Page 87 - A Brief History of Time - Stephen Hawking
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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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