Tau's modern technical uses span physics, biology and a minority position in mathematics, none connected to the letter's ancient sound and numeral value, covered elsewhere in this atlas. Physicists named the tau lepton, a much heavier relative of the electron and the muon discovered in experiments at Stanford in 1975, directly after the letter, continuing the same naming pattern as the earlier muon, discussed elsewhere in this atlas; engineers and physicists also use lowercase tau for a time constant describing how quickly a system responds to change.
In biology and medicine, tau denotes a protein that stabilises the internal skeleton of nerve cells; abnormal tau protein forms tangles strongly associated with Alzheimer's disease and several related neurodegenerative disorders, making tau one of the most closely studied proteins in contemporary dementia research. A small but vocal minority of mathematicians has also proposed since 2010, in what its authors call the Tau Manifesto, replacing pi with a constant called tau, defined as twice pi, arguing it simplifies many formulas involving circles; the proposal has an informal following, marked by an annual Tau Day on June 28, but has not displaced pi in standard mathematical practice.