Werner Heisenberg

Werner Heisenberg (1901-1976), German physicist and Nobel Prize winner, who played a large part in the development of quantum mechanics (see Quantum Theory). Quantum mechanics describes matter in terms of both particles and waves. One of Heisenberg’s best known contributions to quantum theory is the uncertainty principle, which states that the exact position and velocity of a particle cannot both be known at the same time—the more precisely one value is known, the greater the range of possibilities that exist for the other.

Heisenberg was born in Würzburg, Germany. His family moved to Munich in 1910, where Heisenberg received his early education. In the summer of 1920 he graduated from a Munich gymnasium (the German equivalent to a United States high school) and entered the University of Munich. During his first two years of studies, he published four physics research papers, making Heisenberg—at age 20—one of the top contributors to theoretical physics research. Heisenberg finished his undergraduate and graduate work in three years, and in 1923 presented his doctoral dissertation on turbulence in streams of fluid.

In his early career Heisenberg was at the forefront of dramatic changes taking place in the field of quantum mechanics. He studied with three leading quantum theorists at three major centers of quantum research of that time: German physicist Arnold Sommerfeld at the University of Munich; German physicist Max Born at the University of Göttingen in 1923; and, from 1924 to 1927, Danish physicist Niels Bohr at the Institute for Theoretical Physics in Copenhagen.

Heisenberg developed the first version of quantum mechanics, called matrix mechanics, in 1925. His version explained the motion of electrons (tiny negatively charged particles) in an atom in purely mathematical terms (see Atom). His equations showed why electrons behave the way they do, which scientists had been unable to explain before. Heisenberg realized that the laws of classical physics did not govern events on the quantum level. For example, electrons do not follow the laws of classical physics and orbit the nucleus of an atom in a defined path, as planets orbit the Sun.

Heisenberg's matrix mechanics predicted that molecular hydrogen (hydrogen made up of pairs of atoms, sharing their electrons to form molecules) should exist in two distinct forms, called orthohydrogen and parahydrogen. These two forms result from a property of atoms called spin, a kind of angular momentum. In 1925 Heisenberg predicted that the spin of the two hydrogen atoms was the same in parahydrogen, and opposite each other in orthohydrogen. Other scientists soon confirmed his prediction experimentally. Heisenberg won the 1932 Nobel Prize in physics for his development of quantum mechanics and his prediction of the two types of molecular hydrogen.

With the development of matrix mechanics, Heisenberg became one of the founders of quantum mechanics. At about the same time Heisenberg developed matrix mechanics, Austrian physicist Erwin Schrödinger developed a way to describe particles in terms of the probability that any of their characteristics would be a certain value. Schrödinger later showed that both his approach and Heisenberg’s approach yielded the same result.

In 1927 Heisenberg became a professor of theoretical physics at the University of Leipzig. That year he published a paper explaining the uncertainty principle, which stemmed from his matrix mechanics. Using calculations that explain the motion of particles, he showed that it is impossible to know accurately both the velocity and position of a particle at the same time. The more accurately scientists measure one quantity, the more uncertainty exists in the measurement of the other. The consequence of the uncertainty principle is that a description in quantum mechanics is limited to a statement of the relative probability of a value rather than exact numbers.

In 1941 Heisenberg became a professor at the University of Berlin and director of the Kaiser Wilhelm Institute for Physics. During World War II (1939-1945) he chose to remain in Nazi Germany while many of his colleagues fled the country. He was the leader of Germany's atomic research team, despite his opposition to Nazi policies. He worked with Otto Hahn, one of the discoverers of nuclear fission, but the German team failed to develop nuclear weapons.

At the end of the war the United States arrested Heisenberg for his role in the German weapons program and detained him for nine months in England. Following his return to Germany in 1946, he became professor of physics and the director of the Max Planck Institute for Physics and Astrophysics (the former Kaiser Wilhelm Institute) in Göttingen. The institute moved to Munich in 1958, and Heisenberg moved with it, continuing as its director until his death.