Quantum Entaglement and Relativity

[rickolasnice]

OK I think I understand it a bit better now..

Lemme see... So particle A and B are entangled and separated.. once particle A is measured it is an up spin which instantly makes particle B a down spin.. Am I right? ..

But how do we particle A wasn't always an up spin? How do we know that interaction causes the collapse if we don't know what they are doing beforehand?

 

[polymath]

But how do we particle A wasn't always an up spin? How do we know that interaction causes the collapse if we don't know what they are doing beforehand?

That's easier to explain using the 'double slit experiment'... http://en.wikipedia.org/wiki/Double-slit_experiment

In that experiment, a photon can behave as a wave (an object without a definite position) until its position is measured and it collapses to a single point in space. The fact that the photon didn't have a definite position between measurements can be deduced by noting that the photon wave has interfered with itself.

Similarly, some other kind of experiment using particles in magnetic fields could be made to prove that a particle has not been in a definite spin state between measurements.

 

[superelephant]

^nice post count %)

 

[Jabberwocky]

I think we'd need a model for a measurement that gives only incomplete information about an observable, and causes only incomplete collapse of the quantum state...

I believe it is possible to get a complete understanding of the wavefunction without collapsing the wave function. I believe Yakir Aharonov's et al first elucidated the concept called weak measurement. Some have postulated that instead of using quibits to transfer information, a qumode of particles can be used to transfer a bit of information faster than light using a scheme that employs weak measurement. This doesn't violate the no-communication theorem but has never successfully been developed into a superluminal communication protocol.