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Space-warping stars prove Einstein was right. Again

Just over a century ago, Einstein predicted that the gravity of massive objects could warp spacetime.

In the past five years we've seen Einstein's theory of general relativity play out in the detection of gravitational waves, the imaging of a black hole, and the orbit of stars around the supermassive black hole at the centre of our galaxy.

And now, astronomers tracking a quirky pair of stars for 20 years using the Parkes radio telescope have proven him right again.

"This time we're seeing a spinning star at the centre of the system dragging the very fabric of spacetime with it," said astronomer and co-author Matthew Bailes of the Swinburne Institute of Technology.

Dragging the fabric of spacetime

In 1918, two Austrian scientists proposed that if Einstein was right then spinning objects, including Earth, should twist and drag the fabric of spacetime.

The phenomenon, known as the Lense-Thirring effect or frame-dragging, is usually too small to detect.

A tiny effect was first demonstrated in an experiment that measured the subtle movements of gyroscopes placed into space above Earth.

"We've discovered a system where the effect is 100 million times stronger than it is above the Earth," Professor Bailes said.

The discovery, by the international team of astronomers, is reported today in the journal Science.

A unique system

The unusual star system known as PSR J1141-6545 was discovered around 10,000 light years away in the constellation of Musca aka The Fly in 2001.

At the heart is a fast-spinning white dwarf star — the remains of an old star about the size of Earth but 300,000 times its density.

Every five hours it is circled by a neutron star (pulsar) — the core of an exploded star no bigger than a city but about 100 billion times more dense than Earth.

The neutron star sends out regular pulses of high-energy particles like a lighthouse, which the astronomers used to track its orbit.

Over the years, the astronomers noticed the two stars got closer and closer as the white dwarf pulled its neighbour in.

"The orbit shrinks by about 7 millimetres a day," Professor Bailes said.

But as time went on, it became clear that the stars weren't acting as predicted by Einstein's theory.

"I had assumed that we'd done something wrong," Professor Bailes said.

It took four years of detective work by PhD student Vivek Venkatraman Krishnan and team members from the Max Planck Institute for Radio Astronomy to get to the bottom of the puzzle.

"After concluding that this could not have been due to problems with our telescopes, there was a brief period of time that I thought I disproved Einstein's theory," Dr Krishnan said.

In the end they twigged that the orbit of the pulsar was tumbling in space as it was being dragged around by the fast-spinning white dwarf star.

"While we knew that any body that rotates should drag space and time with it according to Einstein, we did not think that this would be measurable for this system," he said.

"This effect is usually expected to be measured only for a select class of heavenly bodies like some neutron stars and black holes."

But this was no ordinary pair of stars.

What makes these stars special?

"The unique formation of this system made the white dwarf spin so fast that we could see its effects in the orbit, for the first time in any binary star system," Dr Krishnan said.

When the pulsar was born, material from the supernova fell onto the white dwarf, making it spin faster and faster.

The cataclysmic explosion also misaligned the spins of the two stars and changed the orbits of the pulsar from a normal circular path to an egg-shaped orbit, Professor Bailes said.

This enabled the astronomers to work out the white dwarf was spinning about once every minute.

"That is causing the fabric of spacetime to be ripped around much more strongly than it would be above the Earth," Professor Bailes said.

This is a very unusual system, said Susan Scott, an astrophysicist at the Australian National University who was not involved in the discovery.

"There's only two confirmed binary systems like that where the white dwarf is known to have formed before the other companion," she said.

But the two stars in this system are much closer together.

Professor Scott said the discovery is "an exciting new example" of testing Einstein's theories in "a different realm of gravity" in the same way the detection of gravitational waves did.

"There are very few things in life where you have to put general relativity in to make them completely accurate. That's because we live in a place of very low gravity.

"But out there in the universe there are places with very high gravity and you need to consider these general relativity effects because they make a difference."

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