The Greek letter τ (tau) was derived from the ancient Phoenician letter 𐤕 (tāw) which, in turn, was derived from an Egyptian hieroglyph ✕ meaning “mark”.
The lower-case τ has many uses in maths, engineering and sciences, such as
- in biochemistry: the τ protein, a protein that stabilise microtubules
- in continuum mechanics: shear stress
- in differential geometry: torsion of a curve
- in engineering and physics: the time constant of a system
- in geometry: the tortuosity
- in mathematics: the Ramanujan tau function, an irrational number with approximate value of 0.41468
- in mechanics: the torque, aka moment of force
- in optics: optical depth of a material, defined as τ = −ln T, where T is the transmittance of that material
- in particle physics: tauon τ− and antitauon τ+
- in physical sciences: relaxation time
- in physics: the lifetime of a spontaneous emission process
- in relativity: proper time
- in spectroscopy: rotational correlation time
- in statistics: the Kendall rank correlation coefficient
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