Attempts to explain Albert Einstein's theory of relativity lead us to proving the fact that light is made of waves and particles and then to quantum mechanics.
However, neither one of Albert Einstein's theory relativity and quantum mechanics had sufficed to fully explain the other theory and the four fundamental forces of nature (the strong force, the weak force, the electromagnetic force, and the gravitational force).
Hence Albert Einstein tried to come up with a unified field theory (UFT) in order to explain all of the four fundamental forces of nature in one theory, but he stopped short of completing his UFT.
As the Big Bang Theory alone could not explain the very beginning of our universe, superstring theory emerged, which explains all of the particles and fundamental forces of nature by modelling them as vibrations of one-dimensional tiny supersymmetric strings, not as point-like particles. The intensity of one-dimensional strings’ vibrations is proportional to the amount of energy and mass. The intensity of strings’ vibrations determines the intensity of force; the patterns of strings’ vibrations determine the types of force, i.e. the strong force, the weak force, the electromagnetic force, and the gravitational force.
With the advent of superstring theory, the four fundamental forces of nature could be explained in one theory. The theory of quantum processes deals with zero-dimensional point-like particles, so the problem of infinities arises. Thus, it is not suitable to serve as a basic theory. Instead, superstring theory avoids the problem of infinities by dealing with one-dimensional objects called strings, which have sizes. In addition, superstring theory naturally encompasses gauge theories and the general theory of relativity.
As such, superstring theory is one of the strongest candidates for a ToE (Theory of Everything). Superstring theory later evolved into membrane theory. Superstring theory, however, leaves us with no other option but to explain the universe using the idea of a multiverse, in which there could be many parallel universes. You could find your doppelgangers in those parallel universes.By Oh hae-young