Is Quantum Entanglement Just Correlation or a Real Physical Process?

[PlayUK]

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?

 

[sokrates]

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.

 

[Nisse]

If that's all that "action at a distance" is then I don't see what all the fuss is about.

 

[sokrates]

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.

 

[meopemuk]

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.

 

[DrChinese]

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.

 

[meopemuk]

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?

 

[DrChinese]

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.

 

[sokrates]

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.

 

[sokrates]

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!

 

[Fredrik]

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.

 

[PlayUK]

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?

 

[alxm]

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)

 

[ajw1]

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.

 

[HallsofIvy]

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.

 

[zenith8]

...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..

 

[PlayUK]

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).

 

[mn4j]

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.
​

 

[mn4j]

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.

 

[DrChinese]

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.​

 

[meopemuk]

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."

 

[Fredrik]

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.

 

[alxm]

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?

 

[zenith8]

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..

 

[sokrates]

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.

 

[sokrates]

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.

 

[WaveJumper]

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)?

 

[fleem]

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.

 

[ajw1]

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.

 

[ThomasT]

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.

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.

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.).

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.

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.

 

[Ilja]

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.

 

[RUTA]

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.

 

[mjames]

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.

 

[mjames]

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.

 

[sokrates]

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.