When a photon from a distant rapidly receeding star is observed on earth it appears red shifted. I understand that red photons have less energy than blue ones. ( E = hf h is Planck's constant )).
What happens to the change in energy ?
I have discussed this problem with some other physics teachers and the proposed explanations have included, not in any order of merit.
1. The energy is used to drive the expansion of the universe
2. Conservation of energy does not apply in quantum physics
3. It is all to do with frames of reference
Can anyone cast some light on this paradox ?