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Big crunch logic

  1. Jun 1, 2003 #1
    i remember reading some old phyiscs question as to whether, when (and if (depending on the mass of the universe)the universe began to contract, if time would also flow backwards(or more accuratly that entropy would decrease). i remember one person (i think this was stephen hawkings actually) said that our lives would move in reverse and we'de "relive" our lives backwards. but i was wondering if this did happen would we realize it? if everything happened in reverse (i.e. chemical reactions in the brain, memory, ect.) then to us we would not realize it. we would see the universe just as it was before-expanding. but if in actuallity it was contracting and we only thought it was expanding then eventually, (if humanity ever lived to see it begin its contraction which i'm fairly certain we won't) we would die in the contracting or the big crunch which we didn't expect. is this a flawed line of thinking? (i know it's sort of pointless to post this because this theory of entropy decrease has been disproved but i'm just wondering about this logic.
  2. jcsd
  3. Jun 1, 2003 #2
    The arrow of time.

    The question of why our perception of the universe runs one way and the nature of the "arrow of time" is complex and fascinating. One of the better semipopular treatments I've read, but still interesting to the pros, is by P.C.W. Davies. There are many factors to consider, not just the flow of energy and matter, but of information too. The universal expansion is the most important factor in defining the arrow, but the fact that there is only positive rest energy is important too.
  4. Jun 1, 2003 #3


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    But, the arrow of time is closely connected to second law of thermodynamics, ie entropy and it has been shown to be violated in the microscopic scale. These are two sites I found concerning the latest experiment that I know of, about this violation:



    I wonder whether this violation can be amplified to include the macroscopic.
  5. Jun 2, 2003 #4
    It's the expansion of the Universe

    which keeps the entropy of the Universe low and allows the second law to operate on the large scale.

  6. Jun 2, 2003 #5
    Stupid question, why not the other way round - increase of entropy is what is perceived as expansion of universe?
  7. Jun 3, 2003 #6


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    I thought the universe's entropy increases with the expansion. How can it be low?
  8. Jun 3, 2003 #7
    No, Hawking said that even if the Universe started contracting again the arrow of time would not reverse.
  9. Jun 3, 2003 #8
    I wouldn't reverse for a long time

    but it eventualy the entropy would be so high that it wouldn't matter, all processes would be essentaily reversible.

  10. Jun 3, 2003 #9
    i know this is the conclusion that he eventually came up with, but i don't understand how it can be so. if the universe were to recollapse into a singularity it would be going to a ordered form therefore at some point, mustn't the entropy decrease? i mean, we do define a singularity as being a very ordered form, don't we? after all don't we call the big bang the ultimate ordered form of the universe.
  11. Jun 12, 2003 #10
    No black holes are very cold and of high entropy. The smaller they are, the higher the temperature and the more they radiate (hawking type).
  12. Jun 18, 2003 #11


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    That's a peculiar statement. The higher the entropy, the LESS reversible processes are.
  13. Jun 18, 2003 #12
    but it eventualy the entropy would be so high that it wouldn't matter, all processes would be essentaily reversible.

    What I mean to say is that when entropy is maximum everything is in equilibrium and all the processes work the same backwards and forwards, so there would be no "arrow of time".
  14. Jun 21, 2003 #13
    If the universe contracted back into a singularity, do you think it'd be possible for the universe to repeat itself again or for an infinite amount of time (that is, once it dies it can start growing again)?
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