In 1964, physicist John Stewart Bell published his paper, "On the Einstein Podolsky Rosen paradox". In the paper, he derived his theorem, Bells Theorem, that states that - No physical theory of local hidden variables can ever reproduce all of the predictions of quantum mechanics. Local hidden variables refers to realism and the local causality theory where combined, it meant that distant events are assumed to have no instantaneous (or at least faster-than-light) effect on local ones.
This meant that classical mechanics cannot explain everything that is going on in quantum mechanics.
This meant that classical mechanics cannot explain everything that is going on in quantum mechanics.