Carlo Rovelli can sure talk up a blizzard (I’m reviewing his conference talk: (The preferred time direction in the dynamics of the full universe). For an Italian native he can really weave a blinding spell in English.
He has my confused when he tries to explain the apparent low entropy Big Bang cosmology. He uses his own brand of relational quantum mechanics I think, but it comes out sounding a bit circular or anthropomorphic. Yet earlier in his lectures he often takes pains to deny anthropomorphic views.
So it is quite perplexing when he tries to explain our perception of an arrow of time by claiming that, “it is what makes us us.” Let me quote him, so you can see for yourself. He starts out by claiming the universe starts in a low entropy state only form our relative point of view. Entropy is an observer dependent concept. It depends on how you coarse grain your physics. OK, I buy that. We couple to the physical external fields in a particular way, and this is what determines how we perceive or coarse grain our slices of the universe. So how we couple to the universe supposedly explains way wee see the apparent entropy we perceive. If by some miracle we coupled more like antiparticles effectively travelling in the reverse time direction then we’d see entropy quite differently, one imagines. So anyway, Rovelli then summarizes:
[On slides: Entropy increase (passage of time) depend on the coarse graining, hence the subsystem, not the microstate of the world.] … “Those depend on the way we couple to the rest of the universe. Why do we couple to the rest of the universe in this way? Because if we didn’t couple to the rest of the universe this way we wouldn’t be us. Us as things, as biological entities that very much live in time coupled in a manner such that the past moves towards the future in a precise sense … which sense? … the one described by the Second Law of Thermodynamics.”
You see what I mean?
Maybe I am unfairly pulling this out of a rushed conference presentation, and to be more balanced and fair I should read his paper instead. If I have time I will. But I think a good idea deserves a clear presentation, not a rush job with a lot of vague wishy-washy babble, or obscuring in a blizzard of words and jargon.
OK, so here’s an abstract from an arxiv paper where Rovelli states things in written English:
” Phenomenological arrows of time can be traced to a past low-entropy state. Does this imply the universe was in an improbable state in the past? I suggest a different possibility: past low-entropy depends on the coarse-graining implicit in our definition of entropy. This, in turn depends on our physical coupling to the rest of the world. I conjecture that any generic motion of a sufficiently rich system satisfies the second law of thermodynamics, in either direction of time, for some choice of macroscopic observables. The low entropy of the past could then be due to the way we couple to the universe (a way needed for us doing what we do), hence to our natural macroscopic variables, rather than to a strange past microstate of the world at large.”
That’s a little more precise, but still no clearer on import. He is still really just giving an anthropocentric argument.
I’ve always thought science was at it’s best when removing the human from the picture. The problem for our universe should not be framed as one of “why do we see an arrow of time?” because, as Rovelli points out, for complex biological systems like ourselves there really is no other alternative. If we did not perceive an arrow of time we would be defined out of existence!
The problem for our universe should be simply, “why did our universe begin (from any arbitrary sentient observer’s point of view) with such low entropy?”
But even that version has the whiff of observer about it. Also, you just define the “beginning” as the end that has the low entropy, then you are done, no debate. So I think there is a more crystalline version of what cosmology should be seeking an explanation for, which is simply, “how can any universe ever get started (from either end of a singularity) in a low entropy state?”
But even there you have a notion of time, which we should remove, since “start” is not a proper concept unless one already is talking about a universe. So the barest question of all perhaps, (at least the barest that I can summon) is, “how do physics universes come to exist?”
This does not even explicitly mention thermodynamics or an arrow of time. But within the question those concepts are embedded. One needs to carefully define “physics” and “physics universes”. But once that is done then you have a slightly better philosophy of physics project.
More hard core physicists however will never stoop to tackle such a question. They will tend to drift towards something where a universe is already posited to exist and has had a Big Bang, and then they will fret and worry about how it could have a low entropy singularity.
It is then tempting to take the cosmic Darwinist route. But although I love the idea, it is another one of those insidious memes that is so alluring but in the cold dead hours of night, when the vampires of popular physics come to devour your life blood seeking converts, seems totally unsatisfying and anaemic. The Many Worlds Interpretation has it’s fangs sunk into a similar vein, which I’ve written about before.
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Going back to Rovelli’s project, I have this problem for him to ponder. What if there is no way for any life, not even in principle, to couple to the universe other than via the way we humans do, through interaction with strings (or whatever they are) via Hamiltonians and mass-energy? If this is true, and I suspect it is, then is not Rovelli’s “solution” to the low entropy Big Bang a bit meaningless?
I have a pithy way of summarising my critique of Rovelli. I would just point out:
The low entropy past is not caused by us. We are the consequence.
So I think it is a little weak for Rovelli to conjecture that the low entropy past is “due to the way we couple to the universe.” It’s like saying, “I conjecture that before death one has to be born.” Well, … duuuuhhh!
The reason my photo is no longer on Facebook is due to the way I coupled to my camera.
I am an X-gener due to the way my parents coupled to the universe.
You see what I’m getting at? I might be over-reaching into excessive sarcasm, but my point is just that none of this is good science. They are not explanations. It is just story-telling. Still, Rovelli does give an entertaining story if you are a physics geek.
So I had a read of Rovelli’s paper and saw the more precise statement of his conjecture:
Rovelli’s Conjecture: “Any generic microscopic motion of a sufficiently rich system satisfies the second law (in either time direction) for a suitable choice of macroscopic observables.“
That’s the sort of conjecture that says nothing. The problem is the “sufficiently rich” clause together with the “suitable choice” clause. You can generate screeds of conjectures with such a pair of clauses. The conjecture only has “teeth” if you define what you mean by “sufficiently rich” and if a “suitable choice” can be identified or motivated as plausible. Because otherwise you are not saying anything useful. For example, “Any sufficiently large molecule will be heavier than a suitably chosen bowling ball.”
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Rovelli does provide a toy example to illustrate his notions in classical mechanics. He has yellow balls and red balls. The yellow balls have an attractor which gives them a natural second law of thermodynamic arrow of time. The same box also has red balls with a different attractor which gives them the opposite arrow of time according to the second law. (Watching the conference video for this is better than reading the arxiv paper.) But “so what?”
Rovelli has constructed a toy universe that has entities that would experience opposite time directions if they were conscious. But there are so many things wrong with this example it cannot be seriously considered as a bulwark for Rovelli’s grander project. For starters, what is the nature of his Red and Yellow attractors? If they are going to act complicated enough to imbue the toy universe with anything resembling conscious life then the question of how the arrow of time arises is not answered, it just gets pushed back to the properties of these mysterious Yellow and Red attractors.
And if you have only such a toy universe without any noticeable observers then what is the point of discussing an arrow of time? It is only a concept that a mind external to that world can contemplate. So I do not see the relevance of Rovelli’s toy model for our much more complicated universe which has internal minds that perceive time.
You could say, in principle the toy model tells us there could be conscious observers in our universe who are experiencing life but in the reverse time direction to ourselves, they remember our future but not our past, we remember their future but not their past. Such dual time life forms would find it incredibly hard to communicate, due to this opposite wiring of memory.
But I would argue that Rovelli’s model does not motivate such a possibility, for the same reason as before. Constructing explicit models of different categories of billiard balls each obeying a second law of thermodynamics in opposite time directions in the same system is one thing, but not much can be inferred from this unless you add in a whole lot of further assumptions about what Life is, metabolism, self-replication, and all that. But if you do this the toy model becomes a lot less toy-like and in fact terribly hard to explicitly construct. Maybe Stephen Wolfram’s cellular automata can do the trick? But I doubt it.
I should stop harping on this. Let me just record my profound dissatisfaction with Rovelli’s attempt to demystify the arrow of time.
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If you ask me, we are not at a sufficiently mature enough juncture in the history of cosmology and physics to be able to provide a suitable explanation for the arrow of time.
So I have Smith’s Conjecture:
“At any sufficiently advanced enough juncture in the history of science, enough knowledge will have accumulated to enable physicists to provide a suitable explanation for the arrow of time.“
Facetiousness aside, I really do think that trying to explain the low entropy big bang is a bit premature. It would be much better to be patient and wait for more information about our universe before attempting to launch into the arrow of time project. The reason I believe so is because I think the ultimate answers about such cosmological questions are external to our observable universe.
But even whether they are external or internal there is a wider problem to do with the nature of time and our universe. We do not know if our universe actually had a beginning, a true genesis, or whether it has always existed.
If the universe had a beginning then the arrow of time problem is the usually low entropy puzzle problem. But if the universe had no beginning then the arrow of time problem becomes a totally different question. There is even a kind of intermediate problem that occurs if our universe had a start but within some sort of wider meta-cosmos. Then the problem is much harder, that of figuring out the laws of this putative metaverse. Imagine the hair-pulling of cosmologists who discover this latter possibility as a fact about their universe (but I would envy them the shear ability to discover the fact, it’d be amazing).
So until we know such a fundamental question I do not see a lot of fruitfulness in pursuing the arrow of time puzzle. It’s a counting your chickens before they hatch situation. Or should I say, counting your microstates before they batch.
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