Friday, September 15, 2017

Bell's Theorem - The Venn Diagram Paradox

Minute Physics is tackling Bell's theorem, with limited success.

It would have been nice if they included Malus' Law description in here, because that is what we knew before QM came around, and that is what we teach students in intro physics.

In any case, I still find it difficult to follow, especially if you didn't pay that much attention to the part when they are doing the counting. They went over this a bit too quickly to let it sink in.

Maybe your brain works faster than mine and can keep up.



Adam said...

I didn't like the fact that they use a purely classical effect (i.e. that one can understand with Maxwell's equations) to describe a purely quantum effect...

ppnl said...

Not sure what you mean. They assumed local realism (A classical view.) to show that it leads to a contradiction.

Adam said...

I didn't like the first half of the video, where they seem to say that the effect is quantum, whereas it can be explained purely classically. (Of course, the second half on entanglement is purely quantum.) I prefer the second video, which makes it clear that they use the polarization story to explain what superposition means in quantum mechanics, using the classical example of polarization of light in Maxwell EM.

ppnl said...

I guess I still don't get the problem.

It is true that they develop Bell's inequality without entanglement. They then state that while this is a strange and unexpected result it does not yet violate local realism. For example there could be some classical realistic mechanism that allows a photon to remember what polarizes it has passed through. Then they introduce entanglement experiments to show that that classical realistic mechanism cannot work.

It seems a simple logical progression that probably follows the historic progression of understanding. First you have Malus' law, then you have light quantized as photons that can be counted, then you can derive Bell's inequality and finally add entanglement to show that we must give up on local realism because of the combination of Bell's inequality and entanglement.

I would have preferred a more explicit explanation of Mallus' law with some math but they call it minute physics for a reason.