Quantum entanglement and the uncertainty principle

[PhysicsDad]

This question may have been posted before, but I didn't have much luck finding it. We can't know the momentum and position of a particle at the same time, but if one were to entangle two particles (I.e. photons), wouldn't it be possible to measure one quality on each and thus deduce both the states for one particle? Or is there some part of these theories that I have missed. If so, could someone please explain why that wouldn't work?

Thanks

 

[DrChinese]

This question may have been posted before, but I didn't have much luck finding it. We can't know the momentum and position of a particle at the same time, but if one were to entangle two particles (I.e. photons), wouldn't it be possible to measure one quality on each and thus deduce both the states for one particle? Or is there some part of these theories that I have missed. If so, could someone please explain why that wouldn't work?

Thanks

Great question, PhysicsDad!

This exact idea was considered by Einstein and 2 others in 1935. Their paper, known as EPR, put forth this same idea - effectively concluding that a more complete specification of the system (than the uncertainty principle allows) must be possible. This conclusion rested on the very reasonable assumption of what is called "local realism". That assumption, reasonable as it was, was hotly debated with no clear answer either way. However, in 1964, J.S. Bell wrote a ground-breaking paper showing that local realism is incompatible with the predictions on Quantum Mechanics.

In 1981, experiments by Aspect et al showed that QM was correct in this particular respect and therefore either locality (c as the speed limit) or realism (pre-existing particle properties) cannot both be correct. Subsequent experiments have confirmed this conclusion many times over.

I would recommed reading the EPR paper, and especially reading about Bell. The Bell paper is not the easiest to follow, although the argument itself is well within grasp. I might suggest you read one of the links from my pages on EPR/Bell. See my tag line to access those.