Wondering about entanglement at it's most basic level?

[jeebs]

Hi,
We just started touching on entanglement on my course this week and I'm struggling to understand the significance of it. Take the wikipedia discussion of it. It talks about two particles being created at once, with the condition that their spins must be in opposite directions. So, separating the particles an arbitrary distance and measuring one of the particles' spin direction means you automatically know what the other particle's spin is.
Apparently this somehow violates relativity by "sending information" instantly, but I don't see this, I don't get what is supposed traveling through space faster than c. We've just drawn a simple conclusion from what our measurement implied. Why is this scenario such a controversial thing?
Is us doing the measurement on one particle somehow making the other particle behave differently or something? If so, how?
Thanks.

 

[G01]

I think the issue is that you are taking the following view of the situation:

Particle A and B each have a definite spin. I can't know it's exact value. Quantum Mechanics tells me the probability for finding spin up and down on either particle, but that is the most information I can get. Nevertheless, the particles each have a definite spin at all times during their travel.

Am I correct that you logic flows something like the above?

If so, the issue is with the statement in bold. The particles do not have definite spins until they are measured. Thus, if the particles are separated in space and still described by the same two body wave function (they are entangled), the measurement of spin A will instantaneously force spin B to take a value even though spin B is separated from the act of measurement by a finite distance.

However, this does not actually violate relativity because there is no way someone can send any useful information via entanglement. (The person observing spin B will only be able to verify correlations in the spin measurements by comparing measurements with the person measuring spin A Obviously this doesn't violate relativity.)

 

[K^2]

If you want to know where entanglement takes a turn for the bizarre, look into quantum teleportation. It uses a pair of entangled particle to send the state of a third particle to a remote location without ever measuring it, allowing to transfer quantum information via classical channels.

Again, it might not mean a lot to you until you understand a little bit about why you can't measure the actual state of a particle, but at least it might give you an example that shows you that entanglement isn't just smoke and mirrors.

(Maybe this will help to visualize it. Imagine you want to send a telegram, so you take a message to the telegraph station in an envelope, and ask them to send it without opening the envelope. They comply. That's the crazy bit about QT.)

 

[jeebs]

The particles do not have definite spins until they are measured. Thus, if the particles are separated in space and still described by the same two body wave function (they are entangled), the measurement of spin A will instantaneously force spin B to take a value even though spin B is separated from the act of measurement by a finite distance.

ahhh, right, got it.

 

[DrChinese]

If you are uncertain that GO1 is correct in saying the entangled pair does not have definite values until measured: be sure to check out Bell's Theorem.