Werner Heisenberg

Werner Heisenberg the German theoretical physicist (1901-1976) who founded matrix mechanics, the first complete formulation of quantum mechanics, and who stated the uncertainty principle that bears his name. He received the 1932 Nobel Prize in Physics and later led a German research programme in nuclear physics.

Early Life and Education

Werner Karl Heisenberg was born on 5 December 1901 in Wuerzburg, Germany, the son of a professor of Greek studies. He studied physics at the University of Munich under Arnold Sommerfeld and completed his doctorate in 1923 with a thesis on turbulence. He then spent a formative period with Max Born at Goettingen and with Niels Bohr in Copenhagen.

Heisenberg soon showed himself to be one of the most original minds in physics. In the summer of 1925, seeking a theory founded only on observable quantities, he devised a mathematical scheme of arrays of numbers that, in collaboration with Born and Pascual Jordan, was recognised as matrix mechanics. This was the first internally consistent formulation of the quantum theory. He was only twenty-three years old, and the work at once established his reputation among the physicists of Europe.

Matrix Mechanics and the Uncertainty Principle

Heisenberg's starting point was the rejection of unobservable concepts such as the definite position and period of an electron in an atom. His scheme reproduced the frequencies and intensities of spectral lines without any picture of orbits. With Schroedinger's wave mechanics, published the following year and shown to be mathematically equivalent, it completed the edifice of quantum mechanics. The two formulations were later shown by Dirac to be special cases of a more general transformation theory.

In 1927 Heisenberg derived the uncertainty principle, according to which the more precisely the position of a particle is known, the less precisely its momentum can be determined, and vice versa. The principle is a consequence of the quantum formalism rather than a limitation on instruments, and it marks a fundamental departure from classical determinism, showing that nature itself sets a limit on the precision of simultaneous measurements. It follows directly from the non-commuting character of the operators that represent position and momentum. The principle has consequences far beyond the atom: it explains why the electron cannot simply fall into the nucleus, why the energy of an excited state has a natural spread, and why the vacuum itself seethes with transient fluctuations of energy, and it shapes the design of instruments from microscopes to gravitational-wave detectors.

The Copenhagen Interpretation and the Solvay Debates

The uncertainty principle became a cornerstone of the Copenhagen interpretation, developed in close dialogue with Bohr, which holds that quantum mechanics describes probabilities and that physical attributes exist only in relation to measurement. Heisenberg's account of the interpretation, and the thought experiments he devised, helped to define the terms of the debates that followed. For many physicists the principle came to stand for the wider claim that the classical ideal of a world fully determined in every detail had to be abandoned.

At the Solvay Conferences of 1927 and 1930 he defended the new theory against the objections of Einstein, who refused to accept that God played dice with the universe. Though Einstein raised ingenious challenges, Heisenberg and Bohr were able to answer them, and the statistical interpretation came to be widely accepted. These exchanges remain among the most celebrated in the history of physics.

Nobel Prize and the War Years

Heisenberg was awarded the 1932 Nobel Prize in Physics, received in 1933, for the creation of quantum mechanics. He was appointed professor at Leipzig at the age of twenty-five and later worked in Berlin and, after the war, in Goettingen and Munich. His interests widened to include the theory of the atomic nucleus, cosmic radiation and the unification of elementary particles. He also gave the first theoretical account of ferromagnetism in terms of the exchange interaction between electron spins, a model that still bears his name.

During the Second World War he led a German research project on nuclear energy and the possibility of nuclear weapons, a role that has been much debated by historians. The programme failed to produce a weapon, and the reasons, whether technical difficulty, lack of resources or reservations about the goal, remain disputed. He was detained by Allied forces at the end of the war and returned to Germany in 1946. The wartime episode and the later controversy over his intentions coloured the last decades of his life.

Influence and Legacy

Heisenberg's formulations gave quantum mechanics its first rigorous shape and his uncertainty principle changed how physicists think about knowledge itself. He wrote influential books on the philosophy of science, exploring the relation between language, observation and physical theory, and he cultivated an ideal of physics as a search for underlying mathematical order.

He spent his last years working on a unified field theory of elementary particles and on philosophical questions. He died in Munich on 1 February 1976. He is remembered as one of the central figures of the quantum revolution, a physicist whose insights were inseparable from the philosophical questions they raised, and whose name is attached to the most famous statement of the limits of measurement.

See also

• Uncertainty principle
• Matrix mechanics
• Copenhagen interpretation
• Quantum revolution