Erwin Schrödinger was the first to use the word “entanglement” (Verschränkung), in a letter to Albert Einstein. His intent was to describe the connection between two particles that first interact and then separate. When physical properties such as position, momentum, spin and polarization are measured for one member of the pair, in accordance with quantum theory, that particle’s status collapses and it is no longer in a state of superposition. The same thing happens to the other member of the pair, though it has not been touched or measured. The effect is immediate regardless of the distance that separates the two particles. This behavior would appear to violate Albert Einstein’s theory of relativity, which proves that nothing including information can travel faster than the speed of light.
Wikipedia gives this example of entanglement in action: a subatomic particle decays into an entangled pair of other particles. The decay obeys the various conservation laws. So the measurement outcomes of one daughter particle must be highly correlated with the measurement outcomes of the other daughter particle. That means the total momenta, angular momenta, energy, and so forth remains roughly the same before and after this process. Specifically, a spin-zero particle could decay into a pair of spin-1/2 particles. Because the total spin before and after this decay must be zero (conservation of angular momentum), whenever the first particle is measured to be spin-up on some axis, the other (when measured on the same axis) is always found to be spin-down.
Einstein, together with Boris Podolsky and Nathan Rosen, contended that because of the so-called EPR paradox, the theory of quantum mechanics as formulated by Werner Heisenberg must be incomplete. They were adhering to a position known as local realism, which requires that distant events be independent.
The rejection by Einstein et al. was itself overturned by Bell’s Inequality. John Stewart Bell (1928-1990) propounded what has come to be known as Bell’s theorem in a 1964 paper titled On the Einstein-Podolsky-Rosen Paradox. The contention was that local realism must conflict with quantum theory and the bottom line was that entanglement is upheld.
Bell’s theoretical investigations were cut short by his unexpected death in Geneva of cerebral hemorrhage. Since his death, numerous experiments have tested the seeming-paradoxical consequences of entanglement. They are in conflict with the ideology of local realism. The experiments appear to support the anti-localist position, but not with absolute certainty. That is because of certain loopholes that exist in the realization of quantum theory validity regarding the counterintuitive consequences of entanglement.
Bell envisaged atoms as objects of experiment. But researchers have instead looked at protons, which exhibit the property of polarization, a parameter that can be readily measured. In recent years, experiments have tended to eliminate the loopholes, vindicating entanglement. Absolute certainty has not been achieved, but most theoreticians are dismissing local realism as an idea that is not compatible with an understanding of the universe as it actually exists.
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