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B Bohr vs Einstein: is the Moon there when we are not looking?

  1. Feb 10, 2016 #1
    For some time now I’ve been intrigued by the famous argument between Bohr and Einstein, and which was apparently settled when Bell’s inequality was tested in various experiments carried out by Alain Aspect. After going around and around the whole issue for a while, I don’t think I’m convinced Bohr won; at least not because of Bell’s inequality and Aspect’s test. I know, I know, you are probably thinking it's silly to still go at it after all these years and all the times QM’s predictions have been confirmed. But for now, can we please stay on Bell’s inequality, Aspect’s tests, entanglement, nonlocality, polarization, faster than light communication, and all those fascinating theories and facts that came out or are related to this discussion?

    Even though most scientists seem to believe everything points towards Bohr winning the argument; as the results of the tested entangled particles is the same ¼ of the times or greater, as QM predicts; instead of 1/3 of the times or greater, as expected if the particles had hidden variables. Would it be unreasonable to ask if it could be possible that the entangled particles still have hidden variables anyway, but not how everyone imagined; maybe, working in a different way; maybe affected by some unknown factor; or maybe even QM’s predictions being a coincidence? I don’t think this last one is so far fetch. Remember that even the greatest physicist of all times was wrong... no, not him; I’m taking about Newton... and yet he was close enough for us to put a man on the moon using his law of gravity; which Einstein proved wasn’t the whole picture after all.

    And with that question, I’m going to stop this first post -as I don’t want to bore you- and continue with the rest of questions and thoughts later on.

    So, what do you think: is the moon there when you are not looking?
    (Do you think the experiments of Bell's tests alone confirmed Bohr was right?)

    All comments and thoughts are welcome...
    Last edited: Feb 10, 2016
  2. jcsd
  3. Feb 10, 2016 #2
    One does need a test of macro-realism (the Leggett-Garg inequality), much like was done in this experiment - http://www.nature.com/nphys/journal/v6/n6/full/nphys1698.html, except closing the "clumsiness loophole" which was not done in that experiment.

    Often people explain the moon is there when you aren't looking because of decoherence, but that is merely entanglement of the moon with the environment. In principle this remains a superposition, see pages 209-210 of 'Quantum Enigma' by Bruce Rosenblum and Fred Kuttner. The measurement problem remains open.

    Then again, QM may be modified to account for hypothesis like GRW theory (https://en.wikipedia.org/wiki/Ghirardi–Rimini–Weber_theory).
    Last edited by a moderator: May 7, 2017
  4. Feb 10, 2016 #3


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    This is an ongoing debate that often comes back to your beliefs more than actual science. But it's unreasonable, at this point, to imagine going back to a local hidden variable scenario. That goes against the science.

    The question about the Moon is also about contextuality - the context of observation shapes reality. Believing the Moon is there when not observed is a "non-contextual" viewpoint. A paper uploaded today addresses this, claiming (as have a number of papers) that non-contextuality cannot be maintained. It is advanced, but the summary will give you the idea. My real point is that much active research is focused on your question.


    "Quantum physics cannot be reconciled with the classical philosophy of noncontextual realism. Realism demands that system properties exist independently of whether they are measured, while noncontextuality demands that the results of measurements do not depend on what other measurements are performed in conjunction with them. The Bell-Kochen-Specker theorem states that noncontextual realism cannot reproduce the measurement statistics of a single three-level quantum system (qutrit). Noncontextual realistic models may thus be tested using a single qutrit without relying on the notion of quantum entanglement in contrast to Bell inequality tests. It is challenging to refute such models experimentally, since imperfections may introduce loopholes that enable a realist interpretation. Using a superconducting qutrit with deterministic, binary-outcome readouts, we violate a noncontextuality inequality while addressing the detection, individual-existence and compatibility loopholes. Noncontextuality tests have been carried out in a range of different physical systems and dimensionalities, including neutrons, trapped ions and single photons, but no experiment addressing all three loopholes has been performed in the qutrit scenario where entanglement cannot play a role. Demonstrating state-dependent contextuality of a solid-state system is also an important conceptual ingredient for universal quantum computation in surface-code architectures, currently the most promising route to scalable quantum computing."

    PS Glad you liked my website. :smile:
  5. Feb 10, 2016 #4
    I really love the fact that you brought up these fascinating theories that non-scientists like me had no idea even existed; and I'm really looking forward to continue reading about them -kind of getting more than I can handle at the moment.... And of course, thank you so much for answering this thread.

    Regarding the moon, I'm kind of glad they are still debating about it; however, I was pointing towards something much more humble, a kind of metaphor for the issue at hand: "Do you think the experiments of Bell's tests alone confirmed Bohr was right?", which I guess is indeed a bit to late to ask, since I'm sure the local hidden variables have been completely discarded far more times than I'd probably even imagine.

    So, I'll leave the next question while I finish reading the links both of you posted; which is:

    If the entangled particles in the experiments mentioned above don't have hidden variables from the moment they were created, I guess the consensus is that they acquire their properties (e.g. spin or polarization) once they interact with the polarized filters, or even the detectors -doesn't really matter-; which, since both particles end up having the same tested property (just in opposite directions) seems like they are in instant communication at the exact moment of the measurement or interaction; which violates nonlocality. However, is it possible that instead, the entangled particles are in a synchronized random state from the beginning; therefore, they don't need any communication at the moment of measurement or interaction, and therefore don't violate nonlocality or relativity?

    Jesus, I thought I was never going to finish that question!
    Last edited: Feb 10, 2016
  6. Feb 10, 2016 #5


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    Not really possible they are in some synchronized state. There are always a few more experiments to drop on you (as people here often do to me). :smile:

    1. The entangled particles do not need to have ever been in contact, nor come from the same source.


    2. Nor do they need to exist at the same time.


    So that should rule out most anything you can come up with. Oh, and they can even be entangled AFTER detection. All of this is stuff that Bell led us to, but took years to understand and be able to test. But the basic theory is the same as 1928.
  7. Feb 10, 2016 #6
    You knocked half of the questions I had prepared, but I still keep the best for the last day: an instant communication system that works at any distance. :smile:

    Now I need to consult with my pillow...

    Thanks for sharing all this.
  8. Feb 10, 2016 #7


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    Can't be done... Many previous threads on this.
  9. Feb 11, 2016 #8


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    What exactly was Bohr's replay to Einstein's statement? I somehow could not find. Or could you explain what do you mean by statement "Bohr was right"?
  10. Feb 11, 2016 #9
    Sorry, should have posted all this at the beginning...

    The argument I was referring to between Bohr and Einstein was on whether or not we can measure the properties of a particle without disturbing it. Einstein said yes, therefore, metaphorically he liked to think that the moon is there when we are not looking; and Bohr said no, that we can never obtain a complete picture of physical reality, and therefore, metaphorically, we can never know if the moon is there when we don't look.

    Einstein later designed a thought experiment, along with Pedolsky and Rosen, to find out if QM could provide a complete description of physical reality. He proposed that by having 2 entangled particles, one could measure the properties on one of them, and therefore know the properties of the other without disturbing it. The experiment was to prove that QM, and therefore Bohr, could not be right; as they predicted that one particle would influence the other at the moment of measurement, no matter how far away it was, and which was in conflict with Einstein's relativity -as no signal can travel faster than the speed of light (the EPR paradox). Instead, Einstein believed that the particles had some properties from the beginning (hidden variables), which we would be measured later on, and which did not conflict with local causality.

    They never resolved the argument. However, John Bell designed a theorem to solve this issue (Bell's inequality), which along with Alain Aspect carrying out Einstein's experiment in Paris 1982, seemed to finally prove Bohr right. But this was years after Einstein and Bohr were dead.

    That's all I know...
  11. Feb 11, 2016 #10


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    In classical physics the existence of things is synonymous to numerical properties attached to them and one can measure them. QM does not say that we have to choose a set of commuting operators to get the value of an attached property.
    It does not mean that observation made real an intrinsic previous property of the system. On the over hand some thing new was created. If you repeat the measurement you get the same result. We can think that after a measurement a new intrinsic value has been attached to the "new" system.
    Maximally entangled epr system have the attached property of nullity for measurement of global momenta, spin and so on.
  12. Feb 11, 2016 #11


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    I don't think this is accurate. Have you some reference for actual quotes of Bohr and Einstein?
    The phrase is about the moon being there when nobody is looking at it (no one is measuring it). So it seems related to question about reality between measurements but you talk about something else.
  13. Feb 11, 2016 #12
    As far as I know the phrase in bold is what the following paper was all about: "Can Quantum-Mechanical Description of Physical Reality Be Considered Complete?" which I just found In Dr. Chinese's website (http://www.drchinese.com/David/EPR.pdf)... cool! I think I'm going to visit that website a lot. :smile:

    In regards to the moon: that's a metaphor; though it's said that Einstein once asked: "If a person, such as a mouse, looks at the universe; does that change the state of the universe?"

    In any case, all this, including the summary above and the metaphors, comes from the following documentary: "Atomic Physics and Reality"; which had me fascinated for the last couple of weeks.

    Last edited: Feb 11, 2016
  14. Feb 11, 2016 #13
    Before I get into that, can I ask a few questions more?

    I probably got this wrong, but I was thinking: It's said that if 2 entangled particles send information between them instantly -across vast distances for example- that would violate Einstein's relativity; as nothing can travel faster than the speed of light, right? However, is it not true that relativity prevents anything from traveling faster than light because of E=mc2, due to mass? I mean, if information between 2 entangled particles either has no mass or its mass must be at least lower than the particles themselves, and these have a low enough mass to allow them to travel at the speed of light, why does it violate relativity?
  15. Feb 11, 2016 #14


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    I don't agree with that. EPR paper is about incompleteness of quantum mechanics. Idea about finding out something about particle without disturbing it is based on QM itself and assumption of locality. There is a bit harsh account of misunderstandings around EPR and Bell: http://arxiv.org/abs/1408.1826 It's not exactly about moon but rather about dice however if you wonder if Bell tests have confirmed Bohr's position it's on topic.
  16. Feb 11, 2016 #15


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    The problem with faster-than-light transmission of information goes much deeper than that. You'll find some threads over in the relativity forum.
  17. Feb 11, 2016 #16
    Until now I saw a relation between it all. But then I guess it's just because I'm looking at it from a layman's point of view.

    Ok. So, what if 2 entangled particles are still somehow connected, no matter how far away they are, as if they were a single system; and therefore, don't need to transmit any information. What I mean is: take the photon from the double slit experiment, while nothing interacts with it is in a wave like state that occupies all the possible locations; it's in all places and at the same time is not in any defined place. So, could the 2 entangled particles behave in the same way; be in the same wave or cloud of possibilities for all combined possible locations, therefore, connected as one single object that doesn't need to transmit any information?
  18. Feb 11, 2016 #17


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    I agree: EPR is arguing that QM is incomplete in that there must be supplemental parameters (the hypothetical hidden variables) that can account for the so called "perfect correlations". EPR relies on the assumptions of locality and observer independence (non-contextuality). Basically these translate to "local realism". If those were demonstrated to be untenable assumptions, the EPR argument fails.

    For those wondering: I would say Maudlin's account is pretty good. There has been a recent spate of authors attempting to steer the Bell result towards an emphasis on rejection of locality. Maudlin follows that to some degree, which I consider a weakness as far as it goes.

    Quantum mechanics itself is non-local, in a manner that is sometimes labeled "quantum nonlocal". When Bell leads to a rejection of local realism, that use of the word "local" is best labeled "EPR local" or "Bell local" (and violation of Bell locality would be Bell nonlocal). So quantum nonlocal and EPR nonlocal are 2 different things. Bohmian Mechanics is nonlocal, and there is action at a distance. This is yet a 3rd type of nonlocality.

    Most often there is no label, and you must figure out the context on your own. There is no sense in which Bell tests prove that there is non-local action at a distance. Bell tests prove that EPR local realism is untenable.
  19. Feb 11, 2016 #18


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    Sure. An entangled system can be considered one quantum system. Such a system is not limited or confined to a local volume of space (or time for that matter). I don't think anyone really has a good picture of how that works. Other than a mathematical one.
  20. Feb 11, 2016 #19
    Let me clarify all this even further; as when I re-read it looked a bit confusing...

    In essence you have two strings of entangled pairs: one for picking values, and the other for transmitting the information. For now forget about instant transmission faster than light; that's the second problem, for which I also have a solution.

    So, from the first string of entangled particles, you would get random values for the property you are trying to measure. You discard the values you don't need (e.g. if you need a 1 you discard all the consecutive pairs with a 0 until you get a 1), and then entangle swap the pair with the 1 with the next entangled pair from the transmission string; which will have the consecutive values you've been choosing from the random pairs.

    Does this makes any sense to anyone?
  21. Feb 11, 2016 #20
    But can you entangle and break the entanglement at will any time you wish?
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