Chris Peterson wrote:Keep in mind that photons are not really massless. They have energy, and therefore a mass equivalent. They also have momentum, which means a mass equivalent. The confusion likely comes from the understanding that a photon has zero rest mass. But that's a theoretical concept, given that a photon is never at rest.
While Art is correct that the kinetic energy of CMB photons is decreasing (because of redshift), this does not violate the law of conservation of energy, because kinetic energy is an observer dependent property- the kinetic energy we measure depends upon our frame of reference.
Can you blame us non scientists for thinking that conservation of energy is a law when scientists commonly call it such? (Just an amusing irony )
The frame of reference energy dependence is interesting. Would it be correct to then assume that the energy of these CMB photons hasn't really decreased, it's just that they look that way due to the expansion of the universe? Wouldn't that mean that the overall energy of the whole Universe is unchanged due to cooling?