# Quantum entanglement (information transmitted )

1. Apr 11, 2013

### shumy

Quantum entanglement (information transmitted !!)

Hello,

In all information I see about quantum entanglement, it's stated that nothing travels faster than light, because no information is transmitted.

I believe that in in the experiments done, you can't detect the exact quantum state of a particle, because if you "read it" you change the state. With this statement in mind it's concluded that no information is transmitted.

What I don't understand is... if it's possible to measure a change in a quantum particle, how can this not be acceptable as information? I believe that the event of changing could be used as information, not just the state itself.

For you that know more physics than me, can you explain me what is wrong in my analysis?

2. Apr 11, 2013

### DrChinese

Welcome to PhysicsForums, shumy!

Entangled particle pairs are in complementary states. Measuring Alice's A tells her something about Bob's B, even if B is far away. True, B is now changed. However, Bob has no way to know that Alice has observed A, nor what the result of that measurement is. In fact, Bob can measure B... But to him, there is nothing he will learn that indicates whether Alice has already made a measurement or not.

Also: For all intents and purposes, Alice and Bob's measurements yield redundant information about the entangled pair (while in the entangled state). Anything more would violate the Heisenberg Uncertainty Principle.

3. Apr 11, 2013

### Naty1

Suppose I have two marbles, one green, one blue. We wrap them up so we don't know which color is where, then we each take one......far apart. I open my package...voila!! it is green'.

What 'information' do you have ...many miles away??

none...., right??

not until we communicate.

4. Apr 11, 2013

### shumy

I accept that measures are random, but if that is so, how experiments like (Bounding the speed of "spooky action at a distance" by Juan Yin) are made? If you can't mesure anything useful, how can you calculate velocities from nothing?

5. Apr 11, 2013

### DrChinese

The answer is that the correlations are discernible when the results from both sides are brought together (classically). The experiment you reference is able to see if there is any speed at which the correlations disappear, indicating a limitation to the effect. So far, at speeds up to 10,000c, nothing has been seen.