What is Bell's theorem: Definition and 98 Discussions
Bell's theorem proves that quantum physics is incompatible with local hidden-variable theories. It was introduced by physicist John Stewart Bell in a 1964 paper titled "On the Einstein Podolsky Rosen Paradox", referring to a 1935 thought experiment that Albert Einstein, Boris Podolsky and Nathan Rosen used to argue that quantum physics is an "incomplete" theory. By 1935, it was already recognized that the predictions of quantum physics are probabilistic. Einstein, Podolsky and Rosen presented a scenario that, in their view, indicated that quantum particles, like electrons and photons, must carry physical properties or attributes not included in quantum theory, and the uncertainties in quantum theory's predictions were due to ignorance of these properties, later termed "hidden variables". Their scenario involves a pair of widely separated physical objects, prepared in such a way that the quantum state of the pair is entangled.
Bell carried the analysis of quantum entanglement much further. He deduced that if measurements are performed independently on the two separated halves of a pair, then the assumption that the outcomes depend upon hidden variables within each half implies a constraint on how the outcomes on the two halves are correlated. This constraint would later be named the Bell inequality. Bell then showed that quantum physics predicts correlations that violate this inequality. Consequently, the only way that hidden variables could explain the predictions of quantum physics is if they are "nonlocal", somehow associated with both halves of the pair and able to carry influences instantly between them no matter how widely the two halves are separated. As Bell wrote later, "If [a hidden-variable theory] is local it will not agree with quantum mechanics, and if it agrees with quantum mechanics it will not be local."Multiple variations on Bell's theorem were proved in the following years, introducing other closely related conditions generally known as Bell (or "Bell-type") inequalities. These have been tested experimentally in physics laboratories many times since 1972. Often, these experiments have had the goal of ameliorating problems of experimental design or set-up that could in principle affect the validity of the findings of earlier Bell tests. This is known as "closing loopholes in Bell test experiments". To date, Bell tests have found that the hypothesis of local hidden variables is inconsistent with the way that physical systems do, in fact, behave.The exact nature of the assumptions required to prove a Bell-type constraint on correlations has been debated by physicists and by philosophers. While the significance of Bell's theorem is not in doubt, its full implications for the interpretation of quantum mechanics remain unresolved.
Starting from this link my understanding of Bell inequality proof goes as follows:
Suppose we have a model of local pre-determinate hidden variables for QM. This amounts to say QM objects are in pre-determinate given states even if we do not measure it. Locality just means that spacelike...
Hi All
I have never been particularly impressed with Tim Mauldin's general take on Bell and QM. I am reading an article of his at the moment. Here is an excerpt (lightly edited by Grammarly to have correct punctuation, etc).
Start Quote
But for expository purposes, the point is most...
I was revisiting @DrChinese 's Bell's Theorem with Easy Math which sparked a few questions, which I am hoping might offer a potential path to a deeper understanding of Bell's Theorem and Quantum Mechanics (QM) in general.
The explanation uses light polarisation experiments to explain how we...
Hi everyone,
I need some help getting the gist of Bell’s theorem and his notion of inequalities.
How would you explain it to someone with limited knowledge of mathematics?
What are the potential implications?
There has been a lot of discussion on Bell's theorem here lately. Superdeterminism as a Bell's theorem loophole has been discussed extensively. But I have not seen discussion about Karl Hess, Hans De Raedt, and Kristel Michielsen's ideas, which essentially suggest that there are several hidden...
I know of Bell's theorem. Kochen-Specker theorem is supposed to be a complement to Bell's theorem. I tried to understand it by reading the Wikipedia article. But I couldn't fully grasp the essential feature of this theorem, in what way it complements Bell's theorem. What are the main...
If correct, as non-physicist, I wonder why the vast jump to "spooky action" is seen as more plausible as some new type of particle faster than the speed of light. Consider the time long before the discovery of radio communication, how weird it must have been to theorize about that. The speed of...
Qutools makes quantum physics kits for educational purposes. Its quED kit is designed to help students learn about entanglement by performing Bell tests. In the manual section 5.1 it describes "the simplest test to verify entanglement of photon pairs."
My question is if the entangled photons...
I just wanted to point out a resource useful for dealing with claims of violating Bell's theorems. You can point the claimant at https://algassert.com/quantum/2015/10/11/Bell-Tests-vs-No-Communication.html and say "I won't believe you unless you can make the 'Write Your Own Classical CHSH...
I’m looking over a recent paper mentioned in another thread. It claims to refute Bell’s theorem. At first glance, the model presented in the paper doesn’t appear consistent with QM. Here’s a simple example.
Suppose we set both polarizers to the same angle ##\alpha = \pi /4##. In the model...
The Paper “On a contextual model refuting Bell’s theorem” has now been published by the journal EPL (Europhysics Letters) and is available under
https://iopscience.iop.org/article/10.1209/0295-5075/134/10004
In this paper a contextual realistic model is presented which correctly predicts...
So I have often heard it argued that "super-determinism" is a loophole to Bell's theorem, that allows a local hidden variable theory. Bell himself alluded to it in a 1980s BBC interview.
But why is this the case? And how is super-determinism different to regular determinism. And the many-world's...
Hello,
Within Griffith's text - chap 12 section 12.2 page 423 - this is a brief summary of Bell's Theorem and description of Bell's 1964 work.
There is a table on page 423 showing the spin of the electron and positron (from pi meson decay) - these would be in the singlet state, one would be...
I've been slowly grinding away with what I can about quantum mechanics and QFT. I'm not sure how far I've gotten but I've come up against a bit of a roadblock concerning how the relativity of simultaneity applies in QFT with specific reference to the outcome of Bell tests.
My misunderstanding...
The CHSH version of Bell's theorem uses a digitization to obtain integer results in the set made of 1,-1, and then sums 4 of them.
Could it be that digitizing after the sum gives a higher result since for example instead of subtracting 1 one would subtract only .2 for example ?
A question about Bell's theorem :
Consider the ##CHSH=AB-AB'+A'B+A'B'##
Then the theorem states : ##-2\leq CHSH\leq 2##
Implying ##|<CHSH>|\leq 2##.
We could repeat the average : ##\langle |\langle CHSH\rangle|\rangle\leq 2##
Now Bell's theorem deals with large numbers average, in order to...
In https://arxiv.org/ftp/arxiv/papers/1409/1409.5158.pdf, the author (Donald A. Graft) concludes that Bell tests cannot refute local realism, because they employ a wrong analysis. He says:
"The quantum joint prediction cannot be recovered in an experiment with separated (marginal) measurements...
Hello, I would like to hear some comments on this:
Recently a paper has been published(Sánchez-Kuntz, N. & Nahmad-Achar, E. Found Phys (2018) 48: 27. https://doi.org/10.1007/s10701-017-0126-z) claiming tha QM has a local realist interpretaion.
In this paper it is asserted that:
"The wave nature...
I've published a paper on local hidden variables with surprising consequences for Bells Theorem. It is available on https://doi.org/10.1515/phys-2017-0106 The journal Open Physics is listed in T/R.
To the background of Bell's argument, the following comments: Since Bell published his theorem...
I am about to do a unit on the wonders of vector decomposition for year 2 in a 3 years science curriculum where the kids follow along with the historical developments of science.
It's the science class I wish I'd had before becoming an engineer at MIT.
In year 3, we are with Einstein and...
Hi All, I am brand new here, still trying to navigate the website so please forgive me if this is not the appropriate space for the following.
I missed the recent topic, "How do you understand EPR & Bell's Theorem? " No new threads permitted now but I would like to express a summary view as my...
This is related to vanhees71 statements (see bottom). I'll explain.
First. The idea is very simple. As summary: Einstein showed that if reality was objective and quantum theory complete, then there had to be nonlocal effects. But since nonlocal effects can violate relativity, then there had to...
Hi. I'm not a physicist, but I’m intrigued by Bell's theorem and I've been stumbling with "superdeterminism." My understanding of the concept is that everything is not just predetermined, but the initial conditions of the universe are fine-tuned and "conspire" so choices of which versions of...
[Mentor's note: This thread has was forked off from another thread because it was a digression there.
This is false. Bell's theorem is based on a certain assumption (the statistical independence or free-will assumption). Some local and realistic theories that contradict this assumption exist...
Most discussions about Bell's theorem meaning get at some point entangled in semantic and philosophic debates that end up in confusion and disagreement. I wonder if it could be possible to avoid this by reducing the premise, the basic assumption to its bare-bones math content in algebraic/group...
Bell (1964) http://cds.cern.ch/record/111654/files/vol1p195-200_001.pdf has 3 unnumbered equations following his equation (14). Let them be (14a)-(14c). Bell then uses his equation (1) to move from (14a)-(14b). It seems to me that he uses this:
[A(b,λ)]2 = 1. (X)...
I just suppose the Bell's Ansatz for the result of measurement to be $$A (\theta,\lambda) $$
Now the parameter lambda could be anything :
-a physical quantity like the polarization angle of the incoming photon
-the coordinate of a 'world'
- the whole wavefunction.
...
In the case of the...
The thread I wanted to post my question on got closed. Recapitulating:
The best (simplest) account I have found to date for the Bell inequality (SPOT stands for Single Photon Orientation Tester):
Imagine that each random sequence that comes out of the SPOT detectors is a coded message. When...
Bell's theorem states that super-luminal communication exists between particles that are separated by space-like separation viz. faster than light transmission of information. There is spontaneity in this. Relativistically this would amount to going back in time. The state of creation of...
In Lecture 5 on quantum entanglement, Susskind calculates the Bell's inequality terms using projection operator (a difficult concept and a tedious derivation). However, I believe the following
I obtained the result on the Bell's inequality using the probability of spin of an electron prepared...
Hi... New to this forum. Be kind!
I did not study physics at university, and consider myself an armchair physicist. I am a computer programmer by trade. I first came across Bells inequalities a few years ago, while working with a fello programmer who did have a PHD in physics. Its pretty...
If correct, Uzan refutes Popescu and Rohrlich.
To me this seems like a big deal.
Super-Quantum, Non-Signaling Correlations Cannot Exist
Pierre Uzan
http://arxiv.org/abs/1602.01135
Comments?
I'm familiar with Bell's Theorem.. have studied it over the years. I'd just like to confirm if my belief is correct. In short. It shows either particles don't exist before measurement or there are hidden variables.. you know all those non-counterfactual and locality arguments.. Specker theorem...
Usually its said that the violation of Bell's inequality means that any theory that contains the assumptions of locality and realism doesn't agree with QM and observations. But sometimes I hear people talk about counter-factual definiteness instead of realism(or maybe the presence of both!) as...
There's something I don't quite get about most illustrations about Bell's inequality theorem. I will explain what:
Consider a pair of entangled photons fired at two arbitrarily oriented polarizers. I most explications, it seems the author suggests that the hidden variable represents the binary...
This just showed up from a team led by Zeilinger, for those interested in loophole-free Bell tests:
http://arxiv.org/abs/1511.03190
A significant-loophole-free test of Bell's theorem with entangled photons
Marissa Giustina, Marijn A. M. Versteegh, Soeren Wengerowsky, Johannes Handsteiner...
I'm trying to follow this mathematical explanation of Bell's theorem.
The problem I find is with the assumption of a probability density for the hidden variable. That implies - and my question is: am I wrong? why? - that you can expect the same distribution of such a variable for any repetition...
[Mentor's note: split off from the thread https://www.physicsforums.com/threads/first-loophole-free-bell-test.829586/ as this is a general question about Bell's Theorem, not the specific experiment discussed in that thread]
It says in the paper ...
... and the 'CHSH-Bell inequality' all...
[this thread was forked from another thread, as it is an interesting topic in its own right]
Yes, a variant of Bell's theorem says that quantum mechanics is truly random as long as we cannot signal faster than light.
http://arxiv.org/abs/quant-ph/0508016
http://arxiv.org/abs/0911.2504
I read with interest the thread here
https://www.physicsforums.com/threads/bells-theorem-and-negative-probabilities.59163/
and was trying to find out more about how a negative probability might be interpreted. I came across this and wondered if anyone could shed more light on it.
"Let us...
I recently understood Bell’s theorem (the inequality and the QM calculation), with the help of you guys. But something still bothers me: assuming for the moment that Bell’s inequalities were NOT violated by experiment, how we would we understand the dependence of the varying correlations on the...
I've been studying Bell's theorem out of curiosity tonight after watching a BBC documentary about quantum mechanics (The secret of quantum physics - 1. Einsteins nightmare).
The episode ended on Bell's theorem disproving locality and showing Einstein to be wrong. So I went and did a little...
If we start with a Bell state
1/Sqrt(2)(|00>+|11>)
and (after moving the second qbit a significant distance away) apply the interferometer transformation
|0> -> 0.5(|0>+|1>)
|1> -> 0.5(|0>-|1>)
to the first qbit, we get
0.5/Sqrt(2)((|0>+|1>)|0>+(|0>-|1>)|1>)
=0.5/Sqrt(2)(|00>+|10>+|01>-|11>)...
A special issue on 50 years of Bell's theorem has been published in Journal of Physics with free access to all articles:
http://iopscience.iop.org/1751-8121/47/42
A GEDANKEN EXPERIMENT REGARDING BELL'S THEOREM AND NONLOCALITYLet’s say I have four boxes with three compartments in each one, and each compartment contains either a white sock or a black sock. This is analogous to photons having spin components either clockwise or counterclockwise (black or...
Quote from this recently posted article.
http://www.nature.com/news/physics-bell-s-theorem-still-reverberates-1.15435
“Quantum theory does not predict the outcomes of a single experiment, but rather the statistics of possible outcomes.”
My question is not in regards to the statistics of...