Superstring Unification

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.

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