and strong nuclear forces could not be described by a field theory. But reports of the death of
field theory proved to be an exaggeration. A new type of field theory was put forward by
Chen Ning Yang and Robert Mills. In 1967 Abdus Salam and Steven Weinberg showed that a
theory of this type could not only describe the weak nuclear forces but could also unify them
with the electromagnetic force. I remember this field theory being treated with great scorn by
most particle physicists. However, it agreed so well with experiments that the 1979 Nobel
Prize was awarded to Salam, Weinberg and Glashow, who had proposed similar unified
theories. The Nobel committee took quite a gamble because the final confirmation of the
theory didn’t come until 1983, with the discovery of the W and Z particles. (That is to say, the
W and Zed particles, for those of us who are British and don’t use an American speech
synthesizer.)
The success sparked a search for a single “grand unified” Yang-Mills theory that would
describe all three kinds of force. Grand unified theories are not very satisfactory. Indeed, their
name is rather an exaggeration. They are not that grand as theories because they contain about
40 numbers that cannot be predicted in advance but have to be adjusted to agree with
experiments. One would hope the ultimate theory of the universe is unique and does not
contain any adjustable quantities. How would those values have been chosen? But the most
powerful objection to the grand unified theories was that they weren’t fully unified. They
didn’t include gravity and there wasn’t any obvious way of extending them so that they did. It
may be that there is no single fundamental theory. Instead there may be a collection of
apparently different theories, each of which works well in certain situations. Different theories
agree with each other where their regions of validity overlap. Thus they can all be regarded as
different aspects of the same theory. But there may be no single formulation of the theory that
can be applied in all situations.
Theoretical physics may be like mapping the Earth. One can
accurately represent a small region of the Earth’s surface, as a map
on a sheet of paper. But if one tries to map a larger region, one gets
distortions because of the curvature of the Earth. It is not possible
to represent every point on the Earth’s surface on a single map.
Instead one uses a collection of maps, which agree in the regions
where they overlap.
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As I said, even if we find a complete unified theory, either in a single formulation, or as a
series of overlapping theories, we will have solved only half the problem. The unified theory
will tell us how the universe evolves in time, given the initial state. But the theory does not in
itself specify the boundary conditions at the edge of space and time that determine the initial
state. This question is fundamental to cosmology. We can observe the present state of the
universe and we can use the laws of physics to calculate what it must have been at earlier
times. But all that tells us is that the universe is as it is now because it was as it was then. We
cannot understand why the universe is the way it is unless cosmology becomes a science, in
the sense it can make predictions. And that requires a theory of the boundary conditions of the
universe.
There have been various suggestions for the initial conditions of the
universe, such as the tunnelling hypothesis and the so-called pre-big
bang scenario. But in my opinion by far the most elegant is what Jim
Hartle and I called the no-boundary proposal. This can be
paraphrased as, the boundary condition of the universe is that it has
no boundary. In other words space and imaginary time together are
curved back on themselves to form a closed surface like the surface of the Earth but with
more dimensions. The surface of the Earth has no boundary, either. There are no reliable
reports of someone falling over the edge of the world.