Answer
Bell's Theorem, proposed by physicist John Bell in 1964, challenges the notion of "local realism" in quantum mechanics. Experimental validations, including the Aspect experiment and subsequent tests, confirmed that the predictions of quantum mechanics violate Bell's inequalities, indicating non-local correlations and challenging classical notions of causality. These experiments provide compelling evidence for non-classical features of the quantum world and have profound implications for our understanding of the foundations of physics.