Special Relativity for everyman - (or woman) - 1. Introduction.

The speed of light. An innocuous phrase. Stating the seemingly obvious fact that light moves at a particular speed. In the same way that anything that moves has a speed. We all know that nothing can travel faster than light and that the speed of light, in a vacuum, is 'c'.

Yet that one simple property, that Light travelling through a vacuum has the speed 'c' wherever and whenever it is measured, threw the whole of Physics into a turmoil..

Why the turmoil?

What is the Speed of Light, but the time it takes for light to travel from A to B, two points fixed in space and time? Or to be more precise, between two events in Spacetime, as scientists refer to it now.

So now we have the time it takes light to travel, in a vacuum, between fixed point A at time T_A and fixed point B at time T_B

Simple enough surely, for any man in the street to understand, so why does it cause a problem for the scientists.

It is only when one examines what that means, that its apparently paradoxical nature is apparent.

Let me illustrate that:

Light from the sun, and all the stars and galaxies in Space, is travelling towards the Earth at 'c'. Fair enough, everyone knows that.

Yet think about it; as our Earth orbits the sun, it is travelling at around 70,000 mph, sometimes towards a star and sometimes away from it. The stars are moving in vast orbits round the centres of their Galaxies, and indeed the Galaxies are moving vast distances on their paths through the heavens. Yet, despite all of this and contrary to all common sense and expectations, the speed of the light from any star relative to the Earth is always measured to be 'c' wherever in the Earth's orbit we measure it, whether the Earth is travelling at 70,000mph toward that light, or 70,000mph away from that light.

Again, imagine two trains speeding along a straight track in opposite directions; imagine that as the trains pass, an observer on each train measures the speed of light from the sun rising at one end of the track. One train is speeding towards the sun and one is speeding away from it yet each observer will still measure the light to be coming towards them, at the same speed.

To the average layman this is not a problem, as the speed of light, 'c', is how fast it travels between two points, the sun and the point where the trains are passing. Indeed the time it takes to do so is the same; yet to the scientist that is anathema, for to him it is the speed that the light is measured to be travelling relative to the observer who is making the measurements on each train. They have to factor in the velocity of the train, +/- v. So, for those observers, it would seem reasonable to claim that the speed of the light would be c + v or c - v, depending on the direction of the train.

Yet, as stated above, it has been found that light is measured to travel at 'c' by any observer; whether that observer is stationary with respect to the points between which the light travels, or is moving with respect to one or both of them.

Einstein's genius led him to understand this; led to the adoption of his two Postulates, that the general Laws of Physics are the same everywhere, in any frame of reference and the constancy of the speed of light from which he developed his Theories of Special and, subsequently, General Relativity.