Schrödinger's Cat (and Wigner's Friend)
To understand and resolve the paradox of Schrödinger's Cat, it helps to understand that Erwin Schrödinger invented his paradox to poke holes in some implications of quantum physics that he regarded as unacceptable. Like Albert Einstein, Max Planck, and others prominent in the new physics, Schrödinger disliked the indeterminacy, the element of chance and apparent loss of strict causality, in quantum mechanics.
Schrödinger's diabolical thought experiment was designed to amplify microscopic quantum uncertainty into the macroscopic world.
The I-PHI key to dissolving the paradox is to focus on the information present at each time in the experiment.
In standard quantum theory, an isolated system is prepared in a known state at time t. This consists of making a quantum measurement on the system and finding the experimental value for some observable quantity S(t). The future development of the system is completely described by a time evolution operator H(t) which yields a complex probability function ψ(t). This is the "wave function" invented by Schrödinger, whose formulation of quantum mechanics is called wave mechanics.
Without any further observation, the best knowledge we have of the system state at later times depends on the (real) square of this (complex) probability amplitude function. If there are a finite number of states, we can calculate the probability of finding the system in each state. Schrödinger's thought experiment imagines two possible states for the cat, alive and dead. His ghoulish Geiger counter apparatus is arranged to have a fifty percent chance of detecting an unpredictable radioactive decay and releasing cyanide to kill the cat in one hour.
So what is known, what information exists in the world, at that time one hour into the experiment?
For an external observer, complete knowledge is the paradoxical superposition of the two ψ-functions for live and dead cats. If we really did this fiendish experiment many times, our estimated probabilities would agree very well with the total observed outcomes.
But what about the cat as a participant observer? For example, if the cat is killed one minute into the experiment, information will be encoded in the universe, information that can be read later in the cat's autopsy. The collapse of the wave function when the atom decayed could also have been registered on a chart recorder monitoring the Geiger counter electrical output. More details on Schrödinger's Cat is in the Experiments section.
These are two examples of how the physical universe can be its own observer. Whenever information is encoded in information structures, we do not need the consciousness of physicists to collapse the wave function and make up the mind of the universe, as Werner Heisenberg, Eugene Wigner, John Wheeler, and others speculated.
In his "Remarks on the Mind-Body Question," Wigner imagines a friend who performs an experiment (it was a randomly generated flash of light, but it could be a Schrödinger's cat experiment) while Wigner is out of the room. When Wigner comes into the room, he learns the outcome. During the time only his friend knew, he wonders whether the state of the system is a superposition of "flash seen" and "flash not seen," or was it known at some previous point?
Clearly the most Wigner could know would be the probabilities he would calculate from the "flash seen" and "flash not seen" wave functions. But equally clearly, Wigner sees that the wave functions have collapsed, since the information has already been encoded in the mind of his friend.
He then substitutes a machine for his friend and decides that the situation is again indeterminate, with just a much larger box. Now it is his consciousness that collapses the wave function. He concludes (incorrectly) not only that mind is necessary to observe the physical universe, but also that mind is not material, a strange contribution to the classic mind-body problem.
Erwin Schrödinger's thought experiment:
One can even set up quite ridiculous cases. A cat is penned up in a steel chamber, along with the following device (which must be secured against direct interference by the cat): in a Geiger counter there is a tiny bit of radioactive substance, so small, that perhaps in the course of the hour one of the atoms decays, but also, with equal probability, perhaps none; if it happens, the counter tube discharges and through a relay releases a hammer which shatters a small flask of hydrocyanic acid. If one has left this entire system to itself for an hour, one would say that the cat still lives if meanwhile no atom has decayed. The psi-function of the entire system would express this by having in it the living and dead cat (pardon the expression) mixed or smeared out in equal parts. It is typical of these cases that an indeterminacy originally restricted to the atomic domain becomes transformed into macroscopic indeterminacy, which can then be resolved by direct observation. That prevents us from so naively accepting as valid a "blurred model" for representing reality. In itself it would not embody anything unclear or contradictory. There is a difference between a shaky or out-of-focus photograph and a snapshot of clouds and fog banks.
Eugene Wigner's Consciousness:
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 without reference to the consciousness. All that quantum mechanics purports to describe are probability connections between subsequent impressions (also called ‘apperceptions’) of consciousness, and even though the dividing line between the observer, whose consciousness is being affected, and the observed physical object can be shifted towards one or the other to a considerable degree, it cannot be eliminated.
Werner Heisenberg's comments on knowledge of the observer:
The laws of nature which we formulate mathematically in quantum theory deal no longer with the particles themselves but with our knowledge of the elementary particles. The conception of objective reality … evaporated into the … mathematics that represents no longer the behavior of elementary particles but rather our knowledge of this behavior.