Heisenberg, Werner

Heisenberg, Werner (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.

Heisenberg, Werner

born Dec. 5, 1901, Wurzburg, Ger.

died Feb. 1, 1976, Munich

in full Werner Karl Heisenberg German physicist and philosopher who discovered a way to formulate quantum mechanics in terms of matrices (1925). For that discovery, he was awarded the Nobel Prize for Physics for 1932. In 1927 he published his indeterminacy, or uncertainty, principle, upon which he built his philosophy and for which he is best known. He also made important contributions to the theories of the hydrodynamics of turbulence, the atomic nucleus, ferromagnetism, cosmic rays, and elementary particles, and he planned the first post-World War II German nuclear reactor, at Karlsruhe, then in West Germany.

In his philosophical and methodological writings, Heisenberg was much influenced by Niels Bohr and Albert Einstein. From the former he derived the concepts of the social and dialogical character of scientific invention; the principle of correspondence (pragmatic and model-theoretical continuity) between macrophysics and microphysics; the permanence, though not the universality, of classical physics; the “interactive,” rather than passive, role of the scientific observer in microphysics; and, consequently, the contextualized character of microphysical theories. From Einstein he derived the concepts of simplicity as a criterion of the central order of nature; scientific realism (i.e., science describing nature itself, not merely how nature can be manipulated); and the theory-ladenness of scientific observations. He was coauthor with Bohr of the philosophy of complementarity. In his later work he conceived of a central order in nature, consisting of a set of universal symmetries expressible in a single mathematical equation for all systems of particulate matter. As a public figure, he actively promoted the peaceful use of nuclear energy after World War II and, in 1957, led other German scientists in opposing a move to equip the West German Army with nuclear weapons. He was, in 1954, one of the organizers of the Conseil Européen pour la Recherche Nucléaire (CERN; later, Organisation Européene pour la Recherche Nucléaire) in Geneva.

Early life

Heisenberg studied physics, together with Wolfgang Pauli, his lifelong friend and collaborator, under Arnold Sommerfeld at the University of Munich and completed his doctoral dissertation (1923) on turbulence in fluid streams. Heisenberg followed Pauli to the University of Göttingen and studied there under Max Born; then, in the fall of 1924, he went to the Institute for Theoretical Physics in Copenhagen to study under Bohr.

Heisenberg's interest in Bohr's model of the planetary atom and his comprehension of its limitations led him to seek a theoretical basis for a new model. Bohr's concept—after 1913 the centrepiece of what has come to be called the old quantum theory—had been based on the classical motion of electrons in well-defined orbits around the nucleus, and the quantum restrictions had been imposed arbitrarily to bring the consequences of the model into conformity with experimental results. As a summary of existing knowledge and as a stimulus to further research, the Bohr atomic model had succeeded admirably, but the results of new research were becoming more and more difficult to reconcile with it.

In June 1925, while recuperating from an attack of hay fever on Helgoland, an island in the North Sea, Heisenberg solved a major physical problem—how to account for the stationary (discrete) energy states of an anharmonic oscillator. His solution, because it was analogous to that of a simple planetary atom, launched the program for the development of the quantum mechanics of atomic systems. (Quantum mechanics is the science that accounts for discrete energy states—as in the light of atomic spectra—and other forms of quantized energy, and for the phenomenon of stability exhibited by atomic systems.) Heisenberg published his results some months later 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”). In this article he proposed a reinterpretation of the basic concepts of mechanics.

Heisenberg's treatment of the problem departed from Bohr's as much as Bohr's had from 19th-century tenets. Heisenberg was willing to sacrifice the idea of discrete particles moving in prescribed paths (neither particles nor paths could be observed) in exchange for a theory that would deal directly with experimental facts and lead to the quantum conditions as consequences of the theory rather than ad hoc stipulations. Physical variables were to be represented by arrays of numbers; under the influence of Einstein's paper on relativity (1905), he took the variables to represent not hidden, inaccessible structures but “observable” (i.e., measurable) quantities. Born saw that the arrays obeyed the rules of matrix algebra; he, Pascual Jordan, and Heisenberg were able to express the new theory in terms of this branch of mathematics, and the new quantum theory became matrix mechanics. Each (usually infinite-dimensional) matrix of the theory specified the set of possible values for a physical variable, and the individual terms of a matrix were taken to generate probabilities of occurrences of states and transitions among states. Heisenberg used the new matrix mechanics to interpret the dual spectrum of the helium atom (that is, the superposed spectra of its two forms, in which the spins of the two electrons are either parallel or antiparallel), and with it he predicted that the hydrogen molecule should have analogous dual forms. With others, he also addressed many atomic and molecular spectra, ferromagnetic phenomena, and electromagnetic behaviour. Important alternative forms of the new quantum theory were proposed in 1926 by Erwin Schrödinger (wave mechanics) and P.A.M. Dirac (transformation theory).

In 1927 Heisenberg published the indeterminacy, or uncertainty, principle. The form he derived appeared in a paper that tried to show how matrix mechanics could be interpreted in terms of the intuitively familiar concepts of classical physics. If q is the position coordinate of an electron (in some specified state), and p its momentum, assuming that q, and independently, p have been measured for many electrons (all in the particular state), then, Heisenberg proved,

Δq · Δp h,

where Δq is the standard deviation of measurements of q, Δp is the standard deviation of measurements of p, and h is Planck's constant (6.626176 × 10−27 erg-second). Indeterminacy principles are characteristic of quantum physics; they state the theoretical limitations imposed upon any pair of noncommuting (i.e., conjugate) variables, such as the matrix representations of position and momentum; in such cases, the measurement of one affects the measurement of the other. Theenormous significance of the indeterminacy principle is recognized by all scientists; but how it is to be understood physically—whether it depends on using intuitive classical (“complementary”) pictures of a quantum system, or whether it is a principle in (a new kind of quantum) statistics, or whether in some sense through the special properties of the mathematical model it also describes a character of individual quantum systems—has been and still is much disputed. Bohr took the principleto apply to the complementary pictures of a quantum system—as a particle or as a wave pocket in classically intuited space; Heisenberg originally took the principle to apply to the nonintuitive properties of quantum, as distinct from classical, systems.

Bohr and Heisenberg elaborated a philosophy of complementarity to take into account the new physical variables and an appropriate measurement process on which each depends. This new conception of the measurement process in physics emphasized the active role of the scientist, who, in making measurements, interacted with the observed object and thus caused it to be revealed not as it is in itself but as a function of measurement. Many physicists, including Einstein, Schrödinger, and Louis de Broglie, refused to accept the philosophy of complementarity.

Later life

From 1927 to 1941 Heisenberg was professor at the University of Leipzig. For the following four years, he was director of the Kaiser Wilhelm (now Max Planck) Institute for Physics in Berlin. Although he did not publicly oppose the Nazi regime, he was hostile to its policies. During World War II he worked with Otto Hahn, one of the discoverers of nuclear fission, on the development of a nuclear reactor. He failed to develop an effective program for nuclear weapons, probably from want of technical resources and lack of will to do so. After the war he organized and became director of the Max Planck Institute for Physics and Astrophysics at Göttingen, moving with theinstitute, in 1958, to Munich; he was also, in 1954, the German representative for the organizing of CERN.

In the postwar period Heisenberg began working on a fundamental spinor equation (a nonlinear differential equation capable of representing with spinors—complex vectorlike entities—all possible particulate states of matter). His intuitions had led him to postulate that such an equation would exhibit a basic set of universal symmetries in nature (a symmetry is a mathematic form invariant under groups of canonical space-time and other changes in the representing elements), and be capable of explaining the variety of elementary particles generated in high-energy collisions. In this work, the “Platonic” character of which he recognized, he had the support and collaboration of Hans-Peter Dürr and Carl Friedrich von Weizsäcker.

Although he early, and indirectly, came under the influence of Ernst Mach, Heisenberg, in his philosophical writings about quantum mechanics, vigorously opposed the Logical Positivism developed by philosophers of science of the Vienna Circle. According to Heisenberg, what was revealed by active observation was not an absolute datum, but a theory-laden datum—i.e., relativized by theory and contextualized by observational situations. He took classical mechanics and electromagnetics, which articulated the objective motions of bodies in space-time, to be permanently valid, though not applicable to quantum mechanical systems; he took causality to apply in general not to individual quantum mechanical systems but to mathematical representations alone, since particle behaviour could be predicted only on the basis of probability.

Heisenberg married Elisabeth Schumacher in 1937; they had seven children. He loved music in addition to physics and saw a deep affinity between these two interests. He also wrote philosophical works, believing that new insights into the ancient problems of Part and Whole and One and Many would help discovery in microphysics. Widely acknowledged as one of the seminal thinkers of the 20th century, Heisenberg was honoured with the Max Planck Medal, the Matteucci Medal, and the Barnard College Medal of Columbia University in addition to the Nobel Prize.

Patrick Aidan Heelan

Uncertainty Principle

Uncertainty Principle, in quantum mechanics, theory stating that it is impossible to specify simultaneously the position and momentum of a particle, such as an electron, with precision. Also called the indeterminacy principle, the theory further states that a more accurate determination of one quantity will result in a less precise measurement of the other, and that the product of both uncertainties is never less than Planck's constant, named after the German physicist Max Planck. Of very small magnitude, the uncertainty results from the fundamental nature of the particles being observed. In quantum mechanics, probability calculations therefore replace the exact calculations of classical mechanics.

Formulated in 1927 by the German physicist Werner Heisenberg, the uncertainty principle was of great significance in the development of quantum mechanics. Its philosophic implications of indeterminacy created a strong trend of mysticism among scientists who interpreted the concept as a violation of the fundamental law of cause and effect. Other scientists, including Albert Einstein, believed that the uncertainty involved in observation in no way contradicted the existence of laws governing the behavior of the particles or the ability of scientists to discover these laws.

Additional reading

Books by Heisenberg include The Physical Principles of the Quantum Theory (1930, reissued 1950; originally published in German, 1930), his most important work, containing themes of early papers amplified into a treatise, Philosophic Problems of Nuclear Science (1952, reissued 1966; originally published in German, 8th enlarged ed., 1949), a collection of his early essays, Physics and Philosophy: The Revolution in Modern Science (1958, reissued 1989), his Gifford lectures, Physics and Beyond (1971; originally published in German, 1969), a memoir of his early life, and Across the Frontiers (1974, reissued 1990; originally published in German, 1971), collected essays and occasional lectures. Biographical material is found in Armin Hermann, Werner Heisenberg, 1901–1976, trans. from German (1976); Carl Friedrich von Weizsäcker and Bartel Leendert van der Waerden, Werner Heisenberg (1977), in German; Elisabeth Heisenberg, Inner Exile: Recollections of a Life with Werner Heisenberg (1984; originally published in German, 1980); and David C. Cassidy, Uncertainty: The Life and Science of Werner Heisenberg (1992). Heisenberg's role in the German wartime atomic program is chronicled in Leslie R. Groves, Now It Can Be Told: Story of the Manhattan Project (1962, reprinted 1983). Collections of essays in honour of Heisenberg include Fritz Bopp (ed.), Werner Heisenberg und die Physik unserer Zeit (1961); Heinrich Pfeiffer (ed.), Denken und Umdenken: Zu Werk und Wirkung von Werner Heisenberg (1977); and Peter Breitenlohner and H. Peter Dürr (eds.), Unified Theories of Elementary Particles (1982). Studies of Heisenberg's philosophy of science include Patrick A. Heelan, Quantum Mechanics and Objectivity (1965); and Max Jammer, The Philosophy of Quantum Mechanics: The Interpretations of Quantum Mechanics in Historical Perspective (1974), and The Conceptual Development of Quantum Mechanics, 2nd ed. (1989), which provide the most complete study of Heisenberg's contribution to quantum mechanics.