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Is the number of photons in the universe fixed

  1. Mar 28, 2012 #1
    It says clearly at the wiki article regarding the CMB that: "The photons that existed at the time of photon decoupling have been propagating ever since, though growing fainter and less energetic, since exactly the same photons fill a larger and larger universe."

    I don't see how that's true. As I understand things when an electron descends from a higher orbit it releases a photon. When electrons are in a lower orbit they have less energy and that loss of energy is transferred to the photons so that the amount of energy in the universe is conserved. It appears to me then that photons are created from electrons and later they are absorbed by electrons when they ascend to a higher orbit. It seems like I've unwittingly answered my question. Still, I want it to be sure. Is it accurate to say that

    any time an electron releases a photon by descending an orbit that same photon must later be absorbed by a different electron ascending to a higher orbit?
  2. jcsd
  3. Mar 28, 2012 #2

    Vanadium 50

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    I don't see how this possibly follows.

    The statement about one group of photons doing one thing - in this case, traveling in empty space in an expanding universe - says absolutely nothing about what an entirely different set of photons in an entirely different environment are doing.
  4. Mar 28, 2012 #3


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    bobsmith76, I believe the Wikipedia statement is referring specifically to the CMB photons, and simply means that what we detect as the CMB comes from original photons that have never interacted with anything over the 13.75 Gy since their creation.
  5. Mar 28, 2012 #4
    Not at all. It could split into an electron/positron pair or some such as it travels along, and there are many interactions with materials that could occur to convert it into a whole bunch of lower energy photons, and so on. So the number of photons in the universe is nothing like fixed. If you just hold up a big bit of black cardboard to the sun you will be helping to increase the photon population, since you will be converting higher energy visible photons into a larger number of lower energy infrared photons. Or perhaps you will decrease the total population since you store a bunch of those photons as heat before they get re-emitted. Anyway you get the idea.
  6. Mar 28, 2012 #5

    thanks for the reply. i appreciate your help.
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