Schrödinger’s cat is the name given an imaginary experiment proposed in 1935 by the Austrian physicist Erwin Schrödinger . Erwin Schrödinger was one of the founders of quantum mechanics, a branch of physics that describes the smallest units of matter and their behavior. He devised the thought experiment to reveal difficulties with one of the main interpretations of quantum mechanics , known as the Copenhagen interpretation.
The rules of quantum mechanics seem to allow that certain particle systems exist in two contradictory states at the same time. For example, imagine that a set of two elementary particles may exist in one of two states. Now imagine each of the two states is equally likely. We cannot know for sure which state the system is in without making an observation or measurement. According to common sense, we might think that the system already exists in one state or the other, whether we know it or not. But such a system can actually behave as if it is in a mixture of the two states. According to the Copenhagen interpretation, the system exists in a blend of the two states until observed. When an observation is made, the system is said to “collapse” into a single definite state.
In devising his thought experiment, Schrödinger hoped to reveal that the Copenhagen interpretation was absurd by applying it to objects far larger than elementary particles. He imagined a cat in a sealed box. The box also held a flask of poison, a hammer mechanism, a sample of radioactive material, and a Geiger counter. A Geiger counter is a device used to detect radiation . The device was set up so that if the radioactive source emitted (gave off) radiation, the Geiger counter would detect it, triggering the hammer mechanism to break the flask. This action would release the poison, killing the cat.
Schrödinger’s mechanism could be set up so the radioactive sample might or might not emit radiation over a given time. According to the Copenhagen interpretation, such a system could be said to exist in both states until observed—that is, it could either be in a state in which the sample had emitted radiation, leading to the death of the cat, or a state in which the sample had not emitted radiation, leaving the cat alive. Before the box was opened and an observation made, therefore, the cat could be said to be both dead and alive.
Schrödinger hoped that this absurd idea would force scientists to rethink the Copenhagen interpretation and propose other interpretations of quantum mechanics. Some physicists have defended the Copenhagen interpretation by claiming that the Geiger counter, or the cat, could function as the “observer” of the radioactive emission. The system’s state would thus collapse before the box was even unsealed.
The cat experiment was simply a work of imagination. But real experiments have shown that systems larger than particles can be held in a blend of quantum states, collapsing into a single state upon measurement.