248 THE QUANTUM [xviii. 2
the significance of the quantum of action for the elementary
physical processes is made clear in every respect or may be
demonstrated by some simple dynamical model, misunderstands,
I believe, the character and the meaning of the hypothesis of
quanta. It is impossible to express a really new principle in
terms of a model following old laws/’ To illustrate this point
we may take the suggestion that the quantum theory implies
the existence of discrete tubes of magnetic induction. This
supposition standing by itself is not sufficient to explain the
quantum theory. For although the ordinary laws of electro
magnetism be applied to the stationary state of an atomic
system (and even in such a state some modification seems
necessary), we are not entitled to apply them to the catastrophic
change which takes place between two stationary states. We
may, for instance, imagine that such a change involves the
separation (or addition) of one or more discrete magnetic tubes,
but the change cannot be expressed in terms of the classical
electromagnetic laws.
Again, Whittaker’s attempt to account for quanta by postulat
ing the existence of a magnetic structure in the atom, although
extremely suggestive, does not entirely overcome the difficulty
pointed out by Planck.
On the other hand there is the danger of departing too far
from the older ideas, and in this connection Planck says : “ Since
nothing probably is a greater drawback to the successful develop
ment of a new hypothesis than over-stepping its boundaries, I
have always stood for making as close a connection between the
hypothesis of quanta and the classical dynamics as possible,
and for not stepping outside of the boundaries of the latter until
experimental facts leave no other course open.” For this reason
Planck himself has not adopted the more radical assumption
“ that any radiant energy whatever, even though it travel
freely in a vacuum, consists of individual quanta or cells.”
The strange conflict * between the two rival theories of light
has been emphasized repeatedly in this volume. There appears
to be direct contradiction between the wave theory and the
quantum theory of radiation. The former is able to explain
the behaviour of light when it is passing through a telescope or
is concerned with interference phenomena; the latter is able
to account for emission, absorption or photo-electric action.
“ The energy of the radiation behaves as though it possessed at
the same time the opposite properties of extension and
localization.” f
* See an article by O. D. Chwolson in Scientia, vol. 41, p. 31, 1927.
t O- W. Richardson, Electron Theory of Matter (2nd edition), p. 507,
1916.