Is superluminal communication via quantum entanglement impossible?

That joke was somewhat humorous the first 30 times I heard it.

Quantum mechanics is really just a somewhat non-standard, or generalized, probability theory. So while they can be misleading, in cases like these, a classical analogy helps intuition: say I have two slips of paper, one red, and one green. I put them in identical envelopes, shuffle them, give one to you, and keep the other. You then go someplace far away, and open your envelope. Say you have the red slip – instantly, you know I must have the green one. But it’s easy to see that this would only allow communication if you had some way to influence, before you open the envelope, which slip you will find in there; but plainly, this isn’t possible. It’s the same with quantum particles and entanglement – the correlations are stronger (which leads to things like Bell inequalities), but not qualitatively different.

[quote=“rockystone, post:10, topic:605366”]

OK. Here’s a thought I had.

I get that you cant control the head/tail up/down yin/yang properties of the particle-pair that you generate. I don’t see why this is an insurmountable barrier to communication. Just keep sending until the message you want to send accidentally generates itself, then stop.
To simplify why this won’t work, notice that if you stop sending, this won’t become obvious at the other end until the time it would have taken the particles to get there, in other words, no sooner than light could get there.

The same is true, alas, for your retort. Relax and let people have their quips.

You are a weird duck…retort wasn’t meant to be humorous.

Feh.
If you wish to check with my wife and daughters, you could confirm that I repeat the same five “jokes” endlessly. They have now become meta-jokes, with the threat of the re-telling being funnier than the joke itself.

Or, according to the always hilarious Terry Pratchett:

Yes, I am a weird duck… Best doggone kind, ja?

Your retort, in my opinion, was unnecessarily snarky. Relax and let people have their fun.

Grin! Yes yes yes! “For Angus – will you ever forget the bloaters?” Bloaters? Angus?

It is a rather infantile form of humor…but part of the joy of being human is that we retain many of the traits of youth well into our adult years. Watch a parent and child playing peekaboo. Do you really think the child is the only one enjoying it?

(Hm… We need college level peekaboo leagues! Then, with some seeds in place, we could take it pro! I see real competition for the NFL!)

Have you finished The Unstrung Harp yet?

This analogy exactly shows why you can’t use quantum entanglement to send messages. You check the bit and it is up, and so you know that your buddy on Alpha Centauri will see a down bit when he checks. You cannot send messages this way, any more than you can send messages via the above envelope method. You can’t even send the message that you’ve opened the envelope. You can send hundreds of sealed paired envelopes to Alpha Centauri, and the instant you open one on Earth you also instantly know the state of an envelope on Alpha Centauri. You don’t need to wait four years for the radio message from your buddy to arrive. Wow, information about the state of that envelope traveled faster than light! Except it didn’t.

It turns out to be surprisingly easy to extend the analogy in order to show the difference between the classical and quantum cases. The situation has an information content of two bits, encoding the truth values of the two propositions ‘the slip in my envelope is red’ and ‘the slip in your envelope is green’. Equivalently, one could phrase these as: ‘the slip in my envelope is red’ and ‘the slip in your envelope has the opposite color of the slip in my envelope’. Clearly, the second proposition is of a somewhat different kind than the others: it does not refer to any paper slip in particular, but rather, expresses a relation between both slips. This is an example of a classical correlation: information not ‘bound’ to either of the two subsystems, but rather, attributed to the system as a whole. The existence of this correlation allows you to infer the color of my slip once you know the color of yours.

However, this is as far as classical correlations go. Quantum correlations go beyond that, i.e. quantum mechanically, systems may be correlated more strongly than possible in the classical case. In fact, both bits of information contained in the system – i.e. the complete information about it – may be of the ‘relational’ kind. Thus, the two propositions may be taken as: (1) ‘If your slip of paper is green, mine is red’ and (2) ‘if your slip of paper is red, mine is green’ (note that it’s not the case that one implies the other – without further specification about the system, if you are given only proposition (1), then it might still be the case that, e.g., (2’) ‘if your slip of paper is red, mine is red’ holds; thus, (1) and (2) are independent, and both are necessary to fix the system exactly).

The interesting thing now is that both propositions, despite describing the system completely, don’t uniquely specify the state of either slip* – both your finding ‘red’ and ‘green’ are possibilities, and since all the information contained in the system is exhausted by (1) and (2), it must be the case that which one you find is random (any other case would imply the possibility of learning more information about the system – but there isn’t any more information to learn). This gives the behavior observed in quantum mechanics: you may discover either a red or a green slip with equal probability, and will instantly know whether I will discover a red or a green slip; however, it should be as clear as in the classical case that no communication is possible using this method.
*Schrödinger, in his essay ‘The Present Situation in Quantum Mechanics’, put it this way: “Maximal knowledge of a total system does not necessarily include total knowledge of all its parts”.