  Photons in blackbody radiation have non-zero interactions due to their
couplings to virtual electron-positron pairs in the vacuum. For temperatures
much less than the electron mass $m$ these effects can be described by an
effective theory incorporating the Uehling and Euler-Heisenberg interactions as
dominant terms. By a redefinition of the electromagnetic field, the Uehling
term is shown not to contribute. The Stefan-Boltzmann energy is then modified
by a term proportional with $T^8/m^4$ in agreement with the semi-classical
result of Barton. The same effects give a speed of light smaller than one at
non-zero temperatures as has also recently been derived using full QED.
