That's what Einstein argued for, and it's what Bell's inequality proves to not be the case.

I actually remember seeing a Youtube video of Susskind talking about how "FTL" communication is a hack to try to force the quantum state to conform with our notions about the world. (he didn't phrase it this way though; just my interpretation).

I don't know the context it was said, but it seemed to imply that there's a way of thinking about quantum states completely independent of set theory and our classical notions, and this way of thinking should not require FTL communication between entangled particles.

Edit:

I think this was the video: https://www.youtube.com/watch?v=xT1bm1yM8Ow

He seems to imply that Aspect's experiment negates hidden/FTL communication.

While GP's example is not what happens in QM (Bell's inequality shows that the state of the two coins is not predetermined-but-secret), it is akin to that example, in the sense that because each person gets a random (though correlated) bit, they cannot transmit information to one another.

Bell's inequality is a statement about the possible strength of the correlation, rather than about information transmission.

Exactly. It is important to note that there is no information exchange if you do not have any information at all.

BTW: why did internet explorer just capitalize Exchange without my asking?

spell check, exchange is a microsoft product.

From what I've read, the rub is that you don't know which side of the coin the current one is until you observe it, and only at the exact time of observation does the other coin assume the other state.

Apparently, the point at which you observe the one coin is called the collapse of the wave function, and we have ways of directly measuring that the collapse doesn't happen until one of the particles is observed. (But it's never been too clear to me how that's possible... if observing a particle collapses the wave function, how do we observe when the collapse happens, without first observing the particle?)

The problem with the 'collapse' point of view is that it's easy to say "and thus only the observation we saw actually exists". But that's an extra postulate beyond asserting the wavefunction describes all of reality. By just saying the wavefunction describes all of reality, you wind up with Many-worlds: http://en.wikipedia.org/wiki/Many-worlds_interpretation So the coins/paper analogies do make sense, except that there's not a single outcome: there are two possible outcomes, both realized, but each version of you interacting ("observing" is a loaded term) with the system to the point of decoherence (http://en.wikipedia.org/wiki/Quantum_decoherence) only ever get to see one of the worlds, which is the one you find yourself in. This isn't that mysterious given that your brain and everything else in your body are made of the same fundamental particles as everything else.

You highlighted a problem with quantum computers and other things: how to make sure a system is in a quantum state without directly measuring it. It's hard to fight decoherence.

It sounds like the universe may be built on lazy expression evaluation.

A little rusty on my QM but I believe the key to demonstrating when the collapse happens is that while doing a measurement will collapse the wave function, applying a transformation won't necessarily.

An experiment can work something like this. Produce 1000 state that you think are undecided. Apply a transformation that you know will turn the uncollapsed 50/50 state to 1 and the collapsed 0 and 1 states to the uncollapsed 50/50 state. Then measure all 1000 states. If they all yield 1, with high probability they were were indeed undecided to begin with. If they yield random results, with high probability they were collapsed 0 or 1 to begin with.

The difference with entangled particles is that you produce them, don't look at them, and ship them out of the lab. Both particles are initially quantum mechanically identical, in a superposition state, which becomes definite only upon measurement.

The coin analogy supports a hidden variable theory, in that the measurement uncertainty isn't due to a fundamental limitation of nature but to the experimenter's lack of knowledge about the system. If that analogy was correct, one could simply send a piece of paper with the result of the measurement along with Alice OR Bob's "entangled" particle and make Alive OR Bob verify the assertion by taking a measurement.

But then, that's not entanglement. Coin flips obey classical probability laws. Quantum mechanical particles don't. Coin flip probabilities don't interfere.

I think the parent is talking about a response to this point of view. Historically, people first wondered whether QM could be explained by hidden variables. Then they proved it could not, which seemed to imply "spooky action at a distance". But now, Susskind is arguing that actually the logic of the 'hidden variable' gives the right intuition, even though there isn't actually a hidden variable.

Here's how I have come to think of it: When you create the two opposite-spin particles and send them to opposite ends of the galaxy, they exist in a superposition of states which follows QM probabilities. The spins are anti-aligned in all of these possible states , but pointing in a different direction in each state. When one person observes his/her particle, you can imagine it's like they are 'masking out' many of the possible states, leaving just one of the possible "universes". But no matter which "universe" gets masked out, in the "surviving universe" the spins are going to be anti-aligned.

This is a lot like the 'hidden variable' picture where the spins were pre-decided and put into boxes, and when person A opened her box and saw "up" she knew that the other person must have the "down" spin. The difference is that QM probabilities are not consistent with the probabilities you would get from such a boxing procedure (in particular when the observers measure the spin at different angles). But conceptually, opening the box and realizing that you have the "up" spin and therefore the other person has down is a lot like doing a spin measurement and masking out many of the superpositions, and concluding that the other person must have the opposite spin from what you measured.

    When you create the two opposite-spin particles

They are not opposite spin, they are entangled! Their spin is undefined until measured even though they are perfectly anticorrelated, as you mention.

Here's another way to frame this:

Composite systems can exhibit entanglement. The Hilbert space such systems live in (H) is the tensor product of the individual component Hilbert spaces (H1, H2). Now the most general state in H is a linear combination ∑ c_ij |i>_1 ⊗ |j>_2 where the subscripts 1 and 2 refer to the component Hilbert spaces. Some of these superpositions are not separable into tensor products of states from each component space H1 and H2. One such superposition is (omitting c_ij) |00> + |11>, as you can see it is not separable. The state |01> + |11> on the other hand can be separated as (|0> + |1>)⊗|1>. An entangled state is an inseparable state.

So the point is that classical analogies are not to be used when dealing with quantum mechanics. Unless someone proves local realism or something replaces QM, that's how it's got to be!

This is a pretty good analogy for explaining why "communication" is the wrong term.

It does get wrong the hidden variable thing, which others are pointing out. A better analogy would be like I create a pair of coins one of which only turns up heads and the other only turns up tails, but you can't tell which is which until you flip them, and I put them in a bag and they each draw one, travel apart, flip, and deduce. But this is still wrong because the coins had their true state "inside" them the whole time. The part about quantum probability comes in where the coins don't decide which one of them is which until they are flipped (but they decide together, consistently).

Isn't entanglement, more like you take the coins to opposite ends of the universe, and then by flipping one and observing the result, you know what the result will be when the other one is flipped? The whole point of entanglement is that the state of the entangled particles isn't determined yet, until observed, which is not the case with the coin example.

http://en.wikipedia.org/wiki/Bell%27s_theorem

The thing about quantum physics according to Feynman is that quantum objects do not behave in any way like macroscopic objects. You can't use analogies that are rooted in the scale we observe.