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Why Spooky Action?

  1. Dec 31, 2014 #1
    I don't see why the "action" part of this problem is necessary. Two entangled particles are created, each with opposite qualities. One is observed. So, naturally the other has opposite qualities. Nothing has changed. There has been no "action." The observation of the one hasn't changed the other. They had opposite qualities from the beginning. What am I missing?
     
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  3. Dec 31, 2014 #2
    They only can be said to have different values of whatever quality you are measuring when they are observed. When unobserved, each of the two contain all possible values of that quality . Upon measurement, each one "chooses" one of the possible qualities to be, without exchange of information between the two, which was not possible classically. The spooky "action" lies in this choice: I put "action" in quotation marks because there is no "action" in the classical sense, but it was labeled "spooky action" when looked at from a classical point of view.
     
  4. Dec 31, 2014 #3
    OK, I get it. But the "spooky" part isn't the observed results of the second particle. It's the notion that the initially unobserved particles "contain all possible values". That's equivalent to asking the old tree falling in a forest question, or if the moon exists if no one is looking at it. Questioning the existence of objective reality sans observation raises spooky, metaphysical implications. If you don't raise those implications, then the two particles are assumed to have set and opposite qualities ab initio and my posted question makes sense. Comments?
     
  5. Dec 31, 2014 #4

    DrChinese

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    Welcome to PhysicsForums, vegasgeorge!

    Yes, your conclusion is fine and certainly appropriate when discussing Quantum Mechanics. Are you familiar with Bell's Theorem? Most people need this to better understand the issues. Bell demonstrates that a realistic theory cannot be compatible with QM unless it is non-local.
     
  6. Dec 31, 2014 #5

    Nugatory

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    The assumption that the two particles had set and opposite qualities ab initio leads to certain bounds on the statistical distribution of the correlations between measurements of the two particles. Quantum mechanics predicts and experiment confirms that these bounds are violated - google for "Bell's theorem" for details.
     
  7. Dec 31, 2014 #6
    Thanks for the direction. I'm trying to understand the basics of Bell's Theorem. Can someone tell me exactly what is meant by "local hidden variables?" I get the "local" part. But I'm not at all sure what "hidden variables" refers to.
     
  8. Dec 31, 2014 #7

    atyy

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    There is a little controversy over whether the variables must be "hidden". To start, you can ignore the qualification "local hidden variables" and replace it with "local variables".

    The controversy is over whether the wave function itself is addressed by Bell's theorem. If Bell's theorem addresses the wave function, then "local variables" is correct, but if Bell's theorem does not address the wave function, then "local hidden variables" is more strictly correct, since it refers to variables other than the wave function, and which are thus "hidden" from the point of view of the quantum formalism.
     
  9. Dec 31, 2014 #8

    Nugatory

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    Those hypothetical ab initio properties you mentioned are hidden variables. They get this name because the idea was that if we just understood what they were and what laws governed their behavior we would be able to predict the results of measurements without having to resort to the probabilistic description of quantum mechanics. For example, a photon with vertical polarization encounters a polarizer at a 45-degree angle... Quantum mechanics says it passes 50% of the time, is absorbed 50% of the time. The hidden variable hypothesis is that there is something about how that photon was created, something that we don't know/understand yet, that determines whether it will pass a 45-degree polarizer or not.

    You need to go back to Einstein's 1935 EPR paper to follow the subject from the beginning - google for "Einstein EPR" will find it pretty quickly.
     
  10. Jan 1, 2015 #9
    Well put, but one must not lose sight of the fact that there are also assumptions in the way the theorem is set up that restrict it to the statistical analysis you describe, if the starting point didn't lend itself to that kind of analysis, and certain theories might do just that, then you could find that "having set and opposite qualities ab initio" as the OP says, is a valid solution to explain the measurements correlations.
    IOW it is not unconceivable to make up a theory for wich the theorem doesn't apply.
     
  11. Jan 1, 2015 #10

    Nugatory

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    That is true. There is an element of aesthetic preference (de gustibus non est disputandum!) in choosing whether these theories are more or less unpalatable than the mainstream assessment of Bell's theorem and the experimental results that support it.

    However, I think that it is fair to say that none of the theories that plausibly evade the assumptions of Bell's theorem would have satisfied the EPR authors.
     
  12. Jan 1, 2015 #11
    Hmm, I think it is fair to say, by what I know about Einstein, that he would have maybe been philosophically disatisfied, but if the theory worked, surprised and happy to finally beat QM.;-)
     
  13. Jan 1, 2015 #12

    Nugatory

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    Could be.... I've often thought that one of the most intriguing unanswerable questions in the history of science is what would have happened if Einstein had lived to see Bell's theorem and its experimental (but not loophole-free) validation. My bet would be that he would have accepted it and moved on, but that's just me.

    (Of course any speculation about what Einstein would have done suffers from the same flaw as my sure-fire strategy for effortlessly winning at duplicate bridge at any level, which is "If you're not sure what the right play is, just make the same play that Benito Garozzo would make in this situation").
     
  14. Jan 1, 2015 #13

    bhobba

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    That's my bet as well. His views seemed to change during his debates with Bohr. When Einstein put his final challenge to Bohr and Bohr defeated it (although with much effort - evidently Bohr worked all through the night on it) Einstein was seen to literally tip his hat to Bohr and from that moment on never questioned if QM was correct - but rather thought it incomplete.

    If he had lived to see Bells result and experimental confirmation I think he would simply have accepted it and found solace in one of the myriad of interpretations we have that gelled with his intuition on how the world worked.

    Thanks
    Bill
     
  15. Jan 3, 2015 #14
    I'm still digesting Bell's Theorum and Einstein's 1935 EPR paper, and will get back to you all on all that. In the meantime, as I see this discussion developing in my absence, I have to ask, doesn't Occam's Razor come into this? Wouldn't the assumption that the two entangled particles had opposite properties ab initio be the simplest explanation? Is it possible that the math is leading QM theorists on a wild goose chase?
     
  16. Jan 3, 2015 #15
    The point is that this assumption leads to Bell's inequalities, but these inequalities are violated. So, yes, it would be the simplest explanation, but this explanation is falsified by observation. (At least if we ignore the remaining loopholes.)
     
  17. Jan 3, 2015 #16

    Nugatory

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    The problem here is that we have experimental results that falsify the ab initio explanation. Thus, the proposition that we're applying the razor to is not "the particles have opposite properties ab initio" but rather "the particles have opposite properties ab initio and every experiment that has shown a violation of Bell's inequality is somehow flawed in a way that causes it to deliver bogus results that just happen to mimic the quantum mechanical prediction and these flaws have somehow been overlooked by two generations of competent scientists even though they have more to gain from finding a flaw than not finding one".

    This proposition doesn't fare well against "the experiments confirm that Bell's inequality is violated" when the razor is applied.
     
  18. Jan 3, 2015 #17

    ShayanJ

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    I'm beginning to think that we need a FAQ on entanglement and Bell's inequality!
     
  19. Jan 3, 2015 #18
    Let me respond in more common terms.

    Normally (but not normal for QM) you would expect that the one photon of the entangled pair were tilted in some specific direction - for example NNE and 20 degrees up, and the other was pointed in exactly the opposite direction (SSW, 20 down). You would also say that those direction were set at the time that the pair became entangled and remained unchanged until they were measure. This "tilt" is the hidden variable and the expectation that it was set when the two were together and remained unchanged while they were apart is what makes it "local".
    Finally, the normal expectation is that the measurement made when the photon reaches the filter is simply determined by that tilt of the photon, the angle of the filter, and perhaps other local conditions.

    If you always keep the filters opposite of each other, you will discover that you get the same result at both detectors. This is consistent with what you would "normally" expect.

    If you hold one detector (detector A) at a constant angle while varying the angle of the other (detector B), then based on "normal" (local hidden variable) assumptions, you can come to certain (inaccurate) conclusions about the distribution of the tilt of those photons. So at this point, you can still hold onto your "normal" view, but you will see that the distribution seems dependent on what angle you picked for the constant angle detector.

    Next, you can step detector A through several angles - in each case varying B to determine the new distribution of photon tilts. At this point, your "normal" assumptions are in trouble. First, you will discover that the distribution of results at detector B is dependent on the setting of detector A. Worse yet, Bell worked out the arithmetic to show that that conclusion is unavoidable.

    With regards to Occam's Razor: The arithmetic described by Bell is called the "Bell Inequality" and it was published in 1964. I notice that DrChinese has a copy of it here: http://www.drchinese.com/David/Bell_Compact.pdf. Although, on the face of it, it appears airtight, some loopholes have been described and in most cases closed. Most people who understand the problem that the Bell Inequality presents to "Local Hidden Value" models, do not consider those models to satisfy Occam's Razor.
     
  20. Jun 12, 2016 #19
    Nugatory - thanks for your concise and non-mathematical explanation - "The assumption that the two particles had set and opposite qualities ab initio leads to certain bounds on the statistical distribution of the correlations between measurements of the two particles."

    However I still believe in 'ab initio' because from what I can understand the entangled particles travel at the speed of light, so qualities cannot change since no time passes for them (although time does pass for the experimenters). Can entangled particles travel slower than light (ie time passes)? Spooky action seems to me to be due to differences in time frames of the particles and experimenters and therefore is not spooky at all (it's ab initio). Where am I going wrong?
     
  21. Jun 12, 2016 #20
    Here's a video that'll help you get a better grasp of the sort of reasoning that Bell used in his famous theorem:

     
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