The Einstein Podolsky Rosen (EPR) Paradox - A simple explanation bit.ly/1tyNFzh ...The EPR paradox yields a dichotomy that physical reality as described by quantum mechanics is incomplete. It is an early and influential critique levelled against the Copenhagen interpretation of quantum mechanics. Albert Einstein and his colleagues Boris Podolsky and Nathan Rosen (known collectively as EPR) designed a thought experiment[1] which revealed that the accepted formulation of quantum mechanics had a consequence which had not previously been noticed, but which looked unreasonable at the time. The scenario described involved the phenomenon that is now known as quantum entanglement. According to quantum mechanics, under some conditions, a pair of quantum systems may be described by a single wave function, which encodes the probabilities of the outcomes of experiments that may be performed on the two systems, whether jointly or individually. At the time the EPR article was written, it was known from experiments that the outcome of an experiment sometimes cannot be uniquely predicted. An example of such indeterminacy can be seen when a beam of light is incident on a half-silvered mirror. One half of the beam will reflect, the other will pass. If the intensity of the beam is reduced until only one photon is in transit at any time, whether that photon will reflect or transmit cannot be predicted quantum mechanically. The routine explanation of this effect was, at that time, provided by Heisenbergs uncertainty principle. Physical quantities come in pairs called conjugate quantities. Examples of such conjugate pairs are position and momentum of a particle and components of spin measured around different axes. When one quantity was measured, and became determined, the conjugated quantity became indeterminate. Heisenberg explained this as a disturbance caused by measurement. The EPR paper, written in 1935, was intended to illustrate that this explanation is inadequate. It considered two entangled particles, referred to as A and B, and pointed out that measuring a quantity of a particle A will cause the conjugated quantity of particle B to become undetermined, even if there was no contact, no classical disturbance. The basic idea was that the quantum states of two particles in a system cannot always be decomposed from the joint state of the two. Heisenbergs principle was an attempt to provide a classical explanation of a quantum effect sometimes called non-locality. According to EPR there were two possible explanations. Either there was some interaction between the particles, even though they were separated, or the information about the outcome of all possible measurements was already present in both particles. The EPR authors preferred the second explanation according to which that information was encoded in some hidden parameters. The first explanation, that an effect propagated instantly, across a distance, is in conflict with the theory of relativity. They then concluded that quantum mechanics was incomplete since, in its formalism, there was no room for such hidden parameters. Violations of the conclusions of Bells theorem are generally understood to have demonstrated that the hypotheses of Bells theorem*, also assumed by Einstein, Podolsky and Rosen, do not apply in our world.[2] Most physicists who have examined the issue concur that experiments, such as those of Alain Aspect and his group, have confirmed that physical probabilities, as predicted by quantum theory, do exhibit the phenomena of Bell-inequality violations that are considered to invalidate EPRs preferred local hidden-variables type of explanation for the correlations to which EPR first drew attention... Wikipedia, EPR Paradox: bit.ly/143zLP6 *Bells theorem: bit.ly/1A3lkIS
Posted on: Sat, 03 Jan 2015 03:32:54 +0000
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