Quantum theory is the basis of the modern understanding of most branches of physics. At the turn of the twentieth century, many physicists assumed that physics was almost totally figured out, with only a few small problems left to be solved. These "small problems" occurred in situations where particles behaved like waves and light behaved like particles, and also occurred in situations where radiation was important. Exploration of these problems led the way into quantum theory, a true revolution in physics.

The early part of quantum theory was built up out of a series of experiments and their explanations. In 1901, Max Planck managed to make one of the problems work out if he assumed that light only occurred in quantized units, somewhat like particles. Then, in 1905, Albert Einstein explained the photoelectric effect with the same quantization units as Planck had used, postulating that the light actually...