ONE hundred years ago, on November 25th 1915, Albert Einstein presented his freshly finished general theory of relativity to the Prussian Academy of Sciences. It was the outcome of nearly a decade's dedicated work. He showed that the theory solved a 150-year-old problem: each year, Mercury's closest point of approach to the Sun was moving forward more than it was expected to. All manner of explanations had been put forth, including an unseen planet called Vulcan, but relativity did the job perfectly. In 1916, Einstein predicted that relativistic effects would cause the apparent positions of stars to change during an eclipse, as the sun bent the distant stars' rays. That prediction was proved right in 1919, in a widely publicised expedition that propelled Einstein to global fame. But what exactly is his general theory of relativity all about?
