This is the attempt to describe the workings of all four fundamental forces of nature, and the relationships between all elementary particles in a single theoretical framework. In physics, forces can be described by fields that mediate, or carry, the interactions between particles. These are known as field theories. For instance, in 1915 Albert Einstein developed general relativity, a field theory of the force of gravity. At subatomic distances, fields are described by quantum field theories, which apply the ideas of quantum mechanics to the fundamental fields associated with the other three forces: the electromagnetic force and the strong and weak nuclear forces.The aim of researchers now is to discover whether quantum chromodynamics-the field theory of the strong nuclear force-can be unified with the electroweak theory that describes the electromagnetic and weak forces.
The result would be the so-called grand unified theory, or GUT However, a successful GUT will still not include the force of gravity. The problem is that physicists still do not know how to formulate a workable quantum field theory of Einstein's theory of gravity. One possible candidate for such a "theory of everything" is called string theory, but we are a long way from knowing whether it is right or not.by Jim Al-Khalili, 30-Second Theory