The thought experiment created by the
Austrian physicist
Erwin Schrödinger to illustrate, among other things, a practical effect of the
Heisenberg Uncertainty Principle, which is related to the general area of
Quantum Mechanics.
Electrons can orbit atomic nucleii at, and only at, specific radii. Quanta are representations of the discrete amounts of
energy which either are absorbed or emitted when electrons either jump up to a higher orbit or fall back to a lower orbit.
Werner Heisenberg was a pioneer in quantum mechanics. Among his many contributions, he proved that the more precisely you know the position of any object (he was specifically dealing with atomic particles) the less you can know about its momentum. You can know where it is, or where it's going, but not both things. Shortly stated this is because the act of measurement itself will alter one or the other of these properties. Formally this is expressed as principal exclusivity between Dirac delta functions and sine functions.
Schrödinger co-pioneered quantum mechanics with Heisenberg and created the wave form equation which expresses it. He recognized how non-intuitive this concept is. To illustrate this difficulty he wrote:
One can even set up quite ridiculous cases. A cat is penned up in a steel chamber, along with the following diabolical 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 one 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 hydrochloric 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 first atomic decay would have poisoned it. The wave 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.
So. The experiment consisted of placing three things in a sealed box: a cat, a vial of poisonous gas, and a radioactive mineral. The experiment is set up so that two conditions are true:
1. if the radioactive mineral decays it will release the gas in some way and thus kill the cat, and
2. there is a 50/50 chance of the mineral decaying in the limited time the experiment takes up.
According to the wave form equation, you can't tell what will happen, only the probability that some outcome will occur. The probability becomes realized only upon opening the box and observing whether the cat lives (in formal terms, the wave form collapses). Another way of stating this is that the cat is both alive and dead until the observer opens the box, at which time it is either alive or dead. And was so all along!
Precisely the same experiment occurs here:
Let's say you shoot one photon at a photographic plate divided into two regions. There is a 50% chance of it hitting section A, and a 50% chance of it hitting section B. Until you develop and look at the plate, these are the probabilities, and both exist at the same time.
When you look at the plate, one of two things happen, depending on what school of Quantum Physics you belong to. There is the Copenhagen Interpretation (so named because that was Einstein's brand of chewing tobacco), which states that when you look at the plate, the wave form will "collapse" and the probability of the photon hitting section A will "jump" to one, while the probability of the photon hitting section B goes to zero. There is also the Many Worlds Interpretation. Here, when you look at the plate, the universe splits into two parallel universes, one where the photon hits A and one where it hits B. The Many Worlds Interpretation is the basis for the popular television show "Melrose Place" (or is it 90210?).
(I have quoted heavily here from a post by Arthur Rudolph to the website www.galactic-quide.com, who was obviously rather eloquent).
Schrödinger's Cat is not one of the great unsolved mathematical puzzles such as Poincare's Conjecture. The Clay Mathematics Institute (www.claymath.org) has not offered a million-dollar Millenium Prize for its proof, as it has for a proof of a solution to the P versus NP Problem. Nor has anyone offered to feed, or bury, the cat.