# Quantum theory

From AMS Glossary

## quantum theory

(Also quantum mechanics.) A theory of matter and radiation, developed in its essentials mostly between 1900 and 1930, distinguished from classical theory (or classical mechanics, Newtonian mechanics) in two important respects: discreteness and indeterminism.

According to classical theory, measurable physical variables such as energy and momentum can have a continuous set of values; according to quantum theory, however, these variables can have only a discrete set of values (and hence are said to be quantized). The dynamical laws of classical theory are deterministic: Given the exact initial state of a system, its future state is in principle exactly determined by dynamical laws. Quantum theory is inherently probabilistic: Its dynamical laws yield only the probabilities that certain discrete values for physical variables will be measured for an ensemble of similarly prepared systems.

*See*energy level, photon.