Eugene Wigner made quantum physics even more subjective than had John von Neumann and Erwin Schrödinger with his famous Schrödinger's Cat Paradox. Wigner claimed that a quantum measurement requires a conscious observer, without which nothing ever happens in the universe.Although Einstein mentioned conservation in the original EPR paper, it is noticeably absent from most later work. Compare Wigner, writing on the problem of measurement in 1963:
When the province of physical theory was extended to encompass microscopic phenomena, through the creation of quantum mechanics, the concept of consciousness came to the fore again: it was not possible to formulate the laws of quantum mechanics in a fully consistent way without reference to the consciousness. All that quantum mechanics purports to provide are probability connections between subsequent impressions (also called "apperceptions") of the consciousness, and even though the dividing line between the observer, whose consciousness is being affected, and the observed physical object can be shifted towards the one or the other to a considerable degree, it cannot be eliminated. It may be premature to believe that the present philosophy of quantum mechanics will remain a permanent feature of future physical theories; it will remain remarkable, in whatever way our future concepts may develop, that the very study of the external world led to the conclusion that the content of the consciousness is an ultimate reality.In 1961 complicated the problem of the "Schnitt" of von Neumann (or the "shifty split" of John Bell) that forms the dividing line between the quantum world and the classical measurement apparatus. Wigner moved it farther into the conscious mind of the observer. Wigner extended the problem of Schrödinger's Cat, by adding a second observer outside the laboratory who is commonly known as Wigner's Friend. The physicist inside the lab opens the box and observes either a live or dead cat. But Wigner's friend outside the lab does not know the outcome, and is said to leave the world in a superposition of states "dead cat/sad friend" and "live cat/happy friend."
Wigner on the problem of measurement and the EPR experimentWigner was rare among physicists in mentioning conservation laws in his discussion of the Einstein-Podolsky-Rosen experiment.
If a measurement of the momentum of one of the particles is carried out — the possibility of this is never questioned — and gives the result p, the state vector of the other particle suddenly becomes a (slightly damped) plane wave with the momentum -p. This statement is synonymous with the statement that a measurement of the momentum of the second particle would give the result -p, as follows from the conservation law for linear momentum. The same conclusion can be arrived at also by a formal calculation of the possible results of a joint measurement of the momenta of the two particles.
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