This is a linkpost for https://www.youtube.com/watch?v=UNpLfXOfzZ8&t=39m25s
Haven't seen the video. Doe she get onto the problem of consciousness? If so, are we agreeing with him?
Yes, that sounds right.
I'm not a big fan of the next section after that where he suggests that the arrow of time means that the physical laws are wrong. Eliezer's quantum mechanics sequence segment on measurement covers the standard physics argument for that just fine, with nothing unusual added and nothing seeming particularly to be missing.
The arrow of time does not mean that any physical laws are wrong. It does mean that they do not explain everything; the arrow of time follows from the laws given a low entropy at the beginning of the universe. The laws do not explain why there would be a low entropy at the beginning of the universe.
Yes, exactly, and if he'd said that then I'd totally agree. But he presented it as a big challenge and that something must be done to fix it.
the arrow of time follows from the laws given a low entropy at the beginning of the universe.
That is not correct. Entropy is a statistical tendency over ensembles of states, which we use to make probabilistic predictions because we do not know the single true state with precision. But the actual physical world has exactly one state, and it evolves deterministically. There is no reason within Newton's and Maxwell's laws for the world to go from low to high entropy; it could just as well evolve in the other direction.
The correct answer is to notice that the laws of physics (which are not the same as the laws of Newton and Maxwell, or even Einstein, even though none of these names are forgotten) are not, in fact, symmetric in time, but have a T-symmetry-violating component accessible to sufficiently subtle experiment. Now you are done explaining the arrow of time.
So far as anyone knows, the true laws of physics are "CPT-symmetric", meaning that you can reverse T without violating any laws providing you also (1) replace particles with antiparticles and (2) reverse all the spatial coordinates.
I don't think there is an explanation for the arrow of time here.
Entropic considerations can't explain (even if one could find a good way of stating precisely) the alleged observation that time "runs" one way rather than the other; but they can explain why we remember the past and not the future, which is plausibly what's actually meant by saying that there's an arrow of time. An explanation of this sort, of course, leaves open the question of why there's a very-low-entropy state at all to serve as the "beginning" of the universe.
[You can] reverse T without violating any laws providing you also (1) replace particles with antiparticles and (2) reverse all the spatial coordinates.
Well, yes, but we in fact have a universe with a bunch of particles in particular coordinates! Given the particles there is an arrow of time, that is, you can tell the difference between forward and backwards evolution.
Reversed spatial particles look the same to us as unreversed; and the names "matter" and "anti-matter" are arbitrary. So those differences are not helpful in explaining an arrow of time. They will not make any large scale difference in how the universe evolves.
Reversed spatial particles look the same to us as unreversed
No they don't; the neutrinos would change their handedness. (So would our amino acids, but that wouldn't affect their functioning, so far as I know, since everything else would as well.) And chiral-reversed neutrinos don't interact with anything. The laws of physics are in fact just about as P-violating as they can possibly be!
and the names "matter" and "anti-matter" are arbitrary
The names are arbitrary, but the functions aren't; matter consists of particles favoured by the CP asymmetry in the laws. Flip everything to antimatter and after a sufficiently long time you have matter again.
I do not think what most people mean by an explanation of the arrow of time is a way of distinguishing the history of the universe from a T-reversed version, given that under CPT symmetry this is equivalent to a way of distinguishing the history of the universe from a CP-reversed version.
By way of illustration, here's an actual cosmologist: Sean Carroll, in his book "From eternity to here".
The lesson of all this is that the statement "this theory is invariant under time reversal" does not, in common parlance, mean "you can reverse the direction of time and the theory is just as good". It means something like "you can transform the state at every time in some simple way, and then reverse the direction of time, and the theory is just as good".
This seems to me to be moving the goalposts, and additionally to put a lot of work into that word 'simple'. Suppose the symmetry was CPXYZT instead, would required the CPXYZ transformation still be simple? Is there a criterion for deciding other than "Sean Carroll thinks so"?
I agree that this definition is fuzzy. (So does Carroll, as he makes clear in the text immediately following the bit I quoted.) But no, I don't think it's moving the goalposts, though it may not be putting them where you would prefer them to be.
I take the basic arrow-of-time problem to be something like this: The universe appears to be dramatically asymmetric in time: it is expanding in one time direction and contracting in the other; if we trace its evolution in the direction we call "past" according to our best understanding of the physics, we find a "big bang"; if we go in the direction we call "future" we find a "big freeze". These are distinguished not only by density/scale but also by entropy: the big bang is a much lower-entropy state than the big freeze. Furthermore, we see a similar dramatic asymmetry in our everyday lives: it's easy to break an egg or fry one, not so easy to put it together or turn it raw again. But in the fundamental laws of physics as we currently know them, we find nothing to explain any of this. Weak interactions do indeed show a slight violation of CP-symmetry, hence of T-symmetry, but frying eggs doesn't appear to have much to do with weak interactions; CPT-symmetry would appear to turn our universe into one that "looks just the same" but has time running "the other way"; and nothing in all of this shows any sign of explaining why (the history of) the universe should be so dramatically asymmetric in time.
If weak parity violation really explains anything here, I don't see what. Do you have any grounds for suspecting that weak parity violation explains why we see a very dense low-entropy universe in one direction and a very sparse high-entropy universe in the other? Do you have any grounds for suspecting that weak parity violation explains why smashing an egg is easier than putting it together?
Is there a criterion other than "Sean Carroll thinks so"?
I'm not sure whether this question is really directed at Sean Carroll (complaining that the passage I quoted is vague) or at me (complaining that I'm treating him as some sort of authority). If it's directed at him, the answer he gives is that what you choose to call time-symmetry is up to you and is just a question of terminology, and what really matters is what symmetries the universe actually has. (And, I think he implicitly says, questions about the "arrow of time" remain whatever definitions you choose to adopt.) If it's directed at me, then (1) I endorse his answer and (2) no, I was not using him as an authority, I was using his book as an example of the sort of thing people are usually concerned about in this area.
If weak parity violation really explains anything here, I don't see what. Do you have any grounds for suspecting that weak parity violation explains why we see a very dense low-entropy universe in one direction and a very sparse high-entropy universe in the other? Do you have any grounds for suspecting that weak parity violation explains why smashing an egg is easier than putting it together?
So first let me note that the weak parity violations cannot explain the observed matter/antimatter asymmetry; it follows that there is a source of CP violation that we don't know about, and hence also a large T violation.
You keep coming back to entropy, but I think this is the wrong way to look at it. Entropy is a probabilistic framework using multiple states of the same energy, that we apply when we don't have all the information; but the universe does, and is deterministically evolving from one specific state of high density, to another specific state of low density. Humans look at the final state and say "there are a lot of hypothetical states with different specific arrangements, which look a lot like this one; therefore it is high entropy"; but so what? You can't get there from the actual initial conditions; inaccessible states can have no physical effect, and ought to have no philosophical one either. Asking for an explanation of "the evolution from low to high entropy" is meaningless; better to ask for an explanation of where the initial conditions come from.
As for "what does that have to do with frying eggs", I opine that once you have identified a microlevel asymmetry, your work is done; there is no need to go through the tedious steps of finding how it produces a macrolevel asymmetry.
You keep coming back to entropy, but I think this is the wrong way to look at it.
I keep coming back to entropy because the asymmetry in entropy is one of the things that needs explaining, and because some of the other things that need explaining seem to be explicable in terms of entropy.
[...] when we don't have all the information; but the universe does
Given any criterion for distinguishing macrostates, you can (in principle) compute entropy relative to that criterion. E.g., if you care only about macroscopic thermodynamic parameters when distinguishing macrostates, you get the classical Boltzmann entropy. These parameters presumably stop making sense when you consider the early enough universe, but we can still say that the thermodynamic entropy of the universe appears to be surprisingly small early on and much larger later on.
(If the universe is infinite in extent, there are some technical difficulties here. I don't know exactly how they are addressed, but I note that cosmologists who accept the possibility that the universe may be infinite don't thereupon seem to stop talking about entropy, and I infer that the current best way of addressing them doesn't make the time asymmetry of entropy go away. If there are experts in the field reading this who would like to enlighten me further, I'm all ears.)
once you have identified a microlevel asymmetry [...] there is no need to go through the tedious steps of finding how it produces a macrolevel asymmetry.
I'm pretty sure this is just plain wrong, unless you have already established that the microlevel asymmetry is responsible for the macrolevel asymmetry. So far as I am aware, there is no reason to think that weak parity violation is responsible for the familiar macro-scale time asymmetries everyone notices.
I keep coming back to entropy because the asymmetry in entropy is one of the things that needs explaining
Again, why bother with entropy as such? Just say "the initial conditions need explaining" and be done.
Given any criterion for distinguishing macrostates, you can (in principle) compute entropy relative to that criterion.
I do not understand how these two paragraphs are a response to what I said. Can you elucidate?
So far as I am aware, there is no reason to think that weak parity violation is responsible for the familiar macro-scale time asymmetries everyone notices.
Electroweak unification. That aside, the original problem was "there is no asymmetry in the laws of physics that can cause [macrolevel asymmetry]; Newton's and Maxwell's (and Einstein's) laws are the same in either time direction". And then we realised that yes, there is an asymmetry in the laws of physics. Well then, that solves the problem; what more do you want, unfried egg in your barley-that-used-to-be-beer?
why bother with entropy as such?
Because it's one of the more obvious descriptive statistics to look at and it shows the difference nice and clearly. If we just say "the initial conditions need explaining" (or: the differences between initial and final) then the obvious question is what about the initial conditions, and part of the answer to that is going to be the entropy. (Or maybe some other thing that's essentially equivalent.)
Also, because it's a statistic that not only is different between the distant past and the distant future, but also varies in a consistent way at present.
I do not understand how these two paragraphs are a response to what I said. Can you elucidate?
I can try, but if they aren't then my best guess is that I didn't correctly understand what you were saying (which was less than 100% clear to me). So I'll be brief about the elucidation, and then whichever of us turns out to have been misunderstood first can do the next round of elucidating :-).
It looked to me as if you were saying, more or less, that entropy is a silly thing to be looking at at all, because it describes only our state of ignorance and not the actual universe; that when we say "the universe seems to be evolving from a low-entropy state to a high-entropy state" all we really mean is something like "we know a lot more about the past of the universe than about its future".
I, on the other hand, think that is a wrong (i.e., a less than maximally useful) way to look at it. Yes, a notion of entropy depends on some state of knowledge and observational ability. But that doesn't mean it depends on picking ours in particular, and there are not-so-arbitrary ways to do it.
Electroweak unification.
Noun phrase!
Would you like to make your argument a little more explicit? Do you think that weak parity violation is responsible for the familiar macro-scale time asymmetries everyone notices?
Well, then, that solves the problem
Only in so far as it's plausible that the asymmetry-in-the-laws that we found actually causes the asymmetry-in-our-observations that we're trying to explain. I don't see that it is plausible, but perhaps the words "electroweak unification" should have enlightened me?
Yes, a notion of entropy depends on some state of knowledge and observational ability. But that doesn't mean it depends on picking ours in particular, and there are not-so-arbitrary ways to do it.
I don't understand how your suggested calculation is non-arbitrary; you still seem to be picking some criterion and then doing math. My point is that the laws of physics don't do any such thing; they just apply the exact laws of motion to the exact particle locations at every time step. Picking a different criterion for the entropy doesn't help - it's still not going to be what actually happens.
Would you like to make your argument a little more explicit? Do you think that weak parity violation is responsible for the familiar macro-scale time asymmetries everyone notices?
Sorry, I will try to be less brief. The known CP violation occurs, as you point out, in the weak force. (Side note: There is also a large source of CP violation somewhere else in the laws of physics, otherwise we wouldn't observe the matter/antimatter asymmetry we do. But that doesn't change the argument since it must occur at high energies.) When you fry an egg, the interactions are basically electric.
At high energies, the electric and weak force unite into the electroweak force. Now, when you do the quantum-field-theory math encapsulated in Feynman diagrams, you are integrating over all the possible paths from initial to final state; including ones with extremely energetic particles in the intermediate states. (This appears to violate the conservation of energy; the usual explanation given to students is that you can do this because of a Heisenberg uncertainty relation between energy and time. If the time is sufficiently short, "the universe is not aware" that energy conservation was violated. Personally I find this explanation immensely unsatisfying, but I don't understand the underlying math; so I'm taking this on faith. Anyway it's the same phenomenon that causes Hawking radiation around black holes.) Well, with high-energy intermediate states, you can get weak particles in your electric interactions; and then you get time asymmetry. To be sure this is a third-order effect; but then, frying an egg takes several seconds, which is an immense amount of time relative to the characteristic timescale of the weak force. (Which is only 'weak' by comparison to the strong nuclear force.)
I don't understand how your suggested calculation is non-arbitrary; you still seem to be picking some criterion and then doing math.
I don't understand what, if anything, you would consider non-arbitrary.
the laws of physics [...] just apply the exact laws of motion to the exact particle locations at every time step.
And why does that conflict with what anyone says about the "arrow of time"?
with high-energy intermediate states, you can get weak particles in your electric interactions; and then you get time asymmetry.
So you actually are suggesting that weak-interaction parity violation is responsible for the asymmetry between frying and un-frying eggs. OK, then. Do you have any actual evidence that it's so? It seems awfully implausible on the face of it, to me, but since (1) neither of us is a quantum field theorist and (2) so far as I know no one knows how to do the QFT calculations on anything like the scale required to understand what's happening when you fry an egg, I'm not sure that either my intuition or yours is to be trusted. So, I dunno: has anyone done the back-of-envelope calculations to figure out whether this works in some sort of toy model? have any actual quantum field theory experts given opinions on how plausible this is?
I don't understand what, if anything, you would consider non-arbitrary.
I'm not sure this is actually an important disagreement; I'm ok with dropping it if you want. However, you are the one who suggested that entropy could be calculated in a non-arbitrary way; but I don't think you've offered an example of such a calculation.
And why does that conflict with what anyone says about the "arrow of time"?
It conflicts with the notion that entropy is a good way to consider the problem; entropy is a non-full-information heuristic that doesn't appear in the actual laws of physics.
neither of us is a quantum field theorist
Well, I'm not a theorist, no. I do have a PhD in experimental particle physics. I will admit that the QFT classes tended to fry my brain like an egg, which is one reason I went experimental.
so far as I know no one knows how to do the QFT calculations on anything like the scale required to understand what's happening when you fry an egg
That's true. I do think, however, that an intuitive understanding is sufficient to get a grasp of how a microlevel asymmetry can become macrolevel.
Do you have any actual evidence that it's so?
It seems that such evidence would have to be in the form of simulations or calculations, since you can't very well turn off the weak interaction and see what happens when you fry an egg without it. I am not aware of any such calculation, no. But, again, there's such a thing as a qualitative insight.
you are the one who suggested that entropy could be calculated in a non-arbitrary way
All I actually said was "not-so-arbitrary". I think that's pretty much all one can say about anything, which is why I asked what if anything you would consider non-arbitrary.
It conflicts with the notion that entropy is a good way to consider the problem; entropy is a non-full-information heuristic that doesn't appear in the actual laws of physics.
I don't see the connection between the two halves of that sentence. There seems to be some implicit premise along these lines: "When contemplating the 'arrow of time' we should not consider anything that doesn't explicitly appear in the laws of physics." but I don't see any reason to accept such a premise.
an intuitive understanding is sufficient to get a grasp of how a microlevel asymmetry can become macrolevel
If you mean that that's enough to appreciate that in principle something of that sort is not entirely ruled out -- yeah, I agree. If you mean that your intuition tells you that weak parity violation really is the reason why we can fry eggs but not un-fry them then, well, I'm afraid I don't trust your intuition as much as you might.
If I talked to a bunch of theoretical physicists -- a group whose intuition in such things I think we should probably trust more than that of either experimentalists like you or pure mathematicians like me -- would you expect them to agree with you, to say "yes, of course, weak parity violation is probably the cause of the familiar macroscopic time-asymmetries we see in the world"? My impression -- which I admit is not based on actually finding lots of theoretical physicists and asking them -- is that they mostly would not say any such thing.
As one example, I'll cite Sean Carroll again; although he is an author of pop-science books he is also a working scientist and this is pretty much in his field of expertise. And he says: Time reversal violation is not the arrow of time.
There seems to be some implicit premise along these lines: "When contemplating the 'arrow of time' we should not consider anything that doesn't explicitly appear in the laws of physics." but I don't see any reason to accept such a premise.
I would say "explicitly or implicitly", and then it seems to me that we have every reason to accept that premise, because where the Devil else are you going to look? Noting that entropy does not appear in the laws of physics even implicitly; it's a heuristic, not a derived quantity.
If I talked to a bunch of theoretical physicists -- a group whose intuition in such things I think we should probably trust more than that of either experimentalists like you or pure mathematicians like me [...]
I would rather phrase it as "micro-level time violation is the cause"; we're talking about weak parity violation only because that's much more easily measured, and implies time violation. That aside, yes, I would expect a poll of theorists to find at least a sizable minority who think micro-level time violation is the cause of macro-scale time asymmetry.
gjm's response to this is correct.
"It could just as well evolve in the other direction." If you mean that you could, if you wanted, call the past "the future," and call the future, "the past," you can do that if you want: but you will remember things in the direction of lower entropy and expect things in the direction of higher entropy. Which as gjm said is what we mean by talking about an arrow of time. In other way, saying that this can happen "just as well" is like saying that when you flip a coin a thousand times, you can just as easily get a thousand heads as any other sequence. But you will actually get a random looking sequence, and you will actually get increasing entropy, not decreasing entropy.
Your second paragraph is simply incorrect: there is no known asymmetry in the laws of physics that might explain the arrow of time. It is explained (in terms of experience) by the fact that time in one direction is vastly different in entropy from the other. We call the low end "the past," because we necessarily remember the low-entropy side of time.
If you assume random conditions to the universe, you will not get either low entropy to high, or high to low (which is really the same thing), but high entropy on both sides, and any low entropy situation like conscious experience would be explained as Boltzmann brains.
Your second paragraph is simply incorrect: there is no known asymmetry in the laws of physics that might explain the arrow of time.
On the other hand, CP violation is one of the Sakharov conditions, and it's not obviously absurd to suspect that the questions "why did the past have so little entropy" and "why does the present have so much more matter than antimatter" might be related to each other.
the questions "why did the past have so little entropy" and "why does the present have so much more matter than antimatter" might be related
Yeah, I wondered (idly -- I don't know enough physics for anything more to be worth while) about that too. I don't suppose anyone reading this is a physicist who can say whether there's anything nontrivial likely to be going on here?
Here is Sean Carroll discussing at least a related question.
His argument seems to be that since CP violation remains a reversible process, it cannot possibly explain why there is less entropy on one side of time than on the other.
[actually, maybe it isn't that clear -- he might just be saying that no one has shown any connection, not that there could not be one]
The bolded text in the article is 'has absolutely nothing to do with that arrow of time'. This is accurately (if excessively tersely) summarizes the article; no hedging is necessary.
Your second paragraph is simply incorrect: there is no known asymmetry in the laws of physics that might explain the arrow of time.
The laws of physics are CPT-invariant, as /u/gjm pointed out; CP symmetry is known to be broken; consequently T symmetry is also broken. The effect has been measured directly: http://physicsworld.com/cws/article/news/2012/nov/21/babar-makes-first-direct-measurement-of-time-reversal-violation.
This is not helpful for explaining the arrow of time, for reasons that Sean Carroll points out in the post I linked.
Just to be clear: In the section you refer to, he is only pointing out that there is a tension between physics's view of time and the intuitive, everyday view of time. He summarizes the view of some continental philosophers who say that this tension means physical laws are wrong. He never claims that he, personally, believes that therefore physical laws are wrong.
Indeed, he notes that physicists have always countered that they can explain, using their theories, why we have the intuitions that we have about time. And actually, David Albert is just such a physicist (turned philosopher). He's spent a large chunk of his career trying to explain how intuitive conceptions of time can be obtained from fundamental physics's conception of time.
He says
[our world] is not even close to being time-reversal symmetric. And once again, there are proposals on the table for how to fiddle around with the theory. Adding a new law governing initial conditions, for example. There are all kinds of proposals about it. This is a very fundamental challenge. … There's a way these laws manifestly get things wrong on the macro level, and we need to figure out what to do about that.
No, not really?
It's not clear that there is such a thing as physics' view of time. One can just do physics with no need for t's; they're pretty much superfluous.