I read a few of the questions suggested to be similar, but I didn't see what I am looking for. Please feel free to point me in the right direction if I just missed it... (there are quite a few).

I am in my mid 30s and never went to college. My understanding is largely from various YouTube sources.

There is one thing I don't understand about entanglement and I can't find any videos that explain it.

If I spin a coin on a table, shoot it with a gun dead center and the coin breaks perfectly into two smaller coins... and the split pushes the coins into spinning in opposite directions of each other... and this was all on a big magic friction-less table that allowed them to spin away from each other until they were a mile away from each other... I could look at one and see that it was spinning clockwise and instantly know that the other coin is spinning counter clockwise without those coins having to communicate faster than light.

I know I'm just some pleb stuck thinking from an intuitive state of mind... but I'm open to expanding that... I just don't understand why entanglement isn't just two objects being in sync because they were created in sync or were synced up at some point. Can anyone explain why this isn't the case?


5 Answers 5


You are asking a good question.

There is a difference between quantum entanglement and classical correlation (like the example of the spinning coin you just gave). This difference however is quite subtle, and it requires some good thought to explain and understand, as it involves some probability theory. The keyword is Bell's Theorem.

Indeed, most popular explanations of entanglement are quite limited and do not get to the point. And that is why you, and many of us asked questions like yours.

If you want to understand, I'd suggest reading David Mermin's classic paper Is the moon there when nobody looks. You will understand the subtle difference between quantum entanglement different and classical correlations -- as well as why it is actually a profound difference.

You can also look at this minutephysics video on the Bell's inequalities.

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    $\begingroup$ Your answer basically is a lengthy link-only answer. $\endgroup$
    – Holger
    Commented Mar 1, 2018 at 8:17
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    $\begingroup$ @Holger There is a subtle difference between my answer and a classical link-only answer. Jokes aside, it would have taken too much effort to explain entanglement to OP's level, knowing te question was going to be close. Still I believe that it is nice to give catered answers to honest questions. So I reassured OP on their doubts, and pointed him to two sources I think are at his level. $\endgroup$
    – Andrea
    Commented Mar 3, 2018 at 14:32

In you example with the coin there is some set of measurable quantities like position, momentum, angular momentum etc. On the scale of everyday life for most purposes you can measure all of those quantities to a higher accuracy than is needed for most applications without affecting the results of similar measurements on other quantities.

Now, suppose an atom that emits a pair of spin entangled electrons in a singlet state. Spin is a kind of angular momentum and it is quantised and comes only in quantities +1 or -1 in suitable units. You can measure the spin along multiple directions like x,y,z or a direction in between those directions. Now suppose you measure each electron in the entangled pair in either the x or y direction chosen randomly. For each electron the probability of getting +1 is 1/2 and the probability of -1 is 1/2. If you measure both electrons in the x direction and compare the results, they will be different: if one electron has +1 the other will have -1. The same is true if you measure both electrons in the y direction. But if you measure one in the x direction and the other in the y direction, then the results will match with probability 1/2. So if you get the result +1 for x spin on one of the pair and compare the result with a measurement of the y spin on the other electron you will find the other electron has +1 with probability 1/2 and -1 with probability 1/2. So the probability of a match depends on both measurements and you can do mathematical calculations to show that classical physics can't match that result.

The standard explanation is either that you don't try to explain this result at all, or you say the electrons somehow interacted non-locally.

There is an explanation that most physicists reject or don't know about for reasons that don't make a lot of sense to me. The gist of the explanation is that both results happen for each electron: there are two versions of the electron. When outside systems such as measuring instruments interact with the electron there are two versions of the measuring instrument after the interaction and there are also two versions of all the systems that receive that information about the electron, including you. The correlations between the results are only established when systems carrying the results interact not when the electrons are measured and they are established entirely locally without any action at a distance. The reason you don't see these other versions of yourself and everything around you is that they are divided into layers that don't interact with one another to a good approximation because quantum mechanics forbids them from exchanging information. The approximation isn't perfect for reasons, e.g. - all the electrons in the atoms in your exist in multiple versions but the scale you would have to measure to see this effect is smaller than that of everyday life. See "The Fabric of Reality" and "The Beginning of Infinity" by David Deutsch and the papers below:





The reason is that quantum phenomena exist. That is, in every other situation but an entangled partner, it should be impossible to know the quantum state.

Because it is considered proven that quantum outcomes are truly unknowable and probabalistic, not governed by any hidden variable, accounting for this seems to imply an instantenous transfer of information between the entangled pair, when the wavefunction of one of them is collapsed. Which Einstein certainly thought impossible. The consensus is that he was wrong, in this specific type of phenomena. Although, it is very likely an area for new physics to resolve.

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    $\begingroup$ An extended discussion has been moved to chat. $\endgroup$
    – rob
    Commented Mar 2, 2018 at 16:46

The two halves of the coin split. You observe one, either through your left eye or your right eye. I observe the other, either through my left eye or my right eye. If we both look through our left eyes, the coins reliably spin the same direction. Ditto if you look through your left eye and I look through my right. Ditto if you look through your right eye and I look through my left.

But whenever we both look through our right eyes, the coins reliably spin in opposite directions.

That's entanglement. (More precisely, it's an extreme violation of Bell's Theorem, even more extreme than entanglement allows.) Your gun story cannot account for these statistics, and (for the same reasons) cannot account for the actual statistics predicted by quantum mechanics (which are also the actual statistics seen in experiments.)

Quantum entanglement is not classical correlation. As for why --- as I said in my comment, you might as well ask why a ray of sunshine is not the same thing as a squirrel. They're not the same thing. Why should they be?

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    $\begingroup$ "That's entanglement. (More precisely, it's an extreme violation of Bell's Theorem, even more extreme than entanglement allows.)" - so in other words it's not entanglement? $\endgroup$ Commented Mar 1, 2018 at 3:26
  • $\begingroup$ @immibis: it is an exaggerated version of entanglement, where the exaggeration is intended to highlight the key feature, much as a map of illinois might contain a zoomed-in map of chicago. Replace "reliably" with "85% of the time" and you'll have an accurate, non-exaggerated description. $\endgroup$
    – WillO
    Commented Mar 1, 2018 at 3:39
  • $\begingroup$ @WillO, I don't see why entanglement is such a problem to explain. If your eyes are performing some sort of XOR operation on two hidden variables - one being in the coin-pieces, the other being in your eyes - then you'd get those results (as written your answer says the results are always the same 100% of the time, but I'm assuming you meant to say they differ in some cases). Also, if your eyes are performing some sort of imperfect coercion of the variable, then you're going to get variability in measurement that reflects it. It still hasn't conveyed to me what is problematic. $\endgroup$
    – Steve
    Commented Mar 1, 2018 at 16:25
  • $\begingroup$ @Steve: Thanks for catching the (critical) typo; I've corrected it. Now as to your question: Try writing down explicitly what those hidden variables are and what that "some sort of XOR" operation is --- I guarantee you can't do it in a way that is consistent with these statistics. Don't forget that in order to know whether to spin the same direction or opposite directions, the coins have to know which eyes <b>both</b> of us have chosen --- but my coin is physically far from your eye, so information can't travel between them fast enough. $\endgroup$
    – WillO
    Commented Mar 1, 2018 at 16:53
  • $\begingroup$ @WillO, can I just clarify if/why they are only different in the "right-eyed-only" case? The impression I had was that identical measurements (i.e. the left-eye-only case too) always produced opposite results. Either way I still don't foresee any problem - it just tells me that there's something non-symmetrical about the situation. $\endgroup$
    – Steve
    Commented Mar 1, 2018 at 17:39

Your coin example is analogous to only one specific state of entanglement. That is referred to as Bell's state. This is a state of perfect anti-correlation. I will only talk about this state. Taking an example of spin - if the two particles of entangled pair are measured along same axis, then they will be always found to be opposite spin. Another way to say "always opposite spin" is perfectly anti correlated.

Perfect anti correlation can be easily explained in terms of your example. You do not even need to shoot a coin, you may just separate a pair of shoes, or socks, or gloves! Perfect anti correlation is a consequence of conservation laws. In case of spin, if one particle gets clockwise spin along X axis, then the other has to get anti-clock wise spin along same axis, in order to conserve angular momentum.

Next level of complexity is that you measure them along any axis, and you will see they have opposite spin. This is complex, but still explainable in terms of conservation laws. Specifically because, there is no measurement at quantum level, it is an alignment which is called "measurement".

Above two, in isolation, can be explained in terms of local hidden variables. Because, the above two involve only one pair at a time for checking the anti correlation.

But there is another level of complexity - Video suggested by Aaron Stevens is one of the best.

Third level of complexity is the one that baffles most people. It is statistical correlation. By definition, statistical correlation, has to involve many pairs.

Suppose you measure one particle of the pair along X axis and the other particle along an axis that is at theta degrees to X axis. There are only two possibilities - 1. they show same spin, 2. they show opposite spins. If they show same spin, let us say they are correlated.

Now you repeat same as above on 2nd, 3rd, .... n pairs. For a very large n, the number of correlated pairs is predicted by quantum mechanics as a formula which I will not write here.

All experiments show that the QM formula is correct.

Then there is a Bell song that you will hear a lot, almost from everyone. If you pay attention to the video, Bell considered all possible outcomes equally likely and came up with an inequality, which obviously, is violated in all experiments.

Bell's theorem as mathematics is just fine. But the issue with applying Bell's inequality to statistical correlations of entangled particles is - All the possible outcomes are not necessarily equally likely.

Even in a process that is as simple as hitting a mountain continuously with a hammer, your past hits have an impact on outcome of subsequent hits.

So having generated and aligned numerous pairs, we can not say the possible outcomes for the next pair are equally likely. And this fact renders application of Bell's inequality to entanglement - void.

So finally, entanglement is baffling, but has been mathematically formulated pretty well. Only thing missing is some realistic explanation which I think will turn out to be much simpler than it seems. Nature is simple at the core.

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    $\begingroup$ You aren't seriously suggesting a local hidden variables explanation of entanglement? $\endgroup$
    – Obie 2.0
    Commented Mar 1, 2018 at 6:56
  • $\begingroup$ @Obie2.0: As Bells inequality only considers the particles of entangled pair to enumerate possible outcomes, considers them all equally likely, and then disproves LHV by violation of inequality, I consider "local" means the ones that are part of the particles only. But if you expand the local, it may include pair source, aligning equipment, and even the space in the vicinity of experiment. $\endgroup$
    – kpv
    Commented Mar 1, 2018 at 7:46
  • $\begingroup$ @Obie2.0: If local means only the two particles of the pair, then I am not suggesting LHV explanation. But if you consider all that I listed as local, then I think the phenomena needs to be scrutinized for some local cause which might build over the duration of experiment. I am refering to the so called "loopholes" need to be scrutinized more, specifically memory loophole. $\endgroup$
    – kpv
    Commented Mar 1, 2018 at 7:48
  • $\begingroup$ @kpv: in which step of the proof of bell's theorem does it appear to you that all outcomes have been assumed equally likely? $\endgroup$
    – WillO
    Commented Mar 2, 2018 at 4:32
  • $\begingroup$ @WillO: I did not question the proof, only its application to statistical correlations of Q entanglement. You may see it in the video from minute 4:30 to 7:00 at youtube.com/watch?v=ZuvK-od647c&t=62s The 5/9 (at 6:25 minutes) is based upon all 9 possibilities being equally likely. May be you need to think how the inequality is first established. That is where the equally likelyhood comes into picture. $\endgroup$
    – kpv
    Commented Mar 2, 2018 at 6:23

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