Physicist, philosopher, and public figure who helped to establish the modern science of quantum mechanics, out of which came the famous indeterminacy, or uncertainty, principle, for which he received the Nobel Prize for Physics in 1932. He also made important contributions to the theories of the hydrodynamics of turbulence, the atomic nucleus, ferromagnetism, cosmic rays, and elementary particles; and he planned the first post-World War II German nuclear reactor, at Karlsruhe, West Germany.
In June 1925, while recuperating from an attack of hay fever on Helgoland, an island in the North Sea, Heisenberg solved a major physical problem--how to account for the stationary (discrete) energy states of an anharmonic oscillator. His solution, because it was analogous to that of a simple planetary atom, launched the program for the development of the quantum mechanics of atomic systems. (Quantum mechanics is the science that accounts for discrete energy states--as in the light of atomic spectra--and other forms of quantized energy, and for the phenomenon of stability exhibited by atomic systems.) Heisenberg published his results some months later in the Zeitschrift fur Physik under the title "Uber quantentheoretische Umdeutung kinematischer und mechanischer Beziehungen" ("About the Quantum-Theoretical Reinterpretation of Kinetic and Mechanical Relationships"). In this article he proposed a reinterpretation of the basic concepts of mechanics.
Heisenberg's treatment of the problem departed from Bohr's as much as Bohr's had from 19th-century tenets. Heisenberg was willing to sacrifice the idea of discrete particles moving in prescribed paths (neither particles nor paths could be observed) in exchange for a theory that would deal directly with experimental facts and lead to the quantum conditions as consequences of the theory rather than ad hoc stipulations. Physical variables were to be represented by arrays of numbers; under the influence of Einstein's paper on relativity (1905), he took the variables to represent not hidden, inaccessible structures but "observable" (i.e., measurable) quantities. Born saw that the arrays obeyed the rules of matrix algebra; he, Pascual Jordan, and Heisenberg were able to express the new theory in terms of this branch of mathematics, and the new quantum theory became matrix mechanics. Each (usually infinite-dimensional) matrix of the theory specified the set of possible values for a physical variable, and the individual terms of a matrix were taken to generate probabilities of occurrences of states and transitions among states. Heisenberg used the new matrix mechanics to interpret the dual spectrum of the helium atom (that is, the superposed spectra of its two forms, in which the spins of the two electrons are either parallel or antiparallel), and with it he predicted that the hydrogen molecule should have analogous dual forms. With others, he also addressed many atomic and molecular spectra, ferromagnetic phenomena, and electromagnetic behaviour. Important alternative forms of the new quantum theory were proposed in 1926 by Erwin Schrodinger (wave mechanics) and P.A.M. Dirac (transformation theory).
In 1927 Heisenberg published the indeterminacy, or uncertainty, principle. The form he derived appeared in a paper that tried to show how matrix mechanics could be interpreted in terms of the intuitively familiar concepts of classical physics. If q is the position coordinate of an electron (in some specified state), and p its momentum, assuming that q, and independently, p have been measured for many electrons (all in the particular state), then, Heisenberg proved,
q p > h,
where q is the standard deviation of measurements of q, p is the standard deviation of measurements of p, and h is Planck's constant (6.626176 10-27 erg-second). Indeterminacy principles are characteristic of quantum physics; they state the theoretical limitations imposed upon any pair of noncommuting (i.e., conjugate) variables, such as the matrix representations of position and momentum; in such cases, the measurement of one affects the measurement of the other. The enormous significance of the indeterminacy principle is recognized by all scientists; but how it is to be understood physically--whether it depends on using intuitive classical ("complementary") pictures of a quantum system, or whether it is a principle in (a new kind of quantum) statistics, or whether in some sense through the special properties of the mathematical model it also describes a character of individual quantum systems--has been and still is much disputed. Bohr took the principle to apply to the complementary pictures of a quantum system--as a particle or as a wave pocket in classically intuited space; Heisenberg originally took the principle to apply to the nonintuitive properties of quantum, as distinct from classical, systems.
Bohr and Heisenberg elaborated a philosophy of complementarity to take into account the new physical variables and an appropriate measurement process on which each depends. This new conception of the measurement process in physics emphasized the active role of the scientist, who, in making measurements, interacted with the observed object and thus caused it to be revealed not as it is in itself but as a function of measurement. Many physicists, including Einstein, Schrodinger, and Louis de Broglie, refused to accept the philosophy of complementarity.
Although he early, and indirectly, came under the influence of Ernst Mach, Heisenberg, in his philosophical writings about quantum mechanics, vigorously opposed the Logical Positivism developed by philosophers of science of the Vienna Circle. According to Heisenberg, what was revealed by active observation was not an absolute datum, but a theory-laden datum--i.e., relativized by theory and contextualized by observational situations. He took classical mechanics and electromagnetics, which articulated the objective motions of bodies in space-time, to be permanently valid, though not applicable to quantum mechanical systems; he took causality to apply in general not to individual quantum mechanical systems but to mathematical representations alone, since particle behaviour could be predicted only on the basis of probability.
Heisenberg married Elisabeth Schumacher in 1937; they had seven children. He loved music in addition to physics and saw a deep affinity between these two interests. He also wrote philosophical works, believing that new insights into the ancient problems of Part and Whole and One and Many would help discovery in microphysics. Widely acknowledged as one of the seminal thinkers of the 20th century, Heisenberg was honoured with the Max Planck Medal, the Matteucci Medal, and the Barnard College Medal of Columbia University. He died in Munich on February 1, 1976.
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