The problem faced by quantum theorists can best be seen in the famous paradox. “Who killed Schrödinger’s cat?” In a hypothetical formulation, a kitten is put in a closed box with a jar of poison and a triphammer poised to smash the jar. The hammer is activated by a counter that records random events, such as radioactive decay. The experiment lasts just long enough for there to be a probability of one-half that the hammer will be released. Quantum mechanics represents the system mathematically by the sum of a live-cat and a dead-cat function, each with a probability of one-half. The question is whether the act of looking (the measurement) kills or saves the cat, since before the experimenter looks in the box both solutions are equally likely. This lighthearted example reflects a deep conceptual difficulty. In more formal terms, a complex system can only be described by using a probability distribution that relates the possible outcomes of an experiment. In order to decide among the various alternatives, a measurement is required. This measurement is what constitutes an event, as distinguished from the probability which is a mathematical abstraction. However, the only simple and consistent description physicists were able to assign to a measurement involved an observer’s becoming aware of the result. Thus the physical event and the content of the human mind were inseparable.

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