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Why is entropy an irreversible process?

  1. Aug 5, 2010 #1
    Our laws that describe entropy to be irreversible using our macroscopic experiments in which they always come out with a certain outcome. However, I think there is no such thing as an irreversible process on a quantum level and the quantum level is the stronghold of everything. Everything is just assigned a probability and the probability of a irreversible process is much lower than a reversible process. So it may seem that entropy is irreversible, but its just extremely improbable that our tests have not shown otherwise. So I believe that this should not be a law which describes the ultimate nature, rather a tendency for higher probabilities.

    What do you think?

    Mentor's note: Fixed the title
    Last edited by a moderator: Aug 5, 2010
  2. jcsd
  3. Aug 5, 2010 #2
    Re: Why is entropy an irreducible process?

    It is a tendency for higher probabilities but for large numbers of particles, the probabilities are so low they are essentially zero. It is like the probability of hitting the mean on a normal distribution: probability is zero but it is still *possible*.

    I don't think we can talk about quantum irreversibility until quantum randomness is figured out.
  4. Aug 5, 2010 #3

    Andy Resnick

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    Re: Why is entropy an irreducible process?

    "Entropy" is not an irreversible process. A system that undergoes an irreversible process gains entropy.

    It is true that quantum mechanics permits spontaneous fluctuations that result in a lowering of the entropy.

    This effect can be made manifest at macroscopic scales as well- I have a copy of a paper (in my office) that shows thsi behavior in a macroscopic system consisting of a packed bed of beads and the capillary rise/flow of a fluid. If I remember, I'll post it tomorrow.

    Statistical mechanics (classical or quantum) is not a substitute for thermodynamics.
  5. Aug 5, 2010 #4
    Re: Why is entropy an irreducible process?

    Entropy, for all practical purposes, increases on the cosmological scale. This is a result of an effectively "irreducible" process when considering all particles, but there probably will be relatively small volumes where entropy diminishes as well. Irreducible because, of the 1081 fermions in the observable universe, the chance of entropy diminishing universally (I believe) will at most be 210-81.
  6. Aug 6, 2010 #5
    Re: Why is entropy an irreducible process?

    So you're saying that entropy can vanish at any moment. Its all just a subject of probability? Also, how exactly did you derive that probability if you did?
  7. Aug 6, 2010 #6
    The physical laws are reversible, but the processes aren't so entropy always increases if you dont intervene and make "order", but with making "disorder" on some other place at the same time. So probability that one system returns in "order" by it self is very low but it depends on the number of the particles involved. If you have two dies the probability that the two dies turn up to be one is much higher than if you have one million dies all of them turn up to be ones.
  8. Aug 7, 2010 #7
    Alan H. Guth in his paper Inflationary Universe: A possible solution to the horizon and flatness problems, Physical Review D, Volume 23, Number 2, estimates the entropy of the universe to be S>1086 (2.14). I believe he uses unitless Planck constants here.

    Earlier I had simplified the universe as a collection of 1081 particles simultaneously undergoing their own binary transition. I attempted to derive the probability that as a whole they might return to their previous state.
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