This quantity is raised to the negative fourth power in the definition of Einstein’s gravitational constant. The slope of a 45 degree line on a Minkowski diagram equals this quantity because Minkowski time is scaled by this quantity. The (10[1])ratio of velocity to this quantity is denoted by beta. By comparing interference patterns, this quantity was found not to depend on the direction of measurement in the Michelson-Morley (-5[1])experiment. The mass-energy equivalence states (10[1])(read slowly) rest energy equals mass times (10[1])this (10[3])quantity squared. For 10 points, name this universal speed limit (10[1])equal to about three times ten to the eight meters (10[1])per second. ■END■