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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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You should really read the following famous article by Mermin: It essentially answers your question in a simple, elegant way. – joshphysics Aug 21 '14 at 22:28
as Feynman said: "Noone understands quantum mechanics", "when expressed in copenhagen interpretation" (last mine) :) – Nikos M. Aug 21 '14 at 23:13
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. – Tanner Swett Aug 22 '14 at 1:13
There is no such thing as "state of the first and second particle". There is only one state, which describes both particles. What you call voodoo is merely the consequence to what happens, when we intuitively assume that physical distance somehow "separates" that one state. It doesn't. – CuriousOne Aug 22 '14 at 1:14
@gpgemini: you are right and the answer has always been in plain sight. The particles do not turn out to be entangled. We know they are entangled at the onset of the experiment, so why should we expect they cease to be entangled at the end? – bright magus Aug 22 '14 at 5:53

4 Answers 4

up vote 6 down vote accepted

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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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. – 171 Aug 22 '14 at 20:57
explained by a "local, real and causal" theory. It is supposed to be choose any two. – Nikos M. Aug 23 '14 at 0:08
@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'. – Emilio Pisanty Aug 23 '14 at 17:18
@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. – Tanner Swett Aug 24 '14 at 14:34
@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. – 171 Aug 25 '14 at 6:38

My understanding is that the Aspect experiment shows that your understanding is wrong.

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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.

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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.

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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. – 171 May 31 at 14:17

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