Atomic Clocks
Atomic clocks measure time by counting the vibrations of atoms or molecules. These are motions that are constant for any particular substance. The concept was introduced by Isidor Rabi [American: 1898-1988] in 1945, and the first atomic clock was built in 1949. Atomic clocks are much more accurate than other measures of time (such as those based on pendulums or vibrations of a quartz crystal). One atomic clock, built in 1999 by the U.S. National Institute of Standards and Technology, would gain or lose less than a second if it ran for 20 million years.
The atomic clock is today's fundamental tool of measurement. Since 1967, the official length of one second has been 9,192,631,770 atomic "beats" of the form of the metal cesium that has 133 protons and neutrons per atom. This second is then used to define a unit of length, the meter; by definition, in one second light travels in a vacuum exactly 299,792,458 meters. Occasionally the length of a year must be adjusted by one second so that time based on the movements of the sun and stars matches the official time measured by atoms.
How it works
A gas of atoms or molecules of exactly one type is fed into a resonator, which responds to a particular frequency (just as a tuning fork responds to a single pitch) and amplifies its energy. The amplified wave is measured to establish the length of a second.