Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Is action at a distance possible as envisaged by the EPR Paradox.

  1. Apr 15, 2010 #1
    Is action at a distance possible as envisaged by the EPR Paradox?
  2. jcsd
  3. Apr 15, 2010 #2


    User Avatar
    Science Advisor

    It's not just possible, it has been experimentally demonstrated. Read up on some of the entanglement threads that have been running on here for a while. Or you could just visit Dr Chinese's website ...
  4. Apr 15, 2010 #3


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    I'll just add that it's not quite what you might expect "action at a distance" to be. You can't use entanglement to remote control your TV, or send any kind of messages.
  5. Apr 15, 2010 #4


    User Avatar
    Science Advisor
    Gold Member

    As Fredrik says, the kind of action at a distance entailed by entanglement is not one which allows signaling or causation. It will always be random, a result of an observation which leads to wave function collapse.
  6. Apr 15, 2010 #5
    No. There's no action at a distance (or paradox) involved in EPR -- just some deductions about one particle based on the experimental preparation and detection of the other particle.
  7. Apr 15, 2010 #6
    Such a thing would be possible if
    1-a non-local hidden variable theory is true.
    2-hidden variables can be locally manipulated.

    Then Alice just has to change the hidden variables on her side and give them the value that causes result +. Bob thus receives the bit -.
  8. Apr 15, 2010 #7
    Sure you could... you just need Classical means to make any sense of it, and then you're limited to 'c' at max. I use IR to control my TV, and that is certainly not FTL. I know you, fredrik understand this, but for the sake of clarity...
  9. Apr 15, 2010 #8


    User Avatar
    Gold Member

    But... even if we cannot use entanglement to send usable information FTL, the particles must clearly be 'communicating' in some way to present the opposite random property, right? And Bell showed there are no local hidden variables involved... or did I miss something?

    MWI is the only 'way out' of this is, as I understand...?
    Last edited: Apr 15, 2010
  10. Apr 15, 2010 #9
    Well, I don't espouse or believe in it, but there is the Bohmian view (Demystifier and Zenith are the dBB'ers here), there is the Transactional Interpretation... and there are the rest of us who just have no idea, don't want to be purely Instrumentalist pretty much just deal with the cognitive dissonance.

    DrChinese is the master of Bell I'd say, and to essentially quote him, Bell shows that no LHV theory can match the prediction of QM. de Broglie-Bohm sidesteps this with NON-Local Hidden Variables, MWI you know about... etc. Bell doesn't really say anything for or against QM, just sets a standard of predictiablity and a test for it.

    What that means... is anyone's guess. I don't think you've missed anything. There is still some kind of "Spukhafte Fernwirking"... or a Hidden Variable.
  11. Apr 16, 2010 #10


    User Avatar
    Gold Member

    Only if you assume that whole sample in experiments can present the opposite random property.
    However if you do not assume that then you can not deduce anything radical out of Bell inequality type experiments.
    That is so called unfair sampling possibility.
  12. Apr 16, 2010 #11


    User Avatar
    Gold Member

    Thanks for the reply Frame Dragger. Very wise, and I think I wanna join your spukhafte-gang of "just-have-no-idea" for the moment... There seems to be more to know about "Spukhafte Fernwirkung"/NLHV... (funny German word :smile:)

    But, if we accept the cognitive dissonance completely, we might never be able to reach "the grapes", that obviously are there. :wink:


    Mitch Hedberg: "Sometimes in the middle of the night, I think of something that's funny, then I go get a pen and I write it down. Or if the pen's too far away, I have to convince myself that what I thought of ain't funny."
    Last edited: Apr 16, 2010
  13. Apr 16, 2010 #12


    User Avatar
    Gold Member

    Okay, that’s a pretty harsh Aspect on the Bells ringing at Einstein’s funeral...

    Are you saying that John Bell was totally wrong, and Alain Aspect was totally stupid spending all this time & money in experimentally verifying that Bell's inequalities are physically violated??

    Isn’t that a little bit too unfair...?
  14. Apr 16, 2010 #13
    Ahhh the grapes... you're right. Then again, I find curiosity drives me, even if I'm unable to buy into a particular Interpretation. It is a luxury I get by not having to produce my own theories or hypotheses, not being in the field of physics. I realize that isn't an option for everyone, and of course many really believe in their view.

    @Zonde: Isn't that a bit of an ongoing debate with you and DrChinese and others (on PF), that is as yet, unsettled here, never mind the world at large?
  15. Apr 16, 2010 #14


    User Avatar
    Gold Member

    Okay, now I definitely wanna join your "spukhafte-gang", where can I buy the member card!? :biggrin:
  16. Apr 16, 2010 #15
    You just did? :wink: To be fair, I think we both join a gang now, not mine... Spukhafte was Einstein... but he didn't believe in QM's predictions. I think most people feel this way to some degree, but I could be wrong...
  17. Apr 16, 2010 #16


    User Avatar
    Gold Member

    Very true, and if I’m not totally wrong, John Bell initially hoped that Einstein’s view was accurate, but had to face the facts his theorem finally showed him. To me, that’s what science is all about; to ask questions – and accept the proven answers.

    I’m not particularly fond of solving one strange "spukhafte", by introducing another amazingly more stranger "spukhafte" – without physical proof... (EPR + MWI = no problem). But then again, this might actually turn out to be the actual solution. We just don’t know yet.

    The future is interesting and not 100% clear!

    PS: If we know that QM & GR is not 100% compatible, then one or both must be (slightly) wrong.
  18. Apr 16, 2010 #17
    No. The properties, motion(s) of the entangled particles that are being jointly analyzed are either identical or closely related in some way due to past interaction(s), a common source, or they're parts of an encompassing system.

    So there really doesn't need to be any communication or causal link of any sort between the separated particles in order to understand why joint detections of them are correlated wrt some global measurement parameter(s).

    That's right, but that statement needs some qualification. In the contexts where joint detection attributes are correlated to global measurement parameters the hidden variable that would, if it were known, allow more precise prediction of individual results is simply not relevant.

    What's relevant in the joint context is the relationship between the two separated particles.

    The oft repeated statement that QM is incompatible with local hidden variables isn't quite true. QM is compatible with lhv formulations of certain setups, such as wrt the individual arms of optical Bell tests. QM is incompatible with lhv formulations of setups where the lhv is irrelevant wrt determining the results, such as wrt the correlations of joint results with some global measurement parameter.

    'Way out' of what -- nonlocality? What nonlocality? If you think that it can be inferred via experimental violations of Bell inequalities or via GHZ inconsistencies, then consider that the physical meaning attributed to BIs and GHZ manipulations associated with Bell tests is rather questionable.

    You might start a separate thread exploring exactly how BIs are derived and exactly how the limits imposed by them are connected with the reality of the experimental setups -- and also exactly how the detection attributes (+1s and -1s) involved in GHZ manipulations are connected to EPR elements of reality.

    It isn't at all a foregone conclusion, nor has it been definitively demonstrated, that experimental violations of BIs or GHZ inconsistencies have the physical meaning that's been attributed to them by some -- that is, quantum entanglement should not be taken as being synonymous with nonlocality or ftl propagations.
  19. Apr 16, 2010 #18


    User Avatar
    Gold Member

    ThomasT, pardon my French, but this reasoning doesn’t convince me in any way. I may be a layman, but I’m not stupid.
    1) Quantum entanglement is a quantum mechanical state of a system of two or more objects.

    2) It is generally accepted that there can be no interpretations of quantum mechanics which use local hidden variables.​

    I’m no expert; I just use 'common sense' to make up my mind of what is plausible (for the moment), and if I add Bell's theorem:
    "No physical theory of local hidden variables can ever reproduce all of the predictions of quantum mechanics."

    To the fact that a lot of serious Bell test experiments have all indicated that Bell's theorem is accurate, and that not one has pointed clearly in the other direction; it’s a no-brainer to make a decision of what’s plausible.

    Now, one could argue there are no "perfect experiments", and this doesn’t prove anything, and so on and so forth...
    First objection: Let’s quit science – we can’t prove anything anymore – there are no perfect experiments.

    Second objection: Yes, there are loopholes in Bell test experiments, as in any experiment. But there are different kind of loopholes, and different kind of performed experiments. The sum of performed experiments, all pointing in the same direction, is IMO much more convincing than current theoretical oppositions.​
    Time will definitely tell – and I hope I’m free to have my own view in the meantime.

    You are free to have yours.

    As a dessert, it would be interesting if some of the pros could comment on this:
  20. Apr 16, 2010 #19
    I would add, Thomas, that a thread such as you describe exists... you were in it, and I believe you and DrChinese et al couldn't come to an agreement. If we're going to continue that discussion, lets, but starting from square one seems silly.
  21. Apr 17, 2010 #20
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook