PRELIMINARIES 359until 1917. Relativistic energy momentum conservation relations involving pho-
tons were not written down till 1923. Einstein's role in these developments is dis-
cussed in Chapter 21, which begins with Einstein's formulation in 1909 of the
particle-wave duality for the case of electromagnetic radiation and also contains
an account of his discovery of the A and B coefficients and of his earliest concern
with the breakdown of classical causality. The Chapter concludes with remarks
on the role of the Compton effect.
The reader may wonder why the man who discovered the relation E = hv for
light in 1905 and who propounded the special theory of relativity in that same
year would not have stated sooner the relation p = hv/c. I shall comment on this
question in Section 25d.
(d) Einstein's work on quantum statistics is treated in Chapter 23, which also
includes a discussion of Bose's contribution.
(e) Einstein's role as a key transitional figure in the discovery of wave
mechanics will be discussed in Chapter 24.
I shall continue the outline of Einstein's contributions to the quantum theory
in Section 18c. First, however, I should like to take leave of our main character
for a brief while in order to comment on the singular role of the photon in the
history of the physics of particles and fields. In so doing, I shall interrupt the
historical sequence of events in order to make some comments from today's van-
tage point.18b. Particle Physics: The First Fifty Years
Let us leave aside the photon for a while and ask how physicists reacted to the
experimental discovery or the theoretical prediction (whichever came first) of other
new particles. No detailed references to the literature will be given, in keeping
with the brevity of my comments on this subject.
The discovery in 1897 of the first particle, the electron, was an unexpected
experimental development which brought to an end the ongoing debate about
whether cathode rays are molecular torrents or aetherial disturbances. The answer
came as a complete surprise: they are neither, but rather are a new form of matter.
There were some initial reactions of disbelief. J. J. Thomson once recalled the
comment of a colleague who was present at the first lecture Thomson gave on the
new discovery: 'I [T.] was told long afterwards by a distinguished physicist who
had been present at my lecture that he thought I had been "pulling their leg" '
[Tl]. Nevertheless, the existence of the electron was widely accepted within the
span of very few years. By 1900 it had become clear that beta rays are electrons
as well. The discoveries of the free electron and of the Zeeman effect (in 1896)
combined made it evident that a universal atomic constituent had been discovered
and that the excitations of electrons in atoms were somehow the sources of atomic
spectra.
The discovery of the electron was a discovery at the outer experimental frontier.