The laws of physics as currently understood—i.e., the laws in the best model we’ ve got—are in fact CPT-symmetric but not T-symmetric. (Because the best model we’ve got is a quantum field theory of the sort that the CPT theorem applies to; and because CP symmetry is violated (1) by that model and (2) in reality, according to the available evidence.)
You understand that the claim is that that is just a historical accident about the way the model was built? The idea that C and P reverse automatically if T is reversed does not make any new predictions that the old model did not. The idea that CPT symmetry is favoured by some kind of experimental evidence seems completely wrong to me. Since the two models are totally equivalent experimentally, this is an issue for Occam.
You understand that the claim is that that is just a historical accident about the way the model was built?
There is a mathematical theorem that says that no model of the same kind can fail to have CPT symmetry. There are robust experimental results that say that the real world doesn’t have CP symmetry. Therefore, it is not an accident that given the general kind of model it is it has CPT symmetry but not T symmetry.
If you are claiming that it’s a historical accident that we have a QFT model and not a CA-type model, then show your evidence. Specifically, the obvious alternative hypotheses are things like “There is no CA-type model that actually fits the data” and “All CA-type models that fit the data as well as the current QFT model are much more complicated than the QFT model”. Given the present state of the art as I understand it, these seem much more likely to me than your hypothesis that it’s just a historical accident. Please feel free to convince me otherwise.
Since the two models are totally equivalent experimentally
What two models? I know of one model, the Standard Model, which fits the data extremely well and is reasonably simple (ha!), and which has CPT symmetry but not T symmetry (and cannot be fudged to have T symmetry without losing its agreement with experiment). What is the other model you propose, that supposedly is totally equivalent experimentally but has T symmetry?
Please be specific about this, because either you have some wonderful physics to show me that I haven’t seen before or you’re failing to distinguish between “some vague intuitions in Tim’s head” and “a very thoroughly worked out and tested physical model”.
There is a mathematical theorem that says that no model of the same kind can fail to have CPT symmetry. There are robust experimental results that say that the real world doesn’t have CP symmetry. Therefore, it is not an accident that given the general kind of model it is it has CPT symmetry but not T symmetry.
This is all a misconception, though. Look carefully at the theorem you mention, and I expect that you will see that it is quite compatible with what I have been saying.
If you are claiming that it’s a historical accident that we have a QFT model and not a CA-type model, then show your evidence.
This doesn’t have anything to do with CAs, really. The two ideas are:
if T is reversed, you would have to manually swap each particle with its anti-particle and reverse parity as well to produce the correct backwards evolution.
T reversal has the effect of automatically swapping each particle with its anti-particle—and reversing its parity—due to these phenomena being implemented using rotating parts, cyclic phenomena, or similar things that do automatically run the other way if T is reversed.
These ideas do not produce different experimental predictions if running forwards normally. Standard physics describes both situations equally well. The second idea (due to Ed Fredkin) suggests that examining the nature of charge and parity might yield reasons why reversing T has that effect. Charge working like a pump is an example of how that might happen. Or maybe these systems will refuse to show us their internal workings.
In this kind of model, charge and parity are made of the same kind of stuff as everything else is. Their properties arise from how matter is patterned, not from them being somehow fundamental. As a result, charge doesn’t even show up in Fredkin’s scheme of fundamental units.
This is all a misconception, though. Look carefully at the theorem you mention, and I expect that you will see that it is quite compatible with what I have been saying.
Would you care to do me (and other readers) the courtesy of explaining what misconception you think I’m actually suffering from, and what about the CPT theorem you think I’ve failed to look at carefully enough, and what specific things it says that would make what you say look sensible?
Would you care to do me (and other readers) the courtesy of explaining what misconception you think I’m actually suffering from, and what about the CPT theorem you think I’ve failed to look at carefully enough, and what specific things it says that would make what you say look sensible?
You don’t seem to understand how a model can lack CPT symmetry and be consistent with theory and observations. You should be aware that others besides Fredkin have seriously proposed this:
The Feynman proposal has the consequence that the electric field flips sign under time reversal, and that the magnetic field does not but it, too, has the consequence that the theory is time reversal invariant.
If I don’t understand how that can happen, then perhaps the problem is either (1) that it can’t or (2) that how it can is a subtle matter which hasn’t been explained well enough for me to understand it. So far, in this discussion, you’ve offered no reason to think #2 more likely than #1, and in any case you haven’t made any attempt to explain how it could happen.
The PDF does not appear to contain the sentence you purport to quote from it. (More specifically, it does not appear to contain the word “flips”. (Neither does the abstract.) In any case, its proposal seems to amount simply to redefining “time reversal”. If all you’re saying is that if you use “T symmetry” to mean what everyone else calls “CPT symmetry” then physics is likely to be T-symmetric but not CPT-symmetric, then (duh!) I agree, but I’m not sure why that’s supposed to be interesting.
The PDF does not appear to contain the sentence you purport to quote from it. (More specifically, it does not appear to contain the word “flips”.
Section 2.4 page 32. Searching for “flips” doesn’t work here either. Copy-n-paste into a text editor shows why—fi and fl are weird ligatures in this PDF.
In any case, its proposal seems to amount simply to redefining “time reversal”. If all you’re saying is that if you use “T symmetry” to mean what everyone else calls “CPT symmetry” then physics is likely to be T-symmetric but not CPT-symmetric, then (duh!) I agree, but I’m not sure why that’s supposed to be interesting.
To quote from the article:
We have articulated the `geometric’ notion of time reversal implicit in Malament’s work, according to which time reversal consists in leaving all [other] fundamental quantities alone, and merely flipping the temporal orientation.
It’s plainly proposing T symmetry. Does that help you to see how such a thing might be possible?
Aha, of course. (I did search for some other substrings, though I forget what. Presumably they also contained ligatures. D’oh.)
plainly proposing T symmetry
… in Malament’s proposal, which is not the same as the Feynman one you cite earlier. The purpose of the paper is to argue for a definitional change whereby we call “T” what is currently generally called “CT”. Everything in the paper is concerned with classical, not quantum, electrodynamics. The paper does not argue that T symmetry (as generally understood or with a revised definition) is plausibly true in quantum electrodynamics.
Does that help you to see how such a thing might be possible?
It would make this discussion more pleasant for me if you’d be less patronizing. Whether you care about that is, of course, up to you.
The behaviour of Fredkin’s model—which he himself says “is grossly less comprehensive while far more inconsistent than conventional physics” really doesn’t seem very important, in comparison with the behaviour of the actual models constructed by actual physicists that make actual predictions that actually fit actual physical data.
We have a very nice theory that seems to describe how the world works with great accuracy and precision. It does not have the property that when you simply T-reverse it C and P get reversed automatically. Nor can it be tweaked to have that property, without breaking its agreement with observation.
You have, so far as I can see, a bunch of handwaving that suggests that there possibly might be some sort of model of some of physics that has the property that T-reversing it brings C and P along automatically. You haven’t actually produced such a model; no one has found one; no one seems to have much idea how to make one.
How on earth can it be reasonable to describe this situation by saying it’s “just a historical accident” that one of the “two ideas” you describe happens to be dominant at the moment?
We have a very nice theory that seems to describe how the world works with great accuracy and precision. It does not have the property that when you simply T-reverse it C and P get reversed automatically.
Only because it doesn’t say anything about that. It’s a model of physics. In physics, you can’t just reverse time, that is not a permitted operation.
Nor can it be tweaked to have that property, without breaking its agreement with observation.
That’s incorrect. If P and C automatically reverse when you reverse T that breaks absolutely nothing.
You haven’t actually produced such a model; no one has found one; no one seems to have much idea how to make one.
The model is as I already described: P and C automatically reverse when you reverse T. This is plausible since P and C might be physically implemented using moving parts. We can discuss how parsimonious that is. That is a discussion based around Occam’s razor.
If I wanted to make a strong case that T symmetry was much more likely than CPT symmetry, then we would have to get into the possible details of hypotheses about why they might reverse. However, that was never my position. We don’t know with much confidence that C and P reverse automatically, but equally we don’t know with much confidence that they won’t. The correct response to such a situation is not to declare CPT symmetry the winner, but to say that there’s uncertainty, and that we don’t know for sure.
Only because it doesn’t say anything about that [...] you can’t just reverse time, that is not a permitted operation.
WTF? Saying that a theory of physics has (say) T-symmetry just means: something is a possible history of the universe iff its time-reversal is.
If P and C automatically reverse when you reverse T
Could you please clarify whether you are saying anything about physics, or whether you are just making the content-free observation that by redefining T-reversal you can interchange the notions of T-symmetry and CPT-symmetry?
Only because it doesn’t say anything about that [...] you can’t just reverse time, that is not a permitted operation.
WTF? Saying that a theory of physics has (say) T-symmetry just means: something is a possible history of the universe iff its time-reversal is.
Uh, I am aware of what “T-symmetry” refers to.
Could you please clarify whether you are saying anything about physics, or whether you are just making the content-free observation that by redefining T-reversal you can interchange the notions of T-symmetry and CPT-symmetry?
As previously discussed, this is about how C and P work—and whether they reverse themselves if you reverse T. This does, inevitably, lead to time reversal not referring to the operation many people use it to refer to today. People think reversing T leaves P and C alone. The idea is that they are wrong about that. This is not a “content-free observation”, it’s about how the operation of parity and charge could depend on the direction of time.
The model is as I already described
What model?
You seem to be asking for more specifics than I, or anyone else, has. However, you snipped my explanation of why specific details are not needed to support my position. So, here it is again:
If I wanted to make a strong case that T symmetry was much more likely than CPT symmetry, then we would have to get into the possible details of hypotheses about why they might reverse. However, that was never my position. We don’t know with much confidence that C and P reverse automatically, but equally we don’t know with much confidence that they won’t. The correct response to such a situation is not to declare CPT symmetry the winner, but to say that there’s uncertainty, and that we don’t know for sure.
Then perhaps you might care to clarify what your point was.
Well, this conversation is pretty tedious for me, and you seem to keep asking me to do more work. Well, OK. So, the context was:
We have a very nice theory that seems to describe how the world works with great accuracy and precision. It does not have the property that when you simply T-reverse it C and P get reversed automatically.
Only because it doesn’t say anything about that. It’s a model of physics. In physics, you can’t just reverse time, that is not a permitted operation.
...and the idea was that the job of physics is mostly to tell us how the temporal evolution of the world works. It’s main job is not to tell us what happens if an impossible physical event—like time running backwards—takes place. So, it is not a terribly big surprise that it doesn’t have too much to say about the issue of whether charge reversal is an automatic consequence of time reversal—or not. That is not really an important part of its job description.
You seem to be asking for more specifics than I, or anyone else, has.
Then you should stop talking about “the model” as if, y’know, you actually have a model.
You understand that the claim is that that is just a historical accident about the way the model was built? The idea that C and P reverse automatically if T is reversed does not make any new predictions that the old model did not. The idea that CPT symmetry is favoured by some kind of experimental evidence seems completely wrong to me. Since the two models are totally equivalent experimentally, this is an issue for Occam.
There is a mathematical theorem that says that no model of the same kind can fail to have CPT symmetry. There are robust experimental results that say that the real world doesn’t have CP symmetry. Therefore, it is not an accident that given the general kind of model it is it has CPT symmetry but not T symmetry.
If you are claiming that it’s a historical accident that we have a QFT model and not a CA-type model, then show your evidence. Specifically, the obvious alternative hypotheses are things like “There is no CA-type model that actually fits the data” and “All CA-type models that fit the data as well as the current QFT model are much more complicated than the QFT model”. Given the present state of the art as I understand it, these seem much more likely to me than your hypothesis that it’s just a historical accident. Please feel free to convince me otherwise.
What two models? I know of one model, the Standard Model, which fits the data extremely well and is reasonably simple (ha!), and which has CPT symmetry but not T symmetry (and cannot be fudged to have T symmetry without losing its agreement with experiment). What is the other model you propose, that supposedly is totally equivalent experimentally but has T symmetry?
Please be specific about this, because either you have some wonderful physics to show me that I haven’t seen before or you’re failing to distinguish between “some vague intuitions in Tim’s head” and “a very thoroughly worked out and tested physical model”.
This is all a misconception, though. Look carefully at the theorem you mention, and I expect that you will see that it is quite compatible with what I have been saying.
This doesn’t have anything to do with CAs, really. The two ideas are:
if T is reversed, you would have to manually swap each particle with its anti-particle and reverse parity as well to produce the correct backwards evolution.
T reversal has the effect of automatically swapping each particle with its anti-particle—and reversing its parity—due to these phenomena being implemented using rotating parts, cyclic phenomena, or similar things that do automatically run the other way if T is reversed.
These ideas do not produce different experimental predictions if running forwards normally. Standard physics describes both situations equally well. The second idea (due to Ed Fredkin) suggests that examining the nature of charge and parity might yield reasons why reversing T has that effect. Charge working like a pump is an example of how that might happen. Or maybe these systems will refuse to show us their internal workings.
In this kind of model, charge and parity are made of the same kind of stuff as everything else is. Their properties arise from how matter is patterned, not from them being somehow fundamental. As a result, charge doesn’t even show up in Fredkin’s scheme of fundamental units.
Would you care to do me (and other readers) the courtesy of explaining what misconception you think I’m actually suffering from, and what about the CPT theorem you think I’ve failed to look at carefully enough, and what specific things it says that would make what you say look sensible?
You don’t seem to understand how a model can lack CPT symmetry and be consistent with theory and observations. You should be aware that others besides Fredkin have seriously proposed this:
For example, according to the PDF of Spacetime symmetries and the CPT theorem Richard Feynman proposed much the same thing:
If I don’t understand how that can happen, then perhaps the problem is either (1) that it can’t or (2) that how it can is a subtle matter which hasn’t been explained well enough for me to understand it. So far, in this discussion, you’ve offered no reason to think #2 more likely than #1, and in any case you haven’t made any attempt to explain how it could happen.
The PDF does not appear to contain the sentence you purport to quote from it. (More specifically, it does not appear to contain the word “flips”. (Neither does the abstract.) In any case, its proposal seems to amount simply to redefining “time reversal”. If all you’re saying is that if you use “T symmetry” to mean what everyone else calls “CPT symmetry” then physics is likely to be T-symmetric but not CPT-symmetric, then (duh!) I agree, but I’m not sure why that’s supposed to be interesting.
Section 2.4 page 32. Searching for “flips” doesn’t work here either. Copy-n-paste into a text editor shows why—fi and fl are weird ligatures in this PDF.
To quote from the article:
It’s plainly proposing T symmetry. Does that help you to see how such a thing might be possible?
Aha, of course. (I did search for some other substrings, though I forget what. Presumably they also contained ligatures. D’oh.)
… in Malament’s proposal, which is not the same as the Feynman one you cite earlier. The purpose of the paper is to argue for a definitional change whereby we call “T” what is currently generally called “CT”. Everything in the paper is concerned with classical, not quantum, electrodynamics. The paper does not argue that T symmetry (as generally understood or with a revised definition) is plausibly true in quantum electrodynamics.
It would make this discussion more pleasant for me if you’d be less patronizing. Whether you care about that is, of course, up to you.
The behaviour of Fredkin’s model—which he himself says “is grossly less comprehensive while far more inconsistent than conventional physics” really doesn’t seem very important, in comparison with the behaviour of the actual models constructed by actual physicists that make actual predictions that actually fit actual physical data.
We have a very nice theory that seems to describe how the world works with great accuracy and precision. It does not have the property that when you simply T-reverse it C and P get reversed automatically. Nor can it be tweaked to have that property, without breaking its agreement with observation.
You have, so far as I can see, a bunch of handwaving that suggests that there possibly might be some sort of model of some of physics that has the property that T-reversing it brings C and P along automatically. You haven’t actually produced such a model; no one has found one; no one seems to have much idea how to make one.
How on earth can it be reasonable to describe this situation by saying it’s “just a historical accident” that one of the “two ideas” you describe happens to be dominant at the moment?
Only because it doesn’t say anything about that. It’s a model of physics. In physics, you can’t just reverse time, that is not a permitted operation.
That’s incorrect. If P and C automatically reverse when you reverse T that breaks absolutely nothing.
The model is as I already described: P and C automatically reverse when you reverse T. This is plausible since P and C might be physically implemented using moving parts. We can discuss how parsimonious that is. That is a discussion based around Occam’s razor.
If I wanted to make a strong case that T symmetry was much more likely than CPT symmetry, then we would have to get into the possible details of hypotheses about why they might reverse. However, that was never my position. We don’t know with much confidence that C and P reverse automatically, but equally we don’t know with much confidence that they won’t. The correct response to such a situation is not to declare CPT symmetry the winner, but to say that there’s uncertainty, and that we don’t know for sure.
WTF? Saying that a theory of physics has (say) T-symmetry just means: something is a possible history of the universe iff its time-reversal is.
Could you please clarify whether you are saying anything about physics, or whether you are just making the content-free observation that by redefining T-reversal you can interchange the notions of T-symmetry and CPT-symmetry?
What model?
Uh, I am aware of what “T-symmetry” refers to.
As previously discussed, this is about how C and P work—and whether they reverse themselves if you reverse T. This does, inevitably, lead to time reversal not referring to the operation many people use it to refer to today. People think reversing T leaves P and C alone. The idea is that they are wrong about that. This is not a “content-free observation”, it’s about how the operation of parity and charge could depend on the direction of time.
You seem to be asking for more specifics than I, or anyone else, has. However, you snipped my explanation of why specific details are not needed to support my position. So, here it is again:
Then perhaps you might care to clarify what your point was.
Then you should stop talking about “the model” as if, y’know, you actually have a model.
Well, this conversation is pretty tedious for me, and you seem to keep asking me to do more work. Well, OK. So, the context was:
...and the idea was that the job of physics is mostly to tell us how the temporal evolution of the world works. It’s main job is not to tell us what happens if an impossible physical event—like time running backwards—takes place. So, it is not a terribly big surprise that it doesn’t have too much to say about the issue of whether charge reversal is an automatic consequence of time reversal—or not. That is not really an important part of its job description.
“Model” can be a pretty general term:
The problem is that you are not using the term in the same sense as me—which leads to communication problems. The results seem kind-of tedious to me.
Yeah, me too. Let’s stop.