Is Quantum Entanglement Just Correlation or a Real Physical Process?

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Coming at this problem from the angle of philosophy/psychology and an unhealthy relationship with the Journal of Consciousness Studies, I'm interested to know how you Physicists interpret the process of collapse, or rather the concept of entanglement. I've read so much new age rubbish all over the place (although I wouldn't call Penrose or Evin Harris Walker new-ager's). Does the violation of Bell's Inequalities demonstrate that quantum "spooky action at a distance" is not merely correlation, but some real physical process? Is this only the case if you try to interpret QM in local realistic terms?

I suppose what I really want to know is how does photon A "connect with" photon B, such that a measurement on A instantaneously acts on B? Is there no fact of the matter at the moment, or is it all down to your particular flavour of philosophical interpretation?
 
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PlayUK, Welcome to PF...

Murray Gell-Mann gives an analogy I like a lot when interpreting "spooky action at a distance":

Professor X has a peculiar habit, he puts on a BLUE sock and a RED sock every day instead of wearing identical pairs like normal people. The foot he chooses to put on these socks, however, is random. Therefore one day he could put on a blue sock on his right foot,but the other day he could do just the opposite. You, as the observant student, cannot know which color will end up in which foot before seeing one of his feet (and no complicated theory will help you predict that because it's really random), but once you see one of his socks, you immediately know the color of the sock you didn't see. There's no mechanism, no spooky action at a distance, when you see the the blue sock, you KNOW where the red sock is.

This is Gell-Mann's interpretation (I think it's originally attributed to someone else but I can't remember it now) and could be found in his book "The Quark and the Jaguar" . So if anybody is going to attack this with their own view on the subject, MGM is the man to talk to.

But I have a feeling his interpretation would be far more convincing than any other that I'll ever see in this forum.
 
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If that's all that "action at a distance" is then I don't see what all the fuss is about.
 
Nisse said:
If that's all that "action at a distance" is then I don't see what all the fuss is about.

There's no fuss among the top physicists . It's perfectly clear. The fuss is more among the public.
 
sokrates said:
There's no fuss among the top physicists . It's perfectly clear. The fuss is more among the public.

sokrates,

I agree with your analysis completely. Thanks for the nice example with professor's socks.

I think the reason why there are still endless discussions of the "spooky action at a distance" is that people (intuitively) try to understand quantum mechanics in the language of 19th century "physical mechanisms". I guess their idea is that the wave function is some kind of material "fluid", that superposition of states is a real thing, and that wavefunction's collapse is an objective physical process. People tend to think that (while not observed) professor's socks *really* exist in a superposition state. So, when the left sock has been observed, there should be some kind of superluminal physical agent/messenger which rushes to the right sock and tells it: "Hey, don't you know that your fellow left sock has been observed and collapsed to red color? You must immediately collapse to blue color. Otherwise you will be found in violation of the laws of quantum mechanics and punished".

The true lesson of quantum mechanics is that it doesn't make sense to think about "physical mechanisms" of events that are not directly observed. In any case, quantum mechanical formalism (superpositions, wave functions, etc) is just a calculational tool, not a physical model.
 
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sokrates said:
PlayUK, Welcome to PF...

Murray Gell-Mann gives an analogy I like a lot when interpreting "spooky action at a distance":

Professor X has a peculiar habit, he puts on a BLUE sock and a RED sock every day instead of wearing identical pairs like normal people. The foot he chooses to put on these socks, however, is random. Therefore one day he could put on a blue sock on his right foot,but the other day he could do just the opposite. You, as the observant student, cannot know which color will end up in which foot before seeing one of his feet (and no complicated theory will help you predict that because it's really random), but once you see one of his socks, you immediately know the color of the sock you didn't see. There's no mechanism, no spooky action at a distance, when you see the the blue sock, you KNOW where the red sock is.

This is Gell-Mann's interpretation (I think it's originally attributed to someone else but I can't remember it now) and could be found in his book "The Quark and the Jaguar" . So if anybody is going to attack this with their own view on the subject, MGM is the man to talk to.

But I have a feeling his interpretation would be far more convincing than any other that I'll ever see in this forum.

Bertlmann’s socks and the nature of reality, Bell, 1981. But this is a simple analogy and does not actually explain Bell test results at all.
 
DrChinese said:
Bertlmann’s socks and the nature of reality, Bell, 1981. But this is a simple analogy and does not actually explain Bell test results at all.

What kind of explanation you want?

We have a theory (quantum mechanics) which predicts experimental results (correlations between different polarizations of separated photons) with great accuracy. Is there anything else needed?
 
meopemuk said:
What kind of explanation you want?

We have a theory (quantum mechanics) which predicts experimental results (correlations between different polarizations of separated photons) with great accuracy. Is there anything else needed?

I'm good with that. I simply pointed out that socks are not a good analogy. As you say, physical mechanisms aren't easily paired with QM.
 
meopemuk said:
sokrates,

I agree with your analysis completely. Thanks for the nice example with professor's socks.

I think the reason why there are still endless discussions of the "spooky action at a distance" is that people (intuitively) try to understand quantum mechanics in the language of 19th century "physical mechanisms". I guess their idea is that the wave function is some kind of material "fluid", that superposition of states is a real thing, and that wavefunction's collapse is an objective physical process. People tend to think that (while not observed) professor's socks *really* exist in a superposition state. So, when the left sock has been observed, there should be some kind of superluminal physical agent/messenger which rushes to the right sock and tells it: "Hey, don't you know that your fellow left sock has been observed and collapsed to red color? You must immediately collapse to blue color. Otherwise you will be found in violation of the laws of quantum mechanics and punished".

The true lesson of quantum mechanics is that it doesn't make sense to think about "physical mechanisms" of events that are not directly observed. In any case, quantum mechanical formalism (superpositions, wave functions, etc) is just a calculational tool, not a physical model.

So true. Maybe we should change the way we think about QM rather than complaining about its unpredictability all the time.
 
  • #10
DrChinese said:
Bertlmann’s socks and the nature of reality, Bell, 1981. But this is a simple analogy and does not actually explain Bell test results at all.

That's it. Thanks Dr. Chinese!
 
  • #11
sokrates said:
Professor X has a peculiar habit, he puts on a BLUE sock and a RED sock every day instead of wearing identical pairs like normal people. The foot he chooses to put on these socks, however, is random.
...once you see one of his socks, you immediately know the color of the sock you didn't see.

This is Gell-Mann's interpretation
This interpretation was completely crushed by Bell. Theories like this one predict correlations in the results of measurements called Bell inequalities. (See e.g. 215, 216). QM predicts that Bell inequalities will be violated. Experiments have confirmed that they are.
 
  • #12
Yes, I liked the socks analogy, but it doesn't square with what I've read, i.e. if this were the case, Bell's Inequalities would hold, as DrChinese et. al. have pointed out. So given Bell's result, I would be right in thinking that this is not merely correlation?
 
  • #13
PlayUK said:
Yes, I liked the socks analogy, but it doesn't square with what I've read, i.e. if this were the case, Bell's Inequalities would hold, as DrChinese et. al. have pointed out. So given Bell's result, I would be right in thinking that this is not merely correlation?

Essentially. It's an interpretation thing, which tends to get debated endlessly here, despite the fact that most working physicists don't really concern themselves with interpretations much at all. It's one of those things that's usually brought up in popular-scientific accounts and introductory textbooks, on how weird and confusing QM is. But once you know about it, most of us just accept the 'weirdness' and get on with it, since it's not relevant to putting QM to practical use. (personally I tend to just think "Who says Nature is obliged to not be 'weird?')

Anyway, so yes, the flaw of the 'socks' argument is that the color is a hidden variable. At all points in time the blue sock was blue and the red sock was red, and the probabilities arose from our lack of knowledge of these variables. Well, the Copenhagen interpretation says there aren't any such hidden variables. They're genuinely indeterminate. Which then raises the question of how, once the blue sock has been observed to be blue, the red sock 'knows' it must be red. And the Bell-test experiments showed that there are no such (local) hidden variables. And there may not be nonlocal ones either, IIRC correctly some more recent results DrChinese pointed out (I'll defer to him on these interpretational matters).

Addressing your original ponderings; I'm more qualified to answer there, being a chemical physicist I supposedly know something about the area where quantum mechanical phenomena become chemical and biochemical phenomena. And my 'professional opinion' about 'quantum consciousness' and its ilk, is that it's basically a load of nonsense. (not least the new-agey stuff which is a horrible pseudoscientific garbage)

(I wrote a somewhat long and detailed criticism here, but opted to delete it as not to bog down the thread with a giant wall of text that may not be of interest. But in short: Bad idea to think it's quantum until 'conventional' explanations fail, chemistry is intrinsically quantum-mechanical, hence they're actually suggesting a hitherto-unknown chemical phenomenon - unlikely since it's generally believed that all biochemistry works just the same as any other chemistry - only the molecules tend to be bigger)
 
  • #14
alxm said:
despite the fact that most working physicists don't really concern themselves with interpretations much at all.
I have seen this argument used several times on this forum. Does somebody have a reference for this?

The fact that most working physicist will probably work on another subject and just have to accept this 'weirdness' of nature for their dayly work doesn't necessairily mean they are happy with the current answers.
 
  • #15
My understanding is that it is not just a matter of observing that electron (not photon) A has positive spin and immediately "knowing" that electron B (which may now be a great distance away from initially paired electron A) has negative spin, as in seeing that one of the professor's socks is red and immediately knowing that the other is blue. The theory says that electron A does NOT HAVE a well defined spin until you observe it. And when you do observe it that causes electron B to immediately have opposite spin to whatever you observe A to have.
 
  • #16
alxm said:
...despite the fact that most working physicists don't really concern themselves with interpretations.

I agree with ajw1 - this is only true in the sense that, for example, most working physicists don't really concern themselves with the physics of nuclear reactors. We all have different fields of study, and we assume that someone somewhere knows how nuclear reactors work.

In fact there is a very large community of physicists who do concern themselves with interpretations. They have conferences and stuff.

To which they don't invite alxm, clearly..
 
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  • #17
alxm said:
And my 'professional opinion' about 'quantum consciousness' and its ilk, is that it's basically a load of nonsense. (not least the new-agey stuff which is a horrible pseudoscientific garbage)

Well I don't want to turn this thread into a metaphysical debate on the nature of Consciousness. I am happy to admit that I don't know the answer here, but would side against the functionalists based on intuition alone (a very bad guide, I know).
 
  • #18
Jaynes explains this in:

Jaynes, E. T., 1989, `Clearing up Mysteries - The Original Goal, ' in Maximum-Entropy and Bayesian Methods, J. Skilling (ed.), Kluwer, Dordrecht, p. 1

Bernouli's Urn Revisited
Define the propositions:
I : "Our urn contains N balls, identical in every respect except that M of them are red, the remaining N-M white. We have no information about the location of particular balls in the urn. They are drawn out blindfolded without replacement."
R_i : "Red on the i'th draw, i = 1; 2; ..."

Successive draws from the urn are a microcosm of the EPR experiment. For the first draw, given only the prior information I , we have

P(R_1|I ) = M/N (16)

Now if we know that red was found on the first draw, then that changes the contents of the urn for the second:
P(R_2|R_1,I ) = (M-1)/(N-1) (17)​
and this conditional probability expresses the causal influence of the first draw on the second, in just the way that Bell assumed.But suppose we are told only that red was drawn on the second draw; what is now our probability for red on the first draw? Whatever happens on the second draw cannot exert any physical influence on the condition of the urn at the first draw; so presumably one who believes with Bell that a conditional probability expresses a physical causal influence, would say that P(R_1|R_2,I) = P(R_1|I).
But this is patently wrong; probability theory requires that
P(R_1|R_2,I) = P(R_2|R_1,I) (18)​
This is particularly obvious in the case M = 1; for if we know that the one red ball was taken in the second draw, then it is certain that it could not have been taken in the first.
In (18) the probability on the right expresses a physical causation, that on the left only an inference. Nevertheless, the probabilities are necessarily equal because, although a later draw cannot physically affect conditions at an earlier one, information about the result of the second draw has precisely the same effect on our state of knowledge about what could have been taken in the first draw, as if their order were reversed.
Eq. (18) is only a special case of a much more general result. The probability of drawing any sequence of red and white balls (the hypergeometric distribution) depends only on the number of red and white balls, not on the order in which they appear; i.e., it is an exchangeable distribution. From this it follows by a simple calculation that for all i and j ,
P(R_i|I ) = P(R_j|I) = M/N (19)​
That is, just as in QM, merely knowing that other draws have been made does not change our prediction for any specified draw, although it changes the hypothesis space in which the prediction is made; before there is a change in the actual prediction it is necessary to know also the results of other draws. But the joint probability is by the product rule,
P(R_i,R_j|I) = P(R_i,|R_j,I )P(R_j|I) = P(R_j|R_i,I)P(R_i|I) (20)​
and so we have for all i and j ,
P(R_i|R_j,I ) = P (R_j|R_i,I) (21)​
and again a conditional probability which expresses only an inference is necessarily equal to one that expresses a physical causation. This would be true not only for the hypergeometric distribution, but for any exchangeable distribution. We see from this how far Karl Popper would have got with his "propensity" theory of probability, had he tried to apply it to a few simple problems.
It might be thought that this phenomenon is a peculiarity of probability theory. On the contrary, it remains true even in pure deductive logic; for if A implies B, then not-B implies not-A. But if we tried to interpret "A implies B" as meaning "A is the physical cause of B", we could hardly accept that "not-B is the physical cause of not-A". Because of this lack of contraposition, we cannot in general interpret logical implication as physical causation, any more than we can conditional probability. Elementary facts like this are well understood in economics (Simon & Rescher, 1966; Zellner, 1984); it is high time that they were recognized in theoretical physics.
 
  • #19
In deriving his inequalities, Bell has several hidden assumptions. Two of them are as follows:
(1) That a conditional probability P(X|Y) expresses a causal influence exerted by Y on X.
(2) Not all local hidden variable theories are included in his equations.

Assumption (1) has been addressed in the my previous post, quoted from Jaynes.
Several other authors have addressed assumption (2) including Jaynes himself in the quoted article above:

See for example:

Exclusion of time in the theorem of Bell
K. Hess et al 2002 Europhys. Lett. 57 775-781

Abstract. The celebrated inequalities of Bell are based on the assumption that local hidden parameters exist. When combined with conflicting experimental results these inequalities appear to prove that local hidden parameters cannot exist. This suggests to many that only instantaneous action at a distance can explain Einstein, Podolsky, Rosen (EPR) type of experiments. We show that Bell-type theories and proofs leading to the well-known inequalities completely exclude a large class of time dependencies in their considerations. Owing to the fact that the electrodynamics of moving bodies cannot be described by time-independent theories or models, we conclude that the Bell theorem cannot describe the physics of EPR experiments. We also show how hidden parameter theories that include time can obtain the quantum result.

Breakdown of Bell's theorem for certain objective local parameter spaces.
Hess and Philipp, PNAS February 17, 2004 vol. 101 no. 7 1799-1805

Abstract: We show that the known proofs of Bell's inequalities contain algebraic manipulations that are not appropriate within the syntax of Kolmogorov's axioms for probability theory without detailed justification. Such justification can be achieved by a variant of the techniques used in Bell-type proofs but only for a subclass of objective local parameter spaces. It cannot be achieved for an extended parameter space that is still objective local and that includes instrument parameters correlated by both time and setting dependencies.

Therefore, it has been shown that the spooky action was due to the mistaken assumption that a
conditional probability must signify a physical influence, and it has also been shown that Bell arguments do not consider all possible local hidden variable theories. Bell's inequalities are only limitations on what can be predicted by Bell-type theories.

Therefore, some of the conclusions of the Aspect-type experiments are premature. At most, such experiments show that Bell-type theories are untenable.
 
  • #20
mn4j said:
Jaynes explains this in:

Jaynes, E. T., 1989, `Clearing up Mysteries - The Original Goal, ' in Maximum-Entropy and Bayesian Methods, J. Skilling (ed.), Kluwer, Dordrecht, p. 1

Bernouli's Urn Revisited
Define the propositions:
I : "Our urn contains N balls, identical in every respect except that M of them are red, the remaining N-M white. We have no information about the location of particular balls in the urn. They are drawn out blindfolded without replacement."
R_i : "Red on the i'th draw, i = 1; 2; ..."
...​


Please, mn4j, this is off topic and belongs in a separate thread. I would be happy to discuss Jaynes' concepts in that context, but this thread is not a "Why Bell is wrong" thread. It is not fair to make every thread mentioning Bell yet another opportunity to confuse newbies with your Local Realist ideas.​
 
  • #21
HallsofIvy said:
The theory says that electron A does NOT HAVE a well defined spin until you observe it. And when you do observe it that causes electron B to immediately have opposite spin to whatever you observe A to have.

No. The theory (quantum mechanics) does not say that. It does not make any statement about the physical state of the system before it is observed. Quantum mechanics gives a mathematical formalism, and within this formalism the state vector of (non-observed) system is indeed a superposition of other state vectors. However, this is quite different from saying that superpositions exist physically. The only way to avoid paradoxes is to refuse answer questions about physical states of non-observed systems. If somebody asks you "Is there Moon when nobody is looking?" you should answer "I don't know, but if you'll decide to look you'll see the Moon there."
 
  • #22
mn4j's anti-Bell rant doesn't belong in this forum at all in my opinion. He isn't making a rational case for anything. He's just spamming the forum with random anti-Bell comments.

I'm not going to waste as much time on him as I did in another thread recently, but I'm going to comment the two specific claims he made here. One of his claims is that not all local hidden variable theories lead to Bell inequalities. The theories that satisfy the assumptions that go into the derivations of Bell inequalities obviously do, so the question is, is there a better definition of "local hidden variable theory" than the one that's represented by those assumptions. mn4j has failed to produce a definition of that concept, and he hasn't been able to give us a good reason for why he thinks that the standard assumptions are too restrictive.

Even if there is a reasonable definition of "local hidden variable theory" that includes theories that don't satisfy Bell inequalities (and I still doubt that), it wouldn't change the fact that the derivation I linked to shows very clearly that we can rule out Bertleman's socks as a valid model of spin.

Regarding the claim that Bell inequalities are based on the assumption that conditional probabilities represent causal influence, I'm just going to refer once more to the derivation I linked to earlier. Do you see any such assumptions there? I don't.
 
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  • #23
zenith8 said:
This is only true in the sense that, for example, most working physicists don't really concern themselves with the physics of nuclear reactors. We all have different fields of study, and we assume that someone somewhere knows how nuclear reactors work.

In fact there is a very large community of physicists who do concern themselves with interpretations. They have conferences and stuff.

To which they don't invite alxm, clearly..

Snooty answers aside, you're missing the point I was addressing: Which is that it's easy to believe, and many apparently do seem to believe, that the question of interpretations is a central and important part of quantum mechanics. Just looking at the number of threads on this board that concern that topic, and you could easily form such an opinion. It's undoubtedly the most written-about topic within QM here.

And my point is that that view is false. Interpretations aren't central to understanding QM. They're not something that's used by the big group of people who are actually using/applying QM. And knowing about different interpretations isn't really necessary to do so.

Do this, if you will. Grab a QM book, say Messiah or Landau-Lifschitz or whichever relatively comprehensive one you've got. How many pages in total? How many dedicated to discussing interpretations? Compare that content to the content of this board. Vastly different. Why is that? Well I already stated my theory - popular-scientific accounts and introductory textbooks that over-emphasize the practical importance of the topic.

I did not say nobody's studying the subject. You decided to infer that - not me. Beat up strawmen much?
 
  • #24
alxm said:
Snooty answers aside, you're missing the point I was addressing: Which is that it's easy to believe, and many apparently do seem to believe, that the question of interpretations is a central and important part of quantum mechanics. Just looking at the number of threads on this board that concern that topic, and you could easily form such an opinion. It's undoubtedly the most written-about topic within QM here.

And you know why that is? Because it's the most interesting topic within QM.

And my point is that that view is false. Interpretations aren't central to understanding QM. They're not something that's used by the big group of people who are actually using/applying QM. And knowing about different interpretations isn't really necessary to do so.

Grin. But you don't understand QM, remember? Nobody understands QM. Feynman et al. said so repeatedly.

Look, my analogy with the physics of nuclear reactors is perfectly apposite. The guy who runs the reactor to generate electricity doesn't need to understand how or why it works in detail, he just needs to run the algorithm. Press the green button - make sure the needle doesn't go into the red zone - electricity comes out the other end.

Or in the case of non-relativistic QM, for example - run the algorithm: supply external potential, solve the Schroedinger equation, expectation values of operators give spectrum of possible results whose probability is given by etc.. etc.. Just because you have learned the algorithm doesn't mean you understand QM, though you may be able to design better transistors or whatever. And good luck to you.

Do this, if you will. Grab a QM book, say Messiah or Landau-Lifschitz or whichever relatively comprehensive one you've got. How many pages in total? How many dedicated to discussing interpretations? Compare that content to the content of this board. Vastly different. Why is that? Well I already stated my theory - popular-scientific accounts and introductory textbooks that over-emphasize the practical importance of the topic.

OK, I did. You're right!

I suppose it's because regurgitating the well-known contents of Landau and Lifschitz doesn't make for good discussion. This is a discussion forum, not a textbook. People naturally like to discuss stuff they don't understand - especially when the better fundamental understanding that serious discussion gives might lead to actual technical progress.

I did not say nobody's studying the subject. You decided to infer that - not me. Beat up strawmen much?

Sure, it's my favourite rhetorical tactic.

Seriously, all the 'real physicists don't bother about interpretations' stuff you and others come out with simply doesn't stand up to examination. Some of the most interesting stuff to come out of physics in the last 50 years has come out of a close study of interpretations - with Bell's work being the most famous example. There is no clear consensus on what non-locality actually means. And when that kind of thing happens, it means you have a handle on the future. Physicists have something to get their teeth into. This is the black-body radiation of the 21st century - try to be a little more courageous.

And before you say it's just philosophy - there is no way to distinguish between interpretations - well that's nonsense too. See, for example, Antony Valentini's recent proposals to test the Bohm interpretation by looking at the cosmic background radiation. A nice article summarizing the current excitement in the field is here:

http://www.sciencemag.org/cgi/content/summary/324/5934/1512"

Of course, I'm aware that what you say is official forum policy. I love the way that the topic of "Philosophy" (where interpretation discussions often get migrated) is sandwiched between "Ph.D. Comics" and "Brain Teasers" right at the end of the list. They must have had a big laugh in the PhysForum Office when they thought of that one..
 
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  • #25
Fredrik said:
This interpretation was completely crushed by Bell. Theories like this one predict correlations in the results of measurements called Bell inequalities. (See e.g. 215, 216). QM predicts that Bell inequalities will be violated. Experiments have confirmed that they are.

Well, I knew this would spark all kinds of controversies in the forum, but as I said, I am not going to confuse it further. And this is in Gell-Mann's book so that's the final word for me.

But let me try to respond you using my own view:

The interpretation I gave is not a theory first of all. It is, as the name speaks for itself, an interpretation. But even if it is a theory, what part of it is contradicting with QM? I never said it's a local hidden variable theory. I don't think it involves anything related to that. So if you are targeting Bell inequalities I think that's kind of off-topic. Just based on what I wrote, could you point out to me what part of this is making a tension with QM? You might counter with the argument that 'color' is a hidden variable, but not necessarily. It may or may not be a hidden variable, that's another discussion we could have, but I don't get this: at the end of the day, what we talk about is what we can observe. Once again: Color is not necessarily a hidden variable here. The fact that we don't KNOW what the color is could just as well be described as a superposition - since you don't know it; it could be red or blue.

And whenever you observe one of the socks, you KNOW the color of the other sock.

End of story for me. I don't care what it was before I measured it because it's not physically observable. If it's not observable, we can argue all day about it, it's like talking about the weather.
I think you guys are making a relatively simple problem much more complicated than it needs to be.

But don't ban me for this, just my 2 cents.
 
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  • #26
zenith8 said:
Just because you have learned the algorithm doesn't mean you understand QM, though you may be able to design better transistors or whatever. And good luck to you.

[...]I suppose it's because regurgitating the well-known contents of Landau and Lifschitz doesn't make for good discussion. This is a discussion forum, not a textbook. People naturally like to discuss stuff they don't understand - especially when the better fundamental understanding that serious discussion gives might lead to actual technical progress.

Your argument is really powerful here. I totally agree. Just because discussing QM's interpretations IS NOT industrially USEFUL at the moment DOESN'T PROVE anything related to its relative importance. In fact it is extremely important for it might come with its own applications based on that new, more sound and conceptual theory.
 
  • #27
Is there another theoretical model in physics in which a non-local and a local universe can co-exist, beside the Universe being a projection/hologram(provided we wished to retain some sort of weak local realism)?
 
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  • #28
Space-time intervals don't exist unless there is an event at each end of that interval. The events are what define, and even what create, that interval. Space-time, itself, is simply a side effect of how particles interact. Specifically, a recording device (like a machine or human) detects no space-time until it transfers information (energy, particles) with the event at each end of that interval. This is because there wasn't any event until the transfer of information took place, and thus there wasn't any space-time until then, as well. So there is no distance between two entangled particles until information is exchanged with that machine or human observer, and thus the two particles are as good as local. And local particles need not worry about causality. Unfortunately this requires that I define the observer, because I can imagine little recording machines at each end of the interval which also don't interact with ME until 'I' trade information with them.
 
  • #29
fleem said:
Space-time intervals don't exist unless there is an event at each end of that interval. The events are what define, and even what create, that interval. Space-time, itself, is simply a side effect of how particles interact. Specifically, a recording device (like a machine or human) detects no space-time until it transfers information (energy, particles) with the event at each end of that interval. This is because there wasn't any event until the transfer of information took place, and thus there wasn't any space-time until then, as well. So there is no distance between two entangled particles until information is exchanged with that machine or human observer, and thus the two particles are as good as local. And local particles need not worry about causality. Unfortunately this requires that I define the observer, because I can imagine little recording machines at each end of the interval which also don't interact with ME until 'I' trade information with them.
I suppose WaveJumper is asking for peer reviewed publications of models.

Your statetements are like the well-known 'is the back of the moon there when we're not observing': because of the way it's defined it can never be falsified.
 
  • #30
PlayUK said:
Does the violation of Bell's Inequalities demonstrate that quantum "spooky action at a distance" is not merely correlation, but some real physical process?
Action at a distance doesn't have any physical meaning. That's why it's called spooky. :smile:

It's natural to assume that there are real (underlying) physical processes involved in producing experimental correlations associated with quantum entangled state representations. However, quantum theory doesn't deal with underlying causes, per se. It's all about correlating data sets with instrumental variables and each other.

PlayUK said:
Is this only the case if you try to interpret QM in local realistic terms?
Quantum theory isn't a causal theory, so it would be a misnomer to call it local or nonlocal. It's realistic only wrt the objective (publicly verifiable) description of experimental preparations (materials, design, etc.).

PlayUK said:
I suppose what I really want to know is how does photon A "connect with" photon B, such that a measurement on A instantaneously acts on B?
There's no particular reason to believe that (and there's no way to know if) a measurement on A instantaneously acts on B in a direct, physically causal sense. No matter what happens at A or B, the results at A and the results at B are always, viewed separately, random sequences.

Appropriately paired however, the joint (A,B) results are correlated to a global variable (the angular difference between the polarizer settings at A and B). Changing the setting at A or B instantaneously changes this global parameter.

Also, the recording of a detection at either A or B directly affects the statistical sample space at the other end via whatever sort of pairing strategy is involved in the experimental design.

PlayUK said:
Is there no fact of the matter at the moment, or is it all down to your particular flavour of philosophical interpretation?
The objective physical fact wrt the experimental production of quantum entanglement is that the spatially separated data accumulations are related due to some common property imparted via local transmission.
 
  • #31
alxm said:
Snooty answers aside, you're missing the point I was addressing: Which is that it's easy to believe, and many apparently do seem to believe, that the question of interpretations is a central and important part of quantum mechanics. Just looking at the number of threads on this board that concern that topic, and you could easily form such an opinion. It's undoubtedly the most written-about topic within QM here.

And my point is that that view is false. Interpretations aren't central to understanding QM. They're not something that's used by the big group of people who are actually using/applying QM. And knowing about different interpretations isn't really necessary to do so.

Which part of the world is interesting for you is your personal choice and your personal problem. There are people interested in computing some physical effects because this is necessary for some applications, for example building some device. Fine, I see no problem here. There are other people who couldn't care less about the minor quantum effects who decide if this device works or not. They care about quantum mechanics because they want to understand how Nature works. I see no problem as well. None of these groups is better than the other one.

Do this, if you will. Grab a QM book, say Messiah or Landau-Lifschitz or whichever relatively comprehensive one you've got. How many pages in total? How many dedicated to discussing interpretations? Compare that content to the content of this board. Vastly different. Why is that? Well I already stated my theory - popular-scientific accounts and introductory textbooks that over-emphasize the practical importance of the topic.

The first thing is one one probably has to care about if one wants to make money as a technician, experimenter and so on in some applied domain of physics, which is more or less by accident complex enough to require QM in some computations. The other one is interesting in itself, it is Faust's desire to understand "was die Welt I am Innersten zusammenhaelt". Given this, it seems reasonable to expect that the two groups are interested in different parts of quantum theory, that different literature is appropriate for them, and that an author of a book about QM has to decide for which of the two groups the book will be written.

It is also quite natural to expect that the first group of people consults specialized books about the particular problems, while the second group discusses these really interesting questions in public forums.

"Shut up and calculate" QM is without doubt a useful tool in applications. But it is of no interest for those interested in the foundations of physics. Different problems, different interests, different groups of people interested in them - none of them better than the other, none of them "central" to "the" quantum mechanics.
 
  • #32
Ilja said:
Which part of the world is interesting for you is your personal choice and your personal problem. There are people interested in computing some physical effects because this is necessary for some applications, for example building some device. Fine, I see no problem here. There are other people who couldn't care less about the minor quantum effects who decide if this device works or not. They care about quantum mechanics because they want to understand how Nature works. I see no problem as well. None of these groups is better than the other one.

The first thing is one one probably has to care about if one wants to make money as a technician, experimenter and so on in some applied domain of physics, which is more or less by accident complex enough to require QM in some computations. The other one is interesting in itself, it is Faust's desire to understand "was die Welt I am Innersten zusammenhaelt". Given this, it seems reasonable to expect that the two groups are interested in different parts of quantum theory, that different literature is appropriate for them, and that an author of a book about QM has to decide for which of the two groups the book will be written.

It is also quite natural to expect that the first group of people consults specialized books about the particular problems, while the second group discusses these really interesting questions in public forums.

"Shut up and calculate" QM is without doubt a useful tool in applications. But it is of no interest for those interested in the foundations of physics. Different problems, different interests, different groups of people interested in them - none of them better than the other, none of them "central" to "the" quantum mechanics.

Exactly. Physicists do physics for different reasons. Some love the beautiful mathematics, some the empiricism, some the experimental challenges, some the applied aspects, etc. There is a subset of physicists who do physics for its ontological implications, i.e., we want to understand the nature of reality and we believe physics is a great way to do that. Generally speaking, we assume there is a unique ontological story to be told so we are disturbed when, say, two working theories possesses incongruous ontological implications, e.g., E&M being Lorentz invariant while Newtonian mechanics is Galilean invariant. Before special relativity reconciled this situation, an instrumentalist might simply have said, “What’s the problem? If you want to do mechanics, use Newton’s laws. If you want to do E&M, use Maxwell’s equations.” The situation with entanglement today is similar in that there is no single ontological story that accommodates all theories of physics, and finding one will, it appears, entail sacrifice so as to subsume EPR-Bell phenomena. For example, Bohmian mechanics violates relativity in that it requires a preferred frame and Many Worlds violates parsimony with its indenumerably infinite “universes.” In my 3 years of QM, a year of QFT and a year of string theory, not one word was ever spoken of EPR or Bell. Granted this is dated (I got my PhD in 1987), but you don’t need to know anything about “quantum weirdness” to do quantum physics. It’s really only a “problem” for those of us who want to use physics to infer metaphysics, although many in the foundations community believe this problem will ultimately motivate new physics.
 
  • #33
sokrates said:
There's no fuss among the top physicists . It's perfectly clear. The fuss is more among the public.

I'm not sure that's entirely fair. EPR caused quite a stir when it was first proposed among some of the brightest minds in the history of physics. It was these kinds of questions that ultimately resulted in it becoming "perfectly clear".

It's true that the general public is several steps behind the top physicists of today. But it took years and a lot of deep thought for physicists to get there originally.
 
  • #34
meopemuk said:
No. The theory (quantum mechanics) does not say that. It does not make any statement about the physical state of the system before it is observed. Quantum mechanics gives a mathematical formalism, and within this formalism the state vector of (non-observed) system is indeed a superposition of other state vectors. However, this is quite different from saying that superpositions exist physically. The only way to avoid paradoxes is to refuse answer questions about physical states of non-observed systems. If somebody asks you "Is there Moon when nobody is looking?" you should answer "I don't know, but if you'll decide to look you'll see the Moon there."

But isn't it true that there are some things of physical meaning you can say about a system before you observe it? For example, the probability of seeing the system in a given state if you do observe it? I would think that's a very real physical attribute that, for example, you might ultimately care about in designing semiconductors. You don't care if a particular electron winds up in a particular state, but you do care about how many of them ultimately wind up in that state.
 
  • #35
mjames said:
I'm not sure that's entirely fair. EPR caused quite a stir when it was first proposed among some of the brightest minds in the history of physics. It was these kinds of questions that ultimately resulted in it becoming "perfectly clear".

It's true that the general public is several steps behind the top physicists of today. But it took years and a lot of deep thought for physicists to get there originally.

I never said we got there easily. But does it matter? EVERYTHING looks complicated before you understand it.

My initial argument was this: Because so many wrong things have been imposed on the minds of the public, they think quantum mechanics is much more difficult than it actually is, involving things like "spooky actions at a distance" and so forth.

I think it's not fair to even dwell on this subject. We must immediately resort to 'Bertlmann's socks' argument whenever we, as the more knowledgeable, need to explain this to somebody.

Even the simplest things caused quite a stir in their times, but this just isn't important.
 
  • #36
sokrates said:
I think it's not fair to even dwell on this subject. We must immediately resort to 'Bertlmann's socks' argument whenever we, as the more knowledgeable, need to explain this to somebody.

Even the simplest things caused quite a stir in their times, but this just isn't important.

Sokrates - as has been pointed out to you several times, you're just wrong about this. Here is a summary of the current state of affairs:

We define nonlocality as a direct influence of one object on another, distant object, contrary to our expectation that an object is influenced directly only by its immediate surroundings.

Consider an EPR experiment, measuring spins. With parallel analyzers, we find that measurement of the spin on one side instantly predicts the result on the other. If you do not believe one side can have a causal influence on the other, you require the results on both sides to be determined in advance (the Bertlmann's socks argument). But this has implications for non-parallel settings (e.g. measure spin on axes 45 degrees apart in the two wings) which conflict with quantum mechanics (Bell).

Bell's analysis showed that any account of quantum phenomena needs to be non-local, not just any 'hidden variables' account i.e. nonlocality is implied by the predictions of standard quantum theory itself. Thus, if nature is governed by these predictions (which it is, according to real experiments) then nature is non-local. This is essentially because the many-particle wave function in the Schroedinger equation is defined on the configuration space of the system, an abstraction which combines or binds distant particles into a single irreducible reality.

So nonlocality - spooky action at a distance if you like - sounds strange and yet it is experimentally verifiable. If you want to refuse to believe it, what are your options?

(1) Loopholes: claim that improving detector efficiencies in the EPR-style experiments will invalidate the results. This is now widely understood to be desperate clutching-at-straws.

(2) Deny, in one way or another, that there is a material world - the description of which is the task of physics. Without objective reality, there is nothing to be objectively nonlocal. Despite its manifest ludicrousness, this has actually become a surprisingly fashionable viewpoint.

However, standard QM is not self-consistent due to the measurement problem. This is solvable only by granting real physical existence to theory objects. Standard QM is thus fundamentally an anti-realist stance - the wave function is just about probabilities, but probabilities of what? Something does travel - of course - along different paths in, say, an interference experiment; to refuse to call it 'real' is merely to play with words. Radical anti-realism can pretend to resolve interpretative paradoxes in virtually any context, e.g. Mach's rejection of grounding 'pressure' and 'temperature' in terms of real microscopic entities obviates the need to understand, say, convergence to thermodynamic equilibrium. More broadly, the philosophical doctrine of solipsism can 'solve' every problem in the history of science by just denying that anything but one's own mental experiences exist. This is ludicrously distant from the kind of solution we are interested in as scientists.

Instrumentalist Copenhagen QM is effectively 'an idea for making it easier to evade the implications of quantum theory for the nature of reality' (Deutsch). The positivist belief that empirical adequacy plus a formalized proof procedure is the best any theory can properly aspire to is - when you think about it - bizarre.

(3) Be a many-worlds person, then everything that can possibly happen happens in both branches of the experiment thus there can be no correlations. This makes two problems - nonlocality and macroscopic superpositions in measurement - go away, at the cost of believing in something apparently ludicrous (bazillions of ontologically-real extra universes) on the basis of assigning an entire universe to each term in a mathematical expansion.

(4) Allow time travel into the past (like in Cramer's transactional interpretation). But then we need to abandon the metaphysical picture of the past generating the future, and this is a step too far for most people.

(5) Make claims along the lines that the universe is necessarily Lorentz invariant and Lorentz invariance of a physical theory requires locality. However, this is just wrong as should be clear from the meaning: 'Lorentz invariance' describes the behaviour of a theory under certain transformations of the reference frame. 'Locality' implies there is no action at a distance. (Nonlocal Lorentz-invariant Wheeler-Feynman electrodynamics good counterexample to this).

(6) Claim that, in quantum field theory, the Equal Time Commutation Relations (operators which represent field quantities at space-like separation all commute) show that the theory contains no superluminal action. However, this is an empty claim. What it actually shows is that measurements on one side do not affect the long-term statistics on the other, that is that no signals can be sent from one region to the other by manipulating the apparatus on one side. However, superluminal influences are still required to explain the correlations between the two regions in an individual experiment.

To summarize finally the connection between non-locality and relativity, remember that although relativity is often taken to imply the existence of some sort of absolute speed limit, this is not actually the case. The fundamental feature of the Lorentz transformations is that they leave the speed of light invariant, not that they render it an insuperable boundary. Note that theories of tachyons and other superluminal transport do exist. We must therefore turn our attention to the question of the compatibility of non-locality with the relativistic picture of space-time. One can say that:

* Violation of Bell's inequality does not require superluminal matter or energy transport

* Violation of Bell's inequality does not entail the possibility of superluminal signalling (unless you believe in Bohm-Valentini style quantum nonequilibrium).

* Violation of Bell's inequality does require superluminal causal connections.

* Violation of Bell's inequality can be accomplished only if there is superluminal information transmission.

An excellent reference is Tim Maudlin's book 'Quantum non-locality and relativity' from where I adapted the previous four starred points.
 
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  • #37
RUTA said:
The situation with entanglement today is similar in that there is no single ontological story that accommodates all theories of physics, and finding one will, it appears, entail sacrifice so as to subsume EPR-Bell phenomena. For example, Bohmian mechanics violates relativity in that it requires a preferred frame and Many Worlds violates parsimony with its indenumerably infinite “universes.”

I disagree. The Bohmian picture can accommodate all theories of physics. It requires a preferred frame, but that means only that it violates relativistic metaphysics, but not relativistic physics.

The problems of BM with spinor fields and BRST quantization seem solvable. For spinor fields, see my approach to spinor quantization in http://ilja-schmelzer.de/papers/clm.pdf" . Spinor fields appear there together with some heavy bosonic partners in a canonically quantized way, which makes the standard BM scheme applicable. Gauge fields and gravity appear there as emergent, non-fundamental fields, thus, they don't need a special Bohmian representation as well as thermodynamic fields need such a representation.

Ilja
 
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  • #39
RUTA said:
For example, Bohmian mechanics violates relativity in that it requires a preferred frame
That is not true. There is a version of Bohmian mechanics that does not require a preferred frame:
http://xxx.lanl.gov/abs/0811.1905 [Int. J. Quantum Inf. 7 (2009) 595-602]
 
  • #40
Demystifier said:
That is not true. There is a version of Bohmian mechanics that does not require a preferred frame:
http://xxx.lanl.gov/abs/0811.1905 [Int. J. Quantum Inf. 7 (2009) 595-602]

Question, Demystifier: As I understood it, some if not most Bohmian versions do require a preferred frame. Is that so?
 
  • #41
DrChinese said:
Question, Demystifier: As I understood it, some if not most Bohmian versions do require a preferred frame. Is that so?

This is true - the standard de Broglie-Bohm pilot-wave theory requires a preferred frame, but so what? As Ilja said this is just a metaphysical question - there is no disagreement with relativistic physics.

The standard 'interpretation' of relativity (all inertial frames are equivalent) is simply a positivistic metaphysical preference. There are other interpretations - such as the Lorentzian one - where there is a preferred frame but nevertheless everything is still entirely observationally equivalent to the standard view. See the recent book 'Einstein, Relativity, and Absolute Simultaneity' edited by William Lane Craig and Quentin Smith which contains a variety of essays by prominent physicists on this topic.

Note that the standard de Broglie-Bohm pilot-wave theory implies a preferred frame essentially because the notion of a configuration, as used in the non-relativistic version, already presupposes simultaneity. Configurations are configurations at some time - they specify where all of the particles in a system are at a given moment. So the very notion of a configuration is not a Lorentz invariant concept.

The reference given by Demystifier is to his own recent paper, which is extremely interesting but not yet incorporated into the Bohmian mainstream (most of them probably haven't read it yet).
 
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  • #42
zenith8 said:
The reference given by Demystifier is to his own recent paper, which is extremely interesting but not yet incorporated into the Bohmian mainstream (most of them probably haven't read it yet).

Yes, not helped by the fact that he is awesomely prolific! :smile:
 
  • #43
Ilja said:
I disagree. The Bohmian picture can accommodate all theories of physics. It requires a preferred frame, but that means only that it violates relativistic metaphysics, but not relativistic physics.

Ilja

The "sacrifice" is the "relativistic metaphysics." Very little interest among relativists (physics) for giving that up.

I suspect no interpretation will win the day unless it yields new physics simply because otherwise it's left to personal preference.
 
  • #44
Demystifier said:
That is not true. There is a version of Bohmian mechanics that does not require a preferred frame:
http://xxx.lanl.gov/abs/0811.1905 [Int. J. Quantum Inf. 7 (2009) 595-602]

Neither the words "preferred" nor "frame" appear in this paper according to my search engine. Relativistic covariance does not mean "no preferred frame." GR and SR both accommodate a preferred frame, in fact, there are arguments for the CMB rest frame as a preferred frame in the context of GR cosmology. Please explain, say, the Mermin device in terms of BM without a preferred frame.
 
  • #45
RUTA said:
The "sacrifice" is the "relativistic metaphysics." Very little interest among relativists (physics) for giving that up.

Whole book full of 'em in the reference I gave in my earlier post #41. How many physicists would be enough for you?

"2005 marked the centenary of one of the most remarkable publications in the history of science, Albert Einstein's 'On the Electrodynamics of Moving Bodies' in which he presented a theory that later came to be known as the Special Theory of Relativity. This 1905 paper is widely regarded as having destroyed the classical conceptions of absolute time and space, along with absolute simultaneity and absolute length, which had reigned in physics from the times of Galileo and Newton to the dawn of the twentieth century. As we embark upon a new century, the Special Theory is now 100 years old, and a great deal has transpired in both philosophy and physics since its first publication. This volume is a timely reappraisal of the theory's central claims, especially concerning the elimination of absolute time and absolute simultaneity.''

"This collection draws together essays by both philosophers and physicists and reflects the cutting edge of research and thought on the question of absolute simultaneity. The issues discussed in the book include Aspect's confirmation of Bell's theorem, de Broglie-Bohm's quantum mechanics, the privileged cosmic time series in a Friedman universe, Lorentz's ideas and neo-Lorentzian theory and other relevant issues. Almost all the contributors are convinced that the received view that simultaneity is not an absolute relation is not only unwarranted but false, and it is hoped that this collection will stimulate discussion among both philosophers and physicists concerning the warrant for and problems with assertions of the relativity of simultaneity on the basis of Einstein's theory."

Keep up!

I suspect no interpretation will win the day unless it yields new physics simply because otherwise it's left to personal preference.

Try http://www.sciencemag.org/cgi/content/short/324/5934/1512" .
 
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  • #46
zenith8 said:
Try http://www.sciencemag.org/cgi/content/short/324/5934/1512" .

This is about Antony Valentini, who is a Bohmian. Humorously, he has a Wiki page which may surprise a number of people on this board (including Demystifier) who are Bohmians: "Currently Antony Valentini is the only person in the world doing research into the De Broglie Pilot Wave theory..."

Apparently, hype rules everywhere. As to the reference above, you will need to have a subscription to view. He writes in the arxiv, for instance:

http://arxiv.org/abs/quant-ph/0506115

And perhaps more relevant to this thread:

Hidden Variables and the Large-Scale Structure of Spacetime:

"We discuss how to embed quantum nonlocality in an approximately classical spacetime background, a question which must be answered irrespective of any underlying microscopic theory of spacetime. We argue that, in deterministic hidden-variables theories, the choice of spacetime kinematics should be dictated by the properties of generic non-equilibrium states, which allow nonlocal signalling. Such signalling provides an operational definition of absolute simultaneity, which may naturally be associated with a preferred foliation of classical spacetime. The argument applies to any deterministic hidden-variables theory, and to both flat and curved spacetime backgrounds. We include some critical discussion of Einstein's 1905 'operational' approach to relativity, and compare it with that of Poincare."
 
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  • #47
DrChinese said:
This is about Antony Valentini, who is a Bohmian. Humorously, he has a Wiki page which may surprise a number of people on this board (including Demystifier) who are Bohmians: "Currently Antony Valentini is the only person in the world doing research into the De Broglie Pilot Wave theory..."

Well, it's true to a first approximation.. :smile:

Especially if you consider the de Broglie theory to be different from the Bohm one.

Apparently, hype rules everywhere. As to the reference above, you will need to have a subscription to view.

Well, I'm not saying you should type the title of the paper into Google (that would be against the rules), but..
 
  • #48
zenith8 said:
Well, it's true to a first approximation.. :smile:

Especially if you consider the de Broglie theory to be different from the Bohm one.

Ha, I hope there are a few more than that... and yes, there are probably a few who Bohmians who are not dBBers, and vice versa. Apparently Valentini is big on labels, as he references Poincare over Einstein in places. I presume he is one of the group who feel Einstein's legacy unjustly overshadows Poincare in certain respects.
 
  • #49
DrChinese said:
This is about Antony Valentini, who is a Bohmian. Humorously, he has a Wiki page which may surprise a number of people on this board (including Demystifier) who are Bohmians: "Currently Antony Valentini is the only person in the world doing research into the De Broglie Pilot Wave theory..."

Though having just looked at this - it's clear his Wikipedia page has recently been heavily vandalized by an illiterate moron - presumably since the Science magazine article came out.
 
  • #50
zenith8 said:
Though having just looked at this - it's clear his Wikipedia page has recently been heavily vandalized by an illiterate moron - presumably since the Science magazine article came out.

I mean, come on, let me quote a bit more:

"Currently Antony Valentini is the only person in the world doing research into the De Broglie Pilot Wave theory, although he has attracted the attention of a reading group of Post Doctoral students, Doctoral Students and Honours Students called "The Cowboy Bebop Experience" who despite their irreverant name, comprise some of the brightest emerging minds in physics who are not yet able to admit their interests lest their careers be stunted forever."

Believe it or not - like they don't have enough trouble with every other quantum physicist hating/ignoring them - there are mutually aggressive factions within the de Broglie-Bohm community. To me this sounds like one of the "Bohmian mechanics" crowd throwing his toys out of the pram because Valentini was getting more attention than them.. (I speculate wildly).

Ha ha - this is so funny.
 
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