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Constant Entropy

  1. Dec 15, 2013 #1
    Good night, maybe this is a dumb question, but: how was the entropy before the Big Bang?

    1. I know, entropy increases or remains constant, so, was the entropy before the Big Bang, and existence of time an space constant?

    2. Why is the entropy in our universe not constant but increasing?

  2. jcsd
  3. Dec 15, 2013 #2


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  4. Dec 15, 2013 #3
    Ok, what is the ANSWER then?
  5. Dec 16, 2013 #4


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    OP, two issues:
    1. Anything regarding "Before the big bang" is not physics. Physics is the study of the real world, based on physical laws revealed by experiments. If your questions involves quantities on which experimental insights is fundamentally impossible, it does not belong into physic's realm.

    2. Whether or not the universe as a whole even has a entropy is not clear at all. Technically, entropy is not a property of a microscopical physical system; it is a property of a macroscopic description of an ensemble of physical systems which are microscopically different but macroscopically equivalent. The entropy is related to the lack of information about the concrete microscopic realization of the ensemble, if only macroscopic descriptors are given. More concretely:

    For example, if you have a box of classical ideal gas particles, and know every particle's position and velocity, then this is a microscopic description of the gas and it does not have a entropy. If, however, you have a box and you only know "there is a classical ideal gas in there, with volume V, pressure p, and temperature T", then the latter is a ensemble description of the gas as it does have an entropy (because there are countless ways of arranging gas particle positions&velocities to realize the given V/p/T combination). So is the universe, as a whole, more like the former or the latter? In principle one could assume that the universe is in a pure state, which would mean that the question of entropy is not warranted.
  6. Dec 16, 2013 #5
    Entropy in our universe

    My question is: Why is the entropy in our universe not constant but increasing?

    What makes entropy increase? and what is necessary for entropy to be constant?
  7. Dec 16, 2013 #6


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    Clearly you need to read up on what entropy IS, then if you're still confused, ask again.
  8. Dec 16, 2013 #7
    The entropy is constant if the system undergoes a reversible process. On the other hand, if the process is irreversible, the entropy increases. One example: If you shuffle a organized deck of cards, it becomes more randomly ordered. But if you shuffle a randomly ordered deck, it does not become more organized. The process was irreversible and entropy has increased.
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