# Quantum

## The Start of Quantum Theory

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=h?*). 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*).