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Is there a theory that we are in an eternal loop? Multiverse?

  1. Apr 24, 2010 #1
    This is my first post and i must apologize for my extreme lack of knowledge of physics compared to your average poster here.

    I feel compelled to post here because about a year ago it suddenly occurred to me that we might be in an eternal loop and I would like to understand better whether is possible or not.

    My original thought was, although likely incorrect, if time is infinite (which i now believe that to be debatable) and matter in the universe is finite (I don't have a good understanding if this is true or not), then wouldn't at some point the state of the universe necessarily repeat itself? And if there is one single repetition in the state if the universe wouldn't that mean that all events in matter would unfold as it did the first time creating an endless loop?

    In talking to a friend about this, I was directed to research multiverse theory which I did to an extent. What I learned about the theory is that it primarily has to do with the existence of an infinite number of universes that exist if traveling enough distance in space. But can that same idea of infinite universes be applied if just looking at time? Or? would the "looped" universe make an infinite number of universes virtually impossible (just looking at time), because a repetition of a state of the universe is much more likely to happen before all imaginable universes gets played out over time.

    In trying to figure some of this stuff out , I learned a little about the big bang , big crunch theory, but I have a real hard time understanding some the stuff that gets written here because of my lack of knowledge of physics. I'm not even sure if i read some of the answers to my questions or not! lol.

    One question that I have that are related to my thoughts on this...Is matter finite or infinite? If matter IS infinite, how can empty space exist? what if a rock suddenly appeared in this universe out of nothingness wouldn't we have a larger amount of matter than before and if so how could we call the lesser amount of matter infinite?

    Also, if both matter and time are infinite would a looped universe even be possible?

    I would like to add on a personal note , I have never before experienced something like the feeling of excitement that I had when thinking up this possible looped reality. The implications would be interesting. All things that happened in my life happened an infinite number of times in the past and will happen an infinite number of times in the future. Which would bring to question...Would the matter that is my body in the other universes be me? Are the eastern religions somewhat onto something very large (if so probably by luck) in their cyclical view of the world? I have been an atheist all my life and I felt how I imagine a born again christian might feel...I had to "tell the news" so to speak..all my friends heard of my new thoughts. The difference between me and the Christians, however, is that I am truly interested in the truth no matter where the truth takes me and i fully understand that my idea is pretty unlikely, especially given my lack of knowledge of physics.

    I know that I brought up a lot of questions here, but even the slightest insight to give me a better understanding of my currently unresolved thoughts on this would be greatly appreciated. And again, I am sorry for my low level of knowledge of physics. Thank you.
  2. jcsd
  3. Apr 25, 2010 #2


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    The primary problem with a simplistic "looped" universe is that it requires that entropy decrease. An entropy decrease is the equivalent of a system spontaneously changing from a highly probable configuration to a highly improbable one. While such a change is, in principle, possible, the argument, when examined in detail, leads to absurdities (it predicts that all observers are fake). This is the old "Boltzmann Brain" argument.

    Personally my favorite way of getting around this has been proposed by Sean Carroll, where the basic idea is that a universe gets started from a pre-existing universe as a vacuum fluctuation. This new baby universe is forever hidden from its parent, and then spawns its own new universes. In this way, entropy always increases, and there is no problem asking about how entropy can decrease.

    He wrote a popular article on the subject for Scientific American, but this is no longer available for free on the web. You can, however, watch this TED talk:
    http://www.ted.com/talks/sean_carroll_on_the_arrow_of_time.html [Broken]
    Last edited by a moderator: May 4, 2017
  4. Apr 25, 2010 #3
    Thank you so much for your response. I just watched both part 1 and 2 of the video of the video you suggested. I believe I understand now about how entropy increases over time in our perceivable universe. I must admit I got a little lost in the "Boltzmann Brain" argument, but I feel like I got the jist of the other stuff. So it is possible that there are baby and parent universes and it is that that could allow for a state of low entropy to be born from high entropy. OK, so i guess so far it is still possible that we are in an eternal loop? Thanks again
  5. Apr 25, 2010 #4


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    Well, the basic idea here is that the eventual fate of our universe is to be extremely large and quite empty. The total entropy for this fate is very, very high, but the entropy density is rather low. By contrast, the start of our universe was very small and very low in entropy.

    So this sort of wiggles out of the problem not by positing that our entire high-entropy universe fluctuates into a new one, but rather that a very small region of our high-entropy universe does. Our universe remains big, empty, and cold, while a new one is born from a teeny tiny bit of ours that, because it was born from a tiny bit, isn't such a large change in entropy.
  6. Apr 25, 2010 #5
    Thank you very much for your response.

    So then you are saying that the majority of the matter of our universe will not be "recycled" to make up other universes, but rather will remain big, cold, empty, and lifeless for eternity? Eternity seems to me like it could be too long of a time for nothing more to happen to our universe. Might not all the matter that is our universe eventually be used to make to make other universes?

    I just checked wikipedia "the end of the universe" and it seems as though there still are some valid theories that include a possible big bang , big crunch scenario. This i suppose would be the easiest way for one to imagine the possibility of the existence of an enternal loop. But, even without the existence of a big crunch , might it still be possible that we are just at one stage of a loop that INCLUDES all these multiverses no matter how complex and incomprehensible the amount of time that it would take for one cycle to complete?

    I believe that humans instinctivly lack an appreciation for large amounts of time and I wonder if we just are too close to our certain point in time to fathom the likelihood of a reoccuring state of the universe. Just what if... everything , including ALL the multiverses ... basically everything that exists has just one reocurrence of a state of being? Wouldn't that make us part of a reoccuring loop? I understand that the details of how this might or might not happen is what you guys are good at theorizing, and I lack cosmological knowledge, but for some reason I can't help but think think that this eternal loop is a possible reality given ifinite time.
  7. Apr 25, 2010 #6


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    Well, no, all the matter within our universe will simply cease to be. It will take an extraordinarily long time for this to happen, of course, but the eventual fate is for our universe to be completely empty.

    The vacuum fluctuations that produce new universes don't actually stem from matter within our own, but rather fluctuations that occur no matter what.

    As for the big crunch, given the current accelerated expansion, that scenario is highly unlikely at this time.
  8. Apr 25, 2010 #7
    Wow very interesting. So what will happen to all the matter though? so matter is not infinite? If the matter is transformed to something else what is it? Thank you for yet another reply.
  9. Apr 25, 2010 #8


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    Well, the basic mechanism is that all protons will eventually decay (we don't know what the lifetime is just yet, just that it must be very long). This leaves a universe that is balanced in matter/anti-matter content, which means that the remaining matter will eventually annihilate, producing radiation. This radiation is eventually redshifted away to nothing.
  10. Apr 25, 2010 #9
    There's a theorem called "Poincare recurrence " theorem. It suggests the history should be "in an eternal loop", but I think it only work in the realm of classical physics, so it probably can't describe the true universe.
  11. Apr 25, 2010 #10


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    Well, that requires energy conservation, which isn't the case here. I'm not sure it's entirely clear that one can recover something similar in the more general conservation laws of General Relativity.
  12. Apr 30, 2010 #11
    OK .. so i have done a bit of research..and true... it seems that the big bang big crunch theory may be less likely than the big freeze. However, despite my lack of knowledge of cosmology and physics, I wonder if the true spirit of my original question is unanswerable. In part, it is because of our relatively limited knowledge of the overall bigger picture... whatever that happens to truly be (including quantum physics and everything else)

    A thought that occurs to me (perhaps directly related to my original question). Does the fact that any particular occurence that has happened, of any given occurence that has happened in our lifetimes, increase the likeliehood that that exact same event has already occured in the past or will occur in the future? It seems less likely that my conciousness (or one exactly like mine) would experience an event...and that event would exist completely uniquely in space and time as opposed to it being just one event in a series of identical events , or even an ifinite number of identical events.

    Thank you very much for those that have contributed to this thread.
  13. May 1, 2010 #12


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    Well, that's basic inductive reasoning. The short answer is yes, the more we see something happen, the more confident we can be that it has happened in the past and will continue to happen in the future. However, experiencing something only once doesn't give any indication as to how common an event is, and so it could as easily be an extraordinarily rare event as one that happens all the time. We have to experience a number of such events to start getting a handle on how common they are.
  14. May 4, 2010 #13
    I have seen the videos, and read these posts. but it leaves me wondering a few things though.

    What causes or why do random fluctations accor?
    When the universe reaches the point of highest entropy, will Dark energy be the highest energy in excistance? Isnt energy relative to objects? wouldnt that mean that the energy in the center will always be higher than that of the outer circle of the universe?

    Thanks for reading and sorry for my sucky english. but do please explain if possible :D
  15. May 4, 2010 #14


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    Which random fluctuations are you talking about? There are many different mechanisms for many different kinds of random fluctuations.

    It will be the highest energy density. Everything else will be diluted away. Galaxies will be really really far from one another, for example.

    Huh? No. Objects are only one possible thing that can have energy.

    Huh? Why? That makes no sense to me.
  16. May 14, 2010 #15


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    Chalnoth & dangerflow, there’s a "back-up copy" here:

    "[URL [Broken] Time Run Backward in Other Universes?[/B]

    ab230924-fa4d-9eac-5e5e8d5152c227b1_1.jpg [/URL]

    Really interesting stuff, especially the part on entropy & gravity.

    (I haven’t forgot our MWI/EPR discussion, but right now time is running away from me... :rolleyes:)
    Last edited by a moderator: May 4, 2017
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