Mu in Modern Use

Mu's modern technical roles cluster almost entirely around smallness and measurement, an association with no connection to the letter's ancient Semitic root mem, meaning water, covered elsewhere in this atlas. The International System of Units uses lowercase mu as the prefix symbol for micro, one millionth, so that a micrometre or a microsecond is written with a mu in scientific notation; statisticians use mu for the mean of a population, distinguishing it from the sample mean written with a Latin letter; and physicists use mu for the coefficient of friction between two surfaces and for magnetic permeability. Particle physicists named the muon, a heavier relative of the electron discovered in cosmic ray studies in 1936, directly after the letter mu, continuing the practice of naming subatomic particles after Greek letters that also produced the later tau lepton, covered elsewhere in this atlas.

None of these 20th-century scientific conventions draws on mu's ancient sound value or its place among the Sefer Yetzirah's mother letters in the Hebrew alphabet it ultimately descends from; the modern uses are a separate, purely notational layer built on the letter's familiar shape. In the Milesian numeral system, discussed elsewhere in this atlas, mu carries the value forty, likewise unconnected to any of its later scientific senses.