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timtyler comments on Quantum Physics, CERN and Hawking radiation - Less Wrong
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Well, T symmetry is favoured by Occam's razor. We have pruned away the momentum quanta from needing to be replaced by their anti-particles. The idea that T-symmetry is true just takes this a bit further, becoming more elegant and neat in the process.
You didn't answer the question, nor did you give any grounds for thinking it doesn't need answering.
Occam's razor applies to theories, not to individual propositions about them. CPT (or T) symmetry isn't something you build into a physical theory by having an axiom like "CPT-symmetry holds"; it arises from the structure of the theory. Do you have any actual reason for believing that theories with T-symmetry but not CPT-symmetry are simpler than theories with CPT-symmetry but not T-symmetry? The CPT theorem seems to me to give good reason not to believe that.
Well, I think so, but maybe not in a format suitable for a short blog post. There are numerous small, simple CA with the property of being symmetrical under T=-T. For example, the BBM. My impression is that other means of reversal are correlated with automaton complexity. Then there's the idea of charge as a pump. That is appealing on other grounds - and pumps tend to have moving parts - which would then reverse automatically if T=>-T . Also, the possibility of simulationverse ideas would seem to favour ease of reversal, to some extent.
IMO, you should really not be counting the CPT theorem as evidence on the issue - one way or the other.
I certainly don't think people should be telling me that the known laws of physics are not symmetrical under T=>-T. IMO, it is more probable that they are symmetrical that way than that they are not. The idea that CPT symmetry illustrates that they are not is simply a popular misconception, with no basis in the facts of the matter.
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.)
Sure, there are plenty of small simple cellular automata with T-symmetry. And also with P-symmetry, which does not hold in the real world. So far as I know, CAs with PT symmetry are just about exactly as easy to make as ones with T symmetry. (And if you have CAs with a property corresponding to C, I bet CPT is as easy to arrange as T.) Why is any of this meant to mean that T-symmetry is simpler than CPT-symmetry?
You may find the idea of "charge as a pump" appealing; fair enough. I am at a loss to see why that is a reason for thinking that T symmetry is simpler than CPT symmetry.
Your argument, so far as you've provided one, seems to go like this: "I expect the universe to work like a cellular automaton. I have the feeling that T symmetry is simpler than CPT symmetry in cellular automata. Therefore the CPT theorem is irrelevant and we should expect T symmetry to prevail but not CPT symmetry". This strikes me as very strange since (1) we have very successful physical theories that are QFTs (to which the CPT theorem applies) and no successful physical theories based on cellular automata, and (2) the CPT theorem is an actual theorem governing QFTs, whereas your intuitions for relative simplicity are merely your intuitions.
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.
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?
Well, if true, it is a reason to think T symmetry or PT symmetry holds - since then charge would reverse itself automatically if T was reversed.
Depending on the nature of the pump. For instance, a pump made out of Standard Model stuff would not exactly reverse itself if T were reversed. Presumably, then, you have something more specific in mind; would you care to say more exactly what it is?
Pumps usually reverse if T is reversed. Most pumps contain something like a rotating fan blade. Reverse T and the momenta all reverse, so the blade turns the other way - and the pump pushes in the opposite direction.
What about C and P you ask? Well, we are talking about a pump inside charged particles, whose action is responsible for charge. C is hardly likely to be relevant. If T symmetry holds, you would just have to reverse T. If PT symmetry holds (but not T symmetry alone), and the components of the pump are sensitive to the sign of P, then you might have to reverse both P and T to get the pump to run backwards.
Whether pumps "usually" reverse exactly when T is reversed depends on whether the universe is actually T-symmetric or not. If you assume that they do then you're begging the question.
Do you have any actual evidence that charged particles such as electrons and quarks are actually likely to contain pumps? This seems, on the face of it, monstrously implausible; of course it might be right -- anything might be right -- but why does the idea even deserve taking seriously, never mind using as the basis for your opinions about whether CPT symmetry is more likely than T symmetry?
It is not "begging the question" it is "explaining one way it could work". If charged particles contain pumps with moving parts, that would make simple T symmetry more plausible, since then there would be a clearer mechanism for explaining why reversing T would lead to all the charges in the universe reversing.
We don't know the details of how electromagnetic and gravitational fields are generated. Something generates fields following an inverse-square law. A pump would be likely to have that effect - and seems about as plausible as anything else. Why do you describe the idea as being "monstrously implausible"?
I think it's monstrously implausible because it requires charged particles to have intricate internal structure of a very curious sort, a thing for which we have no evidence whatever. (Assuming you actually mean a pump, rather than (e.g.) "a source of some substance that can flow", in which case I see no reason whatever for thinking it should be T-symmetric.)
Your justification seems to be that "charged particles contain pumps" would somehow explain the inverse square law, but I don't see that at all. The idea that they are sources/sinks of some substance that spreads out geometrically might (though I don't know how you're going to make that work in the quantum context) but what you're suggesting is both more specific (pumps as such are not required for an inverse square law) and less specific (pumps as such do not imply an inverse square law).