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
a conscious observer, without which nothing ever happens in the universe.
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 he 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."
The information interpretation of quantum mechanics
helps to resolve this paradox as follows,
If the physicist inside the lab clearly looks inside the box and is seen to record the result in a notebook, we can safely conclude that the superposition of states has "collapsed" (been projected) into either the dead cat or live cat state.
New information has been created in the universe. Entropy has been radiated away, so the change is irreversible.
We can assume that an observation has been made, recorded as a measurement, and, to satisfy Wigner, von Neumann, Wheeler, Bell. and others, the measurement has entered the mind of the "conscious observer," though the information interpretation does nor require this step.The collapse occurs with the new information, without the need for observations or measurements.
Since Wigner's friend does not know the actual outcome, he only knows the possibilities and can say, for example, that there is a 75% chance the cat is alive and 25% probability the cat is dead.
But here is the resolution of Wigner's paradox. These probabilities are no longer about superposed quantum states interfering with one another. They are no longer quantum probabilities. The cat is either dead or alive! The chances are no longer ontological. They are epistemic, just human ignorance.
So Wigner is wrong to conclude that the cat remains in a quantum superposition of live and dead cat states.
Nor was the cat ever in such a superposition! It was our calculations of nuclear decay that used the superposition as our best estimates. And it was important to include the possible interference effects while the wave functions (for the decaying nucleus) were still coherent. Once we get information about the nuclear decay, the wave functions decohered and we must switch to "classical" probabilities.
Wigner on the problem of measurement and the EPR experiment
Wigner was rare among physicists in mentioning conservation laws in his discussion of the Einstein-Podolsky-Rosen
Although Einstein mentioned conservation in the original EPR paper, it is noticeably absent from most later work. Compare Wigner, writing on the
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.
One can go even further: instead of measuring the linear momentum of one particle, one can measure its angular momentum about a fixed axis. If this measurement yields the value mℏ, the state vector of the other particle suddenly becomes a cylindrical wave for which the same component of the angular momentum is -mℏ. This statement is again synonymous with the statement that a measurement of the said component of the angular momentum of the second particle certainly would give the value -mℏ. This can be inferred again from the conservation law of the angular momentum (which is zero for the two particles together) or by means of a formal analysis. Hence, a "contraction of the wave packet" took place again.
It is also clear that it would be wrong, in the preceding example, to say that even before any measurement, the state was a mixture of plane waves of the two particles, traveling in opposite directions. For no such pair of plane waves would one expect the angular momenta to show the correlation just described. This is natural since plane waves are not cylindrical waves, or since [the state vector has] properties different from those of any mixture. The statistical correlations which are clearly postulated by quantum mechanics (and which can be shown also experimentally, for instance in the Bothe-Geiger experiment) demand in certain cases a "reduction of the state vector." The only possible question which can yet be asked is whether such a reduction must be postulated also when a measurement with a macroscopic apparatus is carried out. [Considerations] show that even this is true if the validity of quantum mechanics is admitted for all systems.