Page 83 - A Brief History of Time - Stephen Hawking

A Brief History of Time - Stephen Hawking... Chapter 11
sufficient to remove all the infinities. One therefore has a theory that seems to predict that certain quantities,
such as the curvature of space-time, are really infinite, yet these quantities can be observed and measured to
be perfectly finite! This problem in combining general relativity and the uncertainty principle had been
suspected for some time, but was finally confirmed by detailed calculations in 1972. Four years later, a possible
solution, called “supergravity,” was suggested. The idea was to combine the spin-2 particle called the graviton,
which carries the gravitational force, with certain other particles of spin 3/2, 1, ½, and 0. In a sense, all these
particles could then be regarded as different aspects of the same “superparticle,” thus unifying the matter
particles with spin ½ and 3/2 with the force-carrying particles of spin 0, 1, and 2. The virtual particle/antiparticle
pairs of spin ½ and 3/2 would have negative energy, and so would tend to cancel out the positive energy of the
spin 2, 1, and 0 virtual pairs. This would cause many of the possible infinities to cancel out, but it was
suspected that some infinities might still remain. However, the calculations required to find out whether or not
there were any infinities left uncancelled were so long and difficult that no one was prepared to undertake them.
Even with a computer it was reckoned it would take at least four years, and the chances were very high that
one would make at least one mistake, probably more. So one would know one had the right answer only if
someone else repeated the calculation and got the same answer, and that did not seem very likely!

Despite these problems, and the fact that the particles in the super-gravity theories did not seem to match the
observed particles, most scientists believed that supergravity was probably the right answer to the problem of
the unification of physics. It seemed the best way of unifying gravity with the other forces. However, in 1984
there was a remarkable change of opinion in favor of what are called string theories. In these theories the basic
objects are not particles, which occupy a single point of space, but things that have a length but no other
dimension, like an infinitely thin piece of string. These strings may have ends (the so-called open strings) or
they may be joined up with themselves in closed loops (closed strings) Figure 11:1 and Figure 11:2.

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