Reverse Time in Big Crunch: Is It Real or Just a Flawed Logic?

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Discussion Overview

The discussion revolves around the concept of time and entropy in the context of a contracting universe, particularly in relation to the Big Crunch scenario. Participants explore whether time would flow backwards and if entropy could decrease during such a contraction, as well as the implications of these ideas on our perception of reality.

Discussion Character

  • Exploratory
  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • Some participants recall a notion that if the universe began to contract, time might flow backwards, leading to a scenario where individuals would "relive" their lives in reverse, although this is questioned as potentially flawed logic.
  • Others discuss the complexity of the "arrow of time," noting that it is influenced by factors such as the flow of energy, matter, and information, with universal expansion being a key element.
  • There are claims that the second law of thermodynamics, which relates to entropy, is connected to the arrow of time, and some participants mention recent experiments suggesting violations of this law at the microscopic level.
  • One participant suggests that the expansion of the universe keeps entropy low, allowing the second law to operate on a large scale, while another questions whether increasing entropy could instead be perceived as the universe's expansion.
  • Some participants assert that even if the universe were to contract, the arrow of time would not reverse, referencing Stephen Hawking's conclusions, while others express confusion about how entropy could not decrease if the universe collapses into a singularity.
  • There is a discussion about the nature of black holes and their entropy, with some arguing that higher entropy corresponds to less reversible processes, while others suggest that maximum entropy leads to equilibrium where processes can occur in both directions.
  • A question is raised about the possibility of the universe repeating itself after contracting into a singularity, suggesting a cyclical nature of cosmic events.

Areas of Agreement / Disagreement

Participants express a range of views on the relationship between time, entropy, and the universe's expansion or contraction. There is no consensus on whether time would reverse or how entropy behaves in these scenarios, indicating ongoing debate and uncertainty.

Contextual Notes

Participants reference various interpretations of entropy and the arrow of time, with some statements depending on specific definitions and assumptions that are not fully resolved in the discussion.

maximus
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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.
 
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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.
 
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.

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:

http://nanodot.org/article.pl?sid=02/07/20/0445232

http://www.sciencenews.org/20020727/fob1.asp

I wonder whether this violation can be amplified to include the macroscopic.
 
Last edited by a moderator:
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.

Originally posted by jby
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:

http://nanodot.org/article.pl?sid=02/07/20/0445232

http://www.sciencenews.org/20020727/fob1.asp

I wonder whether this violation can be amplified to include the macroscopic.
 
Last edited by a moderator:
Originally posted by Tyger
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.
Stupid question, why not the other way round - increase of entropy is what is perceived as expansion of universe?
 
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.

I thought the universe's entropy increases with the expansion. How can it be low?
 
No, Hawking said that even if the Universe started contracting again the arrow of time would not reverse.
 
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.

Originally posted by AndersHermansson
No, Hawking said that even if the Universe started contracting again the arrow of time would not reverse.
 
Originally posted by AndersHermansson
No, Hawking said that even if the Universe started contracting again the arrow of time would not reverse.

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.
 
  • #10
Originally posted by maximus
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.

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).
 
  • #11
but it eventualy the entropy would be so high that it wouldn't matter, all processes would be essentaily reversible.

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

Originally posted by HallsofIvy
That's a peculiar statement. The higher the entropy, the LESS reversible processes are.

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".
 
  • #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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