Schrodinger’s Cat

In one version of the famous Schrodinger cat gedanken experiment, a healthy cat is placed inside an ideal cat playroom that is isolated from the rest of the world whenever the door is closed. Inside is one deadly object left by mistake. The door is closed. After some time elapses, one wonders whether the cat is alive or dead, the two classical possibilities. Rule 2 of QM tells us, however, that the state of the cat is Ψ = ψ₁ + ψ₂, where ψ₁ means alive and ψ₂ means dead. So QM requires us to consider the cat as being alive and dead simultaneously! However, you are curious. You push a button that opens the door just enough so you could look in to determine the status of the cat. You could peek in but you decide not to. Now what does QM predict for Ψ?

Answer

In QM, it is irrelevant whether you actually peek or not. If in principle you could have determined the status of the cat, QM reduces to the classical result by rule 3. The cat is now either alive or dead, not both. The two QM alternatives reduce to just one possibility.

Note that this example with the cat brings the connection between the non-intuitive behavior of Nature on the microscopic scale up to the macroscopic scale of our everyday experiences. There has been an enormous amount of controversy over this example and its interpretation. Some of the issues are discussed in the references listed below.