Light's speed, Einstein forcefully declared, is 300,000 kilometres per second relative to anything: <a href="http://www.sciencedirect.com/science/article/pii/S0262407913605655">http://www.sciencedirect.com/science/article/pii/S0262407913605655</a> Trouble with physics: The roots of reality, Brian Greene: In the late 1800s, when James Clerk Maxwell realised that light was an electromagnetic wave, his equations showed that light's speed should be about 300,000 kilometres per s