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This question comes from someone who is interested in Physics but with no theoretical background.

In 1936, EPR presented the thought experiment which later came to be known and quantum entanglement.

I understand that the thought experiment reflects the bizarre conclusions of quantum theory, i.e. observation of a state of a particle at one place would let that observer know the state of the correlated particle (light years away) even without observing it. And since quantum theory says that the state of a particle is always in a fuzzy state unless you observe it, this implies that the other particle is getting affected without even observing it...... hence spooky action at a distance ...... which doesn't quite fit with traditional Newtonian physics.

The EPR theory as a thought experiment is quite understandable to me.

What I do not understand is, why did scientists, decades later, build tunnels of several kilometers and sent two entangled particles to each end, and then measure the state of those particles to ascertain quantum entanglement.

I mean, what were they expecting ----- were they expecting the states of the particles to be not in co-relation? How would they explain for that?

As far as I have understood the EPR experiment was a thought experiment that kind of throws quantum theory in an uncomfortable position. But its an experiment that cannot be disproven ---- co-relates are co-relates. It just puts the philosophy of quantum mechanics to doubt.

And you cannot communicate information through entanglement anyway. So my question again : why the experiments?

PS : Please, if possible, provide me with relevant links to learn more about this topic. I don't trust random blogs on the net, and the Wikipedia article is just difficult to understand.

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    $\begingroup$ For the same reasons that you conduct any experiment. $\endgroup$ – Robert Harvey Nov 25 '14 at 16:26
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    $\begingroup$ Because the whole basis of science is testability? $\endgroup$ – Keith Nov 26 '14 at 2:12
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Even for things that seem very clear from the theory, you will want to check them. You asked

I mean, what were they expecting ----- were they expecting the states of the particles to be not in co-relation? How would they explain for that?

Well, of course they were expecting the entanglement. But finding that this is NOT there, would have been a huge thing - Quantum Mechanics needed to be amended!! As much sense as a theory might make, it must be subject to experimental verification in all aspects.

Similarly, most physicists were convinced for decades that the Higgs boson must be there and still we build ever larger experiments looking for it, since if we had NOT found the Higgs boson, we would have to re-think a large bit of what we know about particle physics.

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    $\begingroup$ In contrast to the other answers, this is actually the true, scientific answer. Science relies on empirical tests, and just because we expect something, it doesn't mean we shouldn't test it. Of course, the disproportional expenses to test this one prediction might stem from people still intuitively not believing in entanglement, but that's mere speculation. $\endgroup$ – ACuriousMind Nov 26 '14 at 23:25
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What you seem to not get from the paper is that the EPR-thought experiment actually made a prediction: It predicted that there are correlations within quantum systems that are stronger than in any possible classical system or any local hidden variable theory. The "spooky action at a distance" is just failing classical intuition. Don't read too much into it (I'll comment more below), let's first examine the stronger correlations:

This thought (that bipartite quantum states can exhibit stronger correlations than classically possible) is not really well presented in the EPR paper - and I believe that this is one reason, why experimentalists ignored it for decades. But other people, most prominently perhaps Bell, derived equations that hold for any classical system but do not hold for some entangled states - the easiest example being the CHSH-inequality. This is a testible quantity: You can produce states and test, whether they violate this inequality, if they do, that's a hallmark for a genuinely quantum phenomenon.

But why would you try to show this phenomenon over hundreds of kilometres? A few metres should be enough, shouldn't it? To show the existence of this phenomenon, a few metres would certainly be more than enough. The enterprise of producing entangled pairs over larger and larger distances that has only been tried recently and its due to the already linked to quantum teleportation protocol: While it is not possible to transmit an unknown quantum state via measurements and classical communication (i.e. phone calls), it is possible to transmit it by using entangled states. This opens possibilities for cryptography and information transmission, but for it to work, you'll need entangled states over long distances.

But what about spooky action at a distance and how does this not contradict the theory of relativity, which doesn't allow for instantaneous information transfer? The EPR paper was very much concerned with this "spooky action at a distance", but it is just a term stemming from classical intuition. Entangled states cannot transmit information faster than light (see multiple threads on this topic here, there is a mathematical account e.g. here: The choice of measurement basis on one half of an entangled state affects the other half. Can this be used to communicate faster than light?).

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And you cannot communicate information through entanglement anyway. So my question again : why the experiments?

I think you will find that the technology is important in communication. Quantum cryptography is a way of sending secure messages with, for example, entangled photons used to send a key. If photons are intercepted between the sender and the receiver then the photons are lost and the information never reaches the receiver. The sender and receiver can tell something is amiss. If sender and receiver communicate successfully they can be sure that noone was able to listen in. The unique information passed between the sender and receiver can enables them to, for example share a private key, and communicate securely. The details can be read here and here.

So ... it is not possible to communicate by directly organizing a series of quantum states to be sent from one place to another, but it is possible to send information in the form of a random sequence of information from one place to another securely which can then used by sender and receiver to communicate securely.

I am not sure how much people had in mind quantum cryptography when they started these experiments, but it is an interesting spin-off that has come from this work.

(hope the wikipedia article linked above is helpful)

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    $\begingroup$ I guess what @BlackDagger meant is that you cannot use the apparent "spooky action at a distance" for sending information faster than light. And this is perfectly true. But of course you're right in saying that quantum entanglement has a technical application in quantum cryptography. $\endgroup$ – Emil Nov 25 '14 at 15:51
  • $\begingroup$ -1 deliberately misleading. the OP is correct that you can't send information with EPR-style correlations. $\endgroup$ – innisfree Nov 25 '14 at 16:30
  • $\begingroup$ @innisfree see for example optics.rochester.edu/workgroups/lukishova/QuantumOpticsLab/2010/… and cdn.intechopen.com/pdfs-wm/43793.pdf - quantum entanglement can be used to communicate information securely with regard to key distribution as described in the article. -- so the point I was making in the answer was that this 'technology' (or perhaps better this 'physics') can be used as part of secure quantum cryptography communication. - I take your point that it could be clearer and will edit to make clear. $\endgroup$ – tom Nov 25 '14 at 21:51
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    $\begingroup$ What happens when somebody attempts to attack a quantum key distribution protocol is that the two endpoints no longer get exactly the same bits. There are classical protocols which can be used to verify that the two endpoints have the same bits without leaking the bits. And finally if nobody tampered with the communication, you can send the payload message encrypted with OTP. The message isn't destroyed by tampering, because in case of tampering it wouldn't even be sent. $\endgroup$ – kasperd Nov 25 '14 at 23:27
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    $\begingroup$ @kasperd, thanks for the comments - I have edited and I hpoe that now the description is more accurate as to how this subject relates to communication. $\endgroup$ – tom Nov 25 '14 at 23:57
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A thought experiment is not really an experiment, but an idea. Science requires people to be able to test ideas with reproducible experiments. I can't reproduce a thought "experiment".

A reproducible physical experiment produces physical observations. See "Empirical research".

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  • $\begingroup$ yes - physics is an experimental (and also theoretical) subject $\endgroup$ – tom Nov 25 '14 at 11:41
  • $\begingroup$ @iantresman I think the EPR thought experiment wasn't just an idea because it did not "predict" anything the way an untested theory predicts and requires an experiment for validation. It was an actual experiment which just made use of things that had already been tested --- like co-relates. $\endgroup$ – Black Dagger Nov 25 '14 at 12:11
  • $\begingroup$ -1, this doesn't answer the question or add much to the discussion $\endgroup$ – innisfree Nov 25 '14 at 16:31
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EPR might seems a thought experiment, but it has been translated into an actual physic experiment, especially by physicist Alain Aspect.

You say quantum physic gives bizarre conclusion, that it is in an uncomfortable position, or that "philosophy of quantum mechanic" is at doubt. That's especially why it is important to really perform the "disturbing" experiments.

Many people are dissatisfied of quantum physic because it lacks an explanation compatible with common sense. This gives some idea like Broglie–Bohm theory, that are not very successful. They may hope that quantum theory will be replaced by another theory, a better one and one you can understand.

The thing is, outcomes of Alain Aspect experiment are facts. If another theory is to replace quantum mechanics, it must explain these facts. So this new theory will have some kind of weirdness.

In other words, these experiments shows that quantum theory is not bizarre : the world is bizarre. However, it is bizarre only if you want to stick with the common sense we inherited from our hunter-gatherer ancestors, we can understand it quite well with modern physics.

On the philosophical consequence of quantum physics, I have read and recommend this book by Bernard d'Espagnat "Veiled Reality: An Analysis of Quantum Mechanical Concepts". (I only regret this book doesn't really analyses Everett theory as well as other concepts.)

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