Relativity & Quantum Theory: Is Locality Violated?

[UglyDuckling]

Modern physics is founded on relativity and quantum theory. Since nothing can travel faster than the speed of light, relativity forces a locality constraint on physical theory; spatially remote events cannot influence each other. On the other hand it has in recent years become evident that quantum mechanics violates this restriction. If we assume the validity of Bell’s theorem and the soundness of such experiments as Aspect’s; then no local theory is completely compatible with the predictions of quantum theory. A contradiction exists between the main foundations of physics.

Is this contradiction because relativity itself is fundamentally flawed or is it our interpretation of its consequences that is wrong?

 

[ZapperZ]

Modern physics is founded on relativity and quantum theory. Since nothing can travel faster than the speed of light, relativity forces a locality constraint on physical theory; spatially remote events cannot influence each other. On the other hand it has in recent years become evident that quantum mechanics violates this restriction. If we assume the validity of Bell’s theorem and the soundness of such experiments as Aspect’s; then no local theory is completely compatible with the predictions of quantum theory. A contradiction exists between the main foundations of physics.

Is this contradiction because relativity itself is fundamentally flawed or is it our interpretation of its consequences that is wrong?

In the EPR-type measurement, when you make a measurement of one entangled pair, can you tell me what is this "thing" that travels faster than the speed of light? Was there information being transfered? A superluminal signal? A "ghost particle" perhaps?

If you look carefully, the standard QM makes no mention of anything being transferred from one physical location to another. In other words, no signal moved from one of the entangled particle to its partner. Furthermore, as has been discussed many times on here (see several postings by vanesch and DrChinese), superluminal communication cannot be accomplished via such "quantum teleportation".

So in what way is Special Relativity violated here, and how come all those EPR-type experimental papers make zero mention of such an earth-shattering observation?

Zz.

 

[DaveC426913]

All that being said though, the answer is: yes our understanding of GR and QM is incomplete because the two theories don't mesh together. This not not because of the non-locality issue, it is because of the non-renormalizability issue.

 

[Meir Achuz]

You have to look into relativistic Q Field Theory to discuss this.
Neither E, P, nor R knew about QFT when they sent in EPR, and E was confused about QM. Bell knew and knows QFT, but related to EPR on their own two particle turf. In QFT, space-like surfaces are used, and there is no contradiction with SR.

 

[vanesch]

If we assume the validity of Bell’s theorem and the soundness of such experiments as Aspect’s; then no local theory is completely compatible with the predictions of quantum theory.

There are gazillion discussion threads about this on this very forum. I won't change this thread in yet another MWI thread, no fear If you want to know how this issue is treated there, I'd refer you to the following post:

https://www.physicsforums.com/showpost.php?p=851269&postcount=29

And if you want to discuss this further, I'd ask you to continue the discussion in that thread.

 

[DrChinese]

As ZapperZ correctly points out, there is no superluminal anything going on that we are aware of. Additionally, relativistic QM has been a great success as a theory for decades.

Of course, that does not *prove* absolutely that non-local forces/causes/signals are not possible; but it does help explain why non-local interpretations of QM are not more popular. Such interpretations are generally not Lorentz invariant.

 

[UglyDuckling]

In the EPR-type measurement, when you make a measurement of one entangled pair, can you tell me what is this "thing" that travels faster than the speed of light? Was there information being transfered? A superluminal signal? A "ghost particle" perhaps?

If you look carefully, the standard QM makes no mention of anything being transferred from one physical location to another. In other words, no signal moved from one of the entangled particle to its partner.

Zz.

I don’t think I referred to any physical object traveling faster than light and I agree with your statement about standard QM not describing anything being transferred from one physical location to another. Never-the-less what happens to one system is not independent of what happens to the other system, which is spatially separated from the former. The QM paradigm and now the experimental results indicate that Bell’s inequality is violated. In my opinion once this result is established the QM paradigm becomes something of an irrelevance. The new question is what is it in the nature of space-time that’s enables spatially remote particles to become entangled and does this property contradict special relativity?

Zz.

So in what way is Special Relativity violated here, and how come all those EPR-type experimental papers make zero mention of such an earth-shattering observation?

Zz.

I thought this was a major concern and had been the source of argument for the last seventy years?

UD

 

[ZapperZ]

I don’t think I referred to any physical object traveling faster than light and I agree with your statement about standard QM not describing anything being transferred from one physical location to another. Never-the-less what happens to one system is not independent of what happens to the other system, which is spatially separated from the former. The QM paradigm and now the experimental results indicate that Bell’s inequality is violated. In my opinion once this result is established the QM paradigm becomes something of an irrelevance. The new question is what is it in the nature of space-time that’s enables spatially remote particles to become entangled and does this property contradict special relativity?

But this is exactly my question to you. What exactly is being violated here and in what way? If SR says that information cannot "move" faster than c, where is this "information" moving in an EPR-type experiment? QM doesn't indicate that there's anything moving. As far as I know, no one has detected any information being transferred in between the two entangled objects. So how does this violates SR?

I thought this was a major concern and had been the source of argument for the last seventy years?

UD

Can you please point me those peer-reviewed papers that are voicing such concern? I'm looking here at several papers by Zeilinger, and at no point was there ever a claim of superluminal motion of anything that violates SR.

It would be nice if you can cite specific credible sources rather than basing it on heresay.

Zz.

 

[pervect]

There's no correlation of input - output in the EPR experimnets. There is a correlation between the outputs, but no correlation from input->output.

 

[Hurkyl]

The new question is what is it in the nature of space-time that’s enables spatially remote particles to become entangled and does this property contradict special relativity?

To be honest, I don't think that's the right question at all!

Classically, we impose a very specific mathematical condition on our probabilities. (statistical independence)

Before asking what is it about space-time that violates this very specific mathematical condition, we should first arm ourselves with a good reason why we should think it should hold in the first place!

 

[LnGrrrR]

As far as I know, no one has detected any information being transferred in between the two entangled objects. So how does this violates SR?

Zz.

I think what he meant was the question of how one part of the entangled pair knows to 'collapse' when the other part is measured.

 

[ZapperZ]

I think what he meant was the question of how one part of the entangled pair knows to 'collapse' when the other part is measured.

But that's a separate question than insisting that something is moving at speeds greater than c and thus, SR is violated.

We have MANY questions that are still not answered and still being studied. Why is there a property called "spin", and "charge", and why is so-and-so looks like that? However, just because we don't quite know yet of these answers, doesn't mean we have a free pass to make up our own extrapolation. At best, we can say "we don't know... YET!" We certainly can't say "Oh, obviously, this violates SR!"

Zz.

 

[LnGrrrR]

Zapperz,

Well very true, we certainly can't say something like "It violates SR!" without having a good basis (like experiments, for instance). It seemed to me reading it that he just misspoke, but heck, maybe I'm reading him wrong. :)

However, I think everyone makes up their own extrapolation, or at the least, chooses which explanation they think sounds correct.

 

[UglyDuckling]

But that's a separate question than insisting that something is moving at speeds greater than c and thus, SR is violated.

Zz.

I said was; what happens in on system (particle A) is not independent of what happens in the other system (particle B). But I did not imply that anything physical has moved instantaneously between the particles.

An act of measurement on say particle A, will cause an abrupt change in the super-positioned wave-functions of the particles. This will affect the outcome of any measurements made on particle B; whenever those measurements are taken! The change in the wave-function is super-luminal?

There is no evidence to suggest that the wave-function is physical in nature allowing its remit to fall outside the constraints of special relativity. This appears to remove the contradiction between quantum mechanics and relativity but without actually understanding what’s really going on.

I think what he meant was the question of how one part of the entangled pair knows to 'collapse' when the other part is measured.

Perhaps my question should have been

What is it in the nature of space-time that’s enables spatially remote particles to become entangled and how one part of the entangled pair knows to 'collapse' when the other part is measured?

The validity of relativity may become self evident when this question is answered? As may the Efficacy of QM.

UD

 

[ZapperZ]

I said was; what happens in on system (particle A) is not independent of what happens in the other system (particle B). But I did not imply that anything physical has moved instantaneously between the particles.

An act of measurement on say particle A, will cause an abrupt change in the super-positioned wave-functions of the particles. This will affect the outcome of any measurements made on particle B; whenever those measurements are taken! The change in the wave-function is super-luminal?

There is no evidence to suggest that the wave-function is physical in nature allowing its remit to fall outside the constraints of special relativity. This appears to remove the contradiction between quantum mechanics and relativity but without actually understanding what’s really going on.

EXACTLY!

And if you hold that opinion, it makes your original question at the beginning of this thread even more puzzling. You clearly claim a violation of some kind of SR postulates. Yet, even you admitted here that such a thing, at best, are really unknown.

Zz.

 

[jtbell]

Perhaps my question should have been

What is it in the nature of space-time that’s enables spatially remote particles to become entangled and how one part of the entangled pair knows to 'collapse' when the other part is measured?

The validity of relativity may become self evident when this question is answered? As may the Efficacy of QM.

Most physicists would say that your second question is a matter of experimental test, and that your first one is merely metaphysics, or a matter of interpretation of the theories of relativity and QM. Both theories accurately predict the results of experiments, to date, and to physicists, that is what determines their "validity" or "efficacy".

 

[DrChinese]

1. An act of measurement on say particle A, will cause an abrupt change in the super-positioned wave-functions of the particles.

2. This will affect the outcome of any measurements made on particle B; whenever those measurements are taken!

3. There is no evidence to suggest that the wave-function is physical...

1. I think this is the crux of your issue. This is a specific claim of oQM, and is not strictly prohibited by relativity.

2. This is definitely not correct. You cannot objectively demonstrate that the outcome at B is in any way dependent on a measurement at A. If you could, you could perform superluminal signalling. All you can actually demonstrate is the correlated results follow the HUP.

3. This is really part of the interpretation one adopts.

 

[reilly]

It's good to remember that QED is a relativistically invariant theory. This means that no perturbation to a system can generate information that goes faster than c. As a simple example, suppose a particle is localized with a wave function that is non-zero on a (small) interval. As the wave function evolves according to the Dirac equation, it is always zero at t along any ray if (distance along ray from initial "position" > ct. There's no way a measurement at A can influence a measurement at B until t after the A measurement, where ct is the retarded distance between A and C. If this is not true then something has to be added to QED, and, more generally,to QFT.
Regards,
Reilly Atkinson

 

[UglyDuckling]

2. This is definitely not correct. You cannot objectively demonstrate that the outcome at B is in any way dependent on a measurement at A. If you could, you could perform superluminal signalling. All you can actually demonstrate is the correlated results follow the HUP.

I think you’ve misinterpreted what I meant to say, sorry about the clumsiness of the expression.

The outcome of any measurements on particle B will depend on the state of its wave-function at the point of measurement. Since particle B is super-positioned with particle A, particle B’s wave-function cannot be independent of the state of particle A which will be dependent on any measurements made on particle A. However an observer of particle B has no way of knowing the outcome of any measurements made on particle A. His results will appear random. Therefore no super-luminal information passes between the observers.

It is only by the subsequent analysis of results that non-local correlations can be identified.

3. This is really part of the interpretation one adopts.

This dangerous territory! A physical interpretation puts us right back to violations of SR and contradictory pillars of modern physics.

I think there is communication between the particles but not along the super-luminal route A to B?

 

[LnGrrrR]

The outcome of any measurements on particle B will depend on the state of its wave-function at the point of measurement. Since particle B is super-positioned with particle A, particle B’s wave-function cannot be independent of the state of particle A which will be dependent on any measurements made on particle A. However an observer of particle B has no way of knowing the outcome of any measurements made on particle A. His results will appear random. Therefore no super-luminal information passes between the observers.

It is only by the subsequent analysis of results that non-local correlations can be identified.

Ok, here's the part I'm not getting. Non-locality isn't violated, because particle A can not 'send a message' to particle B faster than light. Correct? Or do they mean that we can't possibly know if non-locality is violated, because any correlation between us can not be faster than light?

If the second case, as a thought experiment, couldn't you separate two entangled photons a light year's distance away (so that a photon that 'collapsed' in one area would only 'collapse' after a year when the message reached the other photon at the speed of light), and put two observers at the ends, who reported to an observer in the middle? (For instance, Jane tests the waveform which collapses it and measures it, and Bob 'sees' the collapse, or tests it himself at the same time) They both then send the information to a 'middleman' who can confirm both measurements before the collapsed photon has time to reach the other. Why wouldn't this work?

(Again, I apologize for all 'newbie' type questions, but specific answers are very hard to find on the web unless you know just where to look.)

 

[UglyDuckling]

Ok, here's the part I'm not getting. Non-locality isn't violated, because particle A can not 'send a message' to particle B faster than light. Correct? Or do they mean that we can't possibly know if non-locality is violated, because any correlation between us can not be faster than light?

Firstly we are talking about the violation of locality not non-locality.

Locality refers to a view of the world where by the speed of light is the ultimate limit that interactions can take place between spatially separated locations. The violation of locality is therefore a violation of the special theory of relativity.

Non-locality refers to a view of the world where the behaviour of a particle is subject not only to what is going on in its immediate vicinity but can also be affected by distant events. These non-local influences affect the particle instantly and are therefore transmitted at infinite speed. What Einstein referred to as “spooky action at a distance”.

The combination of Bell’s inequality and Aspect’s Experiment shows the world does have a non-local flavour to it.

However this non-local flavour does not necessarily mean locality is violated.

Our commonsense encourages to think quantum entities are real and possesses unique properties, that is, they have location, mass, momentum etc. This view of the world is known as physical reality. The combination of “locality” and “physical reality” is called “local reality”. It is this view of the world that that is rejected by Bell’s inequality being exceeded in Aspect’s Experiment.

This demands at least one of the two elements of local reality must be rejected.

If reality is retained then locality is violated. If locality is retained then quantum entities can not possesses specific physical properties. These are merely what we observe and are the result of interactions of quantum entities with the measuring apparatus and ultimately with ourselves. Such observations tell us nothing about what the quantum objects are when they are not being observed. Our commonsense view of the physical reality of the quantum world cannot be substantiated by experimentation. The rejection of reality then opens the possibility of the universe being holistic with an infinite number of unobservable paths of connectivity that provide our measuring apparatus (which are also part of the infinite skein) with the information needed to give us measurable results.

Given the validity of Aspect’s experiment and others I would suggest that determining if the world violates locality or reality is one of the most fundamental questions facing physics today.

If the second case, as a thought experiment, couldn't you separate two entangled photons a light year's distance away (so that a photon that 'collapsed' in one area would only 'collapse' after a year when the message reached the other photon at the speed of light), and put two observers at the ends, who reported to an observer in the middle? (For instance, Jane tests the waveform which collapses it and measures it, and Bob 'sees' the collapse, or tests it himself at the same time) They both then send the information to a 'middleman' who can confirm both measurements before the collapsed photon has time to reach the other. Why wouldn't this work?

Sorry I’m afraid I do not understand your question.

I do not know what you mean by testing the waveform and what is a collapsed photon? What is the objective of the thought experiment? Is it to prove the possibility of super-luminal communication? I’ll answer as best I can.

Testing the waveform

The waveform is an abstract mathematical construct used in QM as part of the method for obtaining probable observed outcomes for a given experimental setup. QM does not give the wave-function any physical reality.

Collapsed Photon

The word collapse usual refers to the fact that once a measurement is taken the waveform for that particular experiment no longer applies to the quantum entity which initiated the observation. Since the waveform is spatially distributed through the experimental set-up and the actual measurement results from a point interaction the waveform is said to collapse.


By observing the polarisation of a single entangled pair of photons you are not likely to get any useful information. Regardless of whether the world is described by quantum mechanics quantum or local reality entities are still subject to the laws of conservation, both QM and RL will predict the same correlation. To tease out the information we need, we run the experiment with beams of photons passing through the filters at different angles. Local reality says altering the angle of detector A can have no influence on the outcome at detector B (set at a different angle) whist non-locality changing the angle at A will alter the probability of the photon passing through filter B.

In your experiment the detectors are separated a light year (Not A very practical choice, still it is a thought experiment!). Incidentally, the apparatus requires a coincidence detector so you can determine which photon pairs with which. This will be held by the middleman who incidentally is likely to be at the source of the paired photons.

The experiment is thus set away by the middleman, Six month later the photons will arrive at the detectors. Bob and Jane carryout the work with the detectors; altering the angles and measuring the counts and feeding back to the coincidence detectors.

On completion Bob will have a set of random results for each of his detector settings, Jane will have a similar are set of random results.

Six months later the results rattle into the coincidence detector and the middleman pairs the results and forms the results into columns for the various setting of the detectors. He runs through the statistical and calculates the correlations. He has his answer a year after he set the experiment away. He sends a message out to Jane and Bob who six month later also know the answer.

Sorry I don’t think your thought experiment allows us to communicate faster than light.



Epilogue

After the excitement of the day Middleman sat down in his comfy chair, lit his pipe, took a few puffs and began to reflect. He thought of President Bush all those millennia ago committing America to the LnGrrrR project, the countless dollars committed to it. He thought of the ancient laboratory that had been his home for the last year and a half. But most of all he thought of the generations of space farers who’d made the journey to the measuring points specified by LnGrrrR half a light year from Earth. These were the ancestors of Bob and Jane, their commitment to the project was total.

He picked up a piece of paper on which he written the message sent to Bob and Jane. He smiled, it was just a number.

43

Yes Bell’s inequality had been exceeded, suddenly he felt very close to Bob and Jane.

 

[UglyDuckling]

Most physicists would say that your second question is a matter of experimental test, and that your first one is merely metaphysics, or a matter of interpretation of the theories of relativity and QM. Both theories accurately predict the results of experiments, to date, and to physicists, that is what determines their "validity" or "efficacy".

The question should have read "the reason for the efficacy of QM?".

Experimentally QM is the most efficacious theory we have and as you say that's what counts. But it would be nice to understand why it is so effective!

 

[-Job-]

If i know that two particles in front of me must have opposite spin, even though we don't know exactly which spin each one has, then, if the distance between these two particles were to increase by an arbitrary amount over time, without their respective spins changing, then at any point, if i check the spin of one of the two particles, i know the value the other one must take.
This only seems weird given the strange assumptions of Quantum Mechanics, that each particle is in a superposition of states, thus not having a definite spin. With this assumption i would be led to believe that by checking the spin of one particle that i am giving the other particle a definite spin, and thus change that other particle at an arbitrarily great distance. There is an obvious "solution" to this strange conclusion which would be that particles are not in a superposition of states, not in a physical way, at least. When i check one particle's spin, collapsing the other particle to a state with a definite spin, is there any actualphysical change taking place in that particle?

 

[LnGrrrR]

UglyDuckling,

First off, thanks for the amusing story at the end, tongue-in-cheek as it is. ;)

Secondly, forgive me if I don't get all the terms right...I'm still very much a 'layman' when it comes to QM, and I sometimes mix them up.

But yes, you got the idea relatively right, with the middleman. The question I was asking though, is this...

Physicists who dont' believe locality is violated...is this because they feel that A) entagled particle A can't send information to particle B at instantaneous speed? or B) WE can't get the information faster than light?

Thanks for the help. :)

 

[Hurkyl]

When i check one particle's spin, collapsing the other particle to a state with a definite spin, is there any actualphysical change taking place in that particle?

Some interpretations interpret the collapse as being a physical change of state, so that there would be a physical change in the other particle.

Other interpretations interpret the collapse as merely being the way we mathematically compute conditional probabilities, so that there is no change in the other particle.

I think there are yet other possibilities as well.

 

[UglyDuckling]

It's good to remember that QED is a relativistically invariant theory. This means that no perturbation to a system can generate information that goes faster than c. As a simple example, suppose a particle is localized with a wave function that is non-zero on a (small) interval. As the wave function evolves according to the Dirac equation, it is always zero at t along any ray if (distance along ray from initial "position" > ct. There's no way a measurement at A can influence a measurement at B until t after the A measurement, where ct is the retarded distance between A and C. If this is not true then something has to be added to QED, and, more generally,to QFT.
Regards,
Reilly Atkinson

I think during this discussion we have tended support the validity of special relativity. There being nothing per se in quantum mechanics which suggests that anything physical is exceeding the speed of light.

On those grounds I would tend to agree with what you and Dirac are saying about QED and QFT i.e. no perturbation can generate information that travels faster than light. And no super-luminal signal can be transmitted from the point where the state of particle A is measured to where particle B is measured.

However, what you haven’t made clear is your view on Bell’s inequality and the result of Aspect’s experiment.

Are you casting doubt on the conclusion that the world is in some respects non-local?

Do you believe in some way physical locality and non-local influences can coexist?

Or do we have to alter our notion of locality to somehow explain how the information about what has happened in some spatially remote part of the world affects the outcome of a local measurement?

 

[UglyDuckling]

UglyDuckling,



Physicists who dont' believe locality is violated...is this because they feel that A) entagled particle A can't send information to particle B at instantaneous speed? or B) WE can't get the information faster than light?

Thanks for the help. :)

Each individual Physicist, who believes that locality is not violated probably has his own view on why this should be.

My personal view is that particle A does not have to send information to particle B in order for the results to be correlated and there is no superluminal activity in the universe.

Quantum mechanics enables us to calculate the probable outcomes from a particular experimental setup. In the case of Aspect’s experiment that includes the correlations between the outcomes. In other words quantum mechanics is saying that all the information regarding the outcomes and their correlations is in place prior to any measurements actual being made.

It seems quantum mechanics regards the whole experimental process as a single complex event and not as a series of causally related individual events. This view is substantiated by the apparent collapse of the wave-function and further substantiated by all the observations being separated from the source by proper intervals of zero magnitude.

 

[UglyDuckling]

If i know that two particles in front of me must have opposite spin, even though we don't know exactly which spin each one has, then, if the distance between these two particles were to increase by an arbitrary amount over time, without their respective spins changing, then at any point, if i check the spin of one of the two particles, i know the value the other one must take.
This only seems weird given the strange assumptions of Quantum Mechanics, that each particle is in a superposition of states, thus not having a definite spin. With this assumption i would be led to believe that by checking the spin of one particle that i am giving the other particle a definite spin, and thus change that other particle at an arbitrarily great distance. There is an obvious "solution" to this strange conclusion which would be that particles are not in a superposition of states, not in a physical way, at least. When i check one particle's spin, collapsing the other particle to a state with a definite spin, is there any actualphysical change taking place in that particle?

Hi Job

Check out Bell's theorem and Aspect's experiment for the means of differentiating between the two possibilities.

You may need a little patience!

ID

 

[UglyDuckling]

When i check one particle's spin, collapsing the other particle to a state with a definite spin, is there any actualphysical change taking place in that particle?

Quantum mechanics predicts the probable outcome of observations. What happens to the particle in flight, if indeed there is such a thing, is not considered.

 

[LnGrrrR]

Uglyduckling,

Couldn't they come up with an experiment to show the validity of locality though?

 

[RandallB]

Couldn't they come up with an experiment to show the validity of locality though?

That's exactly what John Bell was doing when he came up with his Bell's theorem.
Only thing is, so far it has shown that locality, and the action required of an ‘unknown variable’ of any kind requires impossible probabilities. (probability of an event being over 100% or less than 0%).
Indicating a "local" solution can only be non-sense until a different legitimate interpretation of those experiments can be found.

 

[LnGrrrR]

RandallB,

I've read about Bell's experiment, but it seems I can't quite get a grasp on exactly what it MEANS. Do you have perhaps a 'beginner's link' I could use? I've read the specifics of the experiment a few times now, but I can't seem to grasp why it invalidates 'hidden variables'.

Is it your belief that Bell's Inequality makes QM non-local (until a better interpretation is found)? Or am I misunderstanding you?

 

[DrChinese]

Do you have perhaps a 'beginner's link' I could use? I've read the specifics of the experiment a few times now, but I can't seem to grasp why it invalidates 'hidden variables'.

I maintain a page with a lotta links, from beginner on upward. Try:

http://drchinese.com/Bells_Theorem.htm

...and scroll down to the bottom for the links.

-DrC

 

[ttn]

If you look carefully, the standard QM makes no mention of anything being transferred from one physical location to another. In other words, no signal moved from one of the entangled particle to its partner. Furthermore, as has been discussed many times on here (see several postings by vanesch and DrChinese), superluminal communication cannot be accomplished via such "quantum teleportation".

Standard QM has, as part of its dynamics, the collapse postulate. This involves a change to the wave function at one location, due to a measurement that may have been made at a distant location. Standard QM also includes the so-called "completeness doctrine" according to which the wave function provides a complete description of the real physical state of the system. So the change in the wave function at a distant location must be interpreted as a physical change, yes? And, as the original poster suggested, this is in conflict with relativity's prohibition on superluminal causation.

This can all be made rigorous and precise by simply noting that orthodox QM violates a very plausible criterion expressing relativity's prohibition -- what I and others call "Bell Locality." (This is *not* the same thing as the Bell Inequality. Bell Locality is the "local causality" requirement that is one of the premises from which the Inequality is derived.)

You confuse things above by bringing up "signals" and "communication". Don't you think that relativity prohibits any superluminal action-at-a-distance *at all*, not merely such actions that can be harnessed by humans to build telephones (i.e., send "signals", i.e., "communicate")? What the heck does relativity care about humans? Or is your position that superluminal causation is perfectly consistent with relativity so long as the causation can't be harnessed by humans to build a telephone? Because it's just a fact that orthodox QM includes such a superluminal causal mechanism. So either you have to say that this is OK -- that this superluminal mechanism doesn't conflict with relativity because relativity only prohibits superluminal *communication*. Or you have to say, as the original poster suggested, that orthodox QM is inconsistent with relativity.

Saying "orthodox QM doesn't include any superluminal causal mechanisms" just doesn't fly. It just isn't true. (It would be true if you got rid of the collapse postulate and endorsed MWI, but this isn't orthodox QM... but that's crazy for different reasons!)

So in what way is Special Relativity violated here, and how come all those EPR-type experimental papers make zero mention of such an earth-shattering observation?

Because most experimentalists (and most, but not all, theorists) are confused about this stuff.

 

[ttn]

Of course, that does not *prove* absolutely that non-local forces/causes/signals are not possible; but it does help explain why non-local interpretations of QM are not more popular. Such interpretations are generally not Lorentz invariant.

Do you mean to imply that the collapse postulate of orthodox QM *is* Lorentz invariant? The theory's empirical predictions have a certain Lorentz invariance property, but the theory's *dynamics* (half of it, anyway -- the collapse postulate part) is badly Lorentz variant.

 

[RandallB]

Is it your belief that Bell's Inequality makes QM non-local (until a better interpretation is found)? Or am I misunderstanding you?

DrC links are excellent place to start.
QM doesn’t need to be “made” non-local it is because of HUP. That was Einstein’s big problem with it claiming it, QM, had to be incomplete (AKA wrong). Which lead to all those terse exchanges with Niels Bohr.
Being ‘non-local’ w/ HUP, there is no reason for to demand QM not break any logical probability rules.
But a classical local explanation must obey classical logic
– Ref: DrC pages on EPR-Bell.
And remember just because QM is “non-local” does not mean it requires observable FTL speed interactions, just HUP.

 

[quantumcarl]

I really like all this kind of talk:!) .

What I especially like is the fact that all the most experienced physicists have agreed that no one knows what's going on (yet).

Thanks to Dr.Chinese for the links page!

 

[UglyDuckling]

QM also includes the so-called "completeness doctrine" according to which the wave function provides a complete description of the real physical state of the system. So the change in the wave function at a distant location must be interpreted as a physical change, yes?

This assumes there was, in the first place, a real physical state to change. Although the Copenhagen Interpretation regards quantum mechanics as a complete theory “for which the fundamental physical and mathematical hypotheses are no longer susceptible of modification” it does not address the physical state of the unobserved quantum object.

It can only be complete in terms of the consistency of the regularity achieved between the state preparation process and the measurement process. And in this respect QM represents physics most successful paradigm.

But in the words of Bohr “there is no quantum world, 'There is only an abstract quantum physical description. It is wrong to think that the task of physics is to find out how nature is. Physics concerns what we can say about nature.

Still it would be nice to know what its all about?


We still seem to remain with no firm evidence for the violation of relativity, even though Bell’s inequality is exceeded in Aspect’s experiment?

 

[UglyDuckling]

Do you mean to imply that the collapse postulate of orthodox QM *is* Lorentz invariant? The theory's empirical predictions have a certain Lorentz invariance property, but the theory's *dynamics* (half of it, anyway -- the collapse postulate part) is badly Lorentz variant.

But do the laws of special relativity require an abstact mathematical contruct to be compliant?

 

[RandallB]

We still seem to remain with no firm evidence for the violation of relativity, even though Bell’s inequality is exceeded in Aspect’s experiment?

Bell’s inequality & Aspect’s experiment have nothing to do with a violation of relativity. It deals the issue of "local" vs the probablities used in QM. (ref: links above)

SR can be viewed in a "local" world, but QM does use SR in it's "non-local view.
GR expects a "non-local" world, as least for most; I don't see how it could be viewed locally.

 

[ttn]

This assumes there was, in the first place, a real physical state to change. Although the Copenhagen Interpretation regards quantum mechanics as a complete theory “for which the fundamental physical and mathematical hypotheses are no longer susceptible of modification” it does not address the physical state of the unobserved quantum object.

So, according to this, the Copenhagen interpretation provides no ontology for the world at all? It's a mistake to read it as describing literal "happenings" out in the world? I don't agree with this reading of the Copenhagen interpretation, but it is certainly a possible one. The reason I don't agree is that it seems to render the completeness doctrine (which I see as the *core* of Bohr's views, basically defined by his antagonism toward Einstein's ideas for an ensemble interpretation) meaningless. The completeness doctrine says that the wave function alone (without any additional so-called "hidden" variables) provides a complete description. Well... a complete description of what? If it's not supposed to be a description of anything, how can that description be meaningfully claimed to be complete?

But this is all beside the point. There are two possible interpretations. You can take the wave function as a complete description of physical states, or you can say that the wave function isn't a literal description of anything physical and the whole QM formalism is just a black-box recipe for calculating measurement outcomes. Let's just call those "interpretation 1" and "interpretation 2" respectively.

I think it's incontestable that "interpretation 1" is non-local. The collapse postulate describes instantaneous changes to distant parts of the wave function. "Instantaneous" means: lots faster than the speed of light. OK?

As I understand your point, it is that "interpretation 2" is *not* non-local. That's true. But it isn't local, either. It's not local or non-local. Why? Because the whole idea of "locality" is an issue of how fast causal effects (as described by some theory) propagate. And if "interpretation 2" doesn't *say* anything about any causal effects (because it doesn't provide an ontology at all), then it is just meaningless to apply the local/nonlocal terminology to it.

What the EPR argument combined with Bell's Theorem prove is this: no local theory can account for the empirical results. "Interpretation 1" is an example of this, as is (say) Bohm's theory (which is quite explicitly nonlocal, just like "interpretation 1"). "Interpretation 2" on the other hand is *irrelevant* to this claim. It's not a counterexample at all, because "interpretation 2" isn't a theory in the relevant sense. It doesn't *say* anything about what's going on physically.

So the claim is this: there's no way to tell a physical story that is local (i.e., "relativistically causal" to use Bell's terminology) by which you can understand the outcomes of the relevant experiment. Surely this means that nature (the physical world out there) is non-local -- it's just like if, somehow, you had a theorem that proved "there is no way to give a physical account of the data which doesn't involve the existence of stars" then you'd surely conclude: "so nature includes stars." Same deal with EPR/Bell, but replace "stars" with "relativity-violating, superluminal causal influences."

But in the words of Bohr “there is no quantum world, 'There is only an abstract quantum physical description. It is wrong to think that the task of physics is to find out how nature is. Physics concerns what we can say about nature.

Actually, Bohr didn't say this. It was someone else's attempt to summarize Bohr's thought. But that's neither here nor there; just a too-little-known historical tidbit.

Still it would be nice to know what its all about?

I certainly agree about that. I just think it's important to stress that, even though we don't yet know exactly "what it's all about" -- we *do* know (already) that whatever that ultimate physical theory looks like, it has to be nonlocal. We don't know everything about nature, but we do know at least one thing: it contains causal influences that are superluminal. And I think this is a serious problem for relativity. (Bell thought so too.)

We still seem to remain with no firm evidence for the violation of relativity, even though Bell’s inequality is exceeded in Aspect’s experiment?

I don't agree. I think the evidence is extremely firm.

 

[vanesch]

So, according to this, the Copenhagen interpretation provides no ontology for the world at all? It's a mistake to read it as describing literal "happenings" out in the world? I don't agree with this reading of the Copenhagen interpretation, but it is certainly a possible one.

Well, honestly, this is how I always understood the Copenhagen interpretation: there is a classical ontology for macroscopic objects, and there is an UNDESCRIBABLE ontology (whatever that may mean !) ontology for microscopic objects, but whose INFLUENCE on the classical macroscopic objects can be CALCULATED using the QM formalism (which is NOT a representation of any ontology, just a calculational algorithm).

I think that you are more thinking about the von Neumann view, where it is less clear in what way there's an ontology associated with the wavefunction (if you read von Neuman, you have the impression he DOES take it somehow for real) and where he clearly states that collapse happens *somewhere* between the microsystem and the conscious interpretation of the measurement, but that he states that, if it happens late enough in the process, that all places are good enough and are empirically indistinguishable. (in that sense, you could think of von Neuman's vision as a precursor of a many minds view!)

The reason I don't agree is that it seems to render the completeness doctrine (which I see as the *core* of Bohr's views, basically defined by his antagonism toward Einstein's ideas for an ensemble interpretation) meaningless.

I agree with you, but apparently, the idea was that quantum ontology is FUNDAMENTALLY UNDESCRIBABLE and that all you'll ever know of it is the algorithm that calculates the influences on classical objects ; an algorithm we call a quantum-mechanical calculation. No wonder Einstein went beserk over that concept This was a kind of positivist viewpoint, but only limited to the microscopic world.

But this is all beside the point. There are two possible interpretations. You can take the wave function as a complete description of physical states, or you can say that the wave function isn't a literal description of anything physical and the whole QM formalism is just a black-box recipe for calculating measurement outcomes. Let's just call those "interpretation 1" and "interpretation 2" respectively.

Right. I think Copenhagen was interpretation 2.

I think it's incontestable that "interpretation 1" is non-local. The collapse postulate describes instantaneous changes to distant parts of the wave function. "Instantaneous" means: lots faster than the speed of light. OK?

Unless, of course, the wavefunction never collapses and you experience only a part of it... I know, I know... I can't help bringing up MWI each time! Each time it is said that it doesn't exist !

What the EPR argument combined with Bell's Theorem prove is this: no local theory can account for the empirical results.

To keep this politically correct, you should add: "under the assumption that empirical results are ontologically real and unique." If you drop this assumption, MWI is in the run, and that IS a counter example.

So the claim is this: there's no way to tell a physical story that is local (i.e., "relativistically causal" to use Bell's terminology) by which you can understand the outcomes of the relevant experiment.

On the condition that these outcomes are ontologically real and unique...

but we do know at least one thing: it contains causal influences that are superluminal. And I think this is a serious problem for relativity. (Bell thought so too.)

OR it contains parallel worlds we are not aware of, in which case relativity has no problem. So the choice is: accept parallel worlds, or kick out relativity... (or, consider that a local realistic theory is still possible, and that all Bell tests have been circular ; or consider that the hypothesis of an ontology corresponding to a mathematical structure (the reductionist view) is not correct and it is all "just computing" or "emerging")

 

[ttn]

I think that you are more thinking about the von Neumann view, where it is less clear in what way there's an ontology associated with the wavefunction (if you read von Neuman, you have the impression he DOES take it somehow for real) and where he clearly states that collapse happens *somewhere* between the microsystem and the conscious interpretation of the measurement, but that he states that, if it happens late enough in the process, that all places are good enough and are empirically indistinguishable.

Yes, you're right about all this. I probably should have talked about the "orthodox" view rather than the Copenhagen view. The orthodox interpretation (which most people, including apparently sometimes myself, don't bother to distinguish from Copenhagen) is essentially the von Neumann view which, as you say, really does have two distinct dynamical processes ("Process 1" and "Process 2" or whatever), one of which happens when there's no measurement and the other of which happens when there is a measurement.

I think it's arguable whether Bohr agreed with this or not. Frankly, I don't think there's any answer -- Bohr was an obscurantist, not a clear thinker, and, simply put, he wasn't consistent on this very-much black-and-white question. You can find passages that are most naturally interpreted as implying what I called "interpretation 1" before -- and same for "interpretation 2".

So at the end of the day, my attitude is: who cares what Bohr thought. There are two possible consistent views, and neither one of them is any kind of counterexample to the deep conflict that Bell first perceived between quantum physics and relativity.

(in that sense, you could think of von Neuman's vision as a precursor of a many minds view!)

? As you said above, he makes the point that it is very difficult to empirically distinguish versions of the theory which place the "cut" (between "process 1" and "process 2") in different places. FAPP, you can put the cut pretty much anywhere you want and still predict the same things. But i don't see what this point has to do with the many minds view. Maybe you were just joking or something?

Right. I think Copenhagen was interpretation 2.

I lean the other way, but perhaps you're right. But really... who cares?

Unless, of course, the wavefunction never collapses and you experience only a part of it... I know, I know... I can't help bringing up MWI each time! Each time it is said that it doesn't exist !

Is it *really* "said that it doesn't exist" each time? Or perhaps you are just deluded about that because your mind only has access to one very narrow branch of the truth?

My point is: even to discuss anything, you've got to take some things as given. We normally take as given (at least) stuff like the real existence of the macroscopic physical objects around us (e.g., as you did when you accepted the real existence of certain letters spelled out on your computer screen just now). ...which is *all* I'm doing when I neglect to mention your beloved MWI "counterexample" to the claim that nature isn't local. Let me put it this way: if you were right to accuse me of ignoring your counterexample, you've accidently thereby conceded the argument to me -- for your belief about what I wrote is actually premised on the very principle grounding my not taking your counterexample seriously!

To keep this politically correct, you should add: "under the assumption that empirical results are ontologically real and unique." If you drop this assumption, MWI is in the run, and that IS a counter example.

Sure, something like that, though I don't like the precise way you phrased it. How about "under the assumption that our normal everyday perception of the familiar macroscopic external world (of such things as tables, books, and instrument-pointers) isn't delusional."

The point I will keep coming back to forever is this: *without* that assumption, there can be no such thing as science, period. Science can't exist without (among other things) the idea of *evidence* -- if there can be no evidence for a proposition, then there's no way to distinguish the true from the false, and no way to do science (or think generally). And if literally *seeing* something in front of your face doesn't count as evidence, nothing ever will. We *have* to accept the veracity of direct perception, or else (leaving aside crazy mystics) we have no access to reality at all, and there's no more point trying to do physics (or anything else).

OR it contains parallel worlds we are not aware of, in which case relativity has no problem. So the choice is: accept parallel worlds, or kick out relativity... (or, consider that a local realistic theory is still possible, and that all Bell tests have been circular ; or consider that the hypothesis of an ontology corresponding to a mathematical structure (the reductionist view) is not correct and it is all "just computing" or "emerging")

Fine. But I think the people (like, for example, Zapper Z) who were denying that there is any conflict between QM and SR had something other than "parallel universes" in mind. I'm guessing the idea was supposed to be that, even on the assumption that there is only one world (the one we perceive), it's possible for a local theory to account for the empirical results. Patrick, I believe you and I agree that these people would be wrong. Yes? Anyway, I hope some of these dissenters will clarify the basis for their claims so Patrick and I can explain why they're erroneous (or maybe they'll confess that what they had in mind all along was "parallel worlds").

 

[UglyDuckling]

Bell’s inequality & Aspect’s experiment have nothing to do with a violation of relativity. It deals the issue of "local" vs the probablities used in QM. (ref: links above)

QUOTE]
In order to supply a result that distinguishes between hidden variables and quantum mechanics it was necessary to spatially separate the detectors so the interval between measurements of paired photons was space-like. The reason for this is that in the case of the hidden variables there is no way to predict how the results on measured paired photons would correlate. Thus by chance the hidden variable interactions could have provided the same correlations as that predicted by quantum mechanics as long as there was some means of communicating between the detection events.

By isolating the detection events in space-time an upper limit is definitely established for the level of correlation that can be achieved by the hidden variables model.

As the results appear to correspond with those predicted by quantum mechanics a flavour of the super-luminal must be added to our view of the world.

 

[UglyDuckling]

Bell’s inequality & Aspect’s experiment have nothing to do with a violation of relativity. It deals the issue of "local" vs the probablities used in QM. (ref: links above)

QUOTE]
In order to supply a result that distinguishes between hidden variables and quantum mechanics it was necessary to spatially separate the detectors so the interval between measurements of paired photons was space-like. The reason for this is that in the case of the hidden variables there is no way to predict how the results on measured paired photons would correlate. Thus by chance the hidden variable interactions could have provided the same correlations as that predicted by quantum mechanics as long as there was some means of communicating between the detection events.

By isolating the detection events in space-time an upper limit is definitely established for the level of correlation that can be achieved by the hidden variables model.

As the results appear to correspond with those predicted by quantum mechanics a flavour of the super-luminal must be added to our view of the world.

 

[ttn]

In order to supply a result that distinguishes between hidden variables and quantum mechanics it was necessary to spatially separate the detectors...

You have misunderstood Bell's Theorem. The point is not to "distinguish between hidden variables and quantum mechanics." The point is to distinguish between local theories and non-local theories.

The argument is really quite simple. Orthodox QM is a non-local theory. Einstein pointed this out long ago, and noted that (perhaps) a local theory could be constructed by supplementing OQM with "hidden variables." Bell's Theorem then showed that *even* by adding hidden variables, you can't have a local theory that agrees with experiment. So locality cannot be saved, period. The question of the existence of hidden variables simply isn't touched here. Today, there exist both hidden variable theories and non-hidden-variable theories that are empirically viable. What's important and interesting, though, is that both kinds of theories are non-local. That's the lesson of Bell: in order to agree with experiment, a theory has to be non-local, period.

(Assuming we don't go off into parallel-universes MWI la-la-land... )

The reason for this is that in the case of the hidden variables there is no way to predict how the results on measured paired photons would correlate. Thus by chance the hidden variable interactions could have provided the same correlations as that predicted by quantum mechanics as long as there was some means of communicating between the detection events.

By isolating the detection events in space-time an upper limit is definitely established for the level of correlation that can be achieved by the hidden variables model.

If you think about it, this claim (which is a disturbingly widespread misconception) is really quite stupid and obviously wrong. The whole idea of "hidden variables" is to *supplement* the wave-function-description with some other variables, some additional structure. Bohmian Mechanics is of course the nicest example here, where the wave function is supplemented by actual particle positions.

Anyway, here's my point: you have some theory (orthodox QM) which people erroneously think is local; then you're going to *add* some structure to the theory; and suddenly (people erroneously think) the theory has to be made nonlocal in order to make the same predictions the earlier theory was able to make locally without the hidden variables? That's preposterous on its face. Just as an obvious counterexample, you could add deliberately pointless hidden variables -- variables that *did nothing* dynamically and didn't affect in any way the outcomes predicted already by orthodox QM. And yet people think that adding such variables will render a still-local theory in disagreement with experiment all of the sudden? *How* could it possibly do that?

Think of it this way: the basic issue here in all these Bell inequality tests is how strongly correlated outcomes in two places can be. Orthodox QM says they should be correlated a certain way. Now isn't it just obvious that if you *add* some additional variables to orthodox QM (i.e., write down a hidden variable theory) you'll maybe be able to get *stronger* correlations? How could adding structure to the theory somehow force the correlations to be weaker? It just makes no sense. Yet this is implicit in the common (mistaken) view that Bell's theorem is all about "QM vs hidden variables." It isn't, and what these people are missing is the crucial fact that orthodox QM is already non-local. So when you compare the correlations predicted by orthodox QM to those of a local hidden variable theory, you are comparing a non-local theory to a local theory. And then it's no surprise that the non-local theory can predict stronger correlations between distant measurements! But then it's outrageously stupid to conclude that hidden variables are thus refuted.

 

[RandallB]

TNN on you your comment:

But then it's outrageously stupid to conclude that hidden variables are thus refuted.

TNN I don’t think that’s fair.
I love going to Conferences and presentations where I have the chance to more than meet some truly important people in physics. There they often will take the time to actually discuss things with ordinary lay folk like myself. It helps to have rational questions and points. Kip S. Thorne, Wendy Freedman, Sylvester James Gates Jr., Wolfgang Ketterle and even noble-laureate Leon M. Lederman of Fermilab were all so kind. (Although, I did meet Brian Greene the same way, celebrity of TV & Books prevented him entering any real discussions).
My topic is always my same favorite, Einstein’s unknown variable, and are they satisfied it has been refuted. Which once they could see how well I understood the issue they always were careful and considered in their replies. And all the same on two points.
First it is after all near impossible to prove a negative and none claimed that Einstein has been definitely proven wrong on the unknown variable.
But, second that they were (and most felt ‘shouldn’t we all be’) satisfied that a unknown variable was not viable based on EPR-Bell etc.

Now I’ll grant you, they may feel stupid if you or someone can prove as a positive fact that unknown variables can work to resolve EPR-Bell type issues.
But even if that were to occur.
I think it very inappropriate to describe, those satisfied with concluding hidden variables as refuted at this time in physics, as being “outrageously stupid”.
I certainly will not even if they turn out to be wrong, that would just mean they were wrong not stupid.

 

[ttn]

First it is after all near impossible to prove a negative and none claimed that Einstein has been definitely proven wrong on the unknown variable.
But, second that they were (and most felt ‘shouldn’t we all be’) satisfied that a unknown variable was not viable based on EPR-Bell etc.

Yes, I'm very aware that lots of highly-regarded physicists have this view. Nevertheless, it's a wrong (and yes, stupid) view, for the reasons I explained in the previous post.

Now I’ll grant you, they may feel stupid if you or someone can prove as a positive fact that unknown variables can work to resolve EPR-Bell type issues.

I don't know exactly what you mean by "resolve". What's to resolve? What's clear is that there exists a hidden variable theory that makes all the same predictions as orthodox QM. So *clearly* hidden variable theories can't be regarded as non-viable.

But even if that were to occur.
I think it very inappropriate to describe, those satisfied with concluding hidden variables as refuted at this time in physics, as being “outrageously stupid”.
I certainly will not even if they turn out to be wrong, that would just mean they were wrong not stupid.

I don't think all those *people* are stupid. But non-stupid people can hold the occasional stupid view on some isolated issue (usually because they haven't thought that issue through as carefully as they do for other issues). And I stand by what I said before: it's stupid to think that *adding* extra structure to a theory will *reduce* the maximum strength of correlations.

 

[RandallB]

What's to resolve? What's clear is that there exists a hidden variable theory that makes all the same predictions as orthodox QM.

Sorry I didn't see that one in the headlines, Why is the evening news so slow on this.
What "hidden variable theory" does this.
And please don't say BM that would be stupid.
BM is a non-local theroy that just uses a non-local guide wave as a proxy for a hidden variable - that's not the same as a hidden variable.

 

[LnGrrrR]

Ok, after reading some of DocC's articles, I feel a bit better...but can anyone explain to me why hidden variables COULDN'T back up a local theory because of Bell's Experiment? Wasn't that the whole point of hidden variables? And it is impossible to test locality? The idea that 'standard QM' must imply non-locality...how is that?

PLEASE dumb it down a lot for me...as my brain refuses to work with me to jump over this hurdle. If you wish, feel free to message me privately.