WERNER HEISENBERG Nobel Lecture, December 11, 1933

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J.F.

WERNER HEISENBERG Nobel Lecture, December 11, 1933

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http://nobelprize.org/physics/laureates ... ecture.pdf
WERNER HEISENBERG
The development of quantum mechanics
Nobel Lecture, December 11, 1933
Quantum mechanics, on which I am to speak here, arose, in its formal content,
from the endeavour to expand Bohr?s principle of correspondence to
a complete mathematical scheme by refining his assertions. The physically
new viewpoints that distinguish quantum mechanics from classical physics
were prepared by the researches of various investigators engaged in analysing
the difficulties posed in Bohr?s theory of atomic structure and in the radiation
theory of light.
In 1900, through studying the law of black-body radiation which he had
discovered, Planck had detected in optical phenomena a discontinuous phenomenon
totally unknown to classical physics which, a few years later, was
most precisely expressed in Einstein?s hypothesis of light quanta. The impossibility
of harmonizing the Maxwellian theory with the pronouncedly
visual concepts expressed in the hypothesis of light quanta subsequently
compelled research workers to the conclusion that radiation phenomena can
only be understood by largely renouncing their immediate visualization. The
fact, already found by Planck and used by Einstein, Debye, and others, that
the element of discontinuity detected in radiation phenomena also plays an
important part in material processes, was expressed systematically in Bohr?s
basic postulates of the quantum theory which, together with the Bohr-
Sommerfeld quantum conditions of atomic structure, led to a qualitative
interpretation of the chemical and optical properties of atoms. The acceptance
of these basic postulates of the quantum theory contrasted uncompromisingly
with the application of classical mechanics to atomic systems,
which, however, at least in its qualitative affirmations, appeared indispensable
for understanding the properties of atoms. This circumstance was a fresh
argument in support of the assumption that the natural phenomena in which
Planck?s constant plays an important part can be understood only by largely
foregoing a visual description of them. Classical physics seemed the limiting
case of visualization of a fundamentally unvisualizable microphysics, the more
accurately realizable the more Planck?s constant vanishes relative to the
parameters of the system. This view of classical mechanics as a limiting case
DEVELOPMENT OF QUANTUM MECHANICS 291
of quantum mechanics also gave rise to Bohr?s principle of correspondence
which, at least in qualitative terms, transferred a number of conclusions formulated
in classical mechanics to quantum mechanics. In connection with
the principle of correspondence there was also discussion whether the quantum-
mechanical laws could in principle be of a statistical nature; the possibility
became particularly apparent in Einstein?s derivation of Planck?s law
of radiation. Finally, the analysis of the relation between radiation theory
and atomic theory by Bohr, Kramers, and Slater resulted in the following
scientific situation:
According to the basic postulates of the quantum theory, an atomic system
is capable of assuming discrete, stationary states, and therefore discrete
energy values; in terms of the energy of the atom the emission and absorption
of light by such a system occurs abruptly, in the form of impulses. On
the other hand, the visualizable properties of the emitted radiation are described
by a wave field, the frequency of which is associated with the difference
in energy between the initial and final states of the atom by the
relation
E 1 - E2 = h v
To each stationary state of an atom corresponds a whole complex of parameters
which specify the probability of transition from this state to another.
There is no direct relation between the radiation classically emitted by an
orbiting electron and those parameters defining the probability of emission;
nevertheless Bohr?s principle of correspondence enables a specific term of
the Fourier expansion of the classical path to be assigned to each transition
of the atom, and the probability for the particular transition follows qualitatively
similar laws as the intensity of those Fourier components. Although
therefore in the researches carried out by Rutherford, Bohr, Sommerfeld
and others, the comparison of the atom with a planetary system of electrons
leads to a qualitative interpretation of the optical and chemical properties
of atoms, nevertheless the fundamental dissimilarity between the atomic
spectrum and the classical spectrum of an electron system imposes the need
to relinquish the concept of an electron path and to forego a visual description
of the atom.
The experiments necessary to define the electron-path concept also furnish
an important aid in revising it. The most obvious answer to the question
how the orbit of an electron in its path within the atom could be observed

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