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Question about the arrow of time?

  1. Feb 1, 2012 #1
    I'm not sure if this is the correct section but I have a question I would like to be answered, I was thinking since a system will have many more possible disordered states than ordered states, then would this mean that if we were normally moving forward in time, and for example, some glass broke and scattered into 40 pieces, now if the whole universe went back in time, the scattered glass returned back into one piece of glass, and then time went forward again, would the glass necessarily scatter into 40 pieces again? or there is a chance that it will scatter into more or less pieces..?
  2. jcsd
  3. Feb 1, 2012 #2

    Simon Bridge

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    It actually means that you can't go back in time.

    With a lot of Universes, that glass would be unlikely to break the same way in two of them. But if you played space back and forth in time, since the glass already broke, it would play out the same way each pass through the events. It's like playing a recording - before the recording, anything could happen, but afterwards, it's set.

    But to return to the point: since time travel is highly speculative you could also just say that anything could happen. To be useful you need to specify the properties of space-time you are trying to learn about.
  4. Feb 1, 2012 #3


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    Yes, provided that the universe is driven by a deterministic law.

    There is, if the universe is not driven by a deterministic law.
  5. Feb 1, 2012 #4
    But I've read in a book that if the universe contracted back into a singularity and banged again, earth wouldn't necessarily form again, it's like throwing the dice again.. well, at least that's what I understood. And I thought that scientists still aren't sure if we are surrendering to a deterministic universe.. this issue is currently settled or..?
  6. Feb 1, 2012 #5


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    No, it isn't settled yet.
  7. Feb 1, 2012 #6
    demystifier's answer is the best short answer, I think....

    and would likely be the relativistic type (classical) answer.....but from a quantum theory perspective, if one subscribes to a 'many worlds' interpretation of things, meaning an infinite number of parallel worlds, then earth surely would 'appear' again....in fact, it already has, but that's not, I think, the majority view....

    and these conflicting views and theories are why it's 'unsettled'.
  8. Feb 1, 2012 #7


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    Don't be swayed by watching a video clip run backwards and then assuming that if time was running in the reverse direction, then that is how a universe running backwards in time would act - that all events would simply unwind.

    Just sit back for a moment and contemplate a video clip of a a glass falling from a table onto the floor and breaking into 40 pieces next to a ball. Run the video clip backwards and sure enough the pieces somehow re-assemble into a glass which then jumps back up onto the table and stays there. And the ball rests on the floor undisturbed as no glass pieces hit it.

    Doing a little thought experiment you contemplate: Is that is what would occur in a universe with time running backwards. And your scientific mind would ask " How is it that gravity is acting in the upward direction from the floor to the table, but not afterwards from the table. Why is it that the ball on the floor with glass pieces scattered all around had been, still remains on the floor and is not affected by the upwards gravitational force. Something mysterious is going on - reverse gravity acts only on certain objects"

    You next contemplate other scenarios. If the universe running in a time reversal is the same as a movie clip running backwards then heat would have to flow from a cold body to a hot body - how does the heat know which hot body to go to. A light bulb would be absorbing light making the filament hot. A car putting on its brakes would make the car go faster and pressing the accelerator would make the car slow down. That sure would be a strange universe.

    You can only conclude that a universe running backwards in time does not act like a movie running backwards, and 40 pieces of broken glass do not reform into the original whole glass.
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