I was asked (on another non physics site) how an observer can measure the speed of light to be the same as someone who is moving away/ towards the light source. I know that it is a fairly simple idea but I couldn't find the words to answer comprehensively. Here is what I wrote: In physics, there is a wave equation. This describes how a disturbance propogates through space in time. The Maxwell equations give a form that looks like the wave equation (with the disturbance as the electric field and the magnetic field) that predicts that light always travels as 1 divided by the square root of the product of two fundamental physical constants (this is the speed of light). There is no reason that the equations should be different for a man travelling at some speed. Hence, the equations suggest that light will always be measured to travel at about 300 000 000 m/s (or c) regardless of the speed of the observer. As you have obviously considered [hence, the question], this is strange. A man moving away from (or toward) a source of light still sees the light moving towards him at c. The strangeness of the idea can be taken away (and replaced with other strange ideas) when you realize that basic concepts such as absolute space, absolute time and simultaneity are not valid in the Universe in which we live. For example, if you watch a spaceship move away from a light source at close to the speed of light, from your perspective, the spaceship will shorten in the direction of travel and any processes taking place inside will appear to run in slow motion. In this example, you would 'see' the light approach the spaceship at the speed c-v and (naively) infer that the observers in the ship should see the light coming towards them at this speed also. The reason that they don't and that they actually see the light come towards them at c has to do with the fact that they are in a different timeframe and on a different lengthscale. I haven't actually studied special relativity in awhile and you can tell that I am not sure of myself by the length of the text. Is my explanation basically correct (once again, this is not a physics site, just the general chat forum of a football site)? Have I missed a more succinct way of putting it? Any comments would be greatly appreciated.