[lisper]

The issue is not so much with the word "determine" as it is with the word "instantaneously". You could say "Entanglement lets the measurement of one particle instantaneously frob the state of a partner particle" without knowing or specifying what the word "frob" means and you'd have the same problem: you're claiming that measuring one particle does something to the other particle, and does it instantaneously and, moreover, that this is mysterious because the two particles are far apart (hence the slogan "spooky action at a distance"). All this is wrong. There is no "spooky action at a distance" because there is no action. Whether that action is "determining" or "frobbing" the state of the other particle is irrelevant.

The correct story is that measurement and entanglement are the same physical phenomenon. The creation of an EPR pair is the first step in any measurement process. The correlations in EPR measurements derive from exactly the same physical process as the correlations in "ordinary" measurements (I put "ordinary" in scare quotes because, as I said, even "ordinary" measurements start with the creation of an EPR pair). When you "measure" the two halves of an EPR pair what you are really doing is performing two measurements on whatever system produced the EPR pair to begin with. When you look at it that way it is not at all surprising that the measurements should be correlated.

For more details see:

https://www.youtube.com/watch?v=dEaecUuEqfc

http://www.flownet.com/ron/QM.pdf

[Xcelerate]

No one denies that when a measurement is performed, the measuring system and the measured system become entangled (and the aggregate system containing both continues evolving unitarily).

> When you "measure" the two halves of an EPR pair what you are really doing is performing two measurements on whatever system produced the EPR pair to begin with.

Here's the problem with that. Consider the measurements of a pair of space-like separated entangled photons. A choice can be made about the details of these measurements by the experimenters at the last second possible. For example, the rotation angle of a polarizer can be randomly chosen the instant before a photon strikes it, yet correlations with this choice show up in the other measurement that was taken far away. (Note that these correlations still don't allow you to send information faster than light.) But the "which angle" information was never contained in whatever system produced the two photons.

[lisper]

>No one denies that when a measurement is performed, the measuring system and the measured system become entangled (and the aggregate system containing both continues evolving unitarily).

It's not true that no one denies this. Adherents of the Copenhagen interpretation deny it. They claim that measurement involves a non-unitary phenomenon called "wave function collapse."

> I can't quite tell exactly what you all are arguing about.

Exactly this. You may not realize it, but not everyone understands that measurement and entanglement are intimately related (in fact, the exact same physical phenomenon). In fact, some people vehemently deny it. There are even some card-carrying physicists who vehemently deny it.

> entanglement is not a phenomenon that can be explained classically.

That is certainly true (though I've met people who deny this as well).

[Xcelerate]

My apologies, I was editing my post while you were replying to it.

> the aggregate system containing both continues evolving unitarily.

The Copenhagen interpretation is that the measured subsystem collapses in a non-unitary way. That doesn't imply the overall system did. I have no opinion* on whether a measured subsystem collapses unitarily or not — the answer to that question does not currently seem to be experimentally testable. But it is testable that the overall system continues evolving unitarily during a subsystem collapse (well, at least up until the overall system is measured), and this has been tested and verified many times.

(* Ok fine, I do have an opinion. I don't think unitarity breaks — ever. I think it appears to break through the process of decoherence. But I'm certainly not going to claim that as fact unless experiment can prove it somehow.)

[semi-extrinsic]

Sorrt, but you're flat-out wrong, and the sources you link aren't credible. (That's not even a journal paper.) I suggest you read Scott Aaronson's explanation of why you're wrong (he understands this better than me): http://www.scottaaronson.com/blog/?cat=33

[lisper]

Here's the credible citation:

http://arxiv.org/abs/quant-ph/9605002

(It's ironic, BTW, that you should complain that my citation is "not even a journal paper" when yours isn't either.)

And BTW2, Aaronson's blog post doesn't contradict my position. The Bell inequality violations are indeed real. They just aren't caused by "spooky action at a distance." They are caused by the classical correlations that arise when you trace over the quantum wave function of the universe in order to isolate a subsystem.

And BTW3, I actually did submit that paper to Physics Today back in the day. It was rejected, not because it was wrong, but because it wasn't anything new. (It's amazing how the Physics world bifurcates into two camps: those who think that the connection between entanglement and measurement is common knowledge, and those who adamantly deny that it is true.)

[hellofunk]

>It's amazing how the Physics world bifurcates into two camps: those who think that the connection between entanglement and measurement is common knowledge, and those who adamantly deny that it is true.

The fact that there are two legitimate camps means that no one knows for sure what is happening. And you are obviously in one of those camps. So to insist the world is a particular way is to deny how much is still unknown and acknowledged by others in the physics community.

[lisper]

The difference is that one camp has the math solidly on its side, and the other camp has to invoke the extra-physical hypothesis that measurement is somehow different (non-unitary, non-reversible) from everything else.

[semi-extrinsic]

(Not constructive, but OK: Obviously a blog is not a journal paper, but it doesn't try to look like one either. It does have references to the journal papers it discusses though. As for the Cerf and Adami paper you linked above: from my searching it appears this is not published anywhere, it's just a 15-year-old preprint on the arXiv.)

So, to be clear: you are advocating some interpretation of quantum mechanics (for instance, but not limited to, the many worlds interpretation) where the measurement process is really measuring the system that produced the entangled pair?

I believe this interpretation fails to explain experiments where people have entangled particles with timelike separation [1], and then have shown that measurement of the second particle (which has never co-existed with the first particle in any reference frame) collapses the wavefunction of the first particle.

I'm not saying that the Copenhagen interpretation is the absolute truth, in particular many-worlds and superdeterminism are valid alternative interpretations, but I think the one you're advocating doesn't work.

[1] http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.110... (non-paywall: http://arxiv.org/abs/1209.4191 )

[lisper]

> I believe this interpretation fails to explain experiments where people have entangled particles with timelike separation [1]

I don't have time to read that paper right now, but it sounds like simple entanglement transfer, not unlike what is done for "quantum teleportation." In any case, the math behind QIT is simply the math of QM, so anything that QM can explain, QIT can explain.

[UPDATE:] There was a published version of the C&A paper but I can't look it up right now (I'm on the road with very limited internet connectivity). But it turns out that the C&A position is essentially the same as decoherence/many-worlds. There is essentially no dispute over the facts any more. The problem is with the rhetoric in which those facts are wrapped. Even "many-worlds" is highly misleading.

[semi-extrinsic]

I'm not sure what you mean by "There is essentially no dispute over the facts any more." As far as I understand, no-one has been able (yet) to make a prediction based on the many-worlds interpretation that could (even in theory, with infinite resources and time) be tested experimentally. Same goes for the Copenhagen interpretation. Thus neither of these are proper scientific theories (in the sense of Popper), and so we call them "interpretations". In light of this I don't understand how you can claim it is settled in favor of one side or the other?

[lisper]

Read my paper. It describes exactly such an experiment.

[kgwgk]

This message clarifies what you mean. But you said before "A measurement of one entangled particle does NOTHING to the other particle. Its state is exactly the same as it was before.".

By measuring (one of the particles in) the system you are collapsing its wave function. Doesn't that mean the state of the system (both particles) is affected?

[lisper]

No. Collapse doesn't actually happen. Collapse is an approximation to the truth. It's a very (very!) good approximation for systems with a large number of mutually entangled degrees of freedom, but it is an approximation nonetheless. See the links I pointed to earlier to understand why.

[kgwgk]

Doesn't "the truth" involve any change in state at all? I'll look at your answer to quantum mysteries when I have time. But I suspect your original comment could have been phrased "the standard interpretation of quantum mechanics has to die"...

[lisper]

My complaint is more about rhetoric and pedagogy than it is about facts. There is essentially no serious dispute about the facts. But yes, it is true that the Copenhagen interpretation (by which I mean the idea that measurement involves some mysterious non-unitary process called "collapse") is untenable except as an approximation. There is no serious dispute over this.