RELATIVITY THEORY AND QUANTUM THEORY 33group. In most grand unified theories the proton is unstable. News about the pro-
ton's fate is eagerly awaited at this time.
Superunification, the union of all four forces, is the major goal. Some believe
that it is near and that supergravity will provide the answer. Others are not so
sure.
All modern work on unification may be said to represent a program of geo-
metrization that resembles Einstein's earlier attempts, although the manifold sub-
ject to geometrization is larger than he anticipated and the quantum framework
of the program would not have been to his liking.
In the search for the correct field theory, model theories have been examined
which reveal quite novel possibilities for the existence of extended structures
(solitons, instantons, monopoles). In the course of these investigations, topological
methods have entered this area of physics. More generally, it has become clear in
the past decade that quantum field theory is much richer in structure than was
appreciated earlier. The renormalizability of non-Abelian gauge fields with spon-
taneous symmetry breakdown, asymptotic freedom, and supersymmetry are cases
in point.
The proliferation of new particles has led to attempts at a somewhat simplifed
underlying description. According to the current picture, the basic constituents of
matter are: two classes of spin-% particles, the leptons and the quarks; a variety
of spin-1 gauge bosons, some massless, some massive; and (more tentatively) some
fundamental spin-zero particles. The only gauge boson observed so far is the pho-
ton. To date, three kinds of charged leptons have been detected. The quarks are
hypothetical constituents of the observed hadrons. To date, at least five species of
quarks have been identified. The dynamics of the strong interactions are supposed
to prohibit the creation of quarks as isolated, free particles. This prohibition, con-
finement, has not as yet been implemented theoretically in a convincing way. No
criterion is known which enables one to state how many species of leptons and of
quarks should exist.
Weak, electromagnetic, and strong interactions have distinct intrinsic symmetry
properties, but this hierarchy of symmetries is not well understood theoretically.
Perhaps the most puzzling are the small effects of noninvariance under space
reflection and the even smaller effects of noninvariance under time reversal. It
adds to the puzzlement that the latter phenomenon has been observed so far only
in a single instance, namely, in the K° - K° system. (These phenomena were first
observed after Einstein's death. I have often wondered what might have been his
reactions to these discoveries, given his 'conviction that pure mathematical con-
struction enables us to discover the concepts and the laws connecting them' [E7].)
It is not known why electric charge is quantized, but it is plausible that this
will be easily explicable in the framework of a future gauge theory.
In summary, physicists today are hard at work to meet Einstein's demands for
synthesis, using methods of which he probably would be critical. Since about 1970,
there has been much more promise for progress than in the two or three decades