  As Einstein's $E = mc^{2}$ unifies the energy-momentum relation for massive
and massless particles, Wigner's little group unifies their internal space-time
symmetries. It is pointed out that translational symmetries play essential
roles both in formulating the problem, and in deriving the conclusions. The
input translational symmetry is the space-time translation, and the output
translational symmetry is built in the E(2)-like little group for massless
particles. The translational degrees of freedom in this little group are gauge
degrees of freedom. It should be noted that a number of condensed matter
physicists played important supporting and supplementary roles in Wigner's
unification of internal space-time symmetries of massive and massless
particles.
