326 THE LATER JOURNEYphysical. In order to appreciate this, we must first have a brief look at the physics
of particles and fields around 1920.During the second decade of the twentieth century, there were advances in the-
oretical physics of the highest calibre. Rutherford discovered the atomic nucleus,
Bohr the quantum theory of the atom, Einstein general relativity. It was also the
time that provided one of the most striking examples of how physicists can tem-
porarily be led astray by the selection of complexes from nature on grounds of
simplicity. The case in point is the model of the nucleus built of protons and
electrons.
Rutherford had discovered the proton (so baptized in 1919), the nucleus of the
lightest atom. Bohr had been the first to realize that beta decay is a process in
which electrons are ejected from the nucleus. What then was more obvious than
to assume that the nuclear weight was almost entirely due to a number of con-
stituent protons equal to the mass number, with the difference between mass num-
ber and charge number equal to the number of constituent electrons? The nucleus
must be considered 'as a very complex structure ... consisting of positively-
charged particles and electrons, but it is premature (and would serve no useful
purpose) to discuss at the present time the possible structure of the nucleus itself
[Rl]. Thus Rutherford expressed himself on the structure of the atom during a
Royal Society meeting held on March 19, 1914. Even the cautious Rutherford
had but one choice for the nature of the internuclear forces. Again in 1914 he
wrote, 'The nucleus, though of minute dimensions, is in itself a very complex
system consisting of positively and negatively charged bodies bound closely
together by intense electrical forces' [R2] (my italics). Nuclear binding energy, he
conjectured, is an electromagnetic effect. 'As Lorentz has pointed out, the electrical
mass of a system of charged particles, if close together, will depend not only on
the number of these particles, but on the way their fields interact. For the dimen-
sions of the positive and negative electrons considered [a positive electron being a
proton], the packing must be very close in order to produce an appreciable alter-
ation in the mass due to this cause. This may, for example, be the explanation of
the fact that the helium atom has not quite four times the mass of the hydrogen
atom' [R3].
Thus all forces within the atom, whether peripheral or in its core, were initially
perceived to be electrical. This was a natural thought, especially since the nucleus
had been discovered to begin with by the observation that the scattering of alpha
particles on atoms was dominated by a coulomb interaction between the alpha
particle and a near-pointlike atomic core. Not until 1919 did these scatterings give
a first intimation that all was not electrical [R4]. Not until 1921 did experiments
show beyond doubt that the 1/r^2 force law breaks down at small distances. 'It is
our task to find some field of force which will represent these effects.. .. The
present experiments... show that the forces are of very great intensity'' [Cl].