1. Oct 15, 2011

### mirhagk

Okay so everyone is probably familiar with the twin paradox. Basically my question is what would happen if 2 entangled particles somehow underwent a similar scenario? Would they become untangled, or would they remain tangled, and somehow experience time at a different rate then their surrounding?

This has stumped me, but I'm not even sure if it's possible, if anyone knows can they explain, and if not, I'd love to hear anyone's ideas, or suggestions as to what would happen. Please feel free to discuss what would happen.

2. Oct 17, 2011

### DrChinese

Welcome to PhysicsForums, mirhagk!

In principle: you could accelerate an entangled electron and take it somewhere and return it to be with its twin. If you could find a way to do that without causing collapse (which would probably be nearly impossible), then yes, their time experience would be different.

Not sure that would change anything observable about it however. Electrons don't age, even though I seem to.

3. Oct 17, 2011

### mirhagk

First thank you for replying, second, wouldn't that allow you to alter the one, changing the other, even when they are under different times? That would allow for inter time communication, which seems to defy causality.

4. Oct 17, 2011

### DrChinese

Although observing one (Alice) appears to force the other (Bob) to instantaneously take on a specific value of an observable, there is no way to send a message that way. As to causality, there is no way to determine if it is Alice doing something to Bob, or vice versa. The ordering does not change the outcomes in any discernible manner.

5. Oct 17, 2011

### mirhagk

Really? Lol my physics teachers all lied to me. I was under the impression that you could do some sort of communication with entanglement, but apparently I was mistaken. I guess it has to do with not being able to make it fall into a specific value, while the teachers were probably convinced that in the future you could.

Thank you very much for helping me out. This is a great site.

6. Oct 18, 2011

### DrChinese

You're welcome! And you are right about the specific value deal, each person would see some random eigenvalue - which cannot be manipulated in advance.

7. Oct 19, 2011

### Couchyam

You "can" actually communicate with entanglement, but the only real benefit from it is that you can always tell when someone is listening in on your conversation, which you cannot do with classical forms of communication. This does not involve faster-than light signal propagation though, so there is no causality problem.

When you observe an entangled state, the other particle will immediately collapse to a state which you know, which is then observed at some later time t (before you could tell the other observer what you saw). The following question is some food for thought:
which observer is "responsible" for the wave function collapse, if you could just as easily have switched reference frames to one where the (erstwhile) second observer actually observes first? How does one entangled electron "know" what spin its partner is observed to have?

Also, it is worth mentioning that you could actually use entanglement to send signals back in time if it were possible to copy quantum states: if you measure the spin of one electron, and then another person makes many copies of the other electron (which is now in a definite state), then the other person will know if you measured the spin or not. If you instead choose not to measure the spin, then the other person, after copying the electron, will tend to observe an equal number up or down. Fortunately, it is impossible to copy quantum states (a result known as the no-clone theorem).

Last edited: Oct 19, 2011
8. Oct 20, 2011

### DrChinese

There is no known difference in the outcomes due to ordering. Therefore, assigning one observer to be the cause of the collapse is arbitrary.