Disclosed herein is a velocity measuring device to be used in a moving frame to determine the velocity of the moving frame. At least one beam of light is emitted from a site in the moving frame and travels to a mirror disposed in the moving frame and back to the site at which the emission occurred, after which the beam is detected by a light beam detector. By measuring the round trip time of the light beam from emission to detection, a factor gamma can be determined from which the velocity of the moving frame can be computed.