Ian Rodger, of Westport, asks :-

Think of a headlight on a car travelling at 100kph. If light is incapable of travelling faster than the speed of light (c), ie at c plus 100kph, would then the light be brighter immediately in front of the filament? Could it be compressed in someway? If not why not?

Geoffrey Stedman, a physicist at the University of Canterbury who has published on various aspects of relativity, responded.

It probably seems obvious to you that light leaving a car headlight is going faster than the car. However that idea is wrong and lot's of 'obvious things' prove to be wrong in relativity.

Light in all its forms from gamma rays to TV signals is some of the strangest stuff in science. It always travels at 299,792,458 metres per second no matter how fast the source is going.

Why so? Briefly, when you measure all the relative speeds carefully that is what we find. However much people might like to argue against the consequences of relativity, science is based on observations of the world and there is no future in demanding that the world be exactly as we imagined it ought to be. We have to live here.

So there is no reason to suppose some bunching up in front of the headlights; the intensity of the beam of light as with all waves depends on the strength of the fields which make it up, in his case the electric and magnetic fields, and on the frequency of oscillation of the wave motion.