It is tempting but hazardous for non-physicists to offer opinions on quantum mechanics. Having succumbed to that temptation in the post ‘Light’ a few days ago, I figure I may as well continue.

Schrödinger’s Cat is a thought experiment that, at least as popularly understood, brings the conceptual problem of a superposition of states at the quantum level into the familiar macroscopic world of classical physics by linking the fate of a cat to a quantum mechanical process. Suppose a radioactive atom is in a superposition of states, having in some sense both decayed and not decayed, until it is observed. Or, perhaps, until its having decayed or not has any potential to affect phenomena beyond itself. Suppose we make a device that will kill a cat if the atom has decayed, or not if it has not. Then we make a box that is perfectly impenetrable to observation. We can know nothing of what goes on inside the box until it is opened. We put the cat and the quantum device into the box, shut the lid, and the cat is in the same superposition of states as the radioactive atom. Although originally intended as a reductio ad absurdum of the Copenhagen Interpretation, it is now generally viewed more as an illustration of the counterintuitive nature of quantum mechanics.

Suppose we simplify the situation. Omit the radioactive atom, omit the device that will kill the cat or not based on the state of the atom. We just make a box, let’s call it Box 1, that prevents us from telling if there is a live or dead cat inside of it. Because I like cats, let’s also put in a feeding, watering, and waste disposal system that is, all else equal, sufficient to keep the cat in good health. We put the cat in the box and close the lid. There is some duration-dependent probability that the cat will die. Over a few hours or days, the probability is very low. If the experiment is extended across decades the probability will reach 1 at some point. Suppose we have an understanding of the cat sufficient to estimate the probability that the cat is alive at any given point in time.

OK, now let’s make Box 2 and put the quantum device in. Just to make the situation a little more straightforward, we’ll calibrate it so that at any given point in time the probability that the the device will be triggered and cause the cat’s death is equal to the probability of the cat having died of natural (well, non-quantum, at least; calling anything in this experiment ‘natural’ seems a bit strained) causes. We find some identical cats, stick one in Box 1 and the other in Box 2. Although they are identical, presumably we put the cat of which we are more fond in Box 1. We close the boxes, wait a month, then we open them. The probability that the cat has died in Box 2 is twice the probability that the cat has died in Box 1.

There’s always some probability of finding a dead cat when you open one of these boxes. Adding the quantum device only changes the probability, it doesn’t introduce a probability when there was none before.

While Box 1 is closed, to imagine that the cat is alive, or even to say that it must be alive or dead right at that moment, is to imagine an observer who can see into the box. We are describing what such an observer would see. This violates the rules of the thought experiment; we are no longer imagining an observationally impenetrable box, but one whose contents can be watched. So long as we are following the rules, there is no internally consistent representation of the situation that includes concurrent knowledge about what’s going on inside the closed box and outside of it. You can’t imagine a box no one can see into and imagine the cat in it. But it’s very difficult not to imagine the cat… it’s hard to get our minds to follow the rules of the thought experiment.

The implied observer has to be forced into the light. But, once seen, through whose eyes?