…The 1935 EPR paper [1] condensed the philosophical discussion into a physical argument. The authors claim that given a specific experiment, in which the outcome of a measurement is known before the measurement takes place, there must exist something in the real world, an “element of reality”, that determines the measurement outcome. They postulate that these elements of reality are local, in the sense that each belongs to a certain point in spacetime. Each element may only be influenced by events which are located in the backward light cone of its point in spacetime (i.e. the past). These claims are founded on assumptions about nature that constitute what is now known as local realism.
…“Acceptable theories” and the experiment
According to the present view of the situation, quantum mechanics flatly contradicts Einstein’s philosophical postulate that any acceptable physical theory must fulfill “local realism”.
In the EPR paper (1935) the authors realised that quantum mechanics was inconsistent with their assumptions, but Einstein nevertheless thought that quantum mechanics might simply be augmented by hidden variables (i.e. variables which were, at that point, still obscure to him), without any other change, to achieve an acceptable theory. He pursued these ideas for over twenty years until the end of his life, in 1955.
In contrast, John Bell, in his 1964 paper, showed that quantum mechanics and the class of hidden variable theories Einstein favored[17] would lead to different experimental results: different by a factor of 3⁄2 for certain correlations. So the issue of “acceptability”, up to that time mainly concerning theory, finally became experimentally decidable.
There are many Bell test experiments, e.g. those of Alain Aspect and others. They support the predictions of quantum mechanics rather than the class of hidden variable theories supported by Einstein.[2] According to Karl Popper these experiments showed that the class of “hidden variables” Einstein believed in is erroneous.[citation needed]
Implications for quantum mechanics
Most physicists today believe that quantum mechanics is correct, and that the EPR paradox is a “paradox” only because classical intuitions do not correspond to physical reality. How EPR is interpreted regarding locality depends on the interpretation of quantum mechanics one uses. In the Copenhagen interpretation, it is usually understood that instantaneous wave function collapse does occur. However, the view that there is no causal instantaneous effect has also been proposed within the Copenhagen interpretation: in this alternate view, measurement affects our ability to define (and measure) quantities in the physical system, not the system itself. In the many-worlds interpretation locality is strictly preserved, since the effects of operations such as measurement affect only the state of the particle that is measured.[15] However, the results of the measurement are not unique—every possible result is obtained.
The EPR paradox has deepened our understanding of quantum mechanics by exposing the fundamentally non-classical characteristics of the measurement process. Prior to the publication of the EPR paper, a measurement was often visualized as a physical disturbance inflicted directly upon the measured system. For instance, when measuring the position of an electron, one imagines shining a light on it, thus disturbing the electron and producing the quantum mechanical uncertainties in its position. Such explanations, which are still encountered in popular expositions of quantum mechanics, are debunked by the EPR paradox, which shows that a “measurement” can be performed on a particle without disturbing it directly, by performing a measurement on a distant entangled particle. In fact, Yakir Aharonov and his collaborators have developed a whole theory of so-called Weak measurement.[citation needed]
Technologies relying on quantum entanglement are now being developed. In quantum cryptography, entangled particles are used to transmit signals that cannot be eavesdropped upon without leaving a trace. In quantum computation, entangled quantum states are used to perform computations in parallel, which may allow certain calculations to be performed much more quickly than they ever could be with classical computers.
