Bell's Theorem - Easy explained

  1. Last week I had a blast with reading explanations on Bell's theorem. It was the first time that I've actually understood it. So I wanted to share some websites that explain the theorem in an easy way:

    1) Spooky Action at a Distance – An Explanation of Bell’s Theorem
    by Gary Felder
    This article is easy to understand and only basic mathematics is used.

    2) Does Bell’s Inequality rule out local theories of quantum mechanics?
    Updated May 1996 by PEG (thanks to Colin Naturman).
    Updated August 1993 by SIC.
    Original by John Blanton.
    This article is more compact than the one before. It introduces a notation often used in discussions, e.g. N(x+, y−).

    3) Einstein-Podolsky-Rosen paradox and Bell’s inequalities
    by Jan Schütz
    A seminar report introducing the CHSH inequality that is used for experiments.

    4) Bell’s Theorem explained
    A post on the blog Skeptic’s Play that uses set theory to explain Bell’s theorem.

    5) Bell’s theorem analogy
    David M. Harrison uses a classroom analogy to derive Bell’s inequality.

    6) Violation of Bell’s theorem
    Lecture notes by Leonard Susskind also using a Venn diagram.

    7) Lecture 17 – Einstein-Podolski-Rosen Experiment and Bell’s Inequality
    An excellent lecture by Prof. James Binney. You can download the lecture notes here. This lecture assumes that you know some quantum mechanics, e.g. how to calculate probabilities using Dirac’s bra-ket notation.
    Note: If you have wondered too (like me) about the probability density function [itex]\rho(\lambda)[/itex] read this wiki article on local hidden variables. It explains that [itex]\rho(\lambda)[/itex] describes the probability that the source emits entangled particles with the hidden variable [itex]\lambda[/itex].

    8) Paradigms in Physics: Quantum Mechanics
    This is an online textbook made available by the Department of Physics, Oregon State University. Have a look at chapter 4 (quantum spookiness). Although they don’t use the term probability density (see note above in 7) it becomes clear now what is meant with [itex]\rho(\lambda)[/itex] . The authors use populations instead.

    9) Bell’s Theorem with Easy Math and Bell’s Theorem and Negative Probabilities
    Two articles by David R. Schneider also known as our DrChinese.

    10) John Bell himself presenting his theorem
    This is a talk given by John Bell at CERN. The youtube video has captions and you can also view a transcript of the talk.

    Feel free to add more links.
  2. jcsd
  3. Thank you, must check it out :) Is there any one specific link out of the above you suggest for the ones among us with little time?
  4. I read them in the order listed above. After reading the first link I've already understood what Bell's theorem is about, so I recommend the first one if you have little time.
  5. Oh I am not sure if you have posted the link of this or not but there's a website of one of the PF users {Dr. Chinese} , his article proved to help me greatly with the understanding of bell's theorem.
  6. Ken G

    Ken G 3,418
    Gold Member

    I like that crystal-clear explanation a lot, and it does demonstrate why complete locality is untenable, but it does have one flaw-- it makes the common mistake of going overboard on the "action at a distance" or "nonlocal effects" picture, which unnecessarily gets people worried about faster-than-light causality violations. There is no such violation, however-- no information can ever be demonstrated to travel FTL even in a Bell-type experiment. The article concludes: "It's one thing to say the electrons must affect each other instantly, but you might still wonder how an electron here instantly knows what is happening millions of miles away. Moreover, in order to explain the results we got, we had to say that the measurement of one electron somehow changed the other one. Why should the electron, either one, be affected at all by my measuring it? My intent was simply to measure a property of the electron, not to change it. This result demonstrates one of the other strange results of modern physics, which is that the act of measuring a property always changes the system you are measuring. In this case the "system" apparently includes not only the electron you are measuring, but also the other one which isn't even there at the time. Physicists have been trying for over fifty years to understand these results, and there is no consensus on how to interpret them. There is clear agreement, however, that the results occur. Spooky action at a distance is part of nature." (my bold)

    But although the article is in all other respects painstakingly careful not to assume more than it can demonstrate, it misses the fact that it is not a requirement of the results of that experiment that the particles must carry with them "instructions" in order for them to present consistent results in the detectors. Of course, if one does assume the presence of such carried instructions, then the nonlocality must appear as a kind of nonlocal "change" to those local instructions, so a kind of faster-than-light "influence". But this is overinterpreted. Better, in my view, is simply to drop the claim that there are any "local instructions" to change in the first place. After all, if we have established the system is nonlocal, who needs local instructions (and FTL "influences") in the first place! As we drop locality, it is much simpler to simply state that the "instructions" are also inherently nonlocal, and do away with any need for "influences" to propagate around. This makes it much clearer why there is in fact no violation of special relativistic notions of causality, as long as we drop locality. The article implies that such SR causality limitations are violated, and that's just not true.
    Last edited: Dec 27, 2011
  7. DrChinese

    DrChinese 5,626
    Science Advisor
    Gold Member

    Glad to be of help! :smile:

    Here are several links to my Bell pages:

    Original references:

    Negative probabilities:

    Overview with additional links:

    Easy Math:
  8. I forgot to say that if you post a link, please enumerate it and add a short description. So, the next link would be 11).

    DrChinese's introduction is link number 9 in the list.
  9. 11) Nonlocal correlations between the Canary Islands
    This is an excellent blogpost on the blog BackReaction. It discusses the role of nonlocality in Bell's theorem, in particular the locality loophole and the freedom-of-choice loophole.
    Prerequisite: Understanding of Light cones. Here are some practice questions on light cones.

    12) Is the moon there when nobody looks? Reality and the quantum theory
    This is a very intuitive article by David Mermin. He introduces machines that have 3 settings and two lamps (red and green) and shows that assigning 3 "real" properties to particles results in a contradiction.

    13) Spooky Actions At A Distance?: Oppenheimer Lecture
    A lecture by David Mermin on EPR and Bell's theorem. Here, he uses three entangled particles (see Greenberger–Horne–Zeilinger state) instead of two.
    I recommend watching this lecture after you have read 12).
  10. 14) Bertlmann's socks and the nature of reality
    by John Stewart Bell

    Here, John Bell derives the d'Espagnat inequality by considering socks that may or may not survive one thousand washing cycles at 45°C, 90°C and 90°C.

    The d'Espagnat inequality is: [itex]N(A,notB) + N(B,notC) \geq N(A,notC)[/itex]
    Bell mentions that this is trivial: Each member in [itex]N(A,notC)[/itex] on the right hand side either doesn't have property B and therefore is in [itex]N(A,notB)[/itex] or has property B and therefore is in [itex]N(B,notC)[/itex]. Thus, the left hand side cannot be less than the right hand side, in other words the left hand side is greater or equal than the right hand side.

    Note: When you read the document don't wonder about the figures. They are not missing but shown in the end.


    By the way, Reinhold Bertlmann was a colleague of John Bell at CERN. He is a professor now and still seems to wear differently coloured socks.

    Bell's original paper (see DrChinese's site) becomes much more understandable with this document.
  11. StevieTNZ

    StevieTNZ 924
    Gold Member

    How exactly do you get into each chapter to read it?
  12. I would recommend Nick Herbert's explanation here. The style of the proof is similar to Mermin's famous essay, but the logic is even simpler than Mermin's. The exact numbers used in Herbert's article, 0, 30, and 60, happen to be the ones used by Bell himself when he used to explain his theorem to popular audiences.
  13. Doc Al

    Staff: Mentor

    Moderator's Note: Thread reopened. Please keep on topic. Per our rules, posts pushing non-mainstream views will be deleted.
  14. I have also just finished reading Nick Herbert's A simple proof of Bell's Theorem.
    I had questions about local and non-local quantum events, now it is clear to me like daylight.
    Thanks for the links, I'll read each of them as I get free time.
  15. Which posts in this thread are "pushing non-mainstream views"? Or, have they already been deleted? Just curious.
  16. I'm curious how reading Nick Herbert's simple proof of Bell's theorem cleared you up on locality/nonlocality.
  17. jtbell

    Staff: Mentor

    They've already been deleted.
  18. Hey, I'm a newbie in quantum entanglement and Bell world. I didn't even know what they meant by quantum locality and non-locality.

    Now I know an event in Anaheim (local) can be affected by an event in Baltimore (non-local). Even then I hoped, instead of showing percentage of mismatch, Herbert showed how measurements in Anaheim are changed by events in Baltimore.
  19. I don't think that's what Herbert showed. What he showed was the simplest realization of Bell's theorem. The problem is that wrt Herbert's demonstration light would be expected, via a local realistic model, to behave in a way that light has never been observed to behave.
  20. I've read the first paper and it is interesting indeed. But I'm thinking about this paradox, based on what is described there related to Mary.

    Suppose that our source emits only one type of electrons. The type that give green on 1 and 3, and red for 2. We agree that only she has the detector on her part and on my part the detector is inactive. Once I want to send an alarm to her, I activate my detector. So, she will start receiving 50% probability instead of 100% (or 0%) like did before.

    Seems like this would be a way to send information instantly.
  21. DrChinese

    DrChinese 5,626
    Science Advisor
    Gold Member

    Welcome to PhysicsForums, igorcov!

    There are no such electrons possible. Entangled particles will not have a known spin (color in the example).
Know someone interested in this topic? Share a link to this question via email, Google+, Twitter, or Facebook