Bell's theorem, formulated by physicist John Stewart Bell in 1964, is a result in quantum physics that addresses the concept of locality and the nature of physical reality. It provides a way to test the predictions of quantum mechanics against the concept of local realism.
Local realism, also known as the "principle of locality," suggests that physical systems have definite properties even before those properties are measured, and that information cannot travel faster than the speed of light (i.e., no instantaneous action at a distance). Quantum mechanics, on the other hand, allows for phenomena such as entanglement, where two particles can become correlated in such a way that their properties are intrinsically linked, even when separated by large distances.
Bell's theorem shows that certain predictions of quantum mechanics are incompatible with the assumptions of local realism. It establishes that if local realism is valid, certain statistical correlations observed between entangled particles should satisfy certain constraints called Bell inequalities. However, experiments based on Bell's theorem have shown violations of these inequalities, indicating that the predictions of quantum mechanics are indeed in conflict with local realism.
In essence, Bell's theorem suggests that if the predictions of quantum mechanics are correct (as experimental evidence supports), then either locality (or realism) must be abandoned, or the concept of a complete description of physical reality must be reconsidered.
As for its truth, Bell's theorem has been tested experimentally through various Bell inequality experiments, including the famous Aspect experiments in the 1980s. These experiments confirmed violations of Bell's inequalities and provided evidence in favor of the predictions of quantum mechanics. Thus, the experimental results support the non-local and non-realistic nature of quantum mechanics, challenging the assumptions of local realism.
It's worth noting that Bell's theorem and its implications have sparked philosophical debates and interpretations within the realm of quantum physics, leading to discussions about the nature of reality, the role of measurement, and the foundations of quantum mechanics.