Alpha

How to read it

Alpha is said AL-fuh: Cambridge Dictionary gives /ˈæl.fə/ for both British and American English. In Greek the letter is άλφα, which the University of Arizona’s Modern Greek guide writes as “Alfa”, with the “ah” vowel. The American Heritage Dictionary traces the name to Phoenician. The spelling Alfa, with an f, is also the word for A in the international (ICAO) spelling alphabet, which the US Federal Aviation Administration’s manual says to pronounce AL-FAH.

Also searched as: alpha letter, alpha symbol, α, Α, α meaning.

Capital, small and look-alike forms

The capital Α looks the same as the Latin A; Cambridge Dictionary gives the letter’s symbols as A and α. Unicode’s chart cross-refers the small α to two look-alikes with their own code points: the Latin small letter alpha ɑ (U+0251) and the proportional-to sign ∝ (U+221D).

Alpha in statistics

In a hypothesis test, α is the probability of a Type I error: rejecting the null hypothesis when it is true (OpenStax, Introductory Statistics). Its partner β is the probability of a Type II error. Jeff Miller’s history of mathematical symbols, published by the University of St Andrews, finds α for the size of a test in a 1936 paper by Jerzy Neyman and Egon Pearson; in their 1933 paper they had used ε.

In AI and machine learning

Alpha has two common jobs, and which one depends on the text. In Stanford’s CS229 lecture notes, α is the learning rate, the size of the step gradient descent takes each time it updates the parameters θ. In scikit-learn, alpha is the regularisation strength: the constant that multiplies the L2 penalty in ridge regression, which Google’s glossary would call λ. The learning rate itself goes by other letters elsewhere: scikit-learn’s guide uses η and PyTorch’s optimisers use γ.

In quantum physics

α is the fine-structure constant, one of the fundamental physical constants NIST publishes. The 2022 CODATA value is 7.297 352 5643 × 10⁻³, and its inverse, α⁻¹, is 137.035 999 177.

Other uses of alpha

Alpha particles are one of the three kinds of nuclear radiation early experimenters named alpha, beta and gamma; OpenStax’s University Physics says alpha rays were later identified with helium nuclei. Alpha software is at the first stage of development (Cambridge Dictionary), before Beta. In astronomy Alpha names the brightest star in a constellation, and in finance alpha is the return on a portfolio that the American Heritage Dictionary attributes to the manager’s skill rather than the market. In the Bible, Revelation 1:8 has “I am Alpha and Omega, the beginning and the ending”, the words behind the Christian symbol Alpha and Omega (see also Omega).