Quantum Theory
Quantum Theory, the theory that radiant energy is emitted (given off) and absorbed in units, or quanta, rather than in a steady stream. The quantum theory revolutionized physics in the early 1900's; its introduction is often considered the beginning of modern physics. The quantum theory has inspired some of the most brilliant scientific work of the 20th century, and a number of Nobel Prizes have been awarded for achievements related to the theory.
Max Planck, a German physicist, originated the quantum theory in 1900. Certain studies that had been made of radiant energy led Planck to the conclusion that radiant energy is not emitted in a steady stream, like water from a hose, but in pieces, or units (which he called quanta), like bullets from a machine gun. His theory startled the scientific world, which had accepted the concept of classical physics that the emission and absorption of energy, like all other physical processes, takes place in a continuous manner.
In 1905 Albert Einstein helped establish the quantum theory with his explanation of the photoelectric effect—the ejection of electrons from a metal surface exposed to light. Einstein showed that the energy of light is concentrated into particle-like quanta. (According to classical physics, the energy of a light wave, like the energy of a water wave, should be evenly spread over the wave.) Today physicists usually treat light as a wave in processes that involve its transmission, and as quanta, called photons, in processes that involve its emission or absorption.
The quantum theory has led to important discoveries about photoelectricity, photochemistry, the specific heats of solids, and the structure of atoms and certain activities of atoms and to many other findings about matter and energy. The science that applies the quantum theory to matter is called quantum, or wave, mechanics.