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3-Jul-2008 14:00 Eastern US Time

Thumbs-up to Einstein

Pulsars are fairly common in the universe, but pairs of them whizzing around each other are not. Only one such system is known - PSR J0737-3039A/B.

This twin-pulsar system lies about 1,700 light years from Earth and the two pulsars orbit each other in just under two and a half hours. It was discovered in 2003. Since then scientists have been making accurate measurements on the tiny system - small enough that the whole thing could fit inside our own sun.

Now they report that it is behaving exactly as predicted by the general theory of relativity. Einstein's theory has not been tested in such extreme conditions before.

The results of the study by scientists at McGill University's Department of Physics – along with colleagues from several other countries – are published on July 3 in the journal Science.

Teaspoons and battleships

A pulsar is a small object with very high density which has been left behind when a massive star reaches the end of its life and explodes as a supernova. The first pulsar was discovered by Jocelyn Bell, a physics graduate of Glasgow University, while she was a postgraduate student at Cambridge. Since then close to 2000 pulsars have been discovered in our galaxy.

A pulsar has a mass greater than that of our Sun. But it is squeezed into an object the size of a city. This means that a teaspoonful of pulsar weighs as much as all the ships on Earth.

Pulsars spin at staggering speeds. They generate huge gravity fields. They emit powerful radio waves along their magnetic poles. These sweep across space like great lighthouse beams, which can be detected by radio-telescopes.

Testing, testing

Einstein's theory predicts that in a close system of two very massive objects, such as neutron stars, one spinning object's gravitational pull makes the spin axis of the other wobble – or precess, as scientists call it.

Studies of other pulsars in binary systems have shown that this does happen. But they could not give a precise measurement of the amount of wobble.

"Measuring the amount of wobbling is what tests the details of Einstein's theory and gives a benchmark that any alternative gravitational theories must meet," said Scott Ransom of the National Radio Astronomy Observatory.

This latest test of Einstein's theory was led by McGill astrophysics PhD candidate René Breton and Dr. Victoria Kaspi, leader of the McGill University pulsar Group. The twin pulsar was studied using the 100-metre Robert C. Byrd Green Bank Radio Telescope at the National Radio Astronomy Observatory in Green Bank, WV. The McGill scientists worked with colleagues in Canada, the United Kingdom, the U.S., France and Italy.

A binary pulsar creates ideal conditions for testing general relativity's predictions, Breton explains. "Because the larger and the closer the masses are to one another, the more important relativistic effects are."

In a spin

Einstein's theory predicts that in a strong gravitational field an object's spin axis should slowly change direction, as the pulsar orbits its companion, says Kaspi. She is McGill's Lorne Trottier Chair in Astrophysics and Cosmology.

"Imagine a spinning top. When it is slightly non-vertical the spin axis slowly changes direction - an elegant motion called 'precession.'"

Pulsars are too small and too distant to allow astronomers to directly observe their spinning. But by chance the orbits of the twin pulsars lie in the same plane as the line-of-sight to Earth. This means that one passes behind a doughnut-shaped region of ionised gas surrounding the other – which therefore eclipses the signal from the pulsar at the back.

"Those eclipses are the key to making a measurement that could never be done before," Breton said.

Relativity rules

Relativistic spin precession has been seen before. But differences between general relativity and alternative theories of gravity might only be observed in extremely strong gravitational fields such as those in the twin pulsar system.

After four years of observations and calculations, the McGill scientists have determined that the pulsars' spin axes are indeed precessing in just the way that relativity predicts.

"I think that if Einstein were alive today, he would have been absolutely delighted with these results," said Prof. Michael Kramer, Associate Director of the Jodrell Bank Centre for Astrophysics at Manchester University. "Not only because it confirms his theory, but also because of the novel way the confirmation came about."

"A system like this, with two very massive objects very close to each other, is precisely the kind of extreme cosmic laboratory needed to test Einstein's prediction," said Victoria Kaspi, leader of McGill University's pulsar Group.

"It's not quite right to say that we have now ‘proven' general relativity," Breton says. "But so far Einstein's theory has passed all the tests that have been conducted, including ours."

Breton, Kaspi and Ransom worked with Michael Kramer of the Jodrell Bank Observatory at the University of Manchester in Great Britain; Maura McLaughlin of West Virginia University and the NRAO; Maxim Lyutikov of Purdue University and other colleagues in Canada, the U.S., France and Italy. The researchers presented their work in an article in the July 4 issue of Science.

Find out more:

An audio podcast interview with the researchers.

An introduction to pulsars

Audio Interview with Jocelyn Bell

Little Green Men by Jocelyn Bell.

Little Green Men by Cambridge University.

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