Almost all resources I've read about Quantum Entanglement speak about how 'amazing' it is that two entangled particles are bound over any distance, and that the state of one particle determines the state of the other.

I believe that there is possibly a profoundly wrong assumption here that doesn't get addressed properly. The assumption is that when the state of one particle is observed, it is then, and only then, determined, exiting it's super position state (and thus the state of the other particle also being determined, over any distance, instantly - which is where most of the focus lies when talking about entanglement).

But here is my problem - why is the assumption that the state of the first particle is being determined on observation so easily accepted ? It seems to me much more logical and absolutely free of unexplained voodoo that:

  1. the particles are entangled (have opposite symmetrical states).
  2. The state of the first and second particles is unknown and unknowable until observed, but is predetermined from the moment of the particle's inception.
  3. Upon observation, nothing in the particle changes, except that our knowledge of the first particle's state leaves a "super-positioned" state into a specific one.
  4. Basic logical consistency dictates that we "instantly" know the state of the second particle, without the need for "spooky action at a distance".

So I guess that my basic premise is - it seems much more reasonable that our (my?) understanding of superposition is wrong, rather than that particles exchange state information instantly across any space.

Please help me understand where I am wrong ?

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    $\begingroup$ You should really read the following famous article by Mermin: web.pdx.edu/~pmoeck/pdf/Mermin%20short.pdf It essentially answers your question in a simple, elegant way. $\endgroup$ Aug 21, 2014 at 22:28
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    $\begingroup$ as Feynman said: "Noone understands quantum mechanics", "when expressed in copenhagen interpretation" (last mine) :) $\endgroup$
    – Nikos M.
    Aug 21, 2014 at 23:13
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    $\begingroup$ This is a great question, by the way. Given the evidence that you were previously aware of, the hypothesis that the states of the particles are determined before observation is much better than the hypothesis that the states are determined upon observation. It happens, though, that we have additional evidence which contradicts the first hypothesis. $\endgroup$ Aug 22, 2014 at 1:13
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    $\begingroup$ As a bit of an aside - you could check out the Kochen–Specker theorem which gives a more mathematical explanation as some specific contradictions in the local hidden variables theorem. en.wikipedia.org/wiki/Kochen%E2%80%93Specker_theorem $\endgroup$
    – Akoben
    Nov 29, 2014 at 12:25
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    $\begingroup$ Possible duplicate of How do we know that entanglement allows measurement to instantly change the other particle's state? $\endgroup$
    – knzhou
    Dec 14, 2018 at 13:16

4 Answers 4


What you are proposing is called a local hidden variable theory. Bell's theorem proves that any such local hidden variable theory is inconsistent with behavior predicted by quantum mechanics. Bell test experiments have been performed, which show that the predictions made by quantum mechanics are correct, in ways that cannot be explained by a local hidden variable theory.

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    $\begingroup$ I've read Mermin's excellent article, Aspect_experiment, EPR paradox and Bell's theorem. This makes no sense. No one has any idea what is going on, except that some very odd experiments don't behave as we would expect them to. $\endgroup$
    – adams
    Aug 22, 2014 at 20:57
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    $\begingroup$ @gpgemini Sort of. Some very odd experiments don't behave as classical intuition would predict, but there is a formalism which predicts their results perfectly (so far). After that, whether we understand it or not mostly depends on your definition of 'understand'. $\endgroup$ Aug 23, 2014 at 17:18
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    $\begingroup$ @gpgemini I'd say that we do know what's going on. If we assume that states are predetermined, then we end up with experiments whose outcomes don't make any sense. If we assume that states aren't predetermined, we have these theories which explain things perfectly. The logical conclusion is that states aren't predetermined. $\endgroup$ Aug 24, 2014 at 14:34
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    $\begingroup$ @TannerSwett, following this logic, if states aren't predetermined, how do you explain that measuring one particle determines it's state, and instantly also the state of it's entangled sibling, which was previously undetermined. The current theories don't explain how this happens, thus we do not know what is going on. $\endgroup$
    – adams
    Aug 25, 2014 at 6:38
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    $\begingroup$ Well, if you ask me, a scientific theory is completely successful if it manages to explain what happens, without explaining how it happens. After all, we can observe the "what", but we can't observe the "how"; how could we be expected to come up with a decent theory about something we can't make any observations of? In any case, I'll certainly grant you that we don't know how quantum superpositions operate behind the scenes. $\endgroup$ Aug 25, 2014 at 23:33

I read the article by Mermin which @joshphysics refers to in his comment, and I have to say it is a very good article. It gets to the heart of Bell's paradox in a very clear way. But it contains two fallacies which I think ought to be pointed out.

The first is the idea that a Stern-Gerlach apparatus can look at a single atom and measure it as either spin-up or spin-down. This is not how magnets actually work. Any magnetic field that is inhomogenous in the x-direction is just as inhomogeneous in the y-direction. So it cannot split a beam into x-up vs x-down components without also splitting into y-up and y-down components. Yes, Stern and Gerlach did something like this, but to a fan-shaped beam, not a pencil beam. (I am assuming the standard Stern-Gerlach experiment with propagation in the z-direction.) Mermin talks about an abstract machine with three switch settings, but at the end, he admits he is talking about Stern-Gerlach. Doesn't work that way. I explain this in more detail in my blogpost Quantisation of Spin Revisited.

A bigger problem is Mermin's "machine" which lets you push a button and then out come two entangled particles. I don't think such a machine exists. Certainly, the experiments which people talk about with spontaneous down-conversion and coincidence counters are much more complicated than this. I know that people who know better than me will say that it comes to exactly the same thing, but I'm not sure about that. I wrote an article about this once called "There Are No Pea-Shooters for Photons". I don't think there is a pea-shooter for photons, and I'm pretty damn sure that Mermin's pea-shooter for entangled spin pairs does not exist.

There is one more problem with Mermin's analysis, which is not exactly a problem because it's a very clever thing that we owe to Bell in terms of logical clarity. But it obscures the real physics. I'm talking about the notion that the real contradiction occurs when we skew the detectors. That there's nothing wrong with Case A, where coincidences are detected with a probability of 100%.

In fact, that's already a hell of a problem if we have both detectors parallel and we get 100% correlation. Yes, I know you think that just means that the particles were created in a correlated state...so what's the problem? The problem is that yes, if you HAD a pea-shooter (which you don't) that created projectiles in pairs, spin-up and spin-down, and if you HAD a Stern-Gerlach machine to positively detect those spins (which you don't)...if you had those things, yes, you should expect 100% correlation at the detectors. But any physically conceivable "pea-shooter" cannot and will not reliably produce projectile pairs correlated in the x-direction only. The pea-shooters you can realistically imagine will produce correlated pairs in all possible orientations. And the ideal Stern-Gerlach machines (the ones that don't exist) will not give you 100% correlation on those types of sources. Because if you produce a y-correlated pair and put them into an x-aligned "ideal" Stern-Gerlach detector, you will get a 50% coincidence rate.

Any realistic source will produce randomly-correlated projectile pairs, so the "ideal" detector pairs cannot be 100% correlated. Bell's paradox provides a brilliant answer to the very esoteric philosophical question of "what if you DID have such a magical source", but from a practical point of view, the horse is already out of the barn if you can show 100% correlation from an ordinary source.

There are people who know more than me about these things, but I do not believe they have answers to my arguments.

  • $\begingroup$ Wow, this is thought-provoking. I'm especially interested in the 50% coincidence rate you refer to. I'd be interested in your thoughts on one of my questions: physics.stackexchange.com/questions/379813/… $\endgroup$
    – philwalk
    Jan 15, 2018 at 16:10
  • $\begingroup$ I've spent the past few hours going through your blog and previous answers. I haven't gone through the math but the general idea ('photon' effects can be competely explained by Maxwell's equations and mixed state charge oscillations) feels right. How does this generalize to matter waves? e.g. how does an alpha particle wave 'know' to only interact with one object? Thanks! $\endgroup$
    – Matt
    Aug 27, 2021 at 0:23
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    $\begingroup$ @Matt I'm glad you liked by blogposts. I can't answer all your questions but the linear tracks in the bubble chamber are a very important issue that was considered very early in the history of QM. Neville Mott showed that paradoxically, even though the particle waves were spherical, the overwhelming probability was that the detection paths would be straight lines. The Wikipedia article is worth checking out: en.wikipedia.org/wiki/Mott_problem $\endgroup$ Aug 28, 2021 at 0:45
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    $\begingroup$ @Matt can't help you with dark matter. That's outside my territory. $\endgroup$ Aug 29, 2021 at 15:16
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    $\begingroup$ @Matt I have my doubts that a mixed state can ever be driven "upwards" into a quasi-stable pure state, but I think it might evolve downwards into a quasi-stable state from a higher mixed state. (Also, the actual definition of "mixed vs pure states" may be different from the way we are using these terms in the present context.) $\endgroup$ Sep 4, 2021 at 17:19

Here are my two cents on this.

In quantum mechanics, one, two,...many particles are described by a state function. The state function is gives a probability distribution that includes all the possible measurable values of the one,two...many particles.

Let us take two for simplicity and because here is where confusion arises. Because of conservation of quantum numbers the possible probability states of two spin a half particles to be created from a spin zero particle are two, either particle_1 can have spin up and the particle_2 spin down, or the particle_2 is up and particle_1 down. It is a limited outcome probability distribution, but a distribution never the less. In the same way that spinning a coin and getting heads gives you the knowledge that the other side is tails, if you measure one particle's spin you know the spin of the other even if it has gone off to infinity. There is nothing more esoteric than conservation of quantum numbers here.

In my opinion all this entanglement navel gazing is not worth the effort to think it through. Dealing with state functions and probabilities is the job of the physicist who measures.


The spin states of the entangled photons could have been pre-determined from the beginning of the experiment, it requires that certain assumptions on which quantum physics is based are either untrue or misleading. The first assumption I discuss is one of Dirac's. In his introduction to his theory of the Principles of Quantum Mechanics (Fourth Edition revised) he states that “Only questions about the results of experiments have real significance for the physicist.” and then “ The foregoing discussion about the result of an experiment with a single obliquely polarized photon incident on a crystal of tourmaline answers all that can be legitimately be asked about what happens to an obliquely polarised photon when it reaches the tourmaline. Questions about what decides whether the photon is to go through or not and how it changes its direction of polarisation when it goes through cannot be investigated by experiment and should be regarded as outside the domain of science.... Nevertheless some further description is necessary in order to correlate the results of this experiment with the results of other experiments that might be performed with photons and to fit them into a general scheme. Such further description should be regarded, not as an attempt to answer questions outside the domain of science, but as an aid to the formulation of rules for expressing concisely the results of large numbers of experiments.” Obviously when Dirac was formulating these statistical rules he was deeply concerned about the practicalities of making precise measurements on individual particles although he explicitly accepted that there may well be deterministic rules guiding things. All of quantum mechanics is now based on Dirac's derivation of statistical rules based on the idea that determinism is impossible to prove so we must adopt a statistical approach. When we interpret the sayings of physicists then, we should bear in mind that QM is only statistical because we do not have the fineness and gentleness of experiment to be able to conduct the experiment deterministically.

To then come to the issue of Bell's inequality which has been mentioned. Bell himself made the statement in a 1985 radio interview (from wiki) “There is a way to escape the inference of superluminal speeds and spooky action at a distance. But it involves absolute determinism in the universe, the complete absence of free will. Suppose the world is super-deterministic, with not just inanimate nature running on behind-the-scenes clockwork, but with our behavior, including our belief that we are free to choose to do one experiment rather than another, absolutely predetermined, including the ‘decision’ by the experimenter to carry out one set of measurements rather than another, the difficulty disappears. There is no need for a faster-than-light signal to tell particle A what measurement has been carried out on particle B, because the universe, including particle A, already “knows” what that measurement, and its outcome, will be.”. I suggest that this super-determinism is excessive and that only each individual photon transfer needs to be pre-determined, which would be the strict answer to what you ask, but it then begs follow on questions such as how such a 'local' pre-determinism maybe achieved in isolation from a more general non-deterministic environment and this is answered as follows:

There is no example of a photon existing independently and outside of an exchange between two atoms. The only way we can detect a photon is through its acceptance and incorporation into the structure of a receiving atom. Therefore we may consider a photon to be a singular exchange of an indivisible quantum of energy between two atoms. Over a large number of photons the pattern of distribution of those exchanges is then determined to fit a statistical pattern as described by what we know as electromagnetic waves. An exchange is a transaction or an instantaneous moment in time occurring between a giver and a receiver. I would suggest that at the moment of exchange of any photon its destination atom is known and 'agreed' with that destination.

But then if this transaction is instantaneous the question then arises as to how the time-delay appears which gives rise to a speed associated with the photon's time of flight from source atom to destination atom. This question also has a complete answer but which is too big for here.

In conclusion I would say that it is possible for the spin of entangled photons to be pre-determined,there is no experiment or theory which precludes this possibility. In Dirac's words what you ask is “outside the domain of science”! What we should really be asking is how the illusion of 'time' occurs in photon transfers.

  • $\begingroup$ Not sure I follow the illusion of time in photon transfer, need to think about it more, but I do relate to what you quoted about questions outside of the domain of science. Thanks for the great response and giving me some more food for thought on this. $\endgroup$
    – adams
    May 31, 2015 at 14:17
  • $\begingroup$ @171: "superdeterminism" means that everything was pre-ordained; there is no free will, and the results were always going to happen just the way they did. It is an inherently empty branch of philosophy - and it has always been pre-ordained that you would ask this question, and that I would make this comment. :-) $\endgroup$ Mar 5, 2016 at 20:44

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