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Pulsar in a unique triple system to test the nature of gravity

Jenny Winder, News Writer
Jan 11, 2014, 8:00 UTC

Sen—Astronomers using the National Science Foundation's Green Bank Telescope (GBT) have discovered a unique system of two white dwarf stars and a superdense neutron star, all packed within a space smaller than Earth's orbit around the Sun.

The closeness of the stars and their nature has allowed the scientists to make the best measurements yet of the gravitational interactions in such a system and may provide a key for resolving the true nature of gravity.

"This triple system gives us a natural cosmic laboratory far better than anything found before for learning exactly how such three-body systems work and potentially for detecting problems with General Relativity that physicists expect to see under extreme conditions," said Scott Ransom of the National Radio Astronomy Observatory (NRAO).

One of the search's discoveries was a millisecond pulsar 4200 light-years from Earth, spinning nearly 366 times per second, which can be used for studying gravitational waves. Subsequent observations showed that the pulsar is in a close orbit with a white dwarf star, and that pair is in orbit with another, more-distant white dwarf.

"This is the first millisecond pulsar found in such a system, and we immediately recognized that it provides us a tremendous opportunity to study the effects and nature of gravity," Ransom said.

By accurately recording the time of arrival of the pulsar's pulses, the scientists were able to calculate the geometry of the system and the masses of the stars with unparalleled precision.

The system gives the scientists the opportunity to study a concept called the Equivalence Principle, which states that the effect of gravity on a body does not depend on the nature or internal structure of that body.

"While Einstein's Theory of General Relativity has so far been confirmed by every experiment, it is not compatible with quantum theory. Because of that, physicists expect that it will break down under extreme conditions," Ransom explained. "This triple system of compact stars gives us a great opportunity to look for a violation of a specific form of the equivalence principle called the Strong Equivalence Principle," he added.

When a massive star explodes as a supernova and its remains collapse into a superdense neutron star, some of its mass is converted into gravitational binding energy that holds the star together. The Strong Equivalence Principle says that this binding energy still reacts gravitationally as if it were mass. Virtually all alternatives to General Relativity hold that it will not.

"This system offers the best test yet of which is the case," Ransom said.

Under the Strong Equivalence Principle, the gravitational effect of the outer white dwarf would be identical for both the inner white dwarf and the neutron star. If the Strong Equivalence Principle is invalid, the outer star's gravitational effect on the inner white dwarf and the neutron star would be slightly different.

"By doing very high-precision timing of the pulses coming from the pulsar, we can test for such a deviation from the strong equivalence principle at a sensitivity several orders of magnitude greater than ever before available," said Ingrid Stairs of the University of British Columbia. "Finding a deviation from the Strong Equivalence Principle would indicate a breakdown of General Relativity and would point us toward a new, correct theory of gravity," she added.