The theory of gravity that describes the large-scale dynamics of the universe was developed by Albert Einstein during the first two decades of the twentieth century. It is referred to as the general theory of relativity. It will be helpful if we now consider some of the ideas behind this theory.

We should begin by considering the speed of light. In 1865 the English physicist James Clerk Maxwell derived equations proving that electromagnetic radiation travels in a vacuum at a constant and finite speed. One of the key assumptions that Einstein later made was to suppose that two observers who are moving at a constant speed relative to one another would measure the same value for the speed of light.

Einstein's assumption goes against our intuition, to say the least. What might we expect? Speed is a relative quantity; we can measure the speed of an object only in terms of its relationship to something else. For example, when we say that a train travels through a station at a constant speed of one hundred kilometres an hour, what we really mean is that the distance between the train and the platform changes at this rate.

Let us consider two trains, A and B, that simultaneously travel through the station at this speed. If the two trains are moving in the same direction, they will appear to be at rest relative to each other.