Testing Relativity with Two Dead Stars

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Death is an inescapable fact of life, and stars are no exception. Eventually stars burn through their nuclear fuel and die — even our sun has an expiration date.

But a pair of burnt-out white dwarf stars are finding a scientific afterlife, enabling astrophysicists to indirectly observe gravitational waves — faint ripples in the fabric of spacetime that travel outward, much like tossing a stone in a still pond.

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Gravitational waves were first predicted by Einstein’s general theory of relativity in 1916. At the time, we didn’t have the technology to detect them, since they are very weak and fade very quickly, although scientists found indirect evidence in the radio wave regime of their existence in observations of a binary pulsar — work that won the Nobel Prize in Physics in 1993.

These waves should be detectable by a a gravitational wave detector — namely, the upgraded Laser Interferometer Gravitational Observatory (LIGO), which has been searching space for gravity waves since it opened in 2002. LIGO is a joint project between scientists at MIT, Caltech, and many other colleges and universities.

Originally, NASA planned to build a similar gravitational wave detector in space, dubbed LISA (Laser Interferometer Space Antenna), but tightened funding led to the agency scrapping those plans last year. A similar fate befell the European Space Agency’s scaled-down version, the New Gravitational Wave Observatory (although Cosmic Variance reports some intriguing rumors that we still may see such a detector in the next wave of ESA projects).

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Until (if) such a project transpires, astrophysicists are looking for alternative methods to detect gravitational waves, such as the merging of binary neutron stars.

Now astronomers have confirmed more indirect evidence in the optical regime of gravitational waves emitting from a pair of dead white dwarf stars. Their results will appear in The Astrophysical Journal Letters.

The binary system was discovered last April, and the two stars are so close together they make full orbits in less than 13 minutes. General relativity predicts that these orbits should show effects of gravitational waves — namely, the stars should slowly be inching closer, and orbiting each other increasingly faster, over time.

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But how could the astronomers confirm this prediction? They needed an accurate celestial clock, and the stars themselves provided it. When viewed from Earth, the two stars eclipse each other every six minutes — like clockwork. “This is a general relativistic effect you could measure with a wrist watch,” Warren Brown of the Smithsonian Astrophysical Observatory, one of the collaborators, commented via press release.

And, indeed, based on more than 200 hours of observational data collected by several terrestrial telescopes around the world, the two stars are getting closer. The eclipses now occur six seconds faster than in April 2011, just as general relativity predicts.

Chalk another point up for Einstein, whose theory has passed yet another empirical test.

Images: (top) Binary white dwarfs spiral together. Credit: D. Berry/NASA GSFC. (bottom) The 2.1-meter (82-inch) Otto Struve Telescope at the University of Texas McDonald Observatory. McDonald Observatory photo.

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