1935: Einstein Podolski and Rosen publish a paper (the EPR paradox) that says quantum mechanics either breaks relativity’s FTL, either has hidden variables, either is a non local theory.

1960: Bell finds a set of equations (Bell’s inequalities), they are a set of measures of correlations of a quantum system that separates cases: if the correlation is under a certain limit then QM has hidden variables, if it’s over then there are no hidden variables: QM must be a non-local theory.

After this, experimental physicit have implemented this experiment with more and more precision. All the experiments point towards non-local reality.