Albert Einstein coined the term "Unified Field Theory," which describes any attempt to unify the fundamental forces of physics between elementary particles into a single theoretical framework. Einstein spent the latter part of his life searching for such a unified field theory, but was unsuccessful.

### Forces That Have Been Unified

In the past, seemingly different interaction fields (or "forces," in less precise terms) have been unified together.

James Clerk Maxwell successfully unified electricity and magnetism into electromagnetism in the 1800s. The field of quantum electrodynamics, in the 1940s, successfully translated Maxwell's electromagnetism into the terms and mathematics of quantum mechanics.

In the 1960s & 1970s, physicists successfully unified the strong nuclear interaction and weak nuclear interactions together with quantum electrodynamics to form the Standard Model of quantum physics.

### The Current Problem

The current problem with a fully unified field theory is in finding a way to incorporate gravity (which is explained under Einstein's theory of general relativity) with the Standard Model that describes the quantum mechanical nature of the other three fundamental interactions. The curvature of spacetime that is fundamental to general relativity leads to difficulties in the quantum physics representations of the Standard Model.

### Different Theories

Some specific theories that attempt to unify quantum physics with general relativity include:

- Quantum Gravity
- String Theory / Superstring Theory / M-Theory
- Loop Quantum Gravity
- Theory of Everything
- Supersymmetry

Unified field theory is highly theoretical, and to date there is no absolute evidence that it is possible to unify gravity with the other forces.

History has shown that other forces could be combined, and many physicists are willing to devote their lives, careers, and reputations to the attempt to show that gravity, too, can be expressed quantum mechanically. The consequences of such a discovery, of course, cannot be fully known until a viable theory is proven by experimental evidence.