The energy density multiplies two instances of this quantity in each component of the energy-momentum tensor of a dust. (-5[1])Two instances of this quantity multiply a Christoffel symbol in the geodesic equation (15[1])of general relativity. The rate of Thomas precession is (-5[1])proportional to acceleration crossed with this quantity. Einstein showed that this quantity adds like the hyperbolic tangent of the sum of the (*) rapidities. (10[1])The relativistic energy-momentum relation is approximated by the sum of the rest mass and the kinetic energy in the limit of low values of this vector quantity. In natural units, the Lorentz gamma factor contains a denominator equal to the square root of 1 minus the square of this quantity. For 10 points, name this quantity whose magnitude cannot exceed (10[1])the speed of light. ■END■ (0[1])