What was Einsteins famous thought experiment with light?
Einstein wondered what you would see if you were travelling at the speed of light, or more precisely what would you measure the speed of light to be. It was by doing various thought experiments and realising that you must always measure the speed of light to be the same that the profound implications came to him from this and indeed the relativity theories are really underpinned by that one core idea of the absolute speed of light.