Our experience with the microscopic world tells us that all physics should be quantum mechanical, but General Relativity and Electromagnetism are classical theories….
Leaving gravity aside for the moment, the path to the Standard Model begins with “quantizing” Electromagnetism. (1940’s)
Quantization was formally a straightforward process , but when physicists tried to do calculations with the resulting theory the answer to every question was always the same - infinity! Even a simple quantity like the energy density of empty space turned out to be infinite.
Feynman, Schwinger and Tomanaga won the Nobel Prize in 1965 for making sense of this situation by inventing a technique called renormalization for extracting physical answers from the theory of Quantum Electrodynamics (QED)
Lecture 2 , slide 19