'Frankly we don't see a need to modify Einstein's
general theory of relativity very much - if ever -
based on these observations to date,' said Dr
Ramesh Bhat
Photo: Michelle Templeton

Swinburne University of Technology astronomers have used an "odd couple" of stars to confirm - with astonishing precision - that Albert Einstein's general theory of relativity, now almost a century old, is still alive and kicking, and provides a pretty accurate explanation of the universe as we know it.

Einstein's general theory of relativity, which explains gravity as a property of the geometry of space and time, was developed by the great scientist in 1915-16. It has passed all experimental tests since with complete success, making it one of the most durable theories in modern physics, where paradigms are regularly demolished by new discoveries.

But general relativity (GR) has one last test to complete its graduation: that of the emission of seemingly inconsequential 'gravitational waves'. Scientists believe that the universe is permeated by these waves, which pass through us with imperceptibly small effect. The stronger gravity, the bigger these effects - but in the solar system, where orbits are measured in months or years, the effects are negligible. To test this aspect of GR astronomers are needed and, in 1993, astronomers from Princeton University were awarded the Nobel Prize for verifying that a pair of neutron stars were spiralling together in accord with GR's gravitational wave predictions.

Then in 1999, two bizarre stars were found orbiting each other almost 10,000 light years away from Earth, not every few years, but every 4.8 hours. A pulsar, the ultra-dense collapsed core of a once-massive star, was found orbiting a white dwarf, the burnt-out embers of a star like our Sun. If Einstein was right, then every orbit the stars should emit gravitational waves that cause the orbit to shrink by just two millimetres a day.

Professor Matthew Bailes, director of Swinburne's Centre for Astrophysics and Supercomputing, has faithfully trekked, with his team, to the giant, 64-metre Parkes Radio Telescope to monitor the system since 2001.

"When I first read about the discovery of this star I knew it would be an invaluable test of Einstein's theories," he says.

Lead author Dr Ramesh Bhat analysed the data: "Some theories predict different emission properties for stars of unequal density, and this pair was unique for these tests - at last we could differentiate between them."

The experiment was far from straightforward. The pulsar itself was entranced by the strong gravity, wobbling up to five degrees which, from the radioastronomer's point of view, makes its signal roam around like the distant lights of a car on a rough track - all of which has to be allowed for in calculations.

Despite these challenges the team was able to carry out the necessary measurements to confirm that, during the six years of observation, the stars' million-kilometre orbit had shrunk by the requisite 4.2 metres - confirmed to an accuracy of six per cent. "Using extremely precise measurements we were able to track this rate of shrinkage. We found it to be exactly the same rate that was predicted by the theory," says Joris Verbiest, a PhD student at Swinburne. "This confirms that Einstein's theory on its own is extremely comprehensive."

Dr Bhat says the team was also testing other, related theories of gravity to see how they complied with the conditions observed. "We are also asking what extensions may need to be made to GR to encompass all the different phenomena observed - and to place limits on the very few alternative theories of gravity that still challenge Einstein's crown, such as the tensor-scalar theory.

"Frankly, we don't see a need to modify Einstein's theory very much  - if ever - based on these observations to date," he says.

The team is not yet satisfied it has extracted all there is to know from the odd couple. "In the future we will be able to perform even more stringent tests using this interesting stellar pair, but Einstein's resilience sometimes makes me wonder if I'll need to get a new job," Professor Bailes says.

The results have recently been published in the world's leading physics journal, Physical Review.

More information and images available here.