The theory of special relativity was well-understood, and yet quantum mechanics, as originally developed, only worked for non-relativistic systems. The big problem was that quantum mechanics, even relativistic quantum mechanics, wasn't quantum enough to describe everything in our Universe. That was the big advance of the idea of quantum field theory, or its related theoretical advance: second quantization. Feynman diagrams are incredibly useful, but they're a perturbative (i.e., approximate) approach to calculating, and quantum field theory often yields fascinating, unique results when you take a non-perturbative approach. With quantum field theory and further advances in their applications, everything from photon-photon scattering to the strong nuclear force was now explicable.

Source: Forbes April 25, 2019 13:52 UTC