In quantum physics, the ancient Greek letter α, alpha, represents an important constant of nature. Alpha is .00729735256… and goes on for an infinite number of decimal places. This is very close to 1/137, which is 00729927…. The fraction 1/137 diverges from alpha at the sixth digit beyond the decimal point. Alpha is also called the “fine-structure constant” or “Sommerfeld’s Constant.”

One example of the use of alpha is in calculations regarding photosynthesis in the leaves of plants. When a photon of sunlight is absorbed by an electron in a leaf, alpha is used to calculate the strength of interaction between the photon and the electron. Alpha is the ratio of the strength with which electromagnetism (for example, sunlight) affects charged particles, like electrons or protons.

Alpha, the strength of the effect of electromagnetism on charged particles, appears in quantum physics equations for calculating many different properties of particles at the atomic and subatomic levels. For example, it is used in calculating the mass of electrons and the strength of their charge.

Alpha is not expressed in measurement units but is simply a number, .007208352…. You are probably familiar with two other constants of nature which are simply numbers: pi and e (Euler’s number). Pi, for example, is simply the number, 3.14159…. We don’t say that pi equals 3.14159 inches or 3.14159 pounds; it’s just a number, 3.14159…. Similarly, alpha is .007208352…. This is called a “bare number” or a “dimensionless number.”

Finding the value of alpha. Quantum electrodynamics, the theory of the interaction of electrons and photons, incorporates alpha as an important constant. However, the theory cannot be used to calculate its value. Instead the value of alpha must be measured.

The strength of alpha has been found experimentally to be slightly larger than .007208352. Due to the Heisenberg Uncertainty Principle, this value of alpha cannot be determined exactly but always includes a certain amount of imprecision. As alpha is very nearly 1/137, it is often expressed as this fraction.

Physicists have not found a reason that 137, a prime number, should arise in this manner. Some physicists, such as Sir Arthur Eddington and Wolfgang Pauli, intrigued, have devoted considerable energy into exploring the number 137, but without finding any special reason that it should have such importance in quantum physics.

[This article is largely based on the audiobook, Frank Close, The Infinities Puzzle; Tantor Audio, 2012; Chapter 1.]

Etymology of alpha. The ancient Greek symbol α is named “alpha.” This symbol evolved into the English letter a, the first letter of our alphabet. The word “alphabet” itself is named for the first two letters of the ancient Greek alphabet, alpha and beta. In English, these evolved into a and b.