Schrödinger's Cat

Schrödinger's Cat is a famous thought experiment conceived by Austrian physicist Erwin Schrödinger (1887–1961) in 1935 to highlight some of the paradoxes of the quantum picture of the subatomic world if applied to everyday experience. Schrödinger was motivated by a paper on the EPR paradox by Albert Einstein, Boris Podolsky, and Nathan Rosen that had appeared earlier that year. Schrödinger opposed the Copenhagen Interpretation of quantum mechanics developed by physicist Niels Bohr (1885–1951) and others, and Schrödinger regarded his thought experiment as a "ridiculous case" that challenged its rationality.

Quantum theory allows only probabilistic statements to be made about the expected outcome of a measurement or observation. We can predict only the probability of finding an electron in a particular state in the future even if we are in possession of all possible information about its present state. Schrödinger imagined observing a cat in a sealed room along with a Geiger counter sitting beside an occasional source of radioactivity. If the Geiger counter records one of these random radioactive decays then it triggers the release of poisonous gas, which kills the cat. If no radioactive decay occurs, the cat survives. The experiment ends after one hour, when we look in the room to see if the cat is dead or alive. According to the Copenhagen Interpretation of quantum mechanics, Schrödinger claims, before we look into the room the cat is described by a wave function that is some mixture of "dead cat" and "live cat." When and where does the half-dead-half-alive mixed cat state turn into the definite dead cat or live cat state that we discover on looking in the room? Who is the observer who produces the definite state? Is it the cat, the Geiger counter, or the person who looks in the room? How do we interpret the state of cat that is half-dead and half-alive before an observation takes place?

See also Copenhagen Interpretation; EPR Paradox; Paradox; Physics, Quantum

Bibliography

herbert, nick. quantum reality beyond the new physics. london: rider, 1985.

mermin, david. "is the moon there when nobody looks? reality and the quantum theory." physics today 38, no. 4 (1985): 38–47.

john d. barrow

Schrödinger's cat a paradox suggested in 1935 by the Austrian theoretical physicist Erwin Schrödinger (1887–1961), to illustrate the conceptual difficulties of quantum mechanics. Schrödinger described an experiment in which a cat is put into a sealed box containing a lethal device triggered by radioactive decay; an outside observer cannot know whether the device has been set off and the cat killed. According to quantum mechanics the cat is in an indeterminate state, some combination of alive and dead, until the box is opened, at which point it will be found to be one or the other.