![]() Consequently, the only way that hidden variables could explain the predictions of quantum physics is if they are "nonlocal", which is to say that somehow the two particles were able to interact instantaneously no matter how widely the two particles are separated. Bell then showed that quantum physics predicts correlations that violate this inequality. This constraint would later be named the Bell inequality. He deduced that if measurements are performed independently on the two separated particles of an entangled pair, then the assumption that the outcomes depend upon hidden variables within each half implies a mathematical constraint on how the outcomes on the two measurements are correlated. In other words, quantum particles, like electrons and photons, must carry some property or attributes not included in quantum theory, and the uncertainties in quantum theory's predictions would then be due to ignorance or unknowability of these properties, later termed "hidden variables".īell carried the analysis of quantum entanglement much further. Therefore, assuming locality, quantum mechanics must be incomplete, because it cannot give a complete description of the particle's true physical characteristics. This suggests that either the measurement of the first particle somehow also interacted with the second particle at faster than the speed of light, or that the entangled particles had some unmeasured property which pre-determined their final quantum states before they were separated. When they choose a measurement and obtain a result, the quantum state of the other particle apparently collapses instantaneously into a new state depending upon that result, no matter how far away the other particle is. The experimenter has a choice of possible measurements that can be performed on one of the particles. Einstein, Podolsky and Rosen presented a scenario that involves preparing a pair of particles such that the quantum state of the pair is entangled, and then separating the particles to an arbitrarily large distance. By 1935, it was already recognized that the predictions of quantum physics are probabilistic. Bell's paper was a response to a 1935 thought experiment that Albert Einstein, Boris Podolsky and Nathan Rosen used to argue that quantum physics is an "incomplete" theory. The term is broadly applied to a number of different derivations, the first of which was introduced by John Stewart Bell in a 1964 paper titled "On the Einstein Podolsky Rosen Paradox". In the words of physicist John Stewart Bell, for whom this family of results is named, "If is local it will not agree with quantum mechanics, and if it agrees with quantum mechanics it will not be local." " Hidden variables" are hypothetical properties possessed by quantum particles, properties that are undetectable but still affect the outcome of experiments. The "local" in this case refers to the principle of locality, the idea that a particle can only be influenced by its immediate surroundings, and that interactions mediated by physical fields can only occur at speeds no greater than the speed of light. ![]() Quantum physics is incompatible with certain types of local hidden-variable theoriesīell's theorem is a term encompassing a number of closely related results in physics, all of which determine that quantum mechanics is incompatible with local hidden-variable theories given some basic assumptions about the nature of measurement. ![]()
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