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
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, email@example.com
Statistical Mechanics: Entropy, Order Parameters, and Complexity, now available at Oxford University Press (USA, Europe).