vĕrˈnər hīˈzənbĕrk, 1901–76, German physicist. One of the founders of the quantum theory, he is best known for his uncertainty principle, or indeterminacy principle, which states that it is impossible to determine with arbitrarily high accuracy both the position and momentum (essentially velocity) of a subatomic particle like the electron. The effect of this principle is to convert the laws of physics into statements about relative probabilities instead of absolute certainties. In 1926, Heisenberg developed a form of the quantum theory known as matrix mechanics, which was quickly shown to be fully equivalent to Erwin Schrödinger's wave mechanics. His 1932 Nobel Prize in Physics cited not only his work on quantum theory but also work in nuclear physics in which he predicted the subsequently verified existence of two allotropic forms of molecular hydrogen, differing in their values of nuclear spin.

Heisenberg was a student of Arnold Sommerfeld, an assistant to Max Born, and later a close associate of Niels Bohr. He taught at the universities of Leipzig (1927–41) and Berlin (1942–45). During World War II he headed German efforts in nuclear fission research, which failed to develop a nuclear reactor or atomic bomb. Although he claimed after the war to have had qualms about building nuclear weapons, it seems likely that the reasons Germany failed to do so were technical and logistical.

In 1958 he became director of the Max Planck Institute for Physics and Astrophysics, now located in Munich. His later work concerned the so-called S-matrix approach to nuclear forces and the possibility that space and time are quantized, or granular, in structure. His Physics and Philosophy (1962) and Physics and Beyond (1971) remain popular accounts of the revolutions in modern physics.

See D. C. Cassidy, Uncertainty: The Life and Science of Werner Heisenberg (1993); R. P. Brennan, Heisenberg Probably Slept Here: The Lives, Times and Ideas of the Great Physicists of the 20th Century (1996).

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