What is EPR paradox?what is the merit's and demerit's of it and drawbacks for it.
pls answer the question with high knowledge about it
Answers
The EPR Paradox (or the Einstein-Podolsky-Rosen Paradox) is a thought experiment intended to demonstrate an inherent paradox in the early formulations of quantum theory. It is among the best-known examples of quantum entanglement. The paradox involves two particles which are entangled with each other according to quantum mechanics. Under the Copenhagen interpretation of quantum mechanics, each particle is individually in an uncertain state until it is measured, at which point the state of that particle becomes certain. At that exact same moment, the other particle's state also becomes certain. The reason that this is classified as a paradox is that it seemingly involves communication between the two particles at speeds greater than the speed of light, which is a conflict with Einstein's theory of relativity.
The Einstein-Podolsky-Rosen Paradox (EPR)
Although I have heard it said that the EPR paradox has been resolved by experiment in favour of quantum mechanics, and against Einstein, it is actually a far more serious a paradox than Schrodinger's cat. It cannot be regarded as solved, because it apparently demonstrates a very deep conflict between relativity and quantum mechanics.
Einstein, Rosen and Podolsky imagined that a quantum mechanical process generates two particles flying in opposite directions with equal momenta. The momenta of the particles is not known, so the rules of quantum mechanics dictate that it is governed by a wave function. The two particles become separated and then an experiment is done to determine the momentum of one particle. According to conservation of momentum, the momentum of the other also becomes known at that precise point in time, so its state has been changed. Yet the separation between the particles implies that no influence can pass from one to the other. Einstein felt that "No reasonable definition of reality can permit this".
The reason the EPR paradox is so severe is that the predictions of quantum mechanics fly in flat contradiction to the laws of relativity, which are so solidly established and so successfully built into the deepest form of quantum mechanics, quantum electrodynamics. Nonetheless recent experiments based on Bell's inequality support these predictions.
Instantaneous action at a distance is prohibited by relativity because if anything were to travel faster than the speed of light, then it would also have to be able to travel backwards in time. If two physicists each measure the momentum of one of the particle, then the one who measures his particle first causes the other physicist's results to change. But relativity tells us that, according to a moving observer, it was the second physicist who affected the results of the first.
Einstein believed that some other process, such as a hidden variable, must dictate the experimental result. A hidden variable is an unknown quantity which is defined but which cannot be known, and which affects the results of the experiment without revealing its own value. Although David Bohm has produced a "hidden variables" theory based on a form of hidden variable, it is also non-relativistic, non-local, and can hardly be taken seriously as an interpretation of quantum mechanics.
For practical reasons, the EPR paradox cannot be tested in exactly the experiment suggested by Einstein, Podolsky and Rosen, but resolving it is of such importance that it has been developed, especially by John Bell, and tested experimentally a number of times, most significantly by Alain Aspect.