Einstein spent a lot of time thinking about light. In the early 1900s, most experiments showed that light acted like a wave. But, in 1905, Einstein suggested that light sometimes acted like a wave and sometimes like a particle. Light, he said, was like a little bundle of energy (we now call this a photon). Each photon had a specific amount, or quantum, of energy.
Building on the work of a German physicist named Max Plank, Einstein figured out an equation for the energy of a photon (E=hν). He used this equation to explain the photoelectric effect -- a puzzling way that light interacts with metal. The photoelectric effect doesn’t make sense if you think of light as a wave, but it does if you think of light as a photon (Ionization Reaction). Einstein received the Nobel Prize in physics for this in 1921.
Einstein’s work on photons laid the foundation for a field called quantum theory. Many important things came out of this field, including the technology to make TVs and computers (Laser, Compact Disc). One important result that surprised even Einstein is that quantum theory does not give definite outcomes for experiments, but only probabilities (Dice in Hands, Cats).
Quantum theory explains how the universe works on a small scale (Spherical Harmonics), but scientists don’t yet know how quantum theory and the theory of gravity fit together. This is an important area of research that scientists are still investigating (String).