A few thoughts on timeline & entropy

[Djordje]

If we agree that universe has a beginning and if we know that the entropy increases over time (a broken window won't fix itself),

I reckon that entropy had to be zero or a constant some point in past. Then if universe is to begin shrinking, that means it's returning to the primal state of higher order and

least entropy - the entropy'd start to decrease- in that case the usual thermodynamic direction of time will reverse - the window pieces will jump together and the window will be whole again.

The psychological direction of time would also reverse: we wouldn't remember past but future - but that also raises paradoxes like "Could we die before we're born?" etc.

These are just some of my thoughts on this subject, I'm 15 and would like to hear your opinion on this.

[michel123456]

That is all about the relation between the arrow of time and entropy. You made it more complicated by linking entropy with expansion.

IIRC the arrow of time is still an open question. I don't know if it is established that turning time backward will have an analog result on entropy (IOW that entropy and time are the same thing). I don't know either if a shrinking universe corresponds to a backward motion in time. As far as I know, the presumed shrinking would not be like that: time would still go forward. It is just like Hubble noticed that galaxies were coming together instead of walking away. But I may be wrong on this.

[Djordje]

My main premise was that at first universe was far less entropic that today and as it was expanding the entropy increased.

However, shrinking might be returning to that first state (with less entropy).

[michel123456]

So your premise was that at Bang time, entropy was zero or close to zero.

[Djordje]

So your premise was that at Bang time, entropy was zero or close to zero.

That's right. And another opinion of mine is that bigger volume equals bigger entropy - entropy is a statistical law, not an absolute like 1st law of thermodynamics. It is based on huge number of cases of lesser order compared to a single case of higher order. More volume must, IMO, imply more cases of lesser order and that means higher entropy. So if at Bang time space had V₀ and at the point of shrinking _Vmaxit's entropy could be also S₀ and S_max . When it starts collapsing the entropy and volume go from V_max and S_max to V₀ and S₀.

So the whole process could be represented like this: V₀(S₀) --------> V_max(S_max) --------> V₀(S₀)

Maybe I'm wrong but anyways I'm open to critics and new ideas and I'm looking forward to your reply.

Sorry if my English sometimes can't keep up with my thoughts

[J.C.MacSwell]

That's right. And another opinion of mine is that bigger volume equals bigger entropy - entropy is a statistical law, not an absolute like 1st law of thermodynamics. It is based on huge number of cases of lesser order compared to a single case of higher order. More volume must, IMO, imply more cases of lesser order and that means higher entropy. So if at Bang time space had V₀ and at the point of shrinking _Vmaxit's entropy could be also S₀ and S_max . When it starts collapsing the entropy and volume go from V_max and S_max to V₀ and S₀.

So the whole process could be represented like this: V₀(S₀) --------> V_max(S_max) --------> V₀(S₀)

Maybe I'm wrong but anyways I'm open to critics and new ideas and I'm looking forward to your reply.

Sorry if my English sometimes can't keep up with my thoughts

 

Didn't we have this discussion 28 billion years ago, except this time we are having it forwards?...or is it backward this time?

Edited March 17, 2011 by J.C.MacSwell

[Djordje]

Didn't we have this discussion 28 billion years ago, except this time we are having it forwards?...or is it backward this time?

 

I joined yesterday -.-

[J.C.MacSwell]

I joined yesterday -.-

 

Sorry. Bad joke.... 28 billion is twice 14 billion which is roughly the time from the apparent Big Bang. If your assumption is correct about expansion and entropy increasing being linked, then we could very well be in a contraction stage but not know it as our brains would be in reverse.

[Djordje]

Sorry. Bad joke.... 28 billion is twice 14 billion which is roughly the time from the apparent Big Bang. If your assumption is correct about expansion and entropy increasing being linked, then we could very well be in a contraction stage but not know it as our brains would be in reverse.

 

Well some estimate it to 15, or 10 billion... We could but we aren't as entropy still increases.

Edited March 18, 2011 by Djordje

[michel123456]

I had a discussion here some time ago. That was about the effects of turning time backward on gravity. Although the "turning time backward" is a very speculative situation, it appeared from the conversation that gravity does not depend on the arrow of time, IOW that gravity remains attractive under time reversal. That would mean that time reversal does not turn everything inside-out.

I don't know about entropy.

Edited March 18, 2011 by michel123456

[Djordje]

I had a discussion here some time ago. That was about the effects of turning time backward on gravity. Although the "turning time backward" is a very speculative situation, it appeared from the conversation that gravity does not depend on the arrow of time, IOW that gravity remain attractive under time reversal. That would mean that time reversal does not turn everything inside-out.

I don't know about entropy.

Gravity is an absolute law, therefore it shouldn't depend on time arrow. I have given an explanation why I think entropy would decrease in one of the posts above.

[J.C.MacSwell]

I had a discussion here some time ago. That was about the effects of turning time backward on gravity. Although the "turning time backward" is a very speculative situation, it appeared from the conversation that gravity does not depend on the arrow of time, IOW that gravity remains attractive under time reversal. That would mean that time reversal does not turn everything inside-out.

I don't know about entropy.

 

Even in Newtonian mechanics the forces remain the same and do not reverse with time reversal. Attractive forces remain attractive, and repulsive forces remain repulsive.

[michel123456]

Even in Newtonian mechanics the forces remain the same and do not reverse with time reversal. Attractive forces remain attractive, and repulsive forces remain repulsive.

 

Yeap. That makes it difficult for the glass that fall out the table to jump back in place under time reversal.

[J.C.MacSwell]

Yeap. That makes it difficult for the glass that fall out the table to jump back in place under time reversal.

 

It does under time reversal.

 

Dissipated energy converges, glass fragments combine into a glass with the combined kinetic energy to leap, still against the force of gravity, to land on the table.

 

If Gravity was repulsive under time reversal the glass (and table) would be heading for the moon, and if other attractive forces were reversed everything would blow apart.

[michel123456]

It does under time reversal.

 

Dissipated energy converges, glass fragments combine into a glass with the combined kinetic energy to leap, still against the force of gravity, to land on the table.

 

If Gravity was repulsive under time reversal the glass (and table) would be heading for the moon, and if other attractive forces were reversed everything would blow apart.

 

You must be right.

So you say that the other forces stay the way they are? under time reversal.

[J.C.MacSwell]

You must be right.

So you say that the other forces stay the way they are? under time reversal.

 

Not absolutely sure about the weak interaction as I know (am aware of) there is a symmetry violation with it, but I think it would have to be as well, assuming time run backwards would get you back to an earlier starting point. (quantum uncertainty aside)

 

http://www.lbl.gov/abc/wallchart/chapters/05/2.html (Interesting Feynman quote about meeting an Alien)

 

 

But in classical mechanics all forces stay the same.

[Iggy]

That's right. And another opinion of mine is that bigger volume equals bigger entropy - entropy is a statistical law, not an absolute like 1st law of thermodynamics. It is based on huge number of cases of lesser order compared to a single case of higher order. More volume must, IMO, imply more cases of lesser order and that means higher entropy.

 

But, you have to account for temperature lowering with the increase in volume. If the universe is an isolated system, meaning it doesn't transfer heat or mass to any kind of surroundings, then simple expansion would be adiabatic. Change of entropy in adiabatic expansion is zero.

 

You might like section 2.3 of Thermodynamic Asymmetry in Time.

 

The universe seems to be uniformly expanding relative to our local frame. Since this temporal asymmetry occupies a rather unique status it is natural to wonder whether it might be the ‘master’ arrow. The cosmologist T. Gold 1962 proposed just this. Believing that entropy values covary with the size of the universe, Gold asserts that at the maximum radius the thermodynamic arrow will ‘flip’ due to the re-contraction. However, as Tolman 1936 has shown in some detail, a universe filled with non-relativistic particles will not suffer entropy increase due to expansion, nor will an expanding universe uniformly filled with blackbody radiation increase its entropy either. Interestingly, Tolman demonstrated that more realistic universes containing both matter and radiation will change their entropy contents. Coupled with expansion, various processes will contribute to entropy increase, e.g., energy will flow from the ‘hot’ radiation to the ‘cool’ matter. So long as the relaxation time of these processes is larger than the expansion time scale, they should generate entropy. We thus have a purely cosmological method of entropy generation.

[keelanz]

Didn't we have this discussion 28 billion years ago, except this time we are having it forwards?...or is it backward this time?

 

=D

 

Didn't we have this discussion 28 billion years ago, except this time we are having it forwards?...or is it backward this time?

 

have you guys not thought that perhaps gravity is the soul of time? such that it is the fundamental causation of change(other than the original outward force (which arguably could be caused by it own pressure?))

 

im sure with an understanding such as yours you could at least philosophise over the question?

 

=D

[michel123456]

have you guys not thought that perhaps gravity is the soul of time? such that it is the fundamental causation of change(other than the original outward force (which arguably could be caused by it own pressure?))

 

im sure with an understanding such as yours you could at least philosophise over the question?

 

=D

 

Yes. Something like that.

 

But there are some incoherent issues about this concept.

[swansont]

have you guys not thought that perhaps gravity is the soul of time? such that it is the fundamental causation of change(other than the original outward force (which arguably could be caused by it own pressure?))

 

im sure with an understanding such as yours you could at least philosophise over the question?

 

!

Moderator Note

This is not the place for philosophising over the question. This is the place for discussing physics.

[Djordje]

But, you have to account for temperature lowering with the increase in volume. If the universe is an isolated system, meaning it doesn't transfer heat or mass to any kind of surroundings, then simple expansion would be adiabatic. Change of entropy in adiabatic expansion is zero.

 

You might like section 2.3 of Thermodynamic Asymmetry in Time.

 

 

 

The universe is cooling, isn't it? Entropy is increasing, isn't it?

[Djordje]

But, you have to account for temperature lowering with the increase in volume. If the universe is an isolated system, meaning it doesn't transfer heat or mass to any kind of surroundings, then simple expansion would be adiabatic. Change of entropy in adiabatic expansion is zero.

 

You might like section 2.3 of Thermodynamic Asymmetry in Time.

 

 

 

The universe is cooling, isn't it? Entropy is increasing, isn't it?

[Iggy]

The universe is cooling, isn't it?

 

Yes.

 

Entropy is increasing, isn't it?

 

I would assume so.

 

This does not, however, mean that expansion is the cause of increased entropy. With adiabatic expansion entropy is constant. In the current model of cosmology,

 

The first equation can be derived also from thermodynamical considerations and is equivalent to the first law of thermodynamics, assuming the expansion of the universe is an adiabatic process (which is implicitly assumed in the derivation of the Friedmann–Lemaître–Robertson–Walker metric).

 

http://en.wikipedia.org/wiki/Friedmann%E2%80%93Lema%C3%AEtre%E2%80%93Robertson%E2%80%93Walker_metric#Interpretation

 

where

 

It can be proven that any reversible adiabatic process is an isentropic process.

 

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

[michel123456]

(...)in that case the usual thermodynamic direction of time will reverse - the window pieces will jump together and the window will be whole again.

(...)

 

I really wonder.

 

That is what we see if we made a movie of the window that breaks in pieces, and turn the movie back.

But I wonder if it is an accurate representation of time reversal.

I strongly suspect that under time reversal, not only the T part of the CPT symmetry reverses, but the whole thing reverses.

 

The pieces of glass jumping together & making a perfect window is not IMHO a correct representation of time reversal.

Under time reversal, the pieces of glass are made of antimatter, the electromagnetic interactions are reversed, the electrons become positrons, the temperatures are reversed, heat is cold and cold is hot, the distance they jump is negative, etcaetera.

[Djordje]

I really wonder.

 

That is what we see if we made a movie of the window that breaks in pieces, and turn the movie back.

But I wonder if it is an accurate representation of time reversal.

I strongly suspect that under time reversal, not only the T part of the CPT symmetry reverses, but the whole thing reverses.

 

The pieces of glass jumping together & making a perfect window is not IMHO a correct representation of time reversal.

Under time reversal, the pieces of glass are made of antimatter, the electromagnetic interactions are reversed, the electrons become positrons, the temperatures are reversed, heat is cold and cold is hot, the distance they jump is negative, etcaetera.

Possibly, yes.