You create two particles that have opposite spin (one have down the other one up) but you don’t measure them to see which is which yet. Then you send them off in different directions. Then some were down the line, one of them hits a spin filter then we know its spin; lets say it’s down. Then the other ones spin has to be up. This may not seem like a paradox. It’s just like if you have two boxes and one has a blue cube in it and the other one a red cube. If you open one of them and find out that it’s a blue cube you know the other one has to be red. But this logic can’t be transferred to the quantum world, since in our world the red cube is always red and the blue always blue, but a particle spin is neither nor until you measure it. So it seems like when you measure one of the particles you immediately measure the other, and this is an immediate effect is faster then light, or that it’s ‘non-local’, so it goes against relativity, which says that nothing can travel faster then light. Such pairs of particles are called ‘entangeled’ particles. So now quantum mechanics had to be wrong. Or had it?
This was resolved in Bell’s theorem or also called Bells inequality. John Bell found that there should be certain differences between a theory, which is local and supposes that everything’s predetermined and in and a theory, which is non-local and where things doesn’t take on a special attribute until measured. Basicly in a world where every thing is determined you should be able to add together results from two experiments to make a prediction. But if that aren’t the case (which is the case in quantum mechanics) then you can’t add together results from different experiments since results aren’t something constant, but has to be measured before they become something real to use. The verdict came in an experiment by Alain Aspect, and it was in the favour of the non-local. So the attributes of particles are uncertain and have to be measure to become real. But this doesn’t mean that quantum mechanics is right, it simply means that our notion of one static world where every thing always exist is wrong, and that we need a new theory.