By Jonathan Amos
Science reporter, BBC News
Scientists have put Einstein's theory of gravity to a remarkable test using two distant, dead stars.
Massive objects such as pulsars will curve space-time
An international team studied a double pulsar system, a pair of dense neutron stars whose properties mean they act like near-perfect space clocks.
The astronomers observed the way each stellar timepiece behaved in the other's curved space-time to run the rule over General Relativity.
The results, reported in Science, match Einstein's predictions to within 0.05%.
"We've found this double pulsar system to be an excellent laboratory for studying a wide range of physical problems," team leader Professor Michael Kramer from the University of Manchester's Jodrell Bank Observatory, UK, told BBC News.
Einstein's Theory of General Relativity (GR) describes mass and gravity as being generated by the curvature of space-time.
Professor Kramer's team discovered the double pulsar system three years ago. It is 2,000 light-years from Earth in the direction of the constellation Puppis.
It consists of the remnant neutron cores of two giant stars that blew themselves apart when they exhausted their nuclear fuel. The cores orbit each other every 2.4 hours at speeds of a million km per hour.
These rapidly rotating objects each weigh more than our Sun, but their masses are compacted into spaces no larger than a big city.
They are referred to as pulsars because both emit lighthouse-like beams of radio waves that are seen as radio "pulses" every time the beams sweep past the Earth.
The pulses are akin to the ticks of a clock; and the properties of the very stable neutron stars make for ultra-precise time-keeping.
By monitoring the pulses' arrival at Earth, the team was able to measure how the beams from each neutron star were being disturbed as they passed through the curved space-time near their companion.
"We see their orbits edge-on, so what happens is that one pulsar will eclipse the other," explained co-researcher Dr Duncan Lorimer from West Virginia University, Morgantown, US.
"We can measure the signal from the [far] pulsar so precisely that we can see the delay of the signal as its beam passes through the curved space-time of the pulsar sitting in the way; the signal has to travel an extra distance. It's called 'Shapiro delay'," he told BBC News.
The team found this delay to be close to 90 millionths of a second and the ratio of the observed and predicted values to be 1.0001, plus or minus 0.0005 - a precision of 0.05%.
Other effects were noted as well. For example, whenever one pulsar sits deeper in the gravitational field of the other, its clock appears to run slower. This phenomenon has been observed with atomic clocks flown in aeroplanes above Earth, but in the extreme environment of the pulsars the effect is far more pronounced.
The team also saw the distance between the pulsars shrink by 7mm a day.
Einstein's theory predicts that the double pulsar system should be emitting gravitational waves - ripples in space-time that spread out across the Universe at the speed of light - as the neutron stars gradually fall in on each other.
"These waves have yet to be directly detected," said team member Professor Dick Manchester from the Australia Telescope National Facility. "But, as a result, the double pulsar system should lose energy causing the two neutron stars to spiral in towards each other by precisely the amount that we have observed - thus our observations give an indirect proof of the existence of gravitational waves."
The study used some of the biggest radio telescopes in the world
Einstein's ideas have stood up extremely well to almost 100 years of scientific scrutiny, but researchers recognise General Relativity is likely to need at least some revision as they seek to unify its description of the large-scale Universe with quantum mechanics, the ideas that explain the realm of sub-atomic particles.
"We probably know GR is not the final answer on gravity, but it's a pretty good one," said Professor Kramer. "By probing further, by going to even more compact systems such as black holes - we would like to find the breaking point, because that will tell us a lot about the underlying unifying theory."
In the future, the Kramer team hopes to use its stellar clocks to directly detect gravitational waves emitted by merging supermassive black holes.
As the waves pass through the curved space-time around a network of nearby pulsars, they should leave their tell-tale imprint in the radio beam signals received on Earth.
The latest research used three radio telescopes to make the pulsar measurements: the Lovell Telescope at Jodrell Bank, UK; the Parkes Radio Telescope in Australia; and the Robert C Byrd Green Bank Telescope in West Virginia, US.