can have a nonzero z-component only within the skin region of this wave [i.e. of the beam]. Having z-components within this region implies the possibility of a nonzero z-component of angular momentum within this region. Since the wave is identically zero outside the skin and constant inside the skin region, the skin region is the only in which the z-component of angular momentum does not vanish.” (p. 226)

Allen et al. wrote [4,5]: “At a particular local point the z-component of angular momentum flux divided by energy flux does not yield a simple value.” “A different amount of angular momentum might be expected to be transferred at different positions in the wavefront”

Unfortunately, I have no answer from André Gsponer

[1] André Gsponer What is spin? arXiv:physics/0308027v3

[2] Allen L., Padgett M.J., M. Babiker M, The orbital angular momentum of light // Progress in Optics XXXIX, E. Wolf, ed. (Elsevier, Amsterdam, 1999)

[3] Simmonds J. W., Guttmann M. J., States, Waves and Photons (Addison-Wesley, Reading, MA, 1970)

[4] Allen L., Padgett M. J., “The Poynting vector in Laguerre-Gaussian beams and the interpretation of their angular momentum density,” Opt. Commun. 184, 67-71 (2000).

[5]. Allen L., Beijersbergen M. W., Spreeuw R. J. C., and Woerdman J. P., “Orbital angular momentum of light and the transformation of Laguerre-Gaussian modes,” Phys. Rev. A45, 8185-8189 (1992)

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