In 1974 Joel Scherk and
I proposed that the problems that string theory had encountered could
be turned into virtues if it were used as a framework for realizing Einstein's
old dream of a ``unified theory'' of fundamental forces and elementary
particles, rather than as a theory of hadrons (the strongly interacting
nuclear particles). [3 ]
Specifically, we pointed
out that it would provide a theory that incorporates general relativity
without the characteristic short-distance infinities of quantum field
theory. The massless spin two particle, which we had tried so hard to
get rid of, would be identified as the graviton -- the quantum of gravitation!
One implication of this
change in viewpoint was that, to account for the observed strength of
the gravitational force, the characteristic size of a string had to be
roughly the Planck length
(the symbol h is Planck's constant.) This was a big change, since
this distance is some 20 orders of magnitude smaller than the characteristic
size of hadrons previously envisaged.
More refined analyses
lead to a string scale Lst that is about two orders
of magnitude larger than the Planck length. In any case, experiments at
existing accelerators cannot resolve distances shorter than about 10-16
cm, which explains why the point-particle approximation of ordinary
quantum field theories is so successful.
The second problem --
the extra dimensions -- could also be addressed in this setting. Once
one has a theory containing gravitation and generalizing general relativity,
one knows that that the geometry of space-time is dynamically determined.
One could imagine that,
as a consequence of the dynamics, the extra six dimensions form a small
compact space attached to each point in ordinary four-dimensional space-time.
If the size of the extra dimensions is sufficiently small, there would
be no conflict with observations.
I found these ideas very
exciting and have been pursuing them ever since. However, for the ten
year period 1974--1984 only a few colleagues and I pursued these ideas.
One who did was Joel Scherk; tragically, he passed away in 1980.