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Measuring the entropy of the universe?

  1. Jul 20, 2015 #1
    Hi all,

    I was recently watching one of Susskind's 'Theoretical Minimum' lectures in which he says that the entropy of the universe may be measured via the number of observable photons, and that somehow these quantities (photon number and total entropy) are somehow linked. Could anybody with knowledge in the area elucidate this connection? Thanks.
  2. jcsd
  3. Jul 21, 2015 #2
    A link is helpful when asking for feedback on specific material.
  4. Jul 23, 2015 #3


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    Photons are a waste product of energetic processes. All such processes increase the entropy of the system within which they occur.
    Last edited: Jul 23, 2015
  5. Jul 25, 2015 #4

    Ken G

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    I suspect the issue is that the vast number of particles in the universe, by perhaps a factor of a billion, are photons in the CMB. So if you want to track where the entropy is, look there. However, this is not a very good way to look at changes in entropy in the universe, because the CMB is a thermalized radiation field, and the entropy per photon in any thermalized radiation field is the same (it's an order unity constant times the Boltzmann factor). Thus, if you know the photon number, you know the entropy, independent of temperature. Since the photon number in the CMB is not changing, the entropy of the universe as a whole is largely not changing. Where the small changes are coming from on top of that has to do with heat transport, like starlight and supernovae, but it must be a small fraction of the total. So the entropy in the CMB is both the dominant entropy in the universe, and is unchanging with time, so it doesn't sound like it is very important. Maybe it comes up in a multiverse kind of setting, where you could imagine comparing the entropies of different universes.
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