Quantum mechanics, the branch of mathematical physics that deals with atomic and subatomic systems and their interaction with radiation in terms of observable quantities. It is an outgrowth of the concept that all forms of energy are released in discrete units or bundles called quanta.

Quantum mechanics is concerned with phenomena that are so small-scale that they cannot be described in classical terms. Throughout the 1800s most physicists regarded Isaac Newton's dynamical laws as sacrosanct, but it became increasingly clear during the early years of the 20th century that many phenomena, especially those associated with radiation, defy explanation by Newtonian physics. It has come to be recognized that the principles of quantum mechanics rather than those of classical mechanics must be applied when dealing with the behaviour of electrons and nuclei within atoms and molecules. Although conventional quantum mechanics makes no pretense of describing completely what occurs inside the atomic nucleus, it has helped scientists to better understand many processes such as the emission of alpha particles and photodisintegration. Moreover, the field theory of quantum mechanics has provided insight into the properties of mesons and other subatomic particles associated with nuclear phenomena.

In the equations of quantum mechanics, Max Planck's constant of action h =
6.626 10^{-34} joule-second plays a central role. This constant,
one of the most important in all of physics, has the dimensions energy
time. The term "small-scale" used to delineate the domain of quantum
mechanics should not be literally interpreted as necessarily relating to
extent in space. A more precise criterion as to whether quantum
modifications of Newtonian laws are important is whether or not the
phenomenon in question is characterized by an "action" (i.e., time
integral of kinetic energy) that is large compared to Planck's constant.
Accordingly, if a great many quanta are involved, the notion that there is
a discrete, indivisible quantum unit loses significance. This fact
explains why ordinary physical processes appear to be so fully in accord
with the laws of Newton. The laws of quantum mechanics, unlike Newton's
deterministic laws, lead to a probabilistic description of nature. As a
consequence, one of quantum mechanics' most important philosophical
implications concerns the apparent breakdown, or at least a drastic
reinterpretation, of the causality principle in atomic phenomena.

The history of quantum mechanics may be divided into three main periods. The first began with Planck's theory of black-body radiation in 1900; it may be described as the period in which the validity of Planck's constant was demonstrated but its real meaning was not fully understood. The second period began with the quantum theory of atomic structure and spectra proposed by Niels Bohr in 1913. Bohr's ideas gave an accurate formula for the frequency of spectral lines in many cases and were an enormous help in the codification and understanding of spectra. Nonetheless, they did not represent a consistent, unified theory, constituting as they did a sort of patchwork affair in which classical mechanics was subjected to a somewhat extraneous set of so-called quantum conditions that restrict the constants of integration to particular values. True quantum mechanics appeared in 1926, reaching fruition nearly simultaneously in a variety of forms--namely, the matrix theory of Max Born and Werner Heisenberg, the wave mechanics of Louis V. de Broglie and Erwin Schrdinger, and the transformation theory of P.A.M. Dirac and Pascual Jordan. These different formulations were in no sense alternative theories; rather, they were different aspects of a consistent body of physical law.

*Excerpt from the Encyclopedia Britannica without permission.*