UNIFIED FIELD THEORY 327These last words (italicized by me) represent the first instance, as best I know,
where it is stated in the literature that there are strong interactions. It was the
second great discovery by James Chadwick. His first one had been made in 1914:
the primary beta spectrum is continuous [C2]. Until well into the 1920s, this con-
tinuity was believed to have secondary causes. The neutrino was not postulated
until 1929.
Thus nuclear physics began with a nucleus without neutrons, beta decay with-
out neutrinos. Matter was made of protons and electrons. There were neither
weak nor strong interactions. In the beginning there was only electromagnetism.
And, of course, there was gravitation.
Which brings us back to unified field theory.
When Einstein, Weyl, and others began their work on unified field theory, it
was natural to assume that this task consisted exlusively of the union of gravitation
with electromagnetism. To be sure, the separateness of these two fields posed no
conflicts or paradoxes. There were no puzzles such as the Michelson-Morley
experiment nor curious coincidences like the equality of the inertial and the grav-
itational mass. Nevertheless, it seemed physically well-motivated and appealing to
ask, Do nature's only two fields of force, both long-range in character, have a
common origin?
Then it came to pass that physics veered toward a different course, neither led
nor followed by Einstein. First quantum mechanics and then quantum field theory
took center stage. New forces had to be introduced. New particles were proposed
and discovered. Amid all these developments, Einstein stayed with the unification
of gravitation and electromagnetism, the final task he set himself. This insistence
brought the ultimate degree of apartness to his life.
After his death, the urge for unification returned and became widespread, but
both the goals and the methods of pursuit are different now. At the end of this
chapter I shall comment further on this new look of the unification program. I
turn next to an account of Einstein's own efforts at unification. It remains to be
seen whether his methods will be of any relevance for the theoretical physics of
the future. Certainly this work of his did not produce any results of physical inter-
est. I therefore believe it will suffice to indicate (omitting details as much as pos-
sible) the two general directions in which he looked for the realization of his aims.
One of these, based on the extension of space-time to a five-dimensional manifold,
is discussed in the Section 17c. The other, based on generalizations of the geometry
of Riemann, is treated in Section 17e. The discussion of this second category is
preceded by a brief excursion into post-Riemannian geometry and a comment on
the influence of Einstein's general relativity on mathematics.
In the early 1920s, the structure of the nucleus was an interesting but secondary
problem and the unification of forces a minor issue. Quantum phenomena posed
the central challenge. Einstein was well aware of this when, at age forty, he began