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Entropy and contracting stuff

  1. Dec 12, 2005 #1
    is there some kind of law that says if something (e.g. the universe) is contracting or expanding something will happen to the entropy? like if its cotracting the entropy increases or something? i seem to remember reading that.
  2. jcsd
  3. Jan 7, 2006 #2
    well, Entropy of the universe is always increasing-no matter what. Never heard about expanding universe has anything to do with it..
  4. Jan 10, 2006 #3
    In chemistry the equations governing the entropy of a reaction state that :

    1) An increase in dispersal of matter favours an increase in enrtopy,
    2) Conversely, a decrease in the dispersal of matter favours a decrease in entropy.

    IE) Solids have lower entropy than Liquids, which have lower entropy than Gases.
    Splitting 2moles of gases into 7 moles of gases will give an increase in entropy.

    By this reasoning, if the universe is expanding, (due to expansion of space inbetween particles, creation of new-heavier particles, etc,) then there is an INCREASE in entropy.
  5. Jan 23, 2006 #4
    There is a problem regarding electromagnetic radiation in my thermodynamics book (Callen) that tells you to assume that the universe's expansion is isentropic. I quote: "Assume the expansion to be isentropic (this being a nonobvious prediction of cosmological model calculations)"

    I asked my professor about it, and he told me that it's explained in Landau's Statistical Mechanics and that it has something to do with gravity and the universe not being in equilibrium or something like that. I did take a look at that book, but not only do I not remember what it said, but it was also mostly outside of my current understanding.

    I've thought about it a bit since then, and have realized a few things, though I'm not certain if I'm right about them.

    First of all, I too initially went by the phrase: "The entropy of the universe is always increasing." I realize now that this is slightly incorrect. Within that context universe refers to the closed system being considered, not to the cosmological universe. Additionally, entropy doesn't necessarily have to increase, it can also remain constant -- the only thing it can't do is decrease.

    Now I've been thinking that since the universe is a closed system, there is no heat exchanged from outside, hence dQ=0. By dQ=TdS, therefore, assuming the universe isn't at absolute zero -- which I'm pretty sure it isn't -- the entropy remains constant. I'm not sure if this is correct though, since it seems like such simple reasoning for such a profound realization. Maybe I'll ask my professor about it sometime. Unless someone who knows about this stuff would like to contribute. ;)
  6. Jan 23, 2006 #5
    The universe is far to vast and complex for us ol' humans too try and measure if the entire universe is at equilibrium. I think its far easier to just assume the universe is not at equilibrium.

    As far as i know, entropy can decrease, as in many spontanious chemical reactions where gas is changed to solid, seven moles of gas reacts to form two moles of gas, etc.

    As spontanious reactions of this nature are quite uncommon, or require extreme temp's/pressures to be pushed to be spontanious, most of the interactions of particles in the universe will naturally cause an increase in entropy.

    We cant really give a single measurement for the temp' of the universe as a system, as smaller portions of this larger system are constantly fluctuating. I mean, at any given time, one side of the earth is darker than the other, and the interactions of particles in these areas will be different, due to differing temp, radiation, etc.
    By the time we have recorded this particular data, the system in question would have shifted.

  7. Jan 23, 2006 #6


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    There is a caveat in the relationship between gravitation and entropy. Without taking into account gravitation, the more homogeneous matter is spread out, the higher the entropy (if you understand by that, the Boltzmann entropy, namely the size of the box in phase space that corresponds to the macrostate). However, if gravity is switched on, the more *inhomogeneous* the matter distribution is, the higher the entropy. The maximum entropy is reached by a distribution of black holes, and a rather low entropy is reached by a homogeneous gas. This in contrast to the usual lab situation, where all gas molecules lumped in one corner of a box is a "low entropy" situation, and the homogeneous gas in the box the "high entropy" situation.
    In cosmological terms, the rather uniform matter distribution just after the big bang is a very LOW entropy situation, and the lumping of matter into clusters *produces* a lot of (Boltzmann) entropy (lumping together in galaxies, stars and eventually black holes).
  8. Jan 31, 2006 #7


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    I think that this contrast exists within the lab too, between low and high temperature gases. At low temperatures the gas will condense, similarly to the black holes.
  9. Mar 20, 2006 #8
    When Poincarre's recurrency principle is valid then ofcourse in principle entropy can decrease. But the chance it does at some places in e.g. a multiverse is relatatively extremely low. But compared to our local time time, the will be enough time to go that direction. I know of no physcal law(s) that things makes easier under special circumstances in a multiverse or even in our even in our observable universe. Nowadays we already have laws concerning the devellopment of blackholes ana also observational experience. It would be nice if our theories could be extended so that also the formation of e.g. white holes can be foreseen and it would be superbe if we could get experimental evidence for their existence. Maybe quantum-gravity will help us? Until now 9as far as i know) there is no experimental evidence that entropy in practice ever decreased. To my opinion it is absolutely necessary that we find evidence for such a process in order to solve the phylosofical problem we have with the (to my opinium) nonexistence's of creation(begin) and (real) annihillation(end). Studying "Black Energy" might help I suppose.
  10. Mar 21, 2006 #9


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    No ! When the gas condenses, it is in a state of LOWER entropy than when it is filling the whole box. While a gravitationally contracted gas has a HIGHER entropy than when it is evenly spread out through space.
  11. Mar 21, 2006 #10
    Yes, this is in agreement what R.Penrose shows in his book The road to reality.
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