Among the great achievements of Einstein was his understanding of the speed of light. The speed of light, in a vacuum, is a constant at ~ 300,000 kilometers/second (the actual velocity is 299,792.458 km/s). The speed of light is essential to the viability of both Einstein’s theories of Special and General Relativity (since the speed of light is a constant it has been given its own mathematical symbol, c). If the speed of light is not constant then neither of Einstein’s theories are credible and would not be accurate in describing physics at the larger-scales of the Universe and objects moving at high velocities close to the speed of light.