Does anyone really believe the clock on the wall of a spaceship will stop if it reaches the speed of light? If it really is so, it would be nice to know why. When Einstein published his Special Theory of Relativity in 1905, time and space were turned upside down. Newton gave us comfort in setting time as constant, but Einstein said no, it is the speed of light that is constant, and that is something we will never reach.

In Very Special Relativity, Sander Bais, a theoretical physicist at the University of Amsterdam, sets out to explain the theory that concluded with E=mc², and ultimately unleashed atomic power. He does so using a cunning mixture of a simple Cartesian grid, with axes for time and space and coloured arrows to depict objects in motion, and a wry conversational style that, as you get deeper into the book, struggles to mask the fact that it's not easy to explain the work of a genius. The author's space-and-time grid is soon overlayed with Einstein's spacetime grid, and that's when things get interesting.

Apart from using simple geometry to great effect when explaining causality and time dilation, Bais also runs through quick mathematical proofs to arrive at Einstein's formulae for the addition of velocities and to show the relationship between a stationary clock and a moving one. Faced with these elegant equations it's easy to see why, when velocity reaches the speed of light, time stands still.

But why is the speed of light constant? While moving at half the speed of light, if you shine a torch ahead of you, why wouldn't that beam of light travel at one-and-a-half times the speed of light? Bais's basic vector diagrams are like a fan which sucks the fog from your head. He should really get a job teaching this stuff. Oh, he already has.