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Definition: Heisenberg, Werner Karl from Philip's Encyclopedia

German physicist, one of the pioneers of quantum mechanics. He studied under Max Born and Niels Bohr, and in 1926 developed a form of quantum theory based on matrix algebra, known as matrix mechanics. Paul Dirac showed that Heisenberg's theory was equivalent to Erwin Schrödinger's notion of wave mechanics. In 1927, Heisenberg published his famous uncertainty principle. He received the 1932 Nobel Prize in physics.

Summary Article: Heisenberg, Werner (Karl) (1901-1976)
From The Hutchinson Dictionary of Scientific Biography

Place: Germany

Subject: biography, physics

German physicist who founded quantum mechanics and the uncertainty principle. In recognition of these achievements, he was awarded the 1932 Nobel Prize for Physics.

Heisenberg was born on 5 December 1901 in Duisberg. He showed an early aptitude for mathematics and when, after leaving school, he came across a book on relativity by Hermann Weyl entitled Space, Time, And Matter (1918), he became interested in the mathematical arguments underlying physical concepts. This led Heisenberg to study theoretical physics under Arnold Sommerfeld at Munich University. There he formed a lasting friendship with Wolfgang Pauli. He obtained his doctor's degree in 1923 and immediately became assistant to Max Born at Göttingen. From 1924 to 1926, Heisenberg worked with Niels Bohr in Copenhagen and then in 1927 he was offered the chair of theoretical physics at Leipzig at the age of only 26.

Heisenberg stayed at Leipzig until 1941 and then from 1942 to 1945 was director of the Max Planck Institute for Physics in Berlin. During World War II he was in charge of atomic research in Germany and it is possible that he may have been able to direct efforts away from military uses of nuclear power. In 1946 Heisenberg was made director of the Max Planck Institute for Physics in Göttingen, which in 1958 was moved to his home city of Munich. He held the post of director until 1970, and died in Munich on 1 February 1976.

In the early 1920s, there were burning questions relating to the quantum theory. Bohr had used the quantum concept of Max Planck to explain the spectral lines of atoms in terms of the energy differences of electron orbits at various quantum states, but there were weaknesses in the results. In 1922 Pauli began to throw doubt on them and Sommerfeld, although an optimist, could see some flaws in Bohr's work.

In 1923 Heisenberg went to Göttingen to work with Born and used a modification of the quantum rules to explain the anomalous Zeeman effect (see Pieter Zeeman), in which single spectral lines split into groups of closely spaced lines in a strong magnetic field. Information about atomic structure can be deduced from the separation of these lines.

Heisenberg was concerned not to try to picture what happens inside the atom but to find a mathematical system that explained the properties of the atom - in this case, the position of the spectral lines given by hydrogen, the simplest atom. Born helped Heisenberg to develop his ideas, which he presented in 1925 as a system called matrix mechanics. By mathematical treatment of values within matrices or arrays, the frequencies of the lines in the hydrogen spectrum were obtained. This was the first precise mathematical description of the workings of the atom and with it Heisenberg is regarded as founding quantum mechanics, which seeks to explain atomic structure in mathematical terms.

The following year, however, Erwin Schrödinger produced a system of wave mechanics that accounted mathematically for the discovery made in 1923 by Louis de Broglie that electrons do not occupy orbits but exist as standing waves around the nucleus. Wave mechanics was a much more convenient system than the matrix mechanics of Heisenberg and Born and rapidly replaced it, though John Von Neumann showed the two systems to be equivalent in 1944.

Nevertheless, Heisenberg was able to predict from studies of the hydrogen spectrum that hydrogen exists in two allotropes - ortho-hydrogen and para-hydrogen - in which the two nuclei of the atoms in a hydrogen molecule spin in the same or opposite directions respectively. The allotropes were discovered in 1929.

In 1927 Heisenberg made the discovery for which he is best known - that of the uncertainty principle. This states that it is impossible to specify precisely both the position and the simultaneous momentum (mass multiplied by velocity) of a particle. There is always a degree of uncertainty in either, and as one is determined with greater precision, the other can only be found less exactly. Multiplying the degrees of uncertainty of the position and momentum yield a value approximately equal to Planck's constant. (This is a consequence of the wave-particle duality discovered by de Broglie.) Heisenberg's uncertainty principle negates cause and effect; it maintains that the result of an action can be expressed only in terms of the probability that a certain effect will occur. The idea was revolutionary and discomforted even Albert Einstein, but it has remained valid.

Another great discovery was made in 1927, when Heisenberg solved the mystery of ferromagnetism. In it he employed the Pauli exclusion principle, which states that no two electrons can have all four quantum numbers the same. Heisenberg used the principle to show that ferromagnetism is caused by electrostatic intereaction between the electrons. This was also a major insight that has stood the test of time.

Heisenberg was a brilliant scientist with a very incisive mind. He will be remembered with gratitude and respect for solving some of the great and complex problems in quantum mechanics that were so crucial at the beginning of the 20th century for our understanding of nuclear physics.

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