Ψ Schrödinger's Cat
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Ψ Schrödinger's cat is a famous illustration of the principle in quantum theory of superposition (1935). Schrödinger's cat serves to demonstrate the apparent conflict between what quantum theory tells us is true about the nature & behavior of matter on the microscopic level & what we observe to be true about the nature & behavior of matter on the macroscopic level.
Ψ Here's Schrödinger's (theoretical) experiment: 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 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 & kill the cat.
Ψ 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, the cat is both dead & alive according to quantum law, in a superposition of states. It is only when we break open the box and learn the condition of the cat that the superposition is lost, and the cat becomes one or the other (dead or alive).
• This situation is sometimes called quantum indeterminacy or the observer's paradox : the observation or measurement itself affects an outcome, so that the outcome as such does not exist unless the measurement is made. (That is, there is no single outcome unless it is observed.)
Ψ We know that superposition actually occurs at the subatomic level, because there are observable effects of interference, in which a single particle is demonstrated to be in multiple locations simultaneously. What that fact implies about the nature of reality on the observable level (cats, for example, as opposed to electrons) is one of the stickiest areas of quantum physics. Schrödinger himself is rumored to have said, later in life, that he wished he had never met that cat.
Ψ Schrödinger invented this experiment, not to explain what happens to the cat, but to show that the existing theory had confusing aspects to it. Scientists understand a lot, but the more they learn the more new unanswered questions occur to them. Link: Schrödinger's Cat (Print Ready Version)
Robert C. Gates