re: #393 LudwigVanQuixote

So true. A nice discussion of Bell’s inequalities would cause heads to pop.

Well, a quick summary is that we know quantum mechanics has the strange property that a system can be in more than one state at once. A photon, for example, can have its spin up and down at the same time. But when a measurement is made, the system “collapses” into one of the states.

You might think the system really was in one of the states all the time, and we didn’t know. This is called a “hidden variable” theory.
John Bell found that for some experiments, the statistics are different for hidden variable theories compared to if the system really was in all the states at once.
Alain Aspect, et al., did such experiments, and have shown that hidden variable theories do not fit the data; quantum mechanics is inherently random.