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Laser stands for "Light Amplification by Stimulated Emission of Radiation." A laser beam is produced when light bounces back and forth between two mirrors with a special medium (gas, liquid, or solid) between them. As it bounces, the light triggers energized atoms in the medium to release more light, some of which leaks out through one of the mirrors to produce the laser beam.

A laser beam is special because all the photons (discrete "particles" of light energy) in the beam are vibrating in exactly the same lockstep way. The beam is tightly focused and perfectly aligned because all the photons are "marching in phase" like soldiers in a troop.

In an ordinary beam of light, the photons vibrate every which way. Because laser photons are in phase, the beam can stay aligned for very long distances and it can be focused down to a very tiny spot without losing its alignment.

Laser Cooling

When you shine light at something, it normally heats up. That is because the light is absorbed by molecules, causing them to move faster. Faster molecular or atomic motion means a higher temperature.

But under the right conditions, laser light will actually cool atoms! In laser cooling, the laser is tuned to such a frequency that atoms traveling toward the beam get slowed down. A number of criss-crossing laser beams create a trap for the atoms, which get slower and slower, and therefore colder and colder.

With this technique, researchers have achieved temperatures within one millionth of a degree above absolute zero. Absolute zero is the coldest possible temperature, and can never be reached exactly. At absolute zero, the atom's motion would be as slow as possible. (Still not perfectly still, due to quantum mechanics. That's a long story.) Laser cooling has resulted in the design of super-accurate atomic clocks, and in a new form of (very cold) matter called Bose-Einstein Condensate.