Everyone knows what time is. We can practically feel it ticking away, marching on in the same direction with horrifying regularity. Time has enslaved the Western world and become our most precious commodity. Turn it over to the physicists however, and it begins to morph, twist and even crumble away. So what is time exactly?
At the heart of modern physics lurks a terrible puzzle: the two main theories that describe the world we live in just won't fit together.