What’s Bell’s Inequality?

Basically if there are three binary traits for each object in a pool made up of these objects, in our “realistic”, intuitively understandable world, the above inequation holds.

Because:

But in quantum world, this does NOT hold proved by experiments!

The fundamental reason why in QM world it behaves so is because probabilities of event 1 and event 2 happening is not simply P1 + P2, but the event1 wave + event2 wave, combination of waves’ probability:

the interference part could be negative.