A brief History of Time pdfdrive com

FIGURE 11.5 AND FIGURE 11.6
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.

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String theories also lead to infinities, but it is thought they will all
cancel out in versions like the heterotic string (though this is not yet
known for certain). String theories, however, have a bigger problem:
they seem to be consistent only if space-time has either ten or twenty-six
dimensions, instead of the usual four! Of course, extra space-time
dimensions are a commonplace of science fiction indeed, they provide an
ideal way of overcoming the normal restriction of general relativity that
one cannot travel faster than light or back in time (see
Chapter 10
). The
idea is to take a shortcut through the extra dimensions. One can picture
this in the following way. Imagine that the space we live in has only two
dimensions and is curved like the surface of an anchor ring or torus (
Fig.
11.7
). If you were on one side of the inside edge of the ring and you
wanted to get to a point on the other side, you would have to go round
the inner edge of the ring. However, if you were able to travel in the
third dimension, you could cut straight across.