Albert Einstein's General Theory of Relativity is possibly the most perfect intellectual achievement in modern physics. Anything that involves gravity, the force that powers everything on the largest, hottest or densest of scales, can be explained by it. From the moment Einstein first proposed the theory in 1915, it was received with enthusiasm yet also with tremendous resistance, and for the following ninety years was the source of a series of feuds, vendettas, ideological battles and international collaborations featuring a colourful cast of characters.