Page 41 - A Brief History of Time - Stephen Hawking

A Brief History of Time - Stephen Hawking... Chapter 5

Figure 5:2

Figure 5:2 shows a photograph of a collision between a high-energy proton and antiproton. The success of the unification
of the electromagnetic and weak nuclear forces led to a number of attempts to combine these two forces with the strong
nuclear force into what is called a grand unified theory (or GUT). This title is rather an exaggeration: the resultant theories
are not all that grand, nor are they fully unified, as they do not include gravity. Nor are they really complete theories,
because they contain a number of parameters whose values cannot be predicted from the theory but have to be chosen
to fit in with experiment. Nevertheless, they may be a step toward a complete, fully unified theory. The basic idea of
GUTs is as follows: as was mentioned above, the strong nuclear force gets weaker at high energies. On the other hand,
the electromagnetic and weak forces, which are not asymptotically free, get stronger at high energies. At some very high
energy, called the grand unification energy, these three forces would all have the same strength and so could just be
different aspects of a single force. The GUTs also predict that at this energy the different spin-½ matter particles, like
quarks and electrons, would also all be essentially the same, thus achieving another unification.

The value of the grand unification energy is not very well known, but it would probably have to be at least a thousand
million million GeV. The present generation of particle accelerators can collide particles at energies of about one hundred
GeV, and machines are planned that would raise this to a few thousand GeV. But a machine that was powerful enough to
accelerate particles to the grand unification energy would have to be as big as the Solar System – and would be unlikely
to be funded in the present economic climate. Thus it is impossible to test grand unified theories directly in the laboratory.
However, just as in the case of the electromagnetic and weak unified theory, there are low-energy consequences of the
theory that can be tested.
The most interesting of these is the prediction that protons, which make up much of the mass of ordinary matter, can
spontaneously decay into lighter particles such as antielectrons. The reason this is possible is that at the grand
unification energy there is no essential difference between a quark and an antielectron. The three quarks inside a proton
normally do not have enough energy to change into antielectrons, but very occasionally one of them may acquire

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