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?
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.
Again, why bother with entropy as such? Just say “the initial conditions need explaining” and be done.
I do not understand how these two paragraphs are a response to what I said. Can you elucidate?
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?
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 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.
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?
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?
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.
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 what, if anything, you would consider non-arbitrary.
And why does that conflict with what anyone says about the “arrow of time”?
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’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.
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.
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.
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.
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.
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.
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.
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.
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.
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.