**Erwin** 1887–1961

Austrian physicist who founded the study of wave mechanics when he developed a mathematical equation that describes the wavelike behavior of subatomic particles. Schrödinger's equation was fundamental to Paul Dirac's development of quantum mechanics, and he and Dirac shared the Nobel Prize for physics in 1933.

*A Closer Look**Schrödinger's cat*is a thought experiment proposed by the physicist and philosopher Erwin Schrödinger. It shows how quantum-mechanical indeterminacy at a microscopic level can cause indeterminacy at a macroscopic level. The indeterminacy can be resolved by observation but entails a paradox. Schrödinger would have us imagine a cat inside a closed box with a tiny bit of a radioactive substance and an apparatus consisting of a Geiger counter, hammer, and flask of cyanide. Over the course of an hour, there is a chance that the radioactive substance might emit an alpha particle, and an equal chance that it might not. If an alpha particle is emitted and hits the Geiger counter, a relay is set in motion whereby the hammer shatters the flask, releasing the gas and killing the cat. Thus, after an hour, there is equal probability of the cat being alive or dead, and an observer can open the box and see which state the cat is in. But until the observation is made, the two possible states, which are mutually exclusive, coexist. In quantum-mechanical terms they are in a condition of

*superposition*. The act of observation changes that, and one state becomes established to the exclusion of the other. In other words, without being observed, the cat does not exist in a particular state at all. It is neither alive nor dead, or it is both alive and dead, depending on how you want to look at it. This thought experiment provides an extreme case of the condition in which small-scale objects always exist. Under the laws of quantum mechanics, electrons, photons, and all other particles are each in a number of superposed states that interact with each other, forming interference patterns, and giving rise to an overall behavior that often seems wavelike. But a measurement made to determine the state that the particle is in destroys the interference patterns and gives rise to different behavior that seems more like that of tiny particles; hence the phenomenon of

*wave-particle duality.*Whether the cat in the box can also be considered an observer in quantum mechanics currently remains unclear.