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.
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.