# Oscillating universe and entropy

## Main Question or Discussion Point

I heard that :
Theoretically, the oscillating universe could not be reconciled with the Second Law of Thermodynamics: Entropy would build up from oscillation to oscillation and cause heat death.
1- Is this accurate: Entropy would build up from oscillation to oscillation ?
2- What's the reason of that build up: does the big crunch conserve the previous entropy or just minimizing it ?
3- Does the universe become bigger in every next cycle in the oscillating universe, if yes why?

Regards,

marcus
Gold Member
Dearly Missed
1- Is this accurate: Entropy would build up from oscillation to oscillation ?
To define entropy you need an observer. At least I do not see how to define it without an observer----someone who can map the phase space and determine what is a macrostate and what is a microstate: who can say which states are indistinguishable and so on.

But at a big crunch all the observers from before crunch cease to exist. So the concept of entropy does not apply---the Second Law ceases to be applicable---at the crunch moment.

For observers after the crunch, they see the entropy start out low again: reset to some nice low value. No one sees a violation of the Second Law---because no one exists at the precise moment of the crunch/bounce.

So in my view the answer to the question is No. It is not accurate. Entropy would not build up because entropy is not even continuously defined throughout the sequence of bounces. There is no single observer who sees it all and can make a map of the phase space determining what is a macro and what is a microstate. No observer sees the Law violated. So it is not violated.

To define entropy you need an observer. At least I do not see how to define it without an observer----someone who can map the phase space and determine what is a macrostate and what is a microstate: who can say which states are indistinguishable and so on.

But at a big crunch all the observers from before crunch cease to exist. So the concept of entropy does not apply---the Second Law ceases to be applicable---at the crunch moment.

For observers after the crunch, they see the entropy start out low again: reset to some nice low value. No one sees a violation of the Second Law---because no one exists at the precise moment of the crunch/bounce.

So in my view the answer to the question is No. It is not accurate. Entropy would not build up because entropy is not even continuously defined throughout the sequence of bounces. There is no single observer who sees it all and can make a map of the phase space determining what is a macro and what is a microstate. No observer sees the Law violated. So it is not violated.
Hi Marcus,

I think there is a problem with the conclusion you reach here. Since there no observers exist at the moment of the big crunch and equally no single observer exists before and after the big crunch, then by that argument, ALL the laws of physics can be violated during the big crunch and the start of a new universe (if there is one). I base my argument on the premise that all the laws of physics are quantum based at their core and one interpretation of quantum mechanics is that nothing exists unless it observed or observable. Eistein countered with " I like to think the moon is still there even if no one is looking at it". The only law of physics that is not yet fully based on quantum theory is gravity and that I think we agree is just a matter of time before that is resolved.

I don't think that applying the principle that that no observers survive the big crunch so the second law can be violated, is a very good route to take as it opens the doors to all the laws of physics being violated and not being able to say anything about after the big crunch, which now that I think about it may be the correct answer :P

Anyway, the point is either we say nothing can be predicted about after the big crunch or we can predict that after the big crunch this and that will happen even if no observers are there to observe it. What we can not logically do is say that one law is not valid during the crunch because there are no observers while other laws are valid despite there being no observers. That is conveniently having your cake and eating it to suit your purpose.

The question is what do mean by an observer and what does quantum theory mean by nothing exists unless it is observered and why is the moon still there even when we are not looking at it?

I like to think that a photon or any other particle is observed when it interacts with another particle and the "observer" particle does not have to be part of a sentient being. When a photon is absorbed its wave function collapses (even if an human is not there to observe it). The interaction and collapse of the wave function is the observation. Once a particle is "observed" it becomes part of reality and remains "observed" and becomes a lasting tangible "thing". The moon is made up of observed (read interacted) particles and once it exists it remains tangible even if no one happens to be looking at it at any one particular instance.

Let's take the example of the big bang. For many billions of years after the big bang there were no living things or sentient observers, yet as far as know or assume, the universe evolved according to the laws of physics (including the second law of thermodynamics) despite the complete lack of sentient observers. Is that not proof enough that the laws of physics do not require living observers to still be valid?

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LURCH
In the process of entropy, energy is lost from places where their is much to places where there is little, within the universe. However, for entropy to effect the Oscilating Universe model, energy would have to go from within the universe to outside it. IOW; to no-place.

This would amount to energy ceasing to exist, which would be a violation of Conservation. For this reason, I do not think entropy can be applied as a problem of the Oscilating Universe model.

Chronos