Einstein: His Life and Universe

by Walter Isaacson

Schrödinger asked in his thought experiment, when does the system stop being in a superposition incorporating both states and snap into being one reality? This question led to the precarious fate of an imaginary creature, which was destined to become immortal whether it was dead or alive, known as Schrödinger’s cat: 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.23 Einstein was thrilled. “Your cat shows that we are in complete agreement concerning our assessment of the character of the current theory,” he wrote back. “A psi-function that contains the living as well as the dead cat just cannot be taken as a description of a real state of affairs.”