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Heisenberg, Werner Karl

theory mechanics quantum position

[hiy znberg] (1901–76) German physicist: developed quantum mechanics and discovered the uncertainty principle.

Heisenberg, son of a professor of Greek at the University of Munich, was educated at Munich and Göttingen. He worked with in Göttingen, and in Copenhagen. In 1927 he returned to a professorship in Germany at Leipzig.

Heisenberg was a major creative figure among those who revolutionized physics by quantum mechanics. At 24 he formulated a non-relativistic form of the theory of quantum mechanics, producing the matrix mechanics version, and received the 1932 Nobel Prize for physics for this work. An equivalent theory called wave mechanics was produced independently by in 1925 and they were shown to be equivalent .

Heisenberg broke away from the visual concept of the atom and avoided problems such as the apparent wave-particle duality by considering only observable quantities of the atom as ‘real’. He separated in the theory the system of interest and operations on that system to produce an observable quantity. Respectively these were expressed as a matrix and a mathematical operation on the matrix to give a value. He used the theory to predict successfully the observed frequencies and intensities of atomic and molecular spectral lines. He concluded that two forms of molecular hydrogen, called ortho-and para-hydrogen, exist with their nuclear spins aligned in the former and opposed in the latter.

In 1927 Heisenberg discovered a further aspect of quantum mechanics, the principle of uncertainty; that it is impossible to determine exactly both the position and momentum of a particle simultaneously. The uncertainty in position ? x and in momentum ? p obey ? x ? p = h /4p where h is the constant. This relation removed absolute determinacy, or cause and effect, from physics for the first time and replaced it with a statistical probability. This deeply troubled and some others, but is now generally accepted claim that the future of the universe could in principle be deduced from the position and velocity of all particles if given at one instant was rejected. For example: to try to locate the accurate position of an electron, radiation of short wavelength (such as gamma rays) might be bounced off it. However, such energetic rays will radically alter the electron’s momentum on collision, so that certainty in its position is attempted at the expense of that in momentum. In 1932, after had discovered the neutron, Heisenberg proposed that a nucleus of protons and neutrons was a more satisfactory model than one of protons and electrons, as had been assumed. The components of the nucleus should be held together by quantum mechanical exchange forces, which was later confirmed by theory of the strong nuclear interaction by which pi-mesons were exchanged. Later Heisenberg put forward a unified field theory of elementary particles (1966) which received little general support.

During the Nazi period, Heisenberg chose to remain and preserve the German scientific tradition, though he was not a Nazi supporter. He was attacked by the Nazis for refusing to reject in any way Einstein’s physics. As a consequence he lost the chance of the professorship at Munich in 1935 as successor. During the war Heisenberg was called to lead the atomic energy and weapons programme, becoming director of the Kaiser Wilhelm Institute in Berlin in 1941. After the war he helped establish the Max Planck Institute at Göttingen and moved with it to Munich in 1955 as its director.

Heisenberg’s wartime role is controversial. He claimed to have had no intention of allowing an atomic bomb to reach Hitler’s hands, and stated that in such a key role he could have diverted the programme if it ever neared success. He claimed to have revealed this to in 1941, but Bohr said that he failed to understand Heisenberg’s guarded comments. The weapon was not produced by Germany probably because of a higher priority for planes and flying bombs.

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