8.1 Schrö­din­ger’s Cat

Schrö­din­ger, apparently not an animal lover, came up with an example illustrating what the conceptual difficulties of quantum mechanics really mean in everyday terms. This section describes the example.

A cat is placed in a closed box. Also in the box is a Geiger counter and a tiny amount of radioactive material that will cause the Geiger counter to go off in a typical time of an hour. The Geiger counter has been rigged so that if it goes off, it releases a poison that kills the cat.

Now the decay of the radioactive material is a quantum-mechanical process; the different times for it to trigger the Geiger counter each have their own probability. According to the orthodox interpretation, measurement is needed to fix a single trigger time. If the box is left closed to prevent measurement, then at any given time, there is only a probability of the Geiger counter having been triggered. The cat is then alive, and also dead, each with a nonzero probability.

Of course no reasonable person is going to believe that she is looking at a box with a cat in it that is both dead and alive. The problem is obviously with what is to be called a measurement or observation. The countless trillions of air molecules are hardly going to miss observing that they no longer enter the cat's nose. The biological machinery in the cat is not going to miss observing that the blood is no longer circulating. More directly, the Geiger counter is not going to miss observing that a decay has occurred; it is releasing the poison, isn't it?

If you postulate that the Geiger counter is in this case doing the “measurement“ that the orthodox interpretation so deviously leaves undefined, it agrees with our common sense. But of course, this Deus ex Machina only rephrases our common sense; it provides no explanation why the Geiger counter would cause quantum mechanics to apparently terminate its normal evolution, no proof or plausible reason that the Geiger counter is able to fundamentally change the normal evolution of the wave function, and not even a shred of hard evidence that it terminates the evolution, if the box is truly closed.

There is a strange conclusion to this story. The entire point Schrö­din­ger was trying to make was that no sane person is going to believe that a cat can be both dead and kicking around alive at the same time. But when the equations of quantum mechanics are examined more closely, it is found that they require exactly that. The wave function evolves into describing a series of different realities. In our own reality, the cat dies at a specific, apparently random time, just as common sense tells us. Regardless whether the box is open or not. But, as discussed further in section 8.6, the mathematics of quantum mechanics extends beyond our reality. Other realities develop, which we humans are utterly unable to observe, and in each of those other realities, the cat dies at a different time.