Relativistic quantum field theory has worked very well to describe the
observed behaviors and properties of elementary particles. But the theory
itself only works well when gravity is so weak that it can be neglected.
Particle theory only works when we pretend gravity doesn't exist.
General
relativity has yielded a wealth of insight into the Universe, the orbits
of planets, the evolution of stars and galaxies, the Big Bang and recently
observed black holes and gravitational lenses. However, the theory itself
only works when we pretend that the Universe is purely classical and that
quantum mechanics is not needed in our description of Nature.
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** eventually 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 be 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.
This
was a very great hurdle that was overcome for late 20th century physics,
which is why so many young people are willing to learn the grueling complex
and abstract mathematics that is necessary to study a quantum theory of
interacting strings. |