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THE QUANTUM [xvm. 4
XII), and as a matter of fact Rutherford and Chadwick have
themselves shown that this law fails when the distance is
sufficiently diminished.* After the anticipatory work of Nichol
son came the bold advance made by Bohr in introducing quantum
conditions for the determination of stationary states, and a
new hypothesis for determining the amount of radiation emitted
or absorbed in a quantum transition.
Although there has been considerable discussion in the past
as to the “ correctness ” of Bohr’s atomic model, such a discussion
is almost certainly entirely superfluous at the present time when
Bohr himself, and many other physicists with him, would probably
say that such a model has, for the time being, a heuristic value
and serves the purpose of a good working hypothesis. There is,
indeed, a tendency in some quarters to go to extremes and
discard all attempts at constructing models of the ultimate
physical entities. Let us express in mathematical language,
say some philosophers, only these results which are capable of
actual observation, and put on one side all such mechanistic
interpretations as require, for instance, a knowledge of the
position in space at a particular instant of time of a particular
electron.
The conflict between the rival theories of light which has
already been described has its counterpart in the two views which
may be taken as to the constitution of matter. These might be
somewhat inadequately described as the static and the dynamic
views. The wave-theory of matter associated with the names
of de Broglie and Schrödinger emphasizes the idea that a mass
particle is associated with or is equivalent to a condition of
vibration. It may be that the solution of our difficulties in
regard to the rival theories, both of radiation and of matter,
will eventually be found in a more general theory which will
include both radiation and matter in an extended wave
mechanics.
4. The New Quantum Mechanics
The new quantum mechanics affords an instructive example
of two contrasted methods of attacking a physical problem. The
matrix mechanics is based on the principle that only such
quantities as are directly open to observation are to be employed
in the mathematical formulation ; the undulatory mechanics
is based on the assumption that an atomic system, composed of
electrified particles, may be treated as equivalent to a wave-
motion determined by a special form of wave-equation. Although
* See the Twelfth Guthrie Lecture, “ Atomic Nuclei and their Trans
formations,” Rutherford, Pvoc. Phys. Soc., vol. 39, p. 359, 1927.