Schrödinger’s cat is a famous illustration of the Copenhagen interpretation of superposition in quantum theory. The illustration was proposed by Nobel Prize-winning Austrian physicist Erwin Schrödinger in 1935 to point out the illogical paradox between what quantum theorists held to be true about the nature and behavior of matter on the microscopic level and what the average person observes to be true about the nature and behavior of matter on the macroscopic level, that which is visible to the unaided human eye.
The Copenhagen interpretation of quantum physics, which was the prevailing theory at the time, proposed that atoms or photons exist in multiple states that correspond with different possible outcomes — and perhaps most importantly, they do not commit to a definite state until they are observed. Schrödinger tried to show what the Copenhagen interpretation would look like if the mathematics and microscopic world typically used to explain superposition were replaced with large objects the average person could understand. His “thought experiment” featured a cat, some poison and a box.
This is a somewhat simplified version of Schrödinger’s analogy, intended to illustrate the flawed interpretation of quantum superposition:
We place a living cat into a steel chamber, along with a device containing a vial of hydrocyanic acid. There is, in the chamber, a very small amount of hydrocyanic acid, a radioactive substance. If even a single atom of the substance decays during the test period, a relay mechanism will trip a hammer, which will, in turn, break the vial and kill the cat.
The observer cannot know whether or not an atom of the substance has decayed, and consequently, cannot know whether the vial has been broken, the hydrocyanic acid released, and the cat killed. Since we cannot know, according to quantum law, the cat is both dead and alive, in what is called a superposition of states.
According to the Copenhagen interpretation, it is only when the box is opened and someone observes the condition of the cat that the superposition is resolved, and the cat becomes one or the other (dead or alive). The cat’s ability to be both alive and dead until it is observed was referred to as quantum indeterminacy or the observer’s paradox. The Copenhagen interpretation maintained that observation/measurement itself affects an outcome, so the final outcome does not exist until the observation/measurement is made.
Schrödinger’s thought experiment was designed to make someone wonder: Is observation really necessary? Wouldn’t the cat be either dead or alive whether or not the box was opened? Schrödinger acknowledged that superposition actually occurs at the microscopic level, because there are observable effects of interference, in which a single particle is demonstrated to be in multiple locations simultaneously. His questioning of what that fact implies about the nature of reality on the observable level (cats, for example, as opposed to electrons) remains one of the stickiest areas of quantum physics — as does the larger question of when do microscopic possibilities resolve into a particular macroscopic state.
Throughout the years, Schrödinger’s cat analogy has been used to illustrate differences between emerging theories in quantum mechanics. In the Many Worlds interpretation, for example, the cat is both alive and dead because the observer and the cat represent two realities, one in which the cat is dead and one in which the cat is alive.
Schrödinger himself is rumored to have said, later in life, that he wished he had never met that cat.