Tau

Greek alphabet
Α α Alpha	Β β Beta
Γ γ Gamma	Δ δ Delta
Ε ε Epsilon	Ζ ζ Zeta
Η η Eta	Θ θ Theta
Ι ι Iota	Κ κ Kappa
Λ λ Lambda	Μ μ Mu
Ν ν Nu	Ξ ξ Xi
Ο ο Omicron	Π π Pi
Ρ ρ Rho	Σ σ Sigma
Τ τ Tau	Υ υ Upsilon
Φ φ Phi	Χ χ Chi
Ψ ψ Psi	Ω ω Omega
obsolete letters
Ϝ ϝ Digamma	Ϻ ϻ San
Ϙ ϙ Qoppa	Ϡ ϡ Sampi

Tau (upper case Τ, lower case τ) is the 19th letter of the Greek alphabet. In the system of Greek numerals it has a value of 300.

The lower-case letter τ is used as a symbol for:

• Torque, the rotational force in mechanics.
• The elementary tauon particle in particle physics.
• The lifetime of a spontaneous emission process.
• The time constant in an RC circuit.
• A coefficient of correlation — see Kendall's tau.
• The Golden ratio 1.618... (although φ (phi) is more common)
• Tau in astronomy is a measure of opacity, or how much sunlight cannot penetrate the atmosphere
• Tau in biochemistry is a protein associated to microtubules

Other uses of Tau:

• Tau is the name of an island near Kilinailau, Greenwich Island, and Ocean Island.
• In Games Workshop's Warhammer 40,000 science fiction gaming universe, the Tau are an alien race.
• Tau is a symbol used from antiquity and is represented by a type of cross. There are those who accuse Christ of being the pirate of this ancient symbol, but it did not become a symbol of Christianity until a later date. As with Christianity, other ancient societies who used the "Tau" symbol also evolved its use to represent resurrection, reincarnation and blood sacrifice.
• The prefix of many cosmic entities, fictional or not. Notably Tau Ceti Five from System Shock 2

See also: Tau, Alien, Alpha (letter), Astronomy, Beta (letter), Biochemistry, Chi (Greek letter)