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

Entanglement spooky action at a distance

  1. Jul 22, 2008 #1
    Entanglement "spooky action at a distance"

    Why can't we think of entanglement as simply committing (without knowledge) to a random outcome, instead of "spooky action at a distance"?
     
    Last edited by a moderator: Feb 4, 2013
  2. jcsd
  3. Jul 22, 2008 #2

    DrChinese

    User Avatar
    Science Advisor
    Gold Member

    Re: Entanglement


    "Spooky Action at a Distance" (nonlocality) is not the only alternative consistent with the facts. But it is probably the more popular one.

    The answer to your question is that Bell's Theorem demonstrates that there is a mathematical relationship with the outcomes of measurements of entangled particles that is inconsistent with the idea that they are independent and random. Of course, the actual outcomes themselves are random when looked at separately. But when the outcome streams are correlated, the pattern becomes clear.

    Specifically: the correlation of the outcomes follows the formula C=cos^2(theta) where theta is the relative angle between the measurement apparati. On the other hand, the formula associated with your hypothesis is C=.25+(cos^2(theta)/2). Experiments support the first formula - the one which is derived from Quantum Mechanics - and unambiguously reject the second.
     
  4. Jul 22, 2008 #3
    Re: Entanglement

    Because in the global experimental design(s) characteristic of EPR-Bell tests the pairing of individual results (A,B) isn't done randomly. There's a very narrow (nanosecond scale) window within which the coincidence circuitry operates to produce pairs. The effect of such synchronization is that for an individual detection in, say, A's datastream, there should be, at most, one candidate (either a detection or a nondetection attribute) for pairing in B's datastream.

    This interdependency between paired detection events at A and B is a function of the experimental designs necessary to produce EPR-Bell type entanglements and has, as far as I can tell, nothing to do with instantaneous or FTL transmissions.

    If FTL transmissions really aren't involved, then any symbolic locality condition becomes simply a statistical independence condition, and this is just a byproduct of the experimental design.

    For this reason, and also simply because there's no physical evidence for FTL transmissions, the best assumption is that FTL transmissions aren't involved in the production of quantum entanglement.
    As Dr. Chinese has pointed out, one doesn't have to attribute the observed correlation between the angular difference of the spatially separated polarizer settings and the rate of coincidental detection to "spooky action at a distance" -- or even to FTL transmissions.

    For example, if, for any given coincidence interval, it's assumed that the polarizers at A and B interacted with the same incident disturbance, then it isn't difficult to understand the cos^2 angular dependence.
     
  5. Jul 23, 2008 #4
    Re: Entanglement

    Hi! I'm new here and interested in physics, as all of you are :) I'm speaking from a point of view of an amateur (hoping to change this in the future :)) sp I hope you won't lough too much at my contributions :D

    The quantum entanglement is a process, bounding two or more particles together through space and time (if I've understood the definition correctly) and every change it the quantum state of the first particle leads simultaneously to the same change in the paired one, regardless of space and time.

    Now I was wondering, if this means transmitting information (by quantum states) with superlight velocity? Can this one day be used for transmitting information more efficiently (actually instantaneously) trough bigger distances? And what does a quantum state represents, which features does it have?

    best regards, Marin
     
  6. Jul 23, 2008 #5
    Re: Entanglement

    No it does not mean FTL communications or signals. One can only say that there is an FTL "influence".
     
  7. Jul 23, 2008 #6
    Re: Entanglement

    I'm not sure I follow your argument perfectly, but it sounds as though you're saying that the coincidence circuitry may be the source of the correlations, and not anything occuring with or between the entangled particles. If that were true, then any two particles would produce the correlations, not just entangled particles.

    Perhaps I'm misunderstanding your argument.
     
  8. Jul 23, 2008 #7
    Re: Entanglement

    I was in a hurry, as I am now. :smile: Sorry for any misunderstanding.

    My understanding is that it's assumed, at least tacitly, that in EPR-Bell experiments the source of the entanglement at the quantum level is, eg., emission from the same atom of two opposite-moving disturbances, or transmission of an identical torque to two spatially separated particles or groups of particles, or direct interaction of two particles, or however else one might produce spatially separated, yet identical, properties to correlate with respect to one global operation or another.

    The data correlations themselves are indeed produced by the experimenters via the experimental design. But yes, it's presumed that the deep cause of the correlations is whatever is happening at the quantum level.
     
  9. Jul 23, 2008 #8

    vanesch

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Re: Entanglement

    It sounds like you are saying that EPR correlations are obtained by a common property of the two emitted particles ("torque", or "disturbance" or whatever). In other words, that in EPR correlations, one simply finds back the correlation of the common properties the source of the two particles has induced in them. A bit like the source is a vegetable chopper, and it cuts vegetables in two pieces to send them off to two different locations. It randomly picks vegetables (say, a salad, a tomato, a cucumber), but then it takes, say, a salad, cuts it in two pieces, and sends off half a salad to Alice and to Bob. Alice by herself sees randomly the arrival of half a salad, half a tomato, half a cucumber, ... and Bob too, but of course when we compare their results, each time Alice had half a salad, Bob also had half a salad etc...
    Is that what you mean ?
     
  10. Jul 23, 2008 #9
    Re: Entanglement

    I don't know if that's what he meant, but that's what I meant.
     
  11. Jul 23, 2008 #10
    Re: Entanglement

    Yes.


    Ok. On one level I agree that the correlations are not evident until the results of measurements are compared using the coincidence circuitry. But the coincidence circuitry merely compares two measurements that have already been made - it does not fabricate the results. If we extrapolate back in time to attempt to discern what happened, we cannot account for the fact that the two measurement events are outside one anotehrs' light cones. How then did the correlation occur?

    The contenders have always been:
    - Superdeterminism: the entire system, including the experimental components, was pre-ordained to act the way it did, and all conspired to produce the results we see.

    - Hidden variables: there was something hidden in the particles that we couldn't detect that determined the outcome. Bell disproved naive hidden variable theories but more sophisticated ones such as Bohm still are popular among some.

    - Many Worlds: Photon A splits in two at the polarizer, and Photon B splits in two at the other polarizer, and when both reach the coincidence counter, a total of four worlds are created, and the odds of being in any one of those four is governed by Malus' law depending on the difference in angles between the polarizers

    - Copenhagen: the two photons going through the polarizers isn't actually a measurement, for the experimenter, because it hasn't been observed yet by him. So the wave function hasn't collapsed, and the system continues to evolve in the superpositioned state until both measurements have been observed by the same observer. (Unfortunately this doesn't account for the fact that two experimenters could independently view the results of their respective photons, meet, and then compare notes - each believes that he caused the other's wavefunction to collapse. Who's right?) The fact that wavefunction collapse has no objective and logically self-consistent definition is CI's greatest failing IMO.

    If I understand you right, you're arguing for superdeterminism?
     
  12. Jul 23, 2008 #11
    Re: Entanglement

    Alice and Bob's Salad instead of Bertlmann's socks, right? :smile:
     
  13. Jul 23, 2008 #12

    vanesch

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Re: Entanglement

    Yup :approve: o:)
     
  14. Jul 24, 2008 #13
    Re: Entanglement

    I thought my statement was pretty clear. :smile: Maybe not. I'm in a hurry just now, but will return to reply to your and peter's questions in an hour or so.
     
  15. Jul 24, 2008 #14
    Re: Entanglement

    I'd say it's more like a Caesar salad without the anchovies. Just kidding. :rolleyes:

    Here's what I said:
    My understanding is that it's assumed, at least tacitly, that in EPR-Bell experiments the source of the entanglement at the quantum level is, eg., emission from the same atom of two opposite-moving disturbances, or transmission of an identical torque to two spatially separated particles or groups of particles, or direct interaction of two particles, or however else one might produce spatially separated, yet identical, properties to correlate with respect to one global operation or another.

    The above statement pertains to the models and experimental designs (that I've seen) involved in producing quantum entanglements.

    This is, and will likely forever remain, an assumption regarding what is actually happening at the quantum level. Nevertheless, this assumption of "common property [or properties] of the two emitted particles [or spatially separated groups of particles]" is an integral part of the designs of the experiments that produce entangled data (eg., correlations of the type gotten via typical optical Bell tests).

    This my conceptual understanding of the nature of quantum entanglement, and it's the way that at least some of the people that do the experiments that produce quantum entanglement think about it. And, there's simply no reason in the first place to entertain the idea that quantum entanglement has anything to do with FTL propagation of anything.
     
  16. Jul 24, 2008 #15
    Re: Entanglement

    Zap two spatially separated groups of atoms in an identical way, and the two groups of atoms are entangled with respect to the common motional properties induced by the common zapping.

    Two opposite-moving optical disturbances emitted at the same time by the same atom are entangled with respect to the motion of the atom at the time of emission.

    The experimental correlations are produced by analyzing the entangled properties with identical instruments via a global experimental design.

    I think the assumption of some sort of determinism underlies all science.

    My understanding of wave function collapse via the CI is that once an individual qualitative result is recorded, then all of the terms of the wave function that don't pertain to that result are discarded. In this sense, the wave function describes, quantitatively, the behavior of the experimental instruments.

    I don't see what isn't objective or self-consistent about the CI.

    The essence of the CI, as I see it, is that statements regarding events and behavior that haven't been observed are speculative. Objective science begins at the instrumental level.
    Hence a fundamental quantum of action, and limitations on what we can ever possibly know.
     
  17. Jul 24, 2008 #16
    Re: Entanglement

    No, you're trapped in a classical understandng of entanglement.

    Bell's theorem proves that there is no actual property that can be common to the entangled particles before their detection that can account for the correlations that we see. It can get close, but not all the way there.

    It turns out that the probability of joint detection is dependent solely on the difference in angle between the two polarizers. Moreover, it works even if the polarizer angles are set a nanosecond before detection. There's nothing about the experimental set up that could cause that. There's no conceiveable hidden variable scheme that could cause the photons to behave that way. It is as though they "know" what the other polarizer angle was.

    ACK. No, that's the whole point! It underlies all of *your* *common* *sense*.

    Define a qualitative result being recorded. By whom? By a computer? A person? A cat? It's subjective. There is no consistent definiton of observer or observation. It all depends on the experiment. Heisenberg even said this. He said the quantum/classical divide depends on the epxeriment. It's not an objective process. And it's not well understood (in CI). The only thing that is well understood is how to calculate the odds.
     
  18. Jul 24, 2008 #17

    vanesch

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Re: Entanglement

    I'm trying to find out whether you understood the difficulty presented by Bell's theorem or not. If you think that the correlations found in the outcomes in EPR experiments are due to a common property to the two particles, in other words, because the two particles are, say, identical copies of one another, determined by the fact that they have been emitted by the same source (the same atom or so), and hence have random, but identical spin each time, or something else, then:
    1) that wouldn't have surprised anybody
    2) you have not understood Bell's theorem.

    The surprise resides in the fact that the correlations found cannot be explained that way: numerically they don't fit. With the half-a-vegetable emitter, you cannot obtain the same correlations as those of an EPR experiment. That's exactly the content of Bell's theorem. Of course you can find the perfect correlations in the case of identical analysis. That's no surprise. That's like "each time bob finds half a salad, alice finds half a salad too". Easy. That's because they came from the same source: the chopper.
    The crazy thing about EPR results is something like: AT THE SAME TIME, we also have: "each time bob finds a salad, the color of Alice's vegetable is random".
    That's kind of impossible with our chopper: each time bob finds a salad, Alice was supposed to find a salad too, so if she decided not to look at the kind of vegetable, but rather at its color, she should have found systematically "green". Well, no. She finds red or dark green/blue also.

    Now, maybe you know this, but then I don't understand your statements, which then sound tautological to me: "particles show entangled behavior because they became entangled at their source". Sure. But that doesn't explain the "paradoxial" correlations AND lack of correlation at the same time.
     
  19. Jul 25, 2008 #18
    Re: Entanglement

    Yes exactly. Each time Alice eats a tomato, Bob is more likely to eat a cucumber. Each time Alice can't finish her broccoli, Bob eats his carrots more often.

    Those are the types of wacky correlations that entanglement produces. Yes, yes I suppose you could construct very elaborate explanations for all that. Fortunately Bell proved mathematically that NO explanation can work.
     
  20. Jul 25, 2008 #19
    Re: Entanglement

    There are some loopholes in Bell's theorem. The most obvious one is the assumption that even though the theory is assumed to be deterministic, we can assume that the observer can choose the experimental set up at will. This is impossible, because if the observer had "free will" that would violate determinism.

    This is discussed in detail in this article.
     
  21. Jul 25, 2008 #20

    DrChinese

    User Avatar
    Science Advisor
    Gold Member

    Re: Entanglement

    Because it is t'Hooft saying it, it gets more visibility that an article like this otherwise would. But there is plenty to criticize, and the idea of "superdeterminism" is not considered a loophole in Bell's Theorem. Keep in mind that the essential question is whether a local deterministic theory can yield predictions equivalent to QM. A superdeterministic theory comes no closer!

    Keep in mind that such a theory comes with its own rather substantial baggage. It would be somewhat like saying that Bell's Theorem is flawed because you believe in God, and Bell's Theorem tacitly assumes there is no God. I think we can all acknowledge that if there is some unseen force that changes the results of only the experiments we perform to have different values than they really are - then we will be blissfully unaware of this and have incorrect scientific theories. Except that these "incorrect" theories will still work and be useful "as if" they were correct all along.

    (Side comment: I guess the Pythagorean Theory is wrong similarly.)

    The fact is that even if our choice of measurements is pre-determined because we don't have free will, that in no ways explains why the results match QM's predictions and not those of local realistic theories.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Entanglement spooky action at a distance
Loading...