I always wondered what happens to quantum entanglement when time dilation is involved. For example if one of the particles falls into a black hole, can we get info out?
But that's not communication. It's like slicing a coin down the middle without looking at it, hiding each half in a box, and giving one of the boxes to someone going to the moon. A week later, you both open your box, and you know exactly which side of the coin the other person has - but that's not communication.
(Someone smarter than me, please correct whatever mistakes I've made, and explain this in a better way.)
http://en.wikipedia.org/wiki/Spin_(physics)
"Although the direction of its spin can be changed, an elementary particle cannot be made to spin faster or slower."
Can you expand on the last part about statistical correlation?
Suppose you and I both get a coin and we flip them. You then turn over your hand and reveal heads. If the two coins are "entangled", there is now a greater than 50% probability that my coin will be tails, which would not be the case if the coins weren't entangled.
Quantum entanglement works essentially the same way. "Flipping the coin" is equivalent to observing some state of the particle, like spin. In both cases, entanglement is evidenced by the correlation between the states, nothing more.
Now, imagine we had entangled coins and we were in separate rooms. We flip our coins and you observe yours, noting a heads. In my room, I can't know what you did or didn't observe. So when I observe my coin and see tails, I don't realize it's a consequence of your observation. From each of our individual perspectives the coin flip looks perfectly random. This is why information transfer isn't possible through quantum entanglement.
Very interesting, thanks a lot to everyone who responded!
Isn't that unknowable? And aren't both situations (fixed vs undetermined) effectively the same from an observer's point of view?
Relativity of course makes a hash of that.
Still it's as close as you'll get with a classical analogy, and if you only have access to a fixed basis is entirely correct. Expanding beyond the classical explanation is only truly necessary when you start measuring in different bases.
Thinking about this makes it clear how communication is impossible. You will always get both answers - for different values of "you".
I'm probably mistaken, because if this were possible, then you could use it as a 1-bit communication. "If you detect that I've opened my box, then blow up the moon. Otherwise, do nothing."
Time to re-study quantum mechanics... It's strange how easy it is to forget almost everything just by not thinking about it for two years. MIT OCW had some pretty great online courses on the subject.
The answer is no, and so the analogy doesn't break down. You can't observe the other person except by speed-of-light information transfer.
If you ran two two-slit experiments on entangled particle pairs, shouldn't you be able to detect if your counterpart was observing theirs?
How would you know without observing the particle yourself? In which case, how do you know if it collapsed because of your observation or the other person's?
Not an expert by any means, but from what I'm told you can't use it to transmit information, so that's how I think of it.
Has a nice analogy, that being two radio listeners do know what each other are hearing which is cool, but can't communicate anything between them.
I would have to think about the effect of creating a one time pad.
It's kind of like when you go to a doctor and have tests done for a disease if the results are bad, positive, they call you and if the results are negative no call.
At some point a lab tech checks the sample and if it's negative for the disease they know at that moment but you know by the lack of information at some point in time; i.e. no phone call or letter.
The negative results are more like the quantum communication due to the lack of interaction than the positive results. Mine may not be not the best analogy.
(That should also make it clear why you can't use entanglement to communicate instantaneously: the other side must get the same results whether they measure before or after you've sent your message. So if entanglement allowed communication instead of just esoteric types of coordination, you could use it to send messages backwards in time.)