A unified field theory is a theory which tries to incorporate the four fundamental forces of nature, and explain them in terms of one underlying force. The electroweak theory, developed in the 1970s, successfully combines electromagnetism with the weak force. The so-called grand unified theories attempt to combine the theory of the strong force (quantum chromodynamics) with electroweak theory; one outcome of this unification may be that the proton is not stable. The final stage or total unification will be to include gravity, in particular as it is described in the general theory of relativity.