RARELY has a single event changed our view of the universe as much as the solar eclipse of May 1919.With the Sun's light blocked by the Moon, Sir Arthur Eddington and colleagues observed that stars close to the Sun appeared to have moved slightly from their usual positions in the sky. This confirmed a prediction of the general theory of relativity – that massive bodies can bend light – which overturned Newton’s 200-year-old theory of gravitation and propelled Albert Einstein to global stardom.
Today, 86 years after these historic observations, general relativity is still passing all observational tests with flying colours. But never satisfied, and perhaps suspicious of "flawless theories", astronomers have been striving to test the theory under ever more extreme conditions. That quest recently received a huge boost when researchers discovered a double pulsar, a pair of cosmic "clocks" that are orbiting one another, that can be used to make stringent tests of gravitational theories.
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