PRIDE CHEKERABefore the string theory emerged, it was discovered that
quantum field theory was the best theory to probe into deepest
scales of nature a framework for the unification of quantum
mechanics and electromagnetism.
It was then in 1970s that weak and strong nuclear forces could
also be described with the quantum field theory. The Standard
Model (SU(3)×SU(2)×U(1)) of particle physics, had been
unified with general relativity yielding a theory consistence with
almost all theories of physics to deep scales of about 10-^16 cm
probed by particle accelerators.
The Standard Model, structurally when including gravity, has a
massless spin-2 graviton a mediator of gravity, spin- 1
SU(3)×SU(2)×U(1) gauge bosons, and can also include a spin- 0
Higgs Boson, leptons, quarks and 15 multiplets of spin-1/2
fermions of three generations of fives.
This theory had been practically, indirectly, tested, for example
with the precision test of quantum electrodynamics, limits on
neutrino masses, limits on axions (light weakly interacting
particles) of astrophysics, search for rare meson decays and
limits on gravitational coupling of massless scalars.
From all these findings, and an attempt to unify general
relativity and quantum theory yielding a nonrenormalizable
quantum field theory, suggested that there was need for new
physics at very high energy.