`  == The edit I reverted ==  said: ``Photons can travel at only one speed, that of light, so that the differences between them are described by just one factor, the energy or its equivalent, the frequency...For a material gas at given temperature, the pressure and energy density can vary independently for different gases, because molecules can travel at a range of speeds``  This is incorrect, or at least very misleading.  For either a photon or a particle of a given mass, the speed is uniquely determined by the energy or momentum, and only one number (the energy or momentum) is needed to describe the relevant differences between different particles.  Perhaps what's meant is that different gases have constituents with different masses, but that's not what's said. Waleswatcher  `