In physics, the fine-structure constant (usually denoted α, the Greek letter alpha) is a fundamental physical constant, namely the coupling constant characterizing the strength of the electromagnetic interaction. The numerical value of α is the same in all systems of units, because α is a dimensionless quantity. Arnold Sommerfeld introduced the fine-structure constant in 1916.
Alpha specifies how strongly electromagnetic waves (like light or x-rays) affect charged particles (like electrons and protons). Alpha is actually a dimensionless ratio-all units cancel out-involving three quantities:
alpha=0.0072973525698
The fine structure constant, alpha (α), describes how electromagnetic radiation affects charged particles. It has the numerical value 0.007297351, with an uncertainty of 6 in the last decimal place, and as such is one of the best-measured numbers in physics.
Note that α is quite a small number, very nearly 1/137. It is much easier to calculate things when the forces are not very strong. The theory of radiation and charged particles, called Quantum Electrodynamics (or QED for short), is probably the best understood and the most successful theory in all of physics. Some of its predictions have been verified experimentally to twelve decimal places.
Another important property of α is that it is dimensionless. It is formed as a ratio of three of the fundamental constants of Nature: the electric charge of the electron, e; the speed of light, c; and Planck's constant, which is usually written as h(bar), and which is necessary to describe quantum phenomena. Each of these 3 constants has dimensions, that is, it changes when you change your system of units. For example, the speed of light is very close to 3x108 meters per second, but if we want to know it in miles an hour, the answer is a different number, 6.7x108. However, when we form alpha by taking the ratio α=e2/h(bar)c, all the dimensions cancel out. The result is a dimensionless number, whose value is 0.00729735256981 no matter what system of units we use.
Value: 7.297 352 5698 x 10-3
Standard uncertainty : 0.000 000 0024 x 10-3
Relative standard uncertainty: 3.2 x 10-10
Concise form: 7.297 352 5698(24) x 10-3