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Arrow of Time universe acceleration


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So to my understanding there's a model out there that says the arrow of time is somehow related to the universe's expansion which can coincide with the ever increase in entropy which apparently has its own arrow of time. Furthermore, it is stated on wikipedia that if the universe were to contract, it would be indicative of traveling backwards in time. So there's a few things.

What's the exact name of this model?

If I could take a cubic meter of space and somehow stop expansion inside of it, and I dropped a ball inside it, would the ball just "freeze" inside the cube because it can't experience the passing of time and thus not change position over time from velocity?

At the event horizon of black holes, time mathematically appears to stop from our limit calculations, so is space not expanding near the event horizon of a black hole?

 

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the thermodynamic arrow has been used in numerous arguments and models, its a consequence of the 2nd law of thermodynamics. Its been around as long as the concept of entropy has. Just about every famous physicist has contemplated its consequences to just about every model that describes cosmology or any other thermodymamic process. Some physicist have even written entire books just on the arrow of time. In and of itself however isn't a model. Sholman tried to portray a universe inside a black hole using this consequence. Though hes certainly not the only one. Numerous articles discusses its consequence in dealing with black holes. Sean Caroll uses it in terms of a multiverse hypothesis (cyclic universes). The list is endless. One of the mysteries in cosmology has always been explaining why the universe started at a low entropy beginning. Every cosmology model must have a low entropy beginning to match up with observational data.

 

A couple of details to keep in mind, any thermodynamic system must start out at a lower entropy, lower being a relative value. Entropy either has to stay constant or increase, it can never decrease (hence arrow of time )

 

Here is one article describing it

http://edge.org/response-detail/25538

here is one that uses inflation to explain it

http://arxiv.org/abs/1212.1087

here is one using Branes

http://cds.cern.ch/record/539984/files/0202104.pdf

 

here is a decent paper discussing it

http://philsci-archive.pitt.edu/4744/1/gravent_archive.pdf

Edited by Mordred
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Yeah I mean I already know that entropy is used in the arrow of time, but how about an actual "cause"?

not sure what you mean by actual cause, the term arrow of time is simply a descriptive of entropy change to measure time. pretty much an analogy. Time itself is not controlled by entropy. Time also doesn't control entropy. The two are unrelated. The arrow of time is simply a model of time like a clock or measuring a second via beta reactions. Change in entropy follows the ideal gas laws in cosmology in the same principles as a gas in a tank.

 

Time being a measure of rate of change of events. its value as a vector only means it has a forward or backward mathematical relation. In the real universe outside of mathematics time can only flow one direction forwards. No process can reverse time.

 

Or another similar usage the use of the Hubble flow aka scale factor in cosmological time. In this case we use the expansion history as our clock. However that does not mean that if the universe is contracting time is running backwards..

Sounds to me that your under a misunderstanding of a multimedia misconception.

 

I recommend you study what entropy really is..

http://en.wikipedia.org/wiki/Entropy

 

a key note is that entropy is a function of state

http://en.wikipedia.org/wiki/Function_of_state

 

now I particularly want you to read this article, as it describes how a decrease in entropy is performed and its relations

http://www.physics.ohio-state.edu/p670/textbook/Chap_6.pdf

 

now after you read how to decrease the entropy in a tank, does it make sense to say time flowed backward in that tank? while time flows normally outside of it?? Now apply that same scenario to your contracting universe.

Edited by Mordred
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not sure what you mean by actual cause, the term arrow of time is simply a descriptive of entropy change to measure time. pretty much an analogy. Time itself is not controlled by entropy. Time also doesn't control entropy. The two are unrelated. The arrow of time is simply a model of time like a clock or measuring a second via beta reactions. Change in entropy follows the ideal gas laws in cosmology in the same principles as a gas in a tank.

 

Time being a measure of rate of change of events. its value as a vector only means it has a forward or backward mathematical relation. In the real universe outside of mathematics time can only flow one direction forwards. No process can reverse time.

 

Or another similar usage the use of the Hubble flow aka scale factor in cosmological time. In this case we use the expansion history as our clock. However that does not mean that if the universe is contracting time is running backwards..

Sounds to me that your under a misunderstanding of a multimedia misconception.

 

I recommend you study what entropy really is..

http://en.wikipedia.org/wiki/Entropy

 

a key note is that entropy is a function of state

http://en.wikipedia.org/wiki/Function_of_state

 

now I particularly want you to read this article, as it describes how a decrease in entropy is performed and its relations

http://www.physics.ohio-state.edu/p670/textbook/Chap_6.pdf

 

now after you read how to decrease the entropy in a tank, does it make sense to say time flowed backward in that tank? while time flows normally outside of it?? Now apply that same scenario to your contracting universe.

I don't know that you understand "cause." Not cause as in "because," cause as in "this moves here, hitting that thing, causing it to move here," what is the reason for time's arrow exists at all in the first place? Why isn't time just stopped?

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A couple of details to keep in mind, any thermodynamic system must start out at a lower entropy, lower being a relative value. Entropy either has to stay constant or increase, it can never decrease (hence arrow of time )

 

 

Every isolated thermodynamic system................

 

The problem with applying this to the Universe is that we have no proof that the universe is an isolated system (or that it is not, we just don't know) in the thermodynamic sense.

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yeah I forgot to correct that, its explained in the last link I posted.

 

Numerous scientists, have tried to show time as an entropy change, entropy representing change. However you can have numerous forms of change that does not entail a change of entropy. A change in an electromagnetic wave, isn't an entropy change for example. A change of location ie one person walking isn't an entropy change.

 

One definition of time I always liked is "time is a rate of change anywhere in the universe, regardless of what the change is."

The beauty of that is it includes every form of possible change. However does time depend on the universe? How does time work for a multiverse then? Does that mean there is no time between one universe and another ie the formation of a new universe in say a cyclic or bounce multiverse model. In order to form a new universe time must exist, otherwise there would be no change. So does time depend on changes within our universe only, in the multi-verse models?

 

The simple answer is we don't know....

 

this is one of my favourite papers on time in regards to multiverse models

"Time before Time"

http://arxiv.org/ftp/physics/papers/0408/0408111.pdf

Edited by Mordred
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All this is really science fiction.

 

In that respect Poul Andersons's book 'Tau Zero' is as good as any, and better than most.

 

:)

Edited by studiot
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I disagree

Understanding how to "properly" define time is not science fiction. The scientific possibility of a multiverse is also not science fiction. We simply have to accept the fact that just because we can not gain any form of measurement of a multiverse does not preclude the possibility of its existence. Any multiverse model due to this fact is conjectural.

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Understanding how to "properly" define time is not science fiction.

 

Well I'm all ears since I could do with a proper definition of Time.

 

 

But lighten up as well. Poul was renouned for getting his science right, as he did in the book I mentioned. The story was obviously a fairy tale, but IMHO he wrote some cracking ones.

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trust me I wish I could give you a proper definition of time as well as how time works. I would be famous lol. I'll have to read 'Tau Zero' sometime. Right now I've been studying N-body codes for gravity and trying to learn ADS/CFT. Penroses "Roads to reality" has been helpful in that. never bothered learning stringy things before so its all new to me I nearly laughed my head off when he described electromagnetic wave functions as the zigzag model

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But if individual universes in the 'multiverse' are related through the time dimension, then they are also causally related. In effect they are not separate universes, but parts of the same single universe.

 

I've always had a problem with the multiverse model. If something is not causally related and can never affect us or be detected in any way, does it make any difference if it exists or not ?

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trust me I wish I could give you a proper definition of time as well as how time works. I would be famous lol.

In this thread someone quoted entropy.

 

Now there are many confusing ways to introduce entropy, but I found the easiest was the traditional path of indicator diagrams.

A function along an axis was found to be useful and given a name.

 

In the case of entropy the desired function was one that paired with temperature to give the dimensions of energy (as an integral on the diagram), in the same way that the intergral of a pressure-volume diagram give energy.

 

 

In this recent question here about the nature of time I offered a similar route to time as the label for an axis that was required to perform an analysis.

See post#62

 

http://www.scienceforums.net/topic/82939-explanation-of-time/page-4

Edited by studiot
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But if individual universes in the 'multiverse' are related through the time dimension, then they are also causally related. In effect they are not separate universes, but parts of the same single universe.

 

I've always had a problem with the multiverse model. If something is not causally related and can never affect us or be detected in any way, does it make any difference if it exists or not ?

both correct and valid points, I don't waste time studying multiverse theories myself. I have enough on my plate understanding this universe lol. There is numerous papers though that question whether two causal disconnected regions of this universe. Our observable portion of the universe is essentially the bounds of our causal connected region (described as lightcone of our worldline). Its been theorized, last paper I read though says no, but other papers says yes. This isn't the same as a multiverse though. We should discuss this question in another thread and keep this one on topic to the arrow of time.

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