Werner (Karl) Heisenberg

Physicist and philosopher, b. 5 December 1901 (Würzburg, Germany), d. 1 February 1976 (Munich)

As a student Werner Heisenberg was keenly interested in nuclear physics and followed the research of the most prestigious physics institutions of his time. He studied at the University of Munich under Arnold Sommerfeld and graduate in 1923 with a thesis on turbulence in fluid streams. One of his co-students was Daniel Pauli, with whom he maintained a lifelong friendship. Both went to the University of Göttingen to study under Max Born and in late 1924 to the Institute for Theoretical Physics in Copenhagen to study under Niels Bohr.

Bohr had developed a model of the atom that from 1913 had become generally accepted as a great step forward but proved unable to explain new observations. One problem in particular was its inability to account for the discrete energy states of atomic particles. Heisenberg found the solution in 1925 and published it in the Zeitschrift für Physik under the title Über quantentheoretische Umdeutung kinematischer und mechanischer Beziehungen ("About the Quantum-Theoretical Reinterpretation of Kinetic and Mechanical Relationships").

Heisenberg's approach was pragmatic. Rather than assuming that some forces yet hidden to human understanding were responsible for the experimental problems (as Bohr and 19th century physics had done) he represented observed values of variables in mathematical arrays and noticed that the arrays obeyed the rules of matrix algebra. This led to matrix mechanics as the successor to classical mechanics. Each matrix (which is usually of infinite dimensions) specified the set of possible values for a physical variable, and the individual terms of a matrix represented probabilities of occurrences of states and transitions among states. Using his new method Heisenberg correctly predicted details of the behaviour of the hydrogen molecule.

Heisenberg's attempt to interpret matrix mechanics with the more intuitive concepts of classical mechanics led him in 1927 to the formulation of the indeterminacy or uncertainty principle that that any arrangement for the measurement of the position of a particle precludes the simultaneous measurement of its momentum and vice versa. Although the mathematical foundations of the principle are solid and its significance beyond doubt, its detailed meaning is still a matter of debate. In the interpretation of Bohr and Heisenberg it meant a new concept of the measurement process in which the scientist, in making measurements, interacts with the observed object; as a consequence an object is never seen in its true nature but only as it appears through the measurement. Many physicists, including Einstein, Schršdinger, and Louis de Broglie, refused to accept the philosophy of complementarity.

In 1927 Heisenberg was appointed professor at the University of Leipzig. He left the university in 1941 to become director of the Kaiser Wilhelm Institute (later renamed Max Planck Institute) for Physics in Berlin. He did not support Adolf Hitler's National; Socialist (Nazi) Party but did not publicly oppose its policies. He continued to work on a project to develop nuclear energy but did not participate in the programme to develop a nuclear bomb.

When the war ended Heisenberg became the director of the Max Planck Institute for Physics and Astrophysics at Göttingen and transferred the institute to Munich in 1958. In 1954 he became the German representative for the establishment of CERN (Conseil Européen pour le Recherche Nucleaire , the European Organization for Nuclear Research, today the world's largest particle physics centre, located in Geneva, Switzerland).

Heisenberg wrote extensively on the philosophical implications of quantum mechanics and is regarded as one of the most influential thinkers of the 20th century. His writings focussed on the question of causality and human ability to reveal the true character of nature through experiment. He saw causality as not applicable to individual quantum mechanical systems, because particle behaviour could be predicted only on the basis of probability; causality could only be found in the mathematical representations.

Reference

Heelan, P. A. (1995) Werner Heisenberg, Encyclopaedia Britannica 15th ed.

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