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Validity of Relativity

  1. Mar 21, 2006 #1
    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?
     
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  3. Mar 21, 2006 #2

    ZapperZ

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    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 transfered 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.
     
  4. Mar 21, 2006 #3

    DaveC426913

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    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.
     
  5. Mar 21, 2006 #4

    Meir Achuz

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    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.
     
  6. Mar 21, 2006 #5

    vanesch

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    There are gazillion discussion threads about this on this very forum. I won't change this thread in yet another MWI thread, no fear :smile: 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.
     
  7. Mar 21, 2006 #6

    DrChinese

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    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.
     
  8. Mar 21, 2006 #7

    I don’t think I referred to any physical object travelling 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.

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

    UD
     
    Last edited by a moderator: Mar 21, 2006
  9. Mar 21, 2006 #8

    ZapperZ

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    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 transfered in between the two entangled objects. So how does this violates SR?

    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.
     
  10. Mar 21, 2006 #9

    pervect

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    There's no correlation of input - output in the EPR experimnets. There is a correlation between the outputs, but no correlation from input->output.
     
  11. Mar 21, 2006 #10

    Hurkyl

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    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!
     
  12. Mar 21, 2006 #11
    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.
     
  13. Mar 21, 2006 #12

    ZapperZ

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    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.
     
  14. Mar 21, 2006 #13
    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.
     
  15. Mar 22, 2006 #14
    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.

    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
     
  16. Mar 22, 2006 #15

    ZapperZ

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    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.
     
  17. Mar 22, 2006 #16

    jtbell

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    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".
     
  18. Mar 22, 2006 #17

    DrChinese

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    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.
     
  19. Mar 22, 2006 #18

    reilly

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    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
     
  20. Mar 23, 2006 #19
    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.


    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?
     
  21. Mar 23, 2006 #20
    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.)
     
  22. Mar 25, 2006 #21
    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 possess 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 possess 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.


    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.
     
  23. Mar 25, 2006 #22
    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 thats what counts. But it would be nice to understand why it is so effective!
     
  24. Mar 25, 2006 #23

    -Job-

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    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?
     
  25. Mar 25, 2006 #24
    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. :)
     
  26. Mar 26, 2006 #25

    Hurkyl

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    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.
     
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