**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.