# Physical degrees of freedom

by JustinLevy
Tags: degrees, freedom, physical
 Sci Advisor HW Helper P: 1,332 The counting is essentially the same at the linearized level. For example, the energy density of a gas of photons is $$\epsilon/T^4 = \pi^2/15$$ while the energy density for a gas of free SU(N) gluons (at high temperature, say) is $$\epsilon/T^4 = (N^2 - 1) \pi^2/15$$. In other words, you get one photon contribution for each gauge boson. In general, you would have 2 times the dimension of the adjoint representation degrees of freedom.