137

http://www.economist.com/node/16930866 http://www.physlink.com/Education/AskExperts/ae186.cfm

RICHARD FEYNMAN, Nobel laureate and physicist extraordinaire, called it a “magic number” and its value “one of the greatest damn mysteries of physics”. The number he was referring to, which goes by the symbol alpha and the rather more long-winded name of the fine-structure constant, is magic indeed. If it were a mere 4% bigger or smaller than it is, stars would not be able to sustain the nuclear reactions that synthesise carbon and oxygen. One consequence would be that squishy, carbon-based life would not exist. Why alpha takes on the precise value it has, so delicately fine-tuned for life, is a deep scientific mystery.

Slightly belying its name, the fine-structure constant is actually a compound of several other physical constants, whose values can be found in any physics textbook. You start with the square of an electron's charge, divide it by the speed of light and Planck's constant, then multiply the whole lot by two pi. The point of this convoluted procedure is that this combination of multiplication and division produces a pure, dimensionless number. The units in which the original measurements were made cancel each other out and the result is 1/137.036, regardless of the measuring system you used in the first place.



Despite its convoluted origin, though, alpha has a real meaning. It characterises the strength of the force between electrically charged particles. As such, it governs—among other things—the energy levels of an atom formed from negatively charged electrons and a positive nucleus. When electrons jump between these energy levels, they absorb and emit light of particular frequencies. These frequencies show up as lines (dark for absorption; bright for emission) in a spectrum. When many different energy levels are involved, as they are in the spectrum of a chemically mixed star, the result is a fine, comb-like structure—hence the constant's name. If it were to take on a different value, the wavelengths of these lines would change.