A brife history of Lasers
The process which makes lasers possible, Stimulated Emission, was proposed in 1917 by Albert Einstein. No one realized the incredible potential of this concept until the 1950's, when practical research was first performed on applying the theory of stimulated emission to making lasers. It wasn't until 1960 that the first true laser was made by Theodore Maimam, out of synthetic ruby. Many ideas for laser applications quickly followed, including some that never worked, like the laser eraser. Still, the early pioneers of laser technology would be shocked and amazed to see the multitude of ways that lasers are used by everyone, everyday, in today's world.
"All it took to invent the laser was some understanding of Einstein's ideas on stimulated emission of radiation."
Some theorists were on the right track, especially Planck, who proposed that nature acted by using "quanta" of energy. But it was the young, unknown Albert Einstein who explained everything and started the field of quantum mechanics with his paper on the photoelectric effect. Einstein showed that light does not consist of continuous waves, nor of small, hard particles. Instead, it exists as bundles of wave energy called photons. Each photon has an energy that corresponds to the frequency of the waves in the bundle. The higher the frequency (the bluer the color), the greater the energy carried by that bundle.
Indeed, Einstein's Nobel Prize was not awarded for either one of his relativity theories - the Nobel Commitee thought them too speculative at the time. Rather Einstein won the prize for explaining the photoelectric effect. Two of Einstein's 1905 papers were on the theory of atoms and molecules, yet there were still many scientists in 1905 who did not believe in atoms or molecules !
In 1924 Einstein received a paper from Indian physicist S.N.Bose that described light as a gas consisting of photons. This photon gas was a strange type of gas, for the particles in it did not obey the commonsense statistical laws that billiard balls do. If you randomly roll a number of perfectly elastic billiard balls on a frictionless table, sooner or later they will all end up in one pocket or another. If you checked by repeated experiments, you would find that all the balls had an equal probability of falling into any one of the pockets. But if the billiard balls behaved like photons, you would find that if one of the pockets already had a ball in it, the rest of the balls would have a tendency to fall into that pocket. In fact, the more balls already in a pocket, the more likely another ball would choose to join its identical mates.