  If Einstein's photon is $E = cp = \hbar\omega$, Wigner's photon is its
helicity which is a Lorentz-invariant concept coming from the E(2)-like little
group for massless particles. In addition, the E(2)-like little group has two
translation-like degrees of freedom. What happens to them? They are associated
with the gauge degree of freedom. Since the physics of polarized light waves
can be formulated within the framework of the Lorentz group, it is now possible
to use polarization experiments to study the E(2)-like little group in terms of
quantities that can be measured in laboratories.
