Paul A. M. Dirac
One of the founders of quantum mechanics, known as a man of few words. He contributed greatly to constructing relativistic quantum electrodynamics, combining both Einstein's relativity and quantum mechanics. In this process, he stumbled over the problem of negative energy of an electron which can be derived from his "spin 1/2 Dirac equation". Eventually, this difficulty led him to predict the existence of "positron", the anti-particle of electron (and, every particle has its own anti-particle, and matter and anti-matter form a pair, although we mostly encounter matter in this world). This prediction was experimentally confirmed later (1932). Dirac shared the Nobel physics prize with Schroedinger in 1933.
The preceding discovery changed the concept of "vacuum" in physics. It is not a state of emptiness, but a state of quantum fluctuation---an electron may materialize together with a positron out of nothing, and soon annihilate into nothingness again; the inherent probabilistic character of quantum mechanics allows this.