  It is an empirical question whether photons always obey Bose-Einstein
statistics, but devising and interpreting experimental tests of photon
statistics can be a challenge. The nonrelativistic cross section for Compton
scattering illustrates how a small admixture nu of wrong-sign statistics leads
to a loss of gauge invariance; there is a large anomalous amplitude for
scattering timelike photons. Nevertheless, one can interpret the observed
transparency of the solar wind plasma at low frequencies as a bound nu <
10^(-25) if Lorentz symmetry is required. If there is instead a universal
preferred frame, the bound is nu < 10^(-14), still strong compared with
previous results.
