The fine-structure constant is defined in terms of the speed of light c, Planck's constant h, and the charge of the electron e. I usually write it as
, is a shorthand for
"Planck's constant h divided by two pi".)
Experimentalists usually use a different scheme of units. (It's like
the Americans used to measure things in feet, and the Europeans in
meters: the formulas are different, but the answers are the same.) The
formula in SI units is
There is an interesting story about the fine-structure constant. Early on, someone had a theory that it was exactly 1/136. When the measurement got better, he produced an improved theory that it was exactly 1/137. His theory didn't make much sense to people, but it took a much better measurement the second time to disprove him!
It's interesting that g
is close to two (it's about 2.001159652188) for an
important reason, while 
is close to 1/137 for no apparent reason at all.
James P. Sethna, sethna@lassp.cornell.edu
Statistical Mechanics: Entropy, Order Parameters, and Complexity,
now available at
Oxford University Press
(USA,
Europe).