Various Grand Unified Theories have been proposed by physicists in recent times, but no Grand Unified Theory has achieved universal acceptance.

What is a Grand Unified Theory?

Particle physicists identify four types of interactions or forces in our physical world(an interaction or force, in the language of particle physicists, is that which not only binds particles in the atomic nucleus but also governs the behavior of elementary particles in their creation, annihilation, radioactive decay, absorption or scattering).

The four forces are gravity, electromagnetic, strong interactions and weak interactions. Gravity is the most familiar force to us because it is the force which Unify  shapes the structure of spacetime in the large scale. For particle physicists, however, gravity is the weakest of the four forces i.e. at the level of elementary particles.

Electromagnetism Is The Force

Electromagnetism is the force which holds atoms together and produces electromagnetic radiation in the broad spectrum of radio, heat, light and ultraviolet radiation. Electromagnetic radiation includes magnetic and electrical forces components unified by the Scottish physicist James Clark Maxwell (1831-1879) into a coherent theory of electromagnetism.

The so-called weak interactions are stronger than gravity but are effective only over very small distances ( a hundredth the size of an atomic nucleus). The weak interaction is involved in radioactive decay and other reactions producing neutrinos in stars.

Strong Interaction

The strong interaction is also effective only on the small scale and it is the force which holds protons and neutrons together in the atomic nucleus.

A Grand Unified Theory is one which seeks to express the four forces in common unifying terms. In a Grand Unified Theory the four forces are different expressions or manifestations of a single “superforce.” An analogy may be found in the Darwinian Evolutionary Theory in which diverse manifestations of biological life are traced back to a single ancestral form.

Grand Unified Theories predict that at sufficiently high temperatures all the four forces merge into one superforce. At lower temperature, however, a process  Unify referred to as spontaneous symmetry breaking differentiates the unified force into four different forces.

In seeking to prove experimentally the predictions of Grand Unified Theories, particle physicists are faced by the formidable difficulty of producing, in the laboratory, sufficiently high temperatures at which the four forces are unified. It is believed, however, that in the early universe, the high temperature required to unify the four forces was achieved.