# Entaglement and Causality

1. Sep 30, 2004

### IvanTheTerrible

Hi everyone, I'm new here and i have to say this place is awesome! I will definatly be poking around here

something that has bugged me ever since i thought about it. Does quantum entanglment (when used to send a signal) violate causality?

2. Sep 30, 2004

### LURCH

Not according to any data yet received. The crux of the matter is your parenthetical statement "(when used to send a signal)". According to all current theories and observations, it can't be used to send a signal.

3. Sep 30, 2004

### IvanTheTerrible

it can't? if one atom is entangled with another and the quantum state is used as an on/off indicator isn't that 1 bit of information?

4. Sep 30, 2004

### pervect

Staff Emeritus
Think about it some more. The outcome of the quantum measurement will be a purely random number. It's not until you compare this random number to another random number that you notice there is a correlation. Measuring the random state of a particle doesn't send any information, any more than tossing a coin and noticing that it comes up heads (50% of the time) or tails (the other 50%) does. To send some information, you need to control what the receiver reciieves. But the receiver receives an apparently random number. It's not until you compare notes, by sending classical information about the state you measured, that anything at all weird happens.

5. Oct 1, 2004

### IvanTheTerrible

but wouldn't that make the problem of sending information a technological restraint? If you could control the quantum state of one atom you could then use it to send information. So i'm assuming controlling the quantum state of an atom is impossible?

6. Oct 1, 2004

### Janus

Staff Emeritus
No, Think of it this way. The state of any two entangled particles are indeterminate until one or the other is measured. Once this measurement is made both particles take on opposite states. In order for the "receiver" to know whether a "message" has been sent he must measure the state of the the particle. At this point he has no way of knowing of whether he is measuring a state caused by measurement at the "transmitting" end or whether he himself, in making the measurement, initiated the process.

7. Oct 2, 2004