The speed of light is always the same. Because this speed is absolute, time is relative: a moving clock is seen to tick more slowly than the same clock standing still with respect to the observer. This is called time dilation, and the equation describing it shows that the speed of light is the maximum speed attainable in the Universe.

Consequently, instantaneous actions or connections over a finite distance are impossible. In our Universe, all things are always under way, whatever they are. No two events in space-time may be linked instantly; the news that something has happened always takes some time before it has reached other places.

Likewise, the properties of space-time itself cannot be linked instantly across finite distances. This implies that space-time properties may vary from place to place and from time to time. If, for some reason, space-time were not exactly uniform, meaning that its properties differed from event to event, it would be impossible to smooth everything out instantaneously.

When the structure of space can vary in the course of time, we are justified in saying that space-time has its own dynamical behaviour, so that space-time may be seen as real stuff with its own structure, like the Oude Kerk in Delft. Unlike that old church, space-time doesn't just stand there, but it is dynamic. Space-time is not some sort of invisible graph paper on which the paths of all things are drawn, the way Newton said it is. Space-time is stuff, with a dynamic structure.

The invention of the mathematical equation that expresses this astonishing result was Einstein's greatest contribution to physics, even greater than his other enormous achievements, according to just about all physicists. The formulae have a stark beauty of their own, but they need not be presented here. Translated into plain language, they state that the structure and dynamics of space-time are determined by the arrangement of mass, energy and momentum.

Astonishing indeed, because it follows from this equation that, first, gravity does not exist; second, it is now clear why Stevin's experiment showed what it did.

If there is no such thing as gravity, what then is the reason why the orbits of the planets are curved?