In physics, a unified field theory is a type of field theory that allows all that is usually thought of as fundamental forces and elementary particles to be written in terms of a single field. There is no accepted unified field theory. It remains an open line of research. The term was coined by Einstein who attempted to unify the general theory of relativity with electromagnetism, hoping to recover an approximation for quantum theory. A theory of everything is closely related to unified field theory, but differs by not requiring the basis of nature to be fields, and also attempts to explain all physical constants of nature.