Atomic clocks measure time by getting atoms to resonate at their specific frequency, but they have to be very cool to do so. They cool them with laser light... but how does shining a light on something cool it down?
http://www.colorado.edu/physics/2000/bec/lascool1.html
http://nobelprize.org/nobel_prizes/physics/laureates/1997/press.html
"...The laser light functions as a thick liquid, dubbed optical molasses, in which the atoms are slowed down. ...a photon that collides with an atom can transfer all its momentum to that atom...What determines the right energy for photons to be able to affect atoms is the inner structure (energy levels) of the atoms...If an atom moves the conditions change because of what is termed the Doppler effect - the same effect that gives a train whistle a higher pitch when the train is approaching than when it is standing still. If the atom is moving towards the light, the light must have a lower frequency than that required for a stationary atom if it is to be "heard" by the atom.
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