The Standard Model of particle physics is a remarkably successful theory that describes three of the four fundamental forces in the universe:
- Electromagnetism,
- The strong nuclear force, and
- The weak nuclear force,
as well as all known elementary particles, including quarks, leptons, bosons, and the Higgs boson.
However, one major force is conspicuously absent from the Standard Model: gravity.
Gravity, the weakest of the four fundamental forces, is described separately by Einstein’s General Theory of Relativity, which is a classical (non-quantum) theory. The Standard Model, on the other hand, is based entirely on quantum field theory. The two frameworks are fundamentally different and mathematically incompatible at very small (Planck) scales.
Physicists have long sought a way to quantize gravity—that is, to describe it using quantum principles like the other forces. The hypothetical quantum particle that would carry the gravitational force is called the graviton, but it has not been detected, and it does not currently fit within the Standard Model’s structure.
This gap leads to several key challenges:
- Unifying gravity with quantum mechanics remains one of the biggest open problems in theoretical physics.
- Theories like string theory and loop quantum gravity attempt to bridge this gap, but none have been experimentally confirmed.
- The absence of gravity limits the Standard Model’s ability to describe extreme environments, such as the interior of black holes or the very early universe.