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

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In summary, the conversation explores the concept of the arrow of time and its connection to the second law of thermodynamics and the expansion of the universe. While the expansion of the universe keeps the entropy low and allows for the operation of the second law on a large scale, there have been experiments showing a violation of the second law on a microscopic scale. The question is raised whether this violation can be amplified to include the macroscopic. Despite initial theories that if the universe were to contract, time would also reverse, it is concluded that even in a contracting universe, the arrow of time would not reverse. This is because as the universe collapses into a singularity, its entropy increases, making all processes essentially reversible. The possibility of the universe repeating itself
  • #1
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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  • #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.
 
  • #3
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 [Broken]

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

I wonder whether this violation can be amplified to include the macroscopic.
 
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  • #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.

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 [Broken]

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

I wonder whether this violation can be amplified to include the macroscopic.
 
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  • #5
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?
 
  • #6
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?
 
  • #7
No, Hawking said that even if the Universe started contracting again the arrow of time would not reverse.
 
  • #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.

Originally posted by AndersHermansson
No, Hawking said that even if the Universe started contracting again the arrow of time would not reverse.
 
  • #9
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)?
 

1. Is it possible for time to reverse in a Big Crunch scenario?

There is currently no scientific evidence or theoretical framework that supports the idea of time reversing in a Big Crunch. The concept of time reversal is not supported by the laws of physics and goes against the arrow of time that is observed in our universe.

2. Can a Big Crunch scenario be considered a form of time travel?

While the collapse of the universe in a Big Crunch may seem like a form of time travel, it is not the same as traveling back in time. In a Big Crunch, the universe is simply collapsing in on itself, rather than reversing time. Time travel involves physically moving through time, which is not possible in a Big Crunch scenario.

3. What is the logic behind the concept of time reversal in a Big Crunch?

The idea of time reversal in a Big Crunch is based on the assumption that the expansion of the universe will eventually reverse and lead to a collapse. However, this assumption is based on flawed logic and does not take into account the many unknown factors and variables that affect the evolution of the universe.

4. Are there any scientific theories that support the concept of time reversal in a Big Crunch?

No, there are currently no scientific theories that support the concept of time reversal in a Big Crunch. While there are theories that suggest the possibility of a Big Crunch, they do not involve time reversal and are not widely accepted in the scientific community.

5. How does the concept of time reversal in a Big Crunch relate to the concept of entropy?

The concept of time reversal in a Big Crunch goes against the principle of entropy, which states that the universe tends towards disorder and chaos. In a Big Crunch scenario, the universe would have to reverse its natural progression towards disorder, which is not supported by the laws of thermodynamics.

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