[md65536]

Iggy, on 10 November 2011 - 11:02 PM, said:

What trips me is thinking that entropy is not time symmetric, but it is nonetheless derived from the equations of motion which are time symmetric. The arrow arises more from the boundary conditions of the system than the underlying physics. I tried to say this earlier in the thread but mangled it horrifically.

I also think this has something to do with gravitation. If I remember correctly, the laws of gravity don't have a meaningful notion of time symmetry in GR. Personally I don't think gravity is time-symmetric.

But even if it were, relativity of simultaneity messes things up.
Suppose you could "reverse the universe" at a particular instant, run it backwards for a time of t, and then forward for t, and suppose it's possible to end up at the same state as the instant you began reversing it.

That instant won't be the same for other observers. Others will see different parts of the universe reverse at different times, for different lengths of time, and as a whole it would never end up in a state that was identical to a previous state. Everyone's observations would be consistent with your observations (in which a true hypothetical time reversal took place), but relativity of simultaneity would give everyone different experiences of it.

I think that because time is relative, time-symmetry must also be relative. So if you're speaking of an arrow of time and whether it has a fixed direction, I don't think you can talk about a universal arrow of time. This is before even needing to consider entropy.

That is, the underlying physics of relativity do imply that a system that can't be universally synchronized, can't be universally time-symmetric, even if entropy is ignored. I think.

This post has been edited by md65536: 13 November 2011 - 07:33 AM

[michel123456]

Appolinaria, on 13 November 2011 - 06:52 AM, said:

naw. i dont get why that sh*ts increasing.

Ok, the universe is an isolated system, and I just read the second law of thermodynamics.. Okay, it's an irreversible process...

However, I still don't understand why it isn't just in equilibrium, if our universe is one system.

Applause.
I agree that the universe must be a system in equilibrium.
Of course there must exist some arguments against that, some other member could develop about this.
But IIRC there are many scientists that argue that total energy of the Universe must be zero, which IMHO is a stretch of the statement that the universe is an state of equilibrium.

md65536, on 13 November 2011 - 07:20 AM, said:

I also think this has something to do with gravitation. If I remember correctly, the laws of gravity don't have a meaningful notion of time symmetry in GR. Personally I don't think gravity is time-symmetric.
(...)

Applause.
I remember another thread on this forum discussing the time-symmetry of gravity. It resulted from the discussion that gravity don't become repulsive under time reversal. I was never convinced. This is getting the thread out of tracks I'am afraid.

[Appolinaria]

Michel, can you maybe clarify if you have a second;



since black holes in the center of galaxies are gaining mass (as opposed to small, hot ones who lose more than they gain with hawking radiation or w/e) aren't they decreasing in temperature? if they're increasing in entropy, according to the second law, shouldn't they be getting hotter?

[Greg Boyles]

IM Egdall, on 3 November 2011 - 02:59 PM, said:

Yes, the second law of thermodynamics gives a forward arrow of time -- but it is a statistical law. It applies only to a large number of things. When we look at the behavior of a small number of particles, they show time symmetry. No arrow of time is revealed.

Emergent property of complexity? Same as 'mind' emerges from biochemistry.
Arrow of time emerges from complex thermodynamic systems but is absent from the simplest of thermodynamic systems?

This post has been edited by Greg Boyles: 13 November 2011 - 09:59 AM

[michel123456]

Appolinaria, on 13 November 2011 - 09:31 AM, said:

Michel, can you maybe clarify if you have a second;



since black holes in the center of galaxies are gaining mass (as opposed to small, hot ones who lose more than they gain with hawking radiation or w/e) aren't they decreasing in temperature? if they're increasing in entropy, according to the second law, shouldn't they be getting hotter?

Oh no, not black holes again!
Hawking radiation makes the things messy, I am not qualified to answer.

i finally found this old thread about gravity under time reversal.

This post has been edited by michel123456: 13 November 2011 - 10:01 AM

[Appolinaria]

give me a break, black holes are weird as hell!

psh, where god divided by zero... more like where god fell asleep on the keyboard and totally effed shit up. it's still here, existing, nonsensically.

ps good thread, reading now

This post has been edited by Appolinaria: 13 November 2011 - 10:13 AM

[Iggy]

md65536, on 13 November 2011 - 07:20 AM, said:

I think that because time is relative, time-symmetry must also be relative.

That seems reasonable to me.

[A Tripolation]

michel123456, on 13 November 2011 - 09:08 AM, said:

I agree that the universe must be a system in equilibrium.

Why? There are many examples of systems that aren't in equilibrium.

[Appolinaria]

A Tripolation, on 13 November 2011 - 10:37 PM, said:

Why? There are many examples of systems that aren't in equilibrium.

And those are open systems in the universe, right?

If the universe is isolated, shouldn't it be in equilibrium?

[Greg Boyles]

Iggy, on 13 November 2011 - 07:54 PM, said:

That seems reasonable to me.

Can you elaborate as I am not clear where you are going with this.

[Iggy]

Greg Boyles, on 13 November 2011 - 10:57 PM, said:

Can you elaborate as I am not clear where you are going with this.

If there is no global notion of time (dr.Rocket explains here) then it would seem to follow that there is no global notion of time symmetry.

I really don't know though. GR is far above my head.

[Mystery111]

Iggy, on 14 November 2011 - 02:00 AM, said:

If there is no global notion of time (dr.Rocket explains here) then it would seem to follow that there is no global notion of time symmetry.

I really don't know though. GR is far above my head.

There are applications of a global time - such as the Wheeler de Witt equation. This global time however, the time derivative containing all the dynamics of the matter field and radiation field vanishes. So in a way, you try and establish a global time, but it fails. Motion in general relativity is also a symmetry of the theory, not even a true time evolution.

It might even be said, that General Relativity (in respect to time) breaks down on large scales. This might be a mathematical discrepency, or it might be something more sinister, like a singularity being implied when speaking about the absence of a global time. In fact, if a singularity arises when speaking about the absence of a global time, then maybe a global time should not be absent!