The quantum theory of gravitation, effective field theories and strings: Past and present

Gravity is one of four fundamental interactions. The most precise description of this force is still provided by Einstein’s General Theory of Relativity, published in 1915, an entirely classical theory. This description sets gravity apart from the other three forces—strong, weak, and electromagnetism—all described by quantum fields. Therefore, any attempt to unify the four forces must depend on a description of gravity that uses the principles of quantum mechanics.