360 THE QUANTUM THEORYIn the first instance, this finding led to the abandonment of the earlier qualitative
concept of the indivisibility of the atom, but it did not require, or at least not at
once, a modification of the established corpus of theoretical physics.
During the next fifty years, three other particles entered the scene in ways not
so dissimilar from the case of the electron, namely, via unexpected discoveries of
an experimental nature at the outer frontier. These are the proton (or, rather, the
nucleus), the neutron,* and—just half a century after the electron—the muon,
the first of the electron's heavier brothers. As to the acceptance of these particles,
it took little time to realize that their coming was, in each instance, liberating.
Within two years after Rutherford's nuclear model, Bohr was able to make the
first real theoretical predictions in atomic physics. Almost at once after the discov-
ery of the neutron, the first viable models of the nucleus were proposed, and
nuclear physics could start in earnest. The muon is still one of the strangest ani-
mals in the particle zoo, yet its discovery was liberating, too, since it made possible
an understanding of certain anomalies in the absorption of cosmic rays. (Prior to
the discovery of the muon, theorists had already speculated about the need for an
extra particle to explain these anomalies.)
To complete the particle list of the first half century, there are four more par-
ticles (it is too early to include the graviton) which have entered physics—but in
a different way: initially, they were theoretical proposals. The first neutrino was
proposed in order to save the law of energy conservation in beta radioactivity. The
first meson (now called the pion) was proposed as the conveyer of nuclear forces.
Both suggestions were ingenious, daring, innovative, and successful—but did not
demand a radical change of theory. Within months after the public unveiling of
the neutrino hypothesis, the first theory of the weak interactions, which is still
immensely useful, was proposed. The meson hypothesis immediately led to con-
siderable theoretical activity as well.
The neutrino hypothesis was generally assimilated long before this particle was
actually observed. The interval between the proposal and the first observation of
the neutrino is even longer than the corresponding interval for the photon. The
meson postulate found rapid experimental support from cosmic-ray data—or so
it seemed. More than a decade passed before it became clear that the bulk of these
observations actually involved muons instead of pions.
Then there was the positron, 'a new kind of particle, unknown to experimental
physics, having the same mass and opposite charge to an electron' [Dl]. This
particle was proposed in 1931, after a period of about three years of considerable
*It is often said, and not without grounds, that the neutron was actually anticipated. In fact, twelve
years before its discovery, in one of his Bakerian lectures (1920) Rutherford spoke [R2] of 'the idea
of the possible existence of an atom of mass one which has zero nuclear charge.' Nor is there any
doubt that the neutron being in the air at the Cavendish was of profound importance to its discoverer,
James Chadwick [Cl]. Even so, not even a Rutherford could have guessed that his 1920 neutron
(then conjectured to be a tightly bound proton-electron system) was so essentially different from the
particle that would eventually go by that name.