changing allows particles to assume other forms, and species changing
among neutrinos accounts for the solar neutrino deficit. But neutrinos
are highly standoffish (a neutrino can travel light-years through solid
lead without interacting); atoms are the stuff of the real world. An atom
that started life as an atom of nitrogen can, through the process known
as beta decay, become an atom of carbon. This is one of the many inter-
esting phenomena associated with radioactivity, and is an action pro-
moted by the weak force.
The weak force is weak when compared with the strong force, the force
that holds the nucleus of an atom together against the electrical repulsion
generated by the protons residing in the nucleus. Although Einstein and
Kaluza certainly knew of the phenomena of beta decay, and also were well
aware that something had to be holding the nucleus together; the weak
and strong forces had not been isolated when they were developing their
theory.
In the half century between 1940 and 1990, remarkable progress was
made in developing the theories of these forces. A theory that combined
the electromagnetic force and the weak force was developed by Sheldon
Glashow, Abdus Salam, and Steven Weinberg. This theory postulates that
at the exceedingly high temperatures that existed in the early universe,
these two forces were actually a single force, and the cooling of the uni-
verse enabled the two forces to establish themselves as separate forces, in
a manner similar to the way different substances in a mixture will pre-
cipitate out as the mixture cools. Quantum chromodynamics, the theory
of the strong force, was in large part developed by David Politzer, Frank
Wilczek, and David Gross. Both these theories, which won Nobel Prizes
for their discoverers, have been subjected to experiment and have so far
survived; together, they help to comprise the Standard Model of the parti-
cles and forces that make up our universe.
The electroweak theory that combines electromagnetism and the weak
force is a significant step forward to realizing Einstein’s dream of a uni-
fied field theory. The current view is that its idea of forces separating as
the universe cools is a template for the ultimate unified field theory—for
one inconceivably brief moment after the big bang, at some inconceivably
high temperature, all the four forces were a single force, and as the uni-
verse cooled, they separated out. First to separate out would have been
gravity, then the strong force, and finally electromagnetism and the weak
force, separated as described by the electroweak theory.
The development of this theory is a work in progress, but it is one
that is encountering a major obstacle. The electroweak theory and quan-
tum chromodynamics are quantum theories, which rely heavily on quan-
148 How Math Explains the World