What Is a Theory of Everything?

There are four fundamental interactions that describe everything in the universe. The first three we understand through what's called quantum mechanics. But gravity is described through a different theory, einstein's theory of relativity. A theory which would be able to combine these two radically different length scales and describe all the interactions in the universe is often called a theory of everything. Einstein spent the last thr years of his life chasing after this grand, unified theory. And we think that we are making breakthroughs in terms of achieving this onece equation.