Big Bang by Simon Singh, chapter 2 gives the background to a thought experiment first performed by Albert Einstein when he was only 16, and now known as Einstein's mirror. This page provides a brief introduction to Einstein's mirror. Before reading on, try this experiment yourself:
- Einstein's mirror is a hand held mirror. To perform the experiment, hold the mirror and look at your reflection. While retaining hold of the mirror, imagine what would happen if you were travelling at the speed of light. (Hint: Would you see your reflection?)
Einstein knew the standard Victorian theory that light was carried by a luminiferous ether. This ether was, supposedly, a static substance permeating the whole universe.
Light waves were thought to be a disturbance in the luminiferous ether, just as sea waves are a disturbance in the ocean. The speed of light was relative to this ether. Einstein wondered, if he moved at the speed of light relative to the ether, how could light move relative to him? How could there be a reflection in his mirror?
"He concluded that light did not travel at some fixed speed relative to the ether, that light was not carried by the ether, and that the ether did not even exist. Unbeknown to Einstein, this is what Michelson and Morley had already discovered." Singh, p.102.
To preserve Galileo's principle of relativity, Einstein suggested that light always moves at a constant velocity relative to the observer. Assume you measure the speed of light to be 300 000 km/s relative to yourself when you are standing still. If you now move at close to the speed of light, you would still measure light waves moving at 300 000 km/s relative to yourself (not 0 km/s as you might have expected). If the light waves are reflected in Einstein's mirror you'll find them coming back to you at exactly the same velocity (not at close to 600 000 km/s as you might have expected).
You would not be able to tell if you were standing still or moving at the speed of light by looking in Einstein's mirror, or by performing any other experiment.