In 1905, at the age of 26, Albert Einstein proposed his special theory of relativity. The theory reconciled the physics of moving bodies developed by Galileo Galilei and Newton with the laws of electromagnetic radiation. It posits that the speed of light is always the same, irrespective of the motion of the person who measures it. Special relativity implies that space and time are intertwined to a degree never previously imagined.
Starting in 1907, Einstein began trying to broaden special relativity to include gravity. His first breakthrough came when he was working in a patent office in Bern, Switzerland. 鈥淪uddenly a thought struck me,鈥� he recalled. 鈥淚f a man falls freely, he would not feel his weight鈥� This simple thought experiment鈥� led me to the theory of gravity.鈥� He realised that there is a deep relationship between systems affected by gravity and ones that are accelerating.
The next big step forward came when Einstein was introduced to the mathematics of geometry developed by the 19th-century German mathematicians Carl Friedrich Gauss and Bernhard Riemann. Einstein applied their work to write down the equations that relate the geometry of space-time to the amount of energy that it contains. Now known as the Einstein field equations, and published in 1916, they supplanted Newton鈥檚 law of universal gravitation and are still used today, nearly a century later.
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Using general relativity, Einstein made a series of predictions. He showed, for example, how his theory would lead to the observed drift in Mercury鈥檚 orbit. He also predicted that a massive object, such as the sun, should distort the path taken by light passing close to it: in effect, the geometry of space should act as a lens and focus the light (see diagram).
Einstein also argued that the wavelength of light emitted close to a massive body should be stretched, or red-shifted, as it climbs out of the warped space-time near the massive object. These three predictions are now called the three classical tests of general relativity.
Read more: Instant Expert: General relativity