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On another thread, users have asked for an explanation of the "red shift" of photons (the apparent loss of energy of photons due to the expansion of the universe.) All they ever got was a GENERAL RELATIVITY explanation.

So, I'll rephrase the question: if a so-called "photon" represents a discrete QUANTUM of energy exchanged between two or more atoms as a result of the the electron shell of one atom changing its energy level, then what happens to the energy of those photons that "arrive" at a lower energy level (lower "frequency") as a result of red shift?

Let me try and clarify my question based on my understanding of how "photons" work. Let say atom A releases a quantum of electromagnetic energy equal to 3 units. One atom B absorbs 1 of those energy units, and atom C absorbs 2 units. We could then say that atom A emitted two "photons." If, however, atom D absorbed all 3 energy units, then we would have to say that Atom A emitted one "photon." I think this I where people get confused--photons aren't "real" PARTICLES you can count like sheep. They're just a mathematical description of chunks of energy that get "exchanged" AS THOUGH that energy were being delivered as chunks.

So, back to my question: if the energy of EACH "photon" being received is less than it was when it was "emitted," where does that energy go? Does it become "more" photons? I know, I know, in RELATIVITY the work of bending space time robs the "photons" of some of their energy (precisely the way the bending of leaf springs in a dragster robs the axle of some of the energy directed toward the wheels.) BUT our question is: What is the QUANTUM explanation? And please, don't just point the relativistic answer we already got.

If you don't HAVE a quantum explanation, just say so.

Thanks!

AccidentalFourierTransform
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Tommy Jonq
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2 Answers2

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Redshift is a kinematical effect, not a dynamical one. Therefore, the explanation of redshift is the same in classical mechanics and in quantum mechanics.

For example, the derivation of the red-shift formula in non-relativistic classical mechanics is valid in non-relativistic quantum mechanics. Similarly, the derivation in (special) relativistic classical mechanics is valid in (special) relativistic quantum mechanics. Finally, the derivation of the formula in curved space-times in classical mechanics is valid in quantum mechanics in curved space-times.

AccidentalFourierTransform
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Quantum mechanics is a scientific theory. As covered in my answer here, theories do not in general pin everything down but rather they create a space for modeling phenomena.

Any quantum mechanical model of redshift will say "those photons smoothly lost energy on their trajectory, therefore they must have changed frequency to satisfy the Einstein-Planck relation $E = h f.$" That is because the latter property is part of the theory of quantum mechanics. However quantum mechanics itself is not going to argue with you about why this happened.

Now, given what we know about general relativity, we can use that to inform our quantum mechanical model, saying "oh, this cannot be a case of one photon becoming two photons to conserve energy; energy is explicitly not conserved here, there is just one photon whose wavelength is changing." You seem to be very distraught by this idea, that we might try to use one theory to inform another, however it is necessary in this case because you're asking about a fundamentally gravitational phenomenon (redshift) and we do not have a theory of quantum gravity which easily can be made to model such things. So any quantum model must incorporate ideas about this phenomenon from relativity: otherwise we will simply not describe the phenomenon.

So the answer to "where does that energy go? does it become "more" photons?" is: "no, you're not understanding the phenomenon properly, the phenomenon of redshift is explicitly one where energy disappears. We can probably phrase some quantum-mechanical models where the energy doesn't disappear but probably these will ultimately give the photon an effective mass in its interaction with the other fields which absorb the photon's energy: and thus the theory will fail because the photon is massless. It's part of this phenomenon that the energy disappears. Therefore you need to model this phenomenon non-conservatively, and quantum mechanics is not going to explain it, it's going to assume it."

CR Drost
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