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Old 11-11-2010, 04:43 PM
Spatial Rift 47 Spatial Rift 47 is offline
Join Date: Dec 2000
Location: Boston, MA
Posts: 3,222
Originally Posted by LouisB View Post
How does one isolate a single atom, determine the two hyperfine levels of that atom, and make whatever measurements one must make in order to determine what the hell one second actually is? Perhaps even more importantly, how does one determine which atom one needs to isolate? There are so many things I wish I knew and so little time left to even begin.
In order: you don't, you use the equations of atomic structure and energy from quantum mechanics, and you use a spectrometer to measure the radiation emitted by a large assembly of cesium-133 atoms as they make that particular transition when you induce it with a magnetic field.

The choice of that particular transition from that particular atom is largely arbitrary, since it serves only to identify photons of a particular frequency. Any quantum mechanically specified transition would serve the same purpose. We could just as solidly define the second as 2.4967x1015 periods of the radiation corresponding to the Lyman-alpha transition of neutral hydrogen atoms (in the absence of any fine or hyperfine splitting).