RUTA said:
DrChinese said:
DevilsAvocado said:
RUTA said:
DevilsAvocado said:
RUTA said:
DevilsAvocado said:This is just marvelous! Thanks again!
This is going to be very interesting and fun, as soon as I have everything ready for posting. Watch out!
DrChinese said:Looking forward to it...
?)DevilsAvocado said:Me too. (why am I suddenly getting 'nervous'... ?
RUTA said:
RUTA said:
DrChinese said:
Now I have a better understanding of the physical meaning of this "meta time". It can be viewed as a generalization of the notion of proper time. It is also formally analogous to the Newton absolute time (even though it is fully relativistic covariant). More details can be found inDemystifier said:
Can this quantum field theory be used to describe entangled particles and predict the results of Aspect-type experiments? Are you claiming that the LR model can violate any Bell inequalities?akhmeteli said:
JesseM said:
JesseM said:
JesseM said:
I haven't read the whole thread, are you just talking about experimental loopholes like the ones discussed here? There have been experiments that closed the detector efficiency loophole and experiments that closed the locality loophole, but no experiment that closed both loopholes simultaneously--still I think most experts would agree you'd need a very contrived local realist model to get correct predictions (agreeing with those of QM) for the experiments that have already been performed, but which would fail to violate Bell inequalities (in contradiction with QM) in an ideal experiment.akhmeteli said:
What do you mean by "mutally contradicting postulates"? Remember, in its basic form QM is nothing more than a recipe for making predictions about experimental results, it doesn't come with any built-in interpretation of the "meaning" of this recipe...the fact that the recipe involves calculating the evolution of the wavefunction between measurements and then using the projection postulate to get the probabilities of different measurement results doesn't imply that either the wavefunction or the "collapse of the wavefunction" have any independent reality outside the fact that when we use this recipe we do get correct statistical predictions. Indeed, the example of Bohmian mechanics proves that we are free to believe there is some underlying model that explains the origin of the probabilities given in the recipe without the need to assume anything special really happens during the measurement process. And the only assumption about ordinary QM used in Bell's proof that QM is incompatible with local realism is the assumption that the recipe does indeed give correct statistical predictions about experimental results, regardless of the underlying explanation for the predicted statistics.akhmeteli said:
akhmeteli said:
JesseM said:
JesseM said:
JesseM said:
JesseM said:
JesseM said:
JesseM said:
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JesseM said:
RUTA said:
This is kind of a strawman, no one is asking you to adopt a general principle along the lines of "if X is true when condition Y but not condition Z holds, and X is also true when condition Z but not condition Y holds, then we can assume X is true when both conditions Y and Z hold simultaneously". Rather, the reason physicists think we can be pretty confident that Bell inequalities would be violated in an experiment where both loopholes were closed simultaneously has to do with specific considerations about the physical situation we're looking at, like the idea I already mentioned that it would require a very contrived local theory that would exploit both loopholes in just the right way that it would perfectly agree with QM in all experiments done to date.akhmeteli said:
JesseM said:
Are you claiming that your model gives correct statistical predictions about the empirical results of all the Aspect-type experiments that have been done to date?akhmeteli said:
That seems like a slightly evasive answer, since you later say that you distinguish the unitary evolution aspect of QM/QFT from the projection postulate, and only claim that your model reproduces the unitary evolution, but isn't the projection postulate the only way to get actual predictions about empirical experiments from QM/QFT? Do you claim that your model can correctly predict actual empirical experimental results in the types of experiments that have been done to date, yes or no?akhmeteli said:
Why is this a "contradiction", if we don't assume that either the wavefunction or its collapse on measurement are in any sense "real", but just treat them as parts of a pragmatic recipe for making quantitative predictions about experimental results? Do you claim there are any situations where the two postulates don't lead to a unique prediction about the statistics we should expect to see in some empirical experiment? If so, what situation would that be?akhmeteli said:
Yes, this is just what the pragmatic recipe says we should do.akhmeteli said:
Personally I believe there are some true set of laws that describe what's "really" going on (I'd favor some type of many-worlds type view) and which work exactly the same for interactions between quantum systems and "instruments" as they do for interactions between individual particles. But again, if QM is treated just as a pragmatic recipe for making predictions which says nothing about the underlying "reality" one way or another, then in practice I don't think there is much ambiguity about what constitutes a "measurement", my understanding is that it's basically synonymous with interactions that involve environmental decoherence. And the types of experiments that physicists do are typically carefully controlled to prevent environmental decoherence from any other system besides the assigned "measuring device" (for example, a double-slit experiment with an electron will be done in a vacuum to prevent decoherence from interactions between the electrons and air molecules).akhmeteli said:
I don't think it's likely to be wrong in reality since I favor some sort of variant of the many-worlds interpretation, but I do think it's hard to get concrete predictions about empirical results using unitary evolution aloneakhmeteli said:
You didn't actually give a link to the paper, but you seem to be talking about this one. Anyway, Schlosshauer seems to be just arguing for the many-worlds interpretation (see the discussion beginning with 'The basic idea was introduced in Everett’s proposal of a relative-state view of quantum mechanics' on p. 1) and against any sort of objective collapse theory (see p. 13 where he talks about 'physical collapse models'--note that such models would actually be empirically distinguishable from ordinary QM in certain situations, like if information could be recorded and then 'erased' in a sufficiently large system completely isolated from environmental decoherence), but this is not the same as arguing that on a pragmatic level there's anything wrong with using the projection postulate to get quantitative predictions about experimental results. And it typically requires a lot of sophisticated argument to show how any many-worlds type interpretation can give concrete predictions in the form of probabilities (see the preferred basis problem), with no complete agreement among many-worlds advocates on how to do this (Schlosshauer discusses the problem on p. 14 of the paper, in the section 'Emergence of probabilities in a relative-state framework'); I think they all agree that the probabilities should be the same as the ones given by the pragmatic recipe involving the projection postulate, though. Indeed, Schlosshauer says at the beginning of that section that "The question of the origin and meaning of probabilities in a relative state–type interpretation that is based solely on a deterministically evolving global quantum state, and the problem of how to consistently derive Born’s rule in such a framework, has been the subject of much discussion and criticism aimed at this type of interpretation." And a bit later he says "The solution to the problem of understanding the meaning of probabilities and of deriving Born’s rule in a relative-state framework must therefore be sought on a much more fundamental level of quantum mechanics."akhmeteli said:
JesseM said:
I don't think Demystifier was actually saying that there'd be situations where Bohmian mechanics would give different predictions about empirical results than the normal QM recipe involving the Born rule; I think he was just saying that in Bohmian mechanics the collapse is not "real" (i.e. the laws governing measurement interactions are exactly the same as the laws governing other interactions) but just a pragmatic way of getting the same predictions a full Bohmian treatment would yield. In section 4 of the Stanford article on Bohmian mechanics, they say:akhmeteli said:
But if it only reproduces unitary evolution, can it reproduce any of the empirical predictions about probabilities made by the standard pragmatic recipe which includes the Born rule? Or can it only predict complex amplitudes, which can't directly be compared to empirical probabilities without making use of the Born rule or some subtle many-worlds type argument?akhmeteli said:
I am happy that you don’t use this argument. But it does not look like a strawman to me. See, e.g., post 7 in this thread. Furthermore, Aspelmeyer and Zeilinger wrote as follows (see the reference in post 385 in this thread):JesseM said:
I believe I addressed this statement in my previous post and I am not sure I have anything to add.JesseM said:
I appreciate that my answer may look evasive, but I was not trying to sweep anything under the carpet, so maybe the question is not quite appropriate? Let me give you an example. Suppose I’d ask you if the Schroedinger equation correctly describes all experiments performed so far? Yes or no? Strictly speaking, the correct answer is “no”, because the equation is not relativistic and does not describe the electronic spin. But perhaps you’ll agree that this “correct” answer is somewhat misleading because this is a damn good equation :-) So if you want a yes or no answer, then no, the model I offer cannot describe all experiments performed so far, e.g., because it does not describe the electronic spin, and I said so in my previous post. However, this is a quite decent model, as it includes the entire scalar electrodynamics, a well-established theory.JesseM said:
According to the projection postulate, after a measurement, the system is in an eigenstate, so another measurement will produce the same result (say, if the relevant operator commutes with the Hamiltonian). According to unitary evolution, though, a measurement cannot turn a superposition of states into a mixture, so there is a probability that the next measurement will return a different result. If this is not a contradiction, what is? Another situation where the two postulates don’t lead to a unique prediction is, I believe, a loophole-free Bell experiment. You cannot get a violation using just unitary evolution.JesseM said:
This is just great, so we pretty much agree with each other. Then what seems to be the problem?:-)JesseM said:
JesseM, again, it looks like we pretty much agree. I could agree, say, that the difference between unitary evolution and the projection postulate can be explained by environmental decoherence, but let us agree first what we are talking about. This thread is not about quantum theory being good or bad, everybody agrees that it is extremely good. The question of this thread is whether local realism has been ruled out or not. You see, you are talking about something “pragmatic”, but the question of this thread is not exactly pragmatic. As I said earlier in this thread, Nature cannot be “approximately local” or “approximately nonlocal”, it is either precisely local or precisely nonlocal. Or, if you disagree, then please explain what “approximate locality” can possibly be, because I don’t have a slightest idea:-) So yes, quantum theory is extremely good, but this is not relevant to the issue at hand.JesseM said:
Again, I agree, but, as I noted in our previous discussion (https://www.physicsforums.com/showpost.php?p=1706652&postcount=78), you may just complement unitary evolution with the Born rule as an operational principle.JesseM said:
That’s correct. Though I did not give a direct link, post 41 referenced post 31, where there is a reference to the article:-) Sorry for the inconvenience:-)JesseM said:
Again, I agree that quantum theory is a great practical value, but we are not discussing practicality. Again, it seems we both seem to agree that unitary evolution is always correct. However, it is worth mentioning that you are both telling me that you favor many worlds interpretation(s) and that there is no “complete agreement” on how “any many-worlds type interpretation can give concrete predictions in the form of probabilities”. This means that “many-worlds” people can actually live without the projection postulate. They may “all agree that the probabilities should be the same as the ones given by the pragmatic recipe involving the projection postulate”, but, strictly speaking, they are just unable to derive these probabilities. And it is good for them that they cannot derive those probabilities, because if they derived them from unitary evolution, that would mean that they made a mistake somewhere, as you cannot derive from unitary evolution something that directly contradicts it – the projection postulate. Let me emphasize that for all practical purposes you don’t need the Born rule or the projection postulate as precise principles – if they are approximately correct, they may be good enough for practice, but not when you’re trying to understand if Nature is local or notJesseM said:
There is no need to guess what he said, as I gave you the reference to what he actually said. He said that the projection postulate is an approximation in Bohmian mechanics. Of course, you are free to disagree with him, with me or anybody else, but if you do, just say so. Do you believe that the projection postulate can be derived in Bohmian mechanics as a precise principle? With all due respect, I strongly doubt that it can (for reasons I explained), so could you give me a reference to such a result? The Born rule is one thing, the projection postulate is something different.JesseM said:
Again, the Born rule is one thing, the projection postulate is something different. In the quote from Stanford encyclopedia (SE), I’d say, the Born rule is an operational principle. Furthermore, everything they say can be applied to the model I offer. Moreover, one can say that this model is a variant of Bohmian mechanics, which just happens to be local.JesseM said:
As I said, your SE quote above applies to this model. If you believe the Bohmian mechanics can reproduce “any of the empirical predictions about probabilities”, then why should you have a problem with this model? If you don’t believe that, well, at least this model is in good company:-)JesseM said:
I see. I am just not sure such radical ideas as many worlds are really necessary. Furthermore, as I said in our previous discussion, I believe unitary evolution implies that no measurement is ever final, so, strictly speaking, there are never any definite outcomes, but they may seem definite, as transitions between different states of a macroscopic instrument can take an eternity.JesseM said:
(My emphasis)
I take for granted that this is the (calamitous) typo??
(My emphasis)
??
?(My emphasis)RUTA said:
akhmeteli said:
?akhmeteli said:
akhmeteli said:
akhmeteli said:
It does not require contrived model to spot the likely source of systematic error in NIST experiment (if that's the one you have on mind as efficient detection experiment).JesseM said:
If prediction of some green alternate theory is found out to be wrong then theory is wrong.RUTA said:
)zonde said:If prediction of some green alternate theory is found out to be wrong then theory is wrong.
If prediction of well established theory with proven usefulness is found out to be wrong then domain of it's applicability is established instead.
Nowhere in that quote do they imply it is true in general that "if X is true when condition Y but not condition Z holds, and X is also true when condition Z but not condition Y holds, then we can assume X is true when both conditions Y and Z hold simultaneously". Rather they refer to the specific conditions of the experiment when they say "It is very unlikely that such an experiment will disagree with the prediction of quantum mechanics, since this would imply that nature makes use of both the detection loophole in the Innsbruck experiment and of the locality loophole in the NIST experiment." It's quite possible (and I think likely) that the reason they consider it "unlikely" is because a theory making use of both loopholes would be very contrived-looking.akhmeteli said:
JesseM said:
You addressed it by suggested your own model was non-contrived, but didn't give a clear answer to my question about whether it can actually give statistical predictions about experiments so far like the Innsbruck experiment and the NIST experiment (or any experiments whatsoever, see below)--if it can't, then it obviously doesn't disprove the claim that any local realist theory consistent with experiments so far would have to be very contrived!akhmeteli said:
JesseM said:
OK, but can your model actually give "correct predictions about statistical results" for any actual experiments, or does it only reproduce the unitary evolution? If it can't predict actual real-valued statistics that are measured empirically, as opposed to complex amplitudes, then it isn't a local realist model that can explain any existing experiments (you may be able to derive probabilities from amplitudes using many-worlds type arguments, but as I said part of the meaning of 'local realism' is that each measurement yields a unique outcome)akhmeteli said:
Suppose we do a Wigner's friend type thought-experiment where we imagine a small quantum system that's first measured by an experimenter in an isolated box, and from our point of view this just causes the experimenter to become entangled with the system rather than any collapse occurring. Then we open the box and measure both the system and the record of the previous measurement taken by the experimenter who was inside, and we model this second measurement as collapsing the wavefunction. If the two measurements on the small system were of a type that according to the projection postulate should yield a time-independent eigenstate, are you claiming that in this situation where we model the first measurement as just creating entanglement rather than collapsing the wavefunction, there is some nonzero possibility that the second measurement will find that the record of the first measurement will be of a different state than the one we find on the second measurement? I'm not sure but I don't think that would be the case--even if we assume unitary evolution, as long as there is some record of previous measurements then the statistics seen when comparing the records to the current measurement should be the same as the statistics you'd have if you assumed the earlier measurements (the ones which resulted in the records) collapsed the wavefunction of the system being measured according to the projection postulate.akhmeteli said:
But there are two aspects of this question--the first is whether local realism can be ruled out given experiments done so far, the second is whether local realism is consistent with the statistics predicted theoretically by QM. Even if you don't use the projection postulate to generate predictions about statistics, you need some real-valued probabilities for different outcomes, you can't use complex amplitudes alone since those are never directly measured empirically. And if we understand local realism to include the condition that each measurement has a unique outcome, then it is impossible to get these real-valued statistics from a local realist model.akhmeteli said:
No idea where you got the idea that I would be talking about "approximate" locality from anything in my posts. I was just talking about QM being a "pragmatic" recipe for generating statistical predictions, I didn't say that Bell's theorem and the definition of local realism were approximate or pragmatic. Remember, Bell's theorem is about any black-box experiment where two experimenters at a spacelike separation each have a random choice of detector setting, and each measurement must yield one of two binary results--nothing about the proof specifically assumes they are measuring anything "quantum", they might be choosing to ask one of three questions with yes-or-no answers to a messenger sent to them or something. Bell's theorem proves that according to local realism, any experiment of this type must obey some Bell inequalities. So then if you want to show that QM is incompatible with local realism, the only aspect of QM you should be interested in is its statistical predictions about some experiment of this type, all other theoretical aspects of QM are completely irrelevant to you. Unless you claim that the "pragmatic recipe" I described would actually make different statistical predictions about this type of experiment than some other interpretation of QM like Bohmian mechanics or the many-worlds-interpretation, then it's pointless to quibble with the pragmatic recipe in this context.akhmeteli said:
But that won't produce a local realist theory where each measurement has a unique outcome. Suppose you have two separate computers, one modeling the amplitudes for various measurements which could be performed in the local region of one simulated experimenter "Alice", another modeling the amplitudes for various measurements which could be performed in the local region of another simulated experimenter "Bob", with the understanding that these amplitudes concerned measurements on a pair of entangled particles that were sent to Alice and Bob (who make their measurements at a spacelike separation). If you want to simulate Alice and Bob making actual measurements, and you must assume that each measurement yields a unique outcome (i.e. Alice and Bob don't each split into multiple copies as in the toy model I linked to at the end of my last post), then if the computers running the simulation are cut off from communicating with one another and neither computer knows in advance what measurement will be performed by the simulated experimenter on the other computer, then there is no way that such a simulation can yield the same Bell-inequality-violating statistics predicted by QM, even if you program the Born rule into each computer to convert amplitudes into probabilities which are used to generate the simulated outcome of each measurement. Do you disagree that there is no way to get the correct statistics predicted by any interpretation of QM in a setup like this where the computers simulating each experimenter are cut off from communicating? (which corresponds to the locality condition that events in regions with a spacelike separation can have no causal effect on one another)akhmeteli said:
The problem is that there is no agreement on how the many-worlds interpretation can be used to derive any probabilities. If we're not convinced it can do so then we might not view it as being a full "interpretation" of QM yet, rather it'd be more like an incomplete idea for how one might go about constructing an interpretation of QM in which measurement just caused the measuring-system to become entangled with the system being measured.akhmeteli said:
See my comments above about the Wigner's friend type thought experiment. I am not convinced that you can actually find a situation where a series of measurements are made that each yield records of the result, such that using the projection postulate for each measurement gives different statistical predictions then if we just treat this as a giant entangled system which evolves in a unitary way, and then at the very end use the Born rule to find statistical expectations for the state of all the records of prior measurements. And as I said there as well, the projection postulate does not actually specify whether in a situation like this you should treat each successive measurement as collapsing the wavefunction onto an eigenstate or whether you should save the "projection" for the very last measurement.akhmeteli said:
JesseM said:
I wasn't guessing what he said, I was guessing what he meant by what he said. What he said was only the very short statement "Yes, it is an approximation. However, due to decoherence, this is an extremely good approximation. Essentially, this approximation is as good as the second law of thermodynamics is a good approximation." I think this statement is compatible with my interpretation of what he may have meant, namely "in Bohmian mechanics the collapse is not 'real' (i.e. the laws governing measurement interactions are exactly the same as the laws governing other interactions) but just a pragmatic way of getting the same predictions a full Bohmian treatment would yield." Nowhere did he say that using the projection postulate will yield different statistical predictions about observed results than those predicted by Bohmian mechanics.akhmeteli said:
I think they are different only if you assume multiple successive measurements, and understanding "the projection postulate" to imply that each measurement collapses the wavefunction onto an eigenstate, and assuming that for some of the measurements the records of the results are "erased" so that it cannot be known later what the earlier result was. If you are dealing with a situation where none of the measurement records are erased, I'm pretty sure that the statistics for the measurement results you get using the projection postulate will be exactly the same as the statistics you get if you model the whole thing as a giant entangled system and then use the Born rule at the very end to find the probabilities of different combinations of recorded measurement results. And once again, the "projections postulate" does not precisely define when projection should occur anyway, you are free to interpret the projection postulate to mean that only the final measurement of the records at the end of the entire experiment actually collapses the wavefunction.akhmeteli said:
JesseM said:
I think you misunderstood what I meant by "any" above, I wasn't asking if your model could reproduce any arbitrary prediction made by the "standard pragmatic recipe" (i.e. whether it would agree with the standard pragmatic recipe in every possible case, as I think Bohmian mechanics does). Rather, I was using "any" in the same sense as it's used in the question priests used to ask at weddings, "If any person can show just cause why they may not be joined together, let them speak now or forever hold their peace"--in other words, I was asking if there was even a single instance of a case where your model reproduces the probabilistic predictions of standard QM, or whether your model only deals with complex amplitudes that result from unitary evolution. The reason I asked this is that the statement of yours I was responding to was rather ambiguous on this point:akhmeteli said:
If your model does predict actual measurement results, then if the model was applied to an experiment intended to test some Bell inequality, would it in fact predict an apparent violation of the inequalites in both experiments where the locality loophole was closed but not the detector efficiency loophole, and in experiments where the efficiency loophole was closed but not the locality loophole? I think you said your model would not predict violations of Bell inequalities in experiments with all loopholes closed--would you agree that if we model such experiments using unitary evolution plus the Born rule (perhaps applied to the records at the very end of the full experiment, after many trials had been performed, so we don't have to worry about whether applying the Born rule means we have to invoke the projection postulate), then we will predict violations of Bell inequalities even in loophole-free experiments? Likewise, would you agree that Bohmian mechanics also predicts violations in loophole-free experiments, and many-worlds advocates would expect the same prediction even if there is disagreement on how to derive it?
zonde said:If prediction of some green alternate theory is found out to be wrong then theory is wrong.
If prediction of well established theory with proven usefulness is found out to be wrong then domain of it's applicability is established instead.
JesseM said:
akhmeteli said:
DrChinese said:
) agree that nonlocality and/or nonseparability is a fact; would that mean that Quantum Mechanics has proven Relativity Theory wrong (or slightly "useless")?RUTA said: