Page "M-theory" Paragraph 16

This last step is best explained first in a certain limit.

In order to describe our world, strings must be extremely tiny objects.

So when one studies string theory at low energies, it becomes difficult to see that strings are extended objects — they become effectively zero-dimensional ( pointlike ).

Consequently, the quantum theory describing the low energy limit is a theory that describes the dynamics of these points moving in spacetime, rather than strings.

Such theories are called quantum field theories.

However, since string theory also describes gravitational interactions, one expects the low-energy theory to describe particles moving in gravitational backgrounds.

Finally, since superstring string theories are supersymmetric for supersymmetry is needed for consistency, one expects to see supersymmetry appearing in the low-energy approximation.

These three facts imply that the low-energy approximation to a superstring theory is a supergravity theory.