Eugene Wigner made quantum physics even more subjective than had John von Neumann or even Erwin Schrödinger with his famous Cat Paradox. Wigner claimed that a quantum measurement requires the mind of a conscious observer, without which wave functions never collapse and nothing ever happens in the universe. He wrote: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:
Until not many years ago, the "existence" of a mind or soul would have been passionately denied by most physical scientists. The brilliant successes of mechanistic and, more generally, macroscopic physics and of chemistry overshadowed the obvious fact that thoughts, desires, and emotions are not made of matter, and it was nearly universally accepted among physical scientists that there is nothing besides matter. The epitome of this belief was the conviction that, if we knew the positions and velocities of all atoms at one instant of time, we could compute the fate of the universe for all future. Even today, there are adherents to this view though fewer among the physicists than — ironically enough — among biochemists. There are several reasons for the return, on the part of most physical scientists, to the spirit of Descartes's "Cogito ergo sum," which recognizes the thought, that is, the mind, as primary. First, the brilliant successes of mechanics not only faded into the past; they were also recognised as partial successes, relating to a narrow range of phenomena, all in the macroscopic domain.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.Wigner 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 is often said to have extended the problem of Schrödinger's Cat, by adding a second observer inside the laboratory who is commonly known as Wigner's Friend. Popular treatments of Wigner's Friend usually describe him as observing a superposition of live and dead cat. Actually, Wigner's example was a photon and whether its wave function collapsed to cause a flash visible to his friend or not. Wigner's goal was to show that only consciousness can collapse a wave function. Let's use the cat example, because it is more vivid. You can see Wigner's original argument on the Wigner's Friend page.. The physicist friend inside the lab opens the box and observes either a live or dead cat. But Wigner is outside the lab and does not know the outcome. Wigner says this seems to leave the world in a superposition of states - "dead cat/sad friend" and "live cat/happy friend." Wigner says that any inanimate material measuring device is left in a superposition of states. This would include his friend and himself, but for human consciousness. He resolves his paradox by saying that consciousness collapses the wave function, both his friend's inside the laboratory and his own. The information interpretation of quantum mechanics helps to resolve this paradox as follows,
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.