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εllipse
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How can one event affect another instantly over a distance if there is no absolute concept of simultaneity? In which reference frame does the cause have a "simultaneous" effect?
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Timbuqtu said:But ultimately it is true that quantum mechanics is flawed and that we need another theory which does obey the rules of special relativity, for instance quantum field theory.
ttn said:The obvious first cut at a way to do this is simply to return to something like the Lorentz ether theory -- a theory which actually predicts all the same formalism (Lorentz transformations, etc.) as standard relativity but does so on the assumption that there exists a preferred (ether) reference frame. What good does this do? It gives a definite *meaning* to the (near?)-simultaneous action-at-a-distance that is involved in quantum non-locality.
εllipse said:How can one event affect another instantly over a distance if there is no absolute concept of simultaneity? In which reference frame does the cause have a "simultaneous" effect?
Sherlock said:To account for (ie., to produce) predictable variable coincidence rates, you need a global, instrumental or observational variable,
like Theta, the angular difference between the analyzing
polarizers.
selfAdjoint said:I don't think you can support an ether on the base of quantum nonlocality. Remember that QED, for example, is "manifestly covariant", i.e it does obey everything that relativity requires, and it also exhibits, as you say, quantum nonlocality. So either QED is inconsistent (which has NOT been shown!) or quantum nonlocality does not violate relativity requirements. What relativity requires is that CAUSE not be transferred FTL, and there is no transfer of cause in quantum nonlocality, just an after the fact correlation that shows up in the shared future light cone of the two particles.
The view that this constitutes non locality in the relativistic sense is based on an unexamined tendency to view QM is a classical theory, to treat it as if it were in fact one of the hidden variable theories that the Bell inequalities ruled out.
ttn said:But the collapse postulate is still lurking as an inelimanble (if unmentioned) part of the theory, at least as long as you want to claim that QED is consistent with the fact that when you go into the lab and actually *do* one of the scattering experiments mentioned above, you get some definite outcome (e.g., a certain electron is detected to have scattered into a certain angle... as opposed to: the electron scatters into all angles simultaneously with the whole array of detectors all flashing "bing!" but in parallel universes).
So... it's just what I said originally: orthodox quantum theory is non-local. It violates Bell's Locality condition ("Bell Locality"). And any attempt to blame this *apparent* non-locality on the non-completeness of the quantum mechanical description (i.e., any attempt to explain the correlations by reference to some local common cause that was un-accounted-for in the wave function) must fail. That's Bell's Theorem. Hence Bell Locality is false. Nature violates Bell Locality.
And so to whatever extent Bell Locality accurately captures relativity's prohibition on superluminal causation (and Bell and I and many others think it captures it just perfectly), relativity is wrong.
DrChinese said:Nonlocality is a very difficult issue. Almost every angle of the discussion involves definitions, and few people will precisely agree about those definitions. So that is often the source of disagreements...
DrChinese said:You definitely do not "need" to hypothesize a global (nonlocal) variable called "theta" to explain the observed results. You need the Heisenberg Uncertainty Principle, which still applies in cases of entanglement. You cannot extract more information about the particles than the HUP allows.
Note that theta only explains about the polarization of entangled photons, and does not explain why the other photon attributes are also entangled. I.e. position, momentum, etc.
DrChinese said:The real question is: how do you explain the physicality of the results? I don't think non-local hidden variables is the answer. In fact, I am not sure there is an answer.
This is what bothers me. If you say that Bell's theorem proves that there is some sort of instantaneous information exchange (of any kind) over vast distances, that would mean that you are talking about some sense of "simultaneity", but of course in special relativity you have to have a reference frame from which to state something is "simultaneous". In another reference frame, such things won't be simultaneous. Is there a specific reference frame from which the exchange can be said to happen simultaneously or is it supposed to apply to all reference frames? If it is the former then there would be reference frames in which causality is violated. If it is the latter then the relativity of simultaneity must be wrong, which is very troubling because Minkowskian spacetime and general relativity are built from a framework in which relativity of simultaneity is true. Perhaps this is the reason string theorists are attempting to find a different approach to gravitation... But if relativity is so obviously wrong why haven't we reverted back to an ether theory as someone suggested? Although it is commonly known that relativity and quantum mechanics don't agree in situations where they are both pushed to their limits (black holes, big bang), this seems to be a bit more of a substantial disagreement. And while black holes and the big bang may point to flaws in general relativity and the need to find a theory of quantum gravity, the disagreement between locality and non-locality seems to show that not even special relativity can be valid, unless of course Bell's theorem is wrong. So it seems to me that before we even try to come up with a theory of quantum gravity, we need to know if special relativity is even correct. And if special relativity is valid, then it seems there must be some vital flaw in quantum theory.DrChinese said:Just as a reminder, Special Relativity is intended to apply within certain constraints. Within a particular reference frame, c is the speed of photons and other force carriers, and less than c is the speed of particles with mass. So just in case someone is bothered by the statement above that "relativity is wrong"...
εllipse said:...But if relativity is so obviously wrong why haven't we reverted back to an ether theory as someone suggested? Although it is commonly known that relativity and quantum mechanics don't agree in situations where they are both pushed to their limits (black holes, big bang), this seems to be a bit more of a substantial disagreement. And while black holes and the big bang may point to flaws in general relativity and the need to find a theory of quantum gravity, the disagreement between locality and non-locality seems to show that not even special relativity can be valid, unless of course Bell's theorem is wrong. So it seems to me that before we even try to come up with a theory of quantum gravity, we need to know if special relativity is even correct. And if special relativity is valid, then it seems there must be some vital flaw in quantum theory.
Sherlock said:Theta isn't a hypothetical variable. It's the actual joint setting of the polarizers, and it determines the variable rate of coincidental
detection. I used this type of setup as an example, because
it's the most common Bell test setup, and a bit easier to visualize
than some others you might be thinking of.
εllipse said:This is what bothers me. If you say that Bell's theorem proves that there is some sort of instantaneous information exchange (of any kind) over vast distances, that would mean that you are talking about some sense of "simultaneity", but of course in special relativity you have to have a reference frame from which to state something is "simultaneous".
In another reference frame, such things won't be simultaneous. Is there a specific reference frame from which the exchange can be said to happen simultaneously or is it supposed to apply to all reference frames?
If it is the former then there would be reference frames in which causality is violated. If it is the latter then the relativity of simultaneity must be wrong, which is very troubling because Minkowskian spacetime and general relativity are built from a framework in which relativity of simultaneity is true.
Perhaps this is the reason string theorists are attempting to find a different approach to gravitation...
But if relativity is so obviously wrong why haven't we reverted back to an ether theory as someone suggested?
ttn said:The objectionable non-locality is not in Schroedinger's equation (or its analog in the context of a relativistic quantum field theory like QED). So it is irrelevant that those equations are "manifestly covariant". The nonlocality in the orthodox theories is in the collapse postulate.
?llipse said:How can one event affect another instantly over a distance if there is no absolute concept of simultaneity? In which reference frame does the cause have a "simultaneous" effect?
DrChinese said:Locality is not ruled out if you accept that reality is observer dependent. Of course, I have no idea what such "non-reality" actually is... but the point is that there is definitely an escape route out of the conflict.
DrChinese said:Theta is a number, I agree with that. But it is not a fundamental observable, it is derived from 2 other fundamental observables. Those two observables are redundant, because the HUP limits information about any particle.
DrChinese said:In some ways, our disagreement is semantic. Theta acts "as if" it were real. But that is not how QM gets to that point. Once you measure particle A, you learn about B. Using that information about B, you measure B at some other polarizer angle but gain absolutely NO information in that process. The resulting stats are no different than if you measured any single photon's spin at 2 angles - which forms the exact same Theta you describe. So Theta has nothing but a tangential relationship to entanglement - it is not required to be fundamental to it.
DrChinese said:So to summarize: HUP applies to single particles and entangled systems. Theta also applies to single particles and entangled systems. But Theta can be derived using the HUP, while the HUP cannot be derived from Theta. So decide for yourself which is more fundamental. Don't forget that the HUP also covers position and momentum, while Theta does not.
εllipse said:This is what bothers me. If you say that Bell's theorem proves that there is some sort of instantaneous information exchange (of any kind) over vast distances, that would mean that you are talking about some sense of "simultaneity", but of course in special relativity you have to have a reference frame from which to state something is "simultaneous".
Rade said:Quantum mechanics is a local theory in configuration space but not in physical space."[/I]
Now, if the above holds, then clearly QM and Relativity will not find conflict in questions dealing with "configuration space", but they may very well be in conflict within "physical space"--which is also called "real space". If I read the above correctly, QM attributes of an entity must have "non-local" affects only within physical space.
But what is configuration space and how does it differ from "real space" ?
Nicky said:I was under the impression that the collapse postulate is only considered an approximation, since it does not define precisely what constitutes "measurement".
So if all nonlocality in QM flows from the collapse postulate, then the nonlocality may not be physically real, just an artifact of the approximation.
In the many-worlds approach, for example, there is no need for the notion of nonlocality.
I am still trying to understand dynamical collapse theories, so I don't know if they make nonlocality go away as well. Maybe someone who understands them better can answer -- do dynamical collapse theories eliminate the apparent nonlocality of orthodox QM?
Sherlock said:I'm not familiar with Bell tests that require HUP. That is, I don't recall HUP being mentioned in the experiments that I have copies
of here (Aspect et al., and a few others). So, I'm not sure
what you're saying.
Theta doesn't "act as if" it were real. It is real.
...
Hence, experimental violations of Bell inequalities are taken to be
an indicator of the presence of a global, emission-imparted property
(a hidden constant), ie. the presence of entanglement. ...
εllipse said:It seems to me that Bell's inequality doesn't lead to non-locality for a hidden variable theory. If you're in a distant galaxy and I go half way in between you and some oberserver on Earth and before hand both of you know that I have two blue balls and two red balls and that I will either fire a red ball or a blue ball at you, but we agree that whatever I fire at you I will also fire at Earth, then if you attempt to describe the "state" of the balls before they reach you as a wave of possibilities which collapses when you get your ball, then you have to resort to non-locality to explain why the distant observer will also get the same color ball. But if you just attribute your lack of knowledge to ignorance, then you don't have to explain why both balls are the same color. They'll be the same color because I fired them both from the same location and chose them both to be the same color.. they have hidden variables.. Is such an explanation not possible for Bell's Theorem?
εllipse said:Haha, right. I'm sorry. I completely forgot what Bell's theorem was during that post and was thinking along the lines of the EPR paradox. My appologies. How embarrasing.
ttn said:If you forget about (or evade) EPR (like so many physicists still do), then the whole Bell's Theorem thing looks like an argument against hidden variable theories! In fact, it's nothing of the kind. It's a proof that *even* hidden variable theories have to be non-local, which is just another way of saying: the apparent non-locality of orthodox QM is unavoidable and hence real.
selfAdjoint said:But QM isn't nonlocal in the same sense that realist theories are nonlocal.
By denying that the "balls" have "color" until they are observed, it avoids having causes travel faster than light.
The one ball is observed to be red, and the other to be blue, and there is no definite order that they do that. Different observers will see different orders due to relativity of simultenaity.
After the fact, at a place and time with both observations in the past light cone, you can do the Bell inequalities and confirm that they are violated.
The supposed "effect" of one ball color on the other over spacelike separation is precisely the realist position that QM denies.
DrChinese said:Assume A=0 degrees, B=67.5 degrees... QM has a simple explanation: there is NO C, just A and B. Guess what? If there is no C, then there is no problem. If there is no C, then the results are observer dependent. If the results are observer dependent, then there is not simultaneous reality to non-commuting variables. And this explains the EPR paradox perfectly.
Hurkyl said:I used http://arxiv.org/abs/quant-ph/0002060 for a reference: QM does satisfy Bell Locality.
Hurkyl said:On the other hand, isn't relativistic QFT locally causal by the definition given by Bell in that paper?
IOW, given a state, isn't its value at a point in space-time completely determined by a slice of the backwards light-cone?