Page 87 - A Brief History of Time - Stephen Hawking

A Brief History of Time - Stephen Hawking... Chapter 11

Figures 11:5 & 11:6

In string theory, this process corresponds to an H-shaped tube or pipe Figure 11:6 (string theory is rather like
plumbing, in a way). The two vertical sides of the H correspond to the particles in the sun and the earth, and the
horizontal crossbar corresponds to the graviton that travels between them.

String theory has a curious history. It was originally invented in the late 1960s in an attempt to find a theory to
describe the strong force. The idea was that particles like the proton and the neutron could be regarded as
waves on a string. The strong forces between the particles would correspond to pieces of string that went
between other bits of string, as in a spider’s web. For this theory to give the observed value of the strong force
between particles, the strings had to be like rubber bands with a pull of about ten tons.

In 1974 Joel Scherk from Paris and John Schwarz from the California Institute of Technology published a paper
in which they showed that string theory could describe the gravitational force, but only if the tension in the string
were very much higher, about a thousand million million million million million million tons (1 with thirty-nine
zeros after it). The predictions of the string theory would be just the same as those of general relativity on
normal length scales, but they would differ at very small distances, less than a thousand million million million
million millionth of a centimeter (a centimeter divided by 1 with thirty-three zeros after it). Their work did not
receive much attention, however, because at just about that time most people abandoned the original string
theory of the strong force in favor of the theory based on quarks and gluons, which seemed to fit much better
with observations. Scherk died in tragic circumstances (he suffered from diabetes and went into a coma when
no one was around to give him an injection of insulin). So Schwarz was left alone as almost the only supporter
of string theory, but now with the much higher proposed value of the string tension.

In 1984 interest in strings suddenly revived, apparently for two reasons. One was that people were not really
making much progress toward showing that supergravity was finite or that it could explain the kinds of particles
that we observe. The other was the publication of a paper by John Schwarz and Mike Green of Queen Mary
College, London, that showed that string theory might be able to explain the existence of particles that have a
built-in left-handedness, like some of the particles that we observe. Whatever the reasons, a large number of
people soon began to work on string theory and a new version was developed, the so-called heterotic string,
which seemed as if it might be able to explain the types of particles that we observe.

String theories also lead to infinities, but it is thought they will all cancel out in versions like the heterotic string

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