Quote:
Originally Posted by sjastro
It's a good question. And there is a simple answer. The laws of physics such as the conservation of energy apply within frames of references not across them. In the photon's frame of reference (if it can be called that), the photon doesn't lose energy, as much as it doesn't lose energy in the observer's frame. The observer in his cosmological frame of reference only sees the photon of a particular frequency and doesn't see a change of the photon's energy in his frame. Energy is therefore conserved in both frames.
Consider relative velocities. Each observer will measure a different velocity in their frame of reference and therefore calculate a different kinetic energy. For kinetic energy to be conserved across frames would imply a baseline common to all the observers (an absolute frame of reference) which of course doesn't exist.
Regards
Steven
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Steven, thanks for this explanation, it sounds plausible to me :-)