Heisenberg, Werner (1901 - 1976)German Physicist who helped to establish quantum mechanics and made important contributions to the theory of turbulence, the atomic nucleus, ferromagnetism, cosmic rays, and elementary particles; co-author with Niels Bohr of the Principle of Complementarity, and noted for the well-known “Heisenberg Uncertainty (or Indeterminacy) Principle”, Heisenberg did not share the widespread sceptical interpretation placed on this principle, which many have sought to use to substantiate subjectivist interpretations of modern physics.

Heisenberg studied physics with his life-ling friend Wolfgang Pauli, under Arnold Sommerfeld at the University of Munich and did his PhD on turbulence. Following Pauli to the University of Göttingen, he studied there under Max Born and in 1924 went to study under Niels Bohr in Copenhagen.

Heisenberg was aware of growing problems with Bohr's model of the atom and wanted to develop a new model to cope with the growing contradictions.

In 1925, Heisenberg solved problem of how to account for the discrete energy states of an anharmonic oscillator which opened the way for an alternative explanation of the discrete energy levels founds in Bohr's model of the atom and a new interpretation of the basic concepts of quantum mechanics. Heisenberg took as unobservable the supposed trajectory of a particle between its interactions in order to be able to construct a theory which deal only with the measurable interaction events. Consequently, physical variables would be represented by discrete

arraysof numbers. Under the influence of Albert Einstein's paper on relativity, he took the variables to represent onlymeasurablequantities. Born showed that these arrays obeyed the rules ofmatrix algebra, and with Heisenberg, named the new quantum theory “matrix mechanics”. Each matrix specified the possible values for a physical variable, and the terms of a matrix were taken to generate probabilities of occurrences of states and transitions. Using the matrix mechanics to interpret the spectrum of the helium atom and other atomic and molecular spectra, ferromagnetic phenomena, and electromagnetic behaviour, Heisenberg demonstrated the validity of the conception experimentally.In 1927, Heisenberg published the Indeterminacy, or Uncertainty, Principle in which he endeavoured to relate the matrix-entities to the intuitively familiar concepts of classical physics. If

qis the position-coordinate of an electron, andpa measurement of its momentum, then delta-q* delta-p>h(Planck's constant), where delta-qand delta-pare the standard deviation of measurements ofpandq. One of the characteristics of matrix algebra is that thelaw of commutation for multiplicationdoes not hold (a*b not= b*a). Momentum and position are thus characterised as "non-commuting variables", from which it follows that the determination of each of the two variables cannot be deemed to make senseindependentlyof one another, the two entities cannot have a separate meaning independent of one another; the Indeterminacy Principle stated above gives a definite quantitative measure to this degree of interdependence. This conception has a close parallel to Einstein's discovery that measurements of space and time cannot be conceived of measuring entitiesindependentof one another.This result has been interpreted by subjectivist writers to mean that in some way what is at issue is the Mind of the person carrying out the measurement which is determining the interaction, but this idea hinges on a total misunderstanding of the issue. The problem for conception of these processes arises from the fact that for everyday, pictorial thinking, position and momentum are

distinctentities; in relation to quantum phenomena, these concepts are meaningful only in relation to interactions of a particle, and quite distinct interactions are implied in the measurement of momentum or position. This is not at all the case in everday experience.Initially, Heisenberg had arrived at the matrix-mechanics through the solution of mathematical problems and did not see the matrices as representations of particulate properties such as momentum and position. It was Bohr who showed how the Indeterminacy Principle expressed the relationship between the wave and particle conceptions of quantum phenomena and gave a quantitative expression to the Complementarity Principle.

Bohr and Heisenberg generalised the principle of complementarity to take account of a range of physical variables and the measurement process on which each depends. The principle and the difficulties which flow from its interpretation, was the subject of intense controversy among all the great physicists of that time, with Einstein, Schrödinger and Louis de Broglie all disputing Bohr and Heisenberg's interpretation of the principle of complementarity.

>From 1927 to 1941 Heisenberg worked at the University of Leipzig and from 1941 to 1945 in Berlin. He never publicly opposed the Nazi regime and worked with Otto Hahn on the development of a nuclear reactor though he failed to develop an effective program for nuclear weapons. After the war he became director of the Max Planck Institute for Physics and Astrophysics at Göttingen, moving with the institute to Munich.

After the War Heisenberg began work on spinors, complex vector-like representations, hoping to find universal symmetries in nature whcih would explain the wide variety of elementary particles.

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, 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 interactions; 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.

Further Reading:See his History of Quantum Theory.