Lasers are machines that can create a highly focused beam of light and, with enough power, intense heat. The word LASER is an acronym for "light amplification by stimulated emission of radiation." The most powerful lasers can burn through steel in seconds, even from great distances.
In his 1917 paper "On the quantum theory of radiation," Albert Einstein was the first to lay down the theoretical principles behind lasers (although the name didn't exist until the 1960s). When an atom is stimulated into an "excited" state, one of its electrons temporarily rises to a higher energy level. When the electron returns to its original state, a photon of energy (particle of light) is emitted that corresponds to that energy change. Normally, the photon of energy shoots off in a random direction at a random wavelength, but not so with lasers [source: Follows].
In a laser, the emission of photons is highly controlled. Using chemical reactions, gasses or solid crystals, atoms are pumped into an excited state within a mirrored chamber. As the electrons in the atoms return to their normal state, they release photons of light in all directions.
Some of those photons, however, bounce straight off of the mirrored surfaces on either end of the chamber. Those bouncing photons stimulate more atoms in the chamber to emit photons in the same direction. After billions of these reactions, the result is a column of light that's vibrating at exactly the same wavelength (monochromatic) in exactly the same direction — that's called "coherent" or laser light.
After the first laser was demonstrated by physicist Theodore Maiman in 1960, lasers quickly entered the scientific and cultural mainstream. Today, low-energy lasers are used to read CDs and bar codes at the grocery store, while higher energy lasers are beamed from observatories to measure the precise distance of planets [source: Martin].
The race to create laser weapons has been raging for decades. The challenge has always been to create a laser that's powerful enough to generate tremendous heat across long distances, but small and stable enough to be mounted on a ship or even an airplane.