# Entanglement Questions

1. Feb 14, 2016

### Physicsunderstand1

I've recently been reading up on quantum entanglement, and I was wondering how the no communication theorem does not rule out non locality. From my understanding the theorm proves that two entangled particles could not communicate to one another, and this is what occurs within the framework of quantum non locality. My second question also pertains to entanglement. I was wondering how assuming that counterfactual definitness is false rids the need of non locality.

Last edited: Feb 14, 2016
2. Feb 14, 2016

### yeezyseason3

Stupid question, but I'm gonna ask it anyway....isn't non-locality referring to "particles" or "photons" communicating with each other? Also isn't the no-communication theorem referring to the actual observation by an outside reference frame?

3. Feb 14, 2016

### Physicsunderstand1

It is my understanding that the no communication theorem proves that the two particles cannot communicate to one another, however an outside source may communicate to both. I'm not sure of the mathmatical proof reguarding the theory, however this is what I've been told by a physicist.

4. Feb 14, 2016

### yeezyseason3

I don't know how relevant it is, but ill use photons as an example. If you upconvert into two complimentary entangled photons, it is (as far as we know) impossible to delay one of the photons without affecting the other photon in some way. This intrinsic "connection" could be one reason on how non-locality can occur in this scenario.

Last edited: Feb 14, 2016
5. Feb 14, 2016

### Jeff Rosenbury

My understanding is that no connection has been shown, only correlation.

In the human realm, we often say, "Correlation doesn't prove causation." But our human experience is otherwise; usually a correlation means someone did something. Yet the quantum world is not the world of human experience, so perhaps the adage is true?

6. Feb 14, 2016

### Staff: Mentor

The no-signalling theorem says that nothing you do to one particle will allow you send a message to someone watching the other particle.
The wikipedia article at https://en.wikipedia.org/wiki/No-communication_theorem is worth reading.