QUESTION #558

# I heard that the speed of light is the same to any observer, no matter how fast he moves, and that the faster you move, speed of light is still the same relative to you. Is this true, and why?

As far as we can tell, it does appear to be true that the speed of light is the same for all observers. This fact was predicted by Maxwell when he united the electric and magnetic forces into one. From his equations it was possible to calculate that the resulting electromagnetic field travels at a constant speed relative to all observers. It was when Maxwell realised that this constant was the very same speed at which light was known to travel that he guessed that light was, in fact, electromagnetic radiation ! There is, as yet, no intuitive explanation to why the universe should act like this. Since Maxwell's work, numerous experiments have been performed to test the prediction that electromagnetic radiation travels at the same speed for all observers - and none have failed. Instead of being a prediction from theory, it now became to be used as an assumption to build theories upon. Einstein was so convinced of its truth that he modified Newton's theory of gravity to encompass the constancy of light. Likewise, in the 1940s, Feynman, Tomomaga, Bethe and others incorporated the idea into Quantum Mechanics. The resulting theories, General Relativity and QED, are probably the most accurately tested to date - and they require that the speed of light is constant.
Answered by: Sally Riordan, M.A., Management Consultant, London

Imagine you are standing on the back of an old fashioned open back train, in the last car, and you look down on the rail track and I am standing beneath you. Feeling bored and tired of waiting for the train to get going we play throw and catch with a ball. You throw the ball at 5 mph, and I catch the ball which is travelling at 5 mph. After a few moments the train starts to depart the station. You find that you have to throw the ball harder for me to catch it. By now the train is moving forwards at 5 mph.. So in order for me to catch the ball at the same speed we played earlier, you must throw the ball at 10 mph. Because if you throw it at 5mph, the ball will not reach me, because you are moving forwards at 5mph, and throwing the ball backwards at 5 mph, means that from my point of view the ball is stationary. Now you can try this, and you will find that this is how the universe works. If we now pretend that we are both scientists, and you shine a torch from the back of the car, and I equipped with my scientific equipment measure the speed of the light, it turns out that no matter how fast the train leaves the station, I always catch the light from your torch, even if your train departs the station near the speed of light. And if we do calculations then we measure the speed of light as being independent of your motion. Why this is so is indeed a great mystery of science, and one that lies at the core of physics. So here is my explanation. Imagine that we live on a wire mesh ,a lattice of intersecting threads of wire, and we are both moving over that mesh at some given speed. Let us now consider that we are back in station and that sending a message by light means putting a ball onto the mesh. And all balls travel on the mesh of wire travel the same speed. So now it doesn't matter how fast you or I am moving, the balls on the mesh travel with the same speed. Not only does this picture explain why light travels at a constant speed independent of the observer, but it also explains why you see the ball changing colour. In this case is becoming bluer the faster the train leaves the station. The mesh that I am talking about is called space-time, and for many years was called the Ether. Funny how these things come back around after a few hundred years, all be it in a different guise.
Answered by: Andrew Couch, Manchester UK