Why did Strings
enter the story?
String theory is believed to close
this gap. Originally,
string theory was proposed as an explanation for the observed
relationship between mass and spin for certain particles
called hadrons, which include the proton and neutron. Things
didn't work out, though, and
Quantum Chromodynamics ventually proved a better
theory for hadrons.
But
particles in string theory arise as excitations of the string,
and included in the excitations of a string in string theory
is a particle with
zero mass and two units of spin.
If
there were a good quantum theory of gravity, then the particle
that would carry the gravitational force would have
zero mass and two units of
spin. This has been known by theoretical
physicists for a long time. This theorized particle is called
the graviton.
This
led early string theorists to propose that string theory be
applied not as a theory of hadronic particles, but as a theory
of quantum gravity,
the unfulfilled fantasy of theoretical physics in the particle
and gravity communities for decades.
But
it wasn't enough that there is a graviton predicted by string
theory. One can add a graviton to quantum field theory by
hand, but the calculations that are supposed to describe
Nature become useless. This is because, as illustrated in the
diagram above, particle interactions occur at a single point
of spacetime, at zero distance between the interacting
particles. For gravitons, the mathematics behaves so badly at
zero distance that the answers just don't make sense. In
string theory, the strings collide over a small but finite
distance, and the answers do make sense.
This
doesn't mean that string theory is not without its
deficiencies. But the zero distance behavior is such that
we can combine
quantum mechanics and gravity, and we can talk
sensibly about a string excitation that carries the
gravitational force.