In special relativity, gamma (γ) is the symbol for a *very* common quantity,

1/sqrt(1 - v^2/c^2)

where v is the

velocity of the reference frame, and c is the

speed of light. It is used in the

Lorentz transformation relating quantities in two inertial reference frames as a

constant of proportionality. As a

function of v, it begins at 1 for v=0 and increases with increasing v such that it is infinite at v=c.

There are many important uses of γ in relativity. It is the constant of proportionality in time dilation and length contraction. The mechanical energy of a body is E = γmc^{2}, which reduces to the famous E=mc^{2} when the particle velocity is zero. Similarly, the relativistic momentum is **p**=γm**v**, which reduces to the familiar Newtonian momentum when v<<c. Along with β=v/c, γ appears in many other relativistic situations, including relativistic electromagnetism.