The Dirac sea

The beginning of the twentieth century was a time of intellectual giants, whose likes will perhaps never again be equaled. Einstein had just invented relativity, Heisenberg and Schrodinger quantum mechanics, but nobody yet knew how to make the two theories consistent with each other. In 1930, a new person tackled the problem. His name was Paul Dirac. He was twenty-eight years old. He succeeded where the others had failed.

His theory was an unprecedented success, except for one small detail. According to Dirac's theory, a particle could have either positive or negative energy. What did this mean, a particle of negative energy? How could something have negative energy? And why don't ordinary-positive energy-particles fall down into these negative energy states, releasing a lot of free energy in the process?

You or I might have merely stipulated that it was impossible for an ordinary positive energy particle to make a transition to negative energy. But Dirac was not an ordinary man. He was a genius, the greatest physicist of all, and he had an answer. If every possible negative energy state was already occupied, a particle couldn't drop into a negative energy state. Ah ha! So Dirac postulated that the entire universe is entirely filled with negative energy particles. They surround us, permeate us, in the vacuum of outer space and in the center of the earth, every possible place a particle could be. An infinitely dense "sea" of negative energy particles.

The Dirac sea.

Geoffrey A. Landis — Ripples in the Dirac Sea (1989)