Popper's experiment
[3] As a matter of fact, Popper greatly intensified his research activities on the foundations of quantum mechanics throughout the 1950s and 1960s developing his interpretation of quantum mechanics in terms of real existing probabilities (propensities),[4] also thanks to the support of a number of distinguished physicists (such as David Bohm).[5] In 1980, Popper proposed perhaps his more important, yet overlooked, contribution to QM: a "new simplified version of the EPR experiment".[6] The experiment was however published only two years later, in the third volume of the Postscript to the Logic of Scientific Discovery.It maintains that observations lead to a wavefunction collapse, thereby suggesting the counter-intuitive result that two well separated, non-interacting systems require action-at-a-distance.Popper argued that such non-locality conflicts with common sense, and would lead to a subjectivist interpretation of phenomena, depending on the role of the 'observer'.The actual realisation of Popper's experiment required new techniques which would make use of the phenomenon of spontaneous parametric down-conversion but had not yet been exploited at that time, so his experiment was eventually performed only in 1999, five years after Popper had died.Popper's experiment of 1980 exploits couples of entangled particles, in order to put to the test Heisenberg's uncertainty principle.The beam's low intensity is "so that the probability is high that two particles recorded at the same time on the left and on the right are those which have actually interacted before emission.The Heisenberg scatter for both the beams of particles going to the right and to the left, is tested "by making the two slits A and B wider or narrower.The coming into play of these counters is indicative of the wider scattering angles which go with a narrower slit, according to the Heisenberg relations.And since it is, according to the Copenhagen interpretation, our knowledge which is described by the theory – and especially by the Heisenberg relations — it should be expected that the momentumIf the Copenhagen interpretation is correct, then such counters on the far side of B that are indicative of a wide scatter (and of a narrow slit) should now count coincidences: counters that did not count any particles before the slit A was narrowed.[11]Popper was inclined to believe that the test would decide against the Copenhagen interpretation, as it is applied to Heisenberg's uncertainty principle.[12][13] Although it pointed to a crucial flaw in Popper's argument, its full implication was not understood.Kripps theoretically analyzed Popper's experiment and predicted that narrowing slit A would lead to momentum spread increasing at slit B. Kripps also argued that his result was based just on the formalism of quantum mechanics, without any interpretational problem.They showed that once the uncertainty in the position of the source is taken into account, the blurring introduced washes out the Popper effect.Popper's experiment was realized in 1999 by Yoon-Ho Kim & Yanhua Shih using a spontaneous parametric down-conversion photon source.They concluded that:"Popper and EPR were correct in the prediction of the physical outcomes of their experiments.However, Popper and EPR made the same error by applying the results of two-particle physics to the explanation of the behavior of an individual particle.Our experimental result is emphatically NOT a violation of the uncertainty principle which governs the behavior of an individual quantum."[17]This led to a renewed heated debate, with some even going to the extent of claiming that Kim and Shih's experiment had demonstrated that there is no non-locality in quantum mechanics.... Popper's experiment and its analysis forces us to radically change the current held view on quantum non-locality."[19]Short criticized Kim and Shih's experiment, arguing that because of the finite size of the source, the localization of particle 2 is imperfect, which leads to a smaller momentum spread than expected.[20] However, Short's argument implies that if the source were improved, we should see a spread in the momentum of particle 2.[citation needed] Sancho carried out a theoretical analysis of Popper's experiment, using the path-integral approach, and found a similar kind of narrowing in the momentum spread of particle 2, as was observed by Kim and Shih.[21] Although this calculation did not give them any deep insight, it indicated that the experimental result of Kim-Shih agreed with quantum mechanics.Popper's conjecture has also been tested experimentally in the so-called two-particle ghost interference experiment.Popper's proposed experiment, if carried out in this way, is incapable of testing the Copenhagen interpretation of quantum mechanics.The expected additional momentum scatter which Popper wrongly attributed to the Copenhagen interpretation would allow faster-than-light communication, which is excluded by the no-communication theorem in quantum mechanics.
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