Albert Einstein used mathematics and physics to prove properties of physics about which he and other scientists theorized. For instance, his work on Brownian motion, which involved vibrations caused when atoms collide with one another, proved that atoms and molecules actually exist.
Because he found Newtonian physics inadequate to explain nature, Einstein used observation to prove his own inferences about physics as well as the conjectures of other experimenters. His work in quantum mechanics, which explains the workings of sub-molecular and sub-atomic physics, attempts to build one unified theoretical framework. For example, once Robert Millikan completed his experiments on the photoelectric effect, which proved that light striking a metal plate released electrons, Einstein could prove that light consisted of particles, which he called "quanta.”
As international interest in atomic theory grew, Einstein and other physicists, including Max Planck and Niels Bohr, built on their own work to explain how atoms function. For instance, Einstein’s early work with wave mechanics helped prove that light has both the properties of waves and particles. Further, because Einstein could prove that the electromagnetic fields of atoms could move other atoms, he could also prove, using observations of the motion of planets in the solar system, that black holes exist by documenting changes in planets’ gravitational fields that proved the presence of an even stronger field.