The 'time oriented coarse graining' hypothesis -- "Rovelli"

[wabbit]

Agreed - I don't claim these observations solve the mistery : ) it just suggests that the "background" initial singularity and accelerating expansion may not be the obstacle in finding alternate perspectives - something that wasn't clear to me initially.

Regarding your comment, I have a question. The history we describe is one of gradual differentiation and structure formation - or order arising out of chaos to put it in wholly unscientific terms. Intuitively there seems to be a lot more "relevant" information in later states than in early ones - the initial state being perhaps some kind of undifferentiated hot soup of unified field quanta (even suggestive of a thermal state). Might there be a choice of coarse graining where this picture translates into decreasing entropy rather than increasing, or is this hopelessly misguided?

 

[wabbit]

Just quicly, another rambling thought - Actually, I don't expect an alternate perspective to be easy to identify (e.g as from the picture above which I do suspect is misguided (it seems too easy not to be naive), it should be truly and completely foreign to us if it exists. But maybe starting with the universe as information, a set of bits, the subdivision into interacting systems corresponds to different interpretations of those bits - the same string of bits encoding two different pictures when decoded differently. If so, an alternate perspective might correspond to such a thorough scrambling of all bits (cryptography?) and coarse graining (steganography?) that it is completely unrelated to any aspect we can perceive, and it might be completely impossible to reconstruct any relevant observable in the alernate view, using ours.

OK I'll shut up before I get thrown out for empty philosophising:)

 

[stevendaryl]

Regarding your comment, I have a question. The history we describe is that of gradual differentiation and structure formation. Intuitively there seems to be a lot more "relevant" information in later states than in early ones - the initial state being perhaps some kind of undifferentiated hot soup of unified field quanta (even suggestive of a thermal state). Might there be a choice of coarse graining where this picture translates into decreasing entropy rather than increasing, or is this hopelessly misguided?

I don't know whether there is a way to make sense of that, or not, but I have had a similar idea. Roughly speaking, the entropy of a system can be thought of as the number of bits needed to completely describe a system. So if you have a cube filled uniformly by hydrogen in its ground state, that is completely described in a few words. If through thermonuclear processes, the hydrogen is turned into helium, nitrogen, carbon, iron, and all the rest of the elements, and those elements are assembled into humans and buildings and cars and planets and so forth, then it takes vastly more words to completely describe the state, in all its detail. So this is a much higher-entropy system. But now imagine trillions of years of history. The buildings crumble into dust, as do the living creatures and cars, etc. Eventually, you have no large-scale structure at all, just microscopic dust, thoroughly mixed, so that each speck of dust is indistinguishable from any other. This situation to our minds seems very uniform, and simple to describe, just as the initial uniform collection of hydrogen did.

So if we could somehow separate macroscopic entropy from microscopic, macroscopic entropy does not increase forever, it starts off low in the early universe, increases for a while, and then decreases again. So it seems roughly time-symmetric.

 

[marcus]

The ideas of the paper http://arxiv.org/abs/1407.3384
which is the topic of the thread were presented and discussed at the September 2014 Tenerife conference

5YqQaWEss74
52 minutes. About 33 minutes of presentation followed by discussion.
Lots of questions and discussion at the end (Robert Wald, George Ellis, Jim Hartle, Simon Saunders, David Alpert...)
Thanks to Fuzzyfelt for pointing this out! It's a really interesting youtube.
In fact the questions raised evidently helped to motivate the next paper on this subject which Demystifier just gave the link to:
http://lanl.arxiv.org/abs/1505.01125
Is Time's Arrow Perspectival?
Carlo Rovelli
(Submitted on 4 May 2015)
We observe entropy decrease towards the past. Does this imply that in the past the world was in a non-generic microstate? I point out an alternative. The subsystem to which we belong interacts with the universe via a relatively small number of quantities, which define a coarse graining. Entropy happens to depends on coarse-graining. Therefore the entropy we ascribe to the universe depends on the peculiar coupling between us and the rest of the universe. Low past entropy may be due to the fact that this coupling (rather than microstate of the universe) is non-generic. I argue that for any generic microstate of a sufficiently rich system there are always special subsystems defining a coarse graining for which the entropy of the rest is low in one time direction (the "past"). These are the subsystems allowing creatures that "live in time" ---such as those in the biosphere--- to exist. I reply to some objections raised to an earlier presentation of this idea, in particular by Bob Wald, David Albert and Jim Hartle.
6 pages, 4 figures.
Here is the schedule of talks (with abstracts) for the Tenerife conference
http://philosophy-of-cosmology.ox.a...acts-for-Final-Conference-12-16-Sept-2014.pdf

 

[wabbit]

So if we could somehow separate macroscopic entropy from microscopic, macroscopic entropy does not increase forever, it starts off low in the early universe, increases for a while, and then decreases again. So it seems roughly time-symmetric.

Indeed, macroscopic and microscopic entropy, didn't quite see that so clearly.
With luck in some toy models at least there might even be a symmetry exchanging the two. That would be nice:)

But maybe the suggestion to follow is that of Rovellis examples, ball colorings and the small ball/big balls: these toy models seem like they could have a lot in store.

 

[marcus]

Personally I suspect that all subsystems are obliterated at cosmological bounce (at near Planckian density).
The problem they are wrestling with is how does it happen that expansion starts at a low entropy state.
The universe begins expanding at what is considered a very unlikely state, that gives the direction to time.
I don't see the problem because I do not see how you can even define subsystems, or macroscopic variables, or entropy at Planckian density. Gravity is repellent at extreme density (quantum effects) which is what causes geometry to bounce.
How do you define a region, or a "thing", or even a locality under such conditions? I think entropy is a meaningless idea at such high density.

Rovelli's point is that the definition of entropy involves interaction with a subsystem in an essential way, he emphasizes that.
Yes there is strong dependence on how the subsystem interacts with the universe.
But he does not make what I think is a reasonable conjecture that therefore (when the idea of a subsystem, or a split of the Hilbert space into a tensor product of two or more factors is itself unrealizable) entropy cannot be defined at bounce.

One would have to wait for some expansion to occur and density to get well below Planckian, gravity stops being repellent, more normal version of geometry, locality settles down. Definite subsystems begin to emerge. *Then* we know what entropy is. This is just my suspicion.

 

[marcus]

Since we've turned a page, I'll bring forward the most relevant links:
The ideas of the paper http://arxiv.org/abs/1407.3384 which is the topic of the thread
were presented and discussed at the September 2014 Tenerife conference

52 minutes. About 33 minutes of presentation followed by discussion.
Lots of questions and discussion at the end (Robert Wald, George Ellis, Jim Hartle, David Alpert...)
In fact the questions raised evidently helped to motivate the next paper on this subject which Demystifier just gave the link to:
http://lanl.arxiv.org/abs/1505.01125
Is Time's Arrow Perspectival?
Carlo Rovelli
(Submitted on 4 May 2015)
We observe entropy decrease towards the past. Does this imply that in the past the world was in a non-generic microstate? I point out an alternative. The subsystem to which we belong interacts with the universe via a relatively small number of quantities, which define a coarse graining. Entropy happens to depends on coarse-graining. Therefore the entropy we ascribe to the universe depends on the peculiar coupling between us and the rest of the universe. ..
... These are the subsystems allowing creatures that "live in time" ---such as those in the biosphere--- to exist. I reply to some objections raised to an earlier presentation of this idea, in particular by Bob Wald, David Albert and Jim Hartle.
6 pages, 4 figures.
Here is the schedule of talks (with abstracts) for the Tenerife conference
http://philosophy-of-cosmology.ox.a...acts-for-Final-Conference-12-16-Sept-2014.pdf