An artist's impression of the double pulsar system used to test Einstein's theory of general relativity.
Credit: John Rowe Animation
SYDNEY, 15 September 2006: Einstein's theory of general relativity is at least 99.95 per cent right according to a new study of radiation pulses emitted by a pair of distant stars.
An international research team including scientists from the radio telescope at Australia's Parkes Observatory measured the radiation from a 'double pulsar' system 2,000 light-years away. They then applied models of general relativity to the system, putting the 91-year-old theory to the test.
The results, published online today in the U.S. journal, Science, show that general relativity is correct to within 0.05 per cent - the most stringent limit to date, according to the researchers.
"It's been very exciting observing this system and trying to work out all the details of what's going on. It's got the whole pulsar community very excited," said George Hobbs, a researcher with the CSIRO Australia Telescope National Facility, and one of the authors of the study.
Pulsars are synonymous with neutron stars - the only difference being that neutron stars are undetectable with radio waves. "Pulsars seem to shoot out a radio beam from their magnetic poles, and if that radio beam just happens to pass in the line of the sight to the Earth, then we can detect it and call it a pulsar," said Hobbs. "A pulsar is simply a neutron star that we can see."
The double pulsar system in the study consists of two highly compact stars, each with a width of about 20 km, orbiting each other at 1 million kilometres an hour. "[Each star is] about one and a half times the mass of our sun, but all compressed into something the size of a city," said Hobbs.
The researchers used three of the world's largest radio telescopes - the Parkes radio telescope, the Lovell Telescope at Jodrell Bank in the U.K., and the Robert C. Byrd Green Bank Telescope in West Virginia in the U.S. - to observe the double pulsar over a period of three years.
"You measure when the pulsars arrived at your telescope, you predict when they should have been - given a theory of relativity - and you see if that theory of relativity is correct or not," said Hobbs.
"If you have two pulsars going around each other, you can increase the number of tests that you can do because you know everything about the system. The reason why this is so exciting is because for the first time, we know the full system completely."
The research group used six parameters to test the theory of general relativity - more than have ever been tested before. The more parameters measured, the more rigorous the test.
In the future, Hobbs said the team would like to measure more parameters. "The reason it's hard to measure the other ones is because the effects are very small. So we need to observe for longer, or we need to improve our ability to observe these pulsars."
Once more parameters have been measured, Hobbs said, "We can find full system geometries, which means we know exactly the orbit of these systems. Then we'll be able to work out the moment of inertia of the neutron star, which no one has ever done before."
Australian researchers are also particularly interested in another prediction of Einstein's theory of general relativity: gravitational waves, said Hobbs. This idea suggests that the whole fabric of space propagates as a wave.
The study confirms that general relativity is still valid, giving hope that gravitational waves may exist, even though no one has seen them, he said.
