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The expectation value of the electric field of a state with defined photon number is zero, so any electromagnetic wave (with non-zero expectation value) must be in a superposition state of definite photon number states. Still, people talk all the time about the number of photons, like the number of photons in the Universe.

I guess this is some type of classical approximation, but aren't photon always quantum? Do we have to take the (semi)-classical limit in some particular way?

Now that I think of it, maybe the paradox can be explained accepting that the universe is not at a photon-number eigenstate, but what we are calculating is the expected value of number of photons.

jinawee
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  • I was just reading this paper, which might be relevant. – Emilio Pisanty Aug 31 '17 at 09:08
  • This is written as a question about cosmology, but I don't see anything about the logic that has anything to do with cosmology. –  Jun 20 '18 at 17:54
  • @Ben it's basically how does this question make sense:https://physics.stackexchange.com/questions/276890/how-to-calculate-how-many-photons-are-in-the-universe/276900. I guess I'm asking, when does it make sense to talk about a definite number of photons, like those emmited from a lightbulb. – jinawee Jun 20 '18 at 18:05
  • @jinawee: My point is that regardless of the context from which you came up with the question, it doesn't seem to me that the cosmology stuff is in any way relevant. –  Jun 20 '18 at 20:35

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