On the 4th November 1964, the physicist John S. Bell published a paper called On the Einstein-Podolsky-Rosen paradox. This was an important paper for both philosophy and physics with implications for our understanding of reality and freedom.
When quantum theory was developed in the early 20th century, the philosophical implications troubled some, including Einstein. The “Copenhagen interpretation” put realism in science under threat. Although the “macro” world (people, planets, plates and platypuses) were argued to be real existing things, electrons and other particles were held not to be. The world was therefore divided into the “classical” and the “quantum” worlds, or as John S. Bell later called them, the “speakable” and the “unspeakable”.
In 1935, Einstein published a paper with Nathan Rosen and Boris Podolsky (known collectively as EPR) arguing that quantum mechanics was not a complete theory, but required additional “hidden” variables to preserve realism and locality. “In the vernacular of Einstein: locality meant no instantaneous (“spooky”) action at a distance; realism meant the moon is there even when not being observed.” (wiki)
Bell also argued for realism, thus rejecting the Copenhagen Interpretation. He worked with realist theories such as de Broglie–Bohm theory, but the theory violated the EPR locality criterion. This fact was used to argue that it was on the wrong track, but Bell’s 1964 paper showed that “any serious version of quantum theory (regardless of whether or not it is based on microscopic realism) must violate locality. This means that if nature is governed by the predictions of quantum theory, the ‘locality principle’ is simply wrong, and our world is nonlocal” (American Scientist)