Boffins have discovered a unique triple star system that could help them to probe one of the most fundamental physics problems of all: the true nature of gravity itself.
Millisecond pulsar, left foreground, is orbited by a hot white dwarf star, centre, both of which are orbited by another, more-distant and cooler white dwarf, top right. Credit: Bill Saxton; NRAO/AUI/NSF
The tightly packed system includes two white dwarf stars and a superdense neutron star, all squidged into an space smaller than the Earth's orbit around the Sun, which has allowed researchers to take the best measurements yet of the complex gravitational interactions in such a system.
Astroboffins can use the unique features of the system to probe an aspect of Albert Einstein's General Theory of Relativity that does not mesh well with quantum theory - the Equivalence Principle. The most famous, although potentially mythical, experiment demonstrating the principle is the story of Galileo dropping two balls of different weights from the Leaning Tower of Pisa and having them arrive on the ground at the same time. The principle states that the effect of gravity on a body has nothing to do with the nature or internal structure of that body.
"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).
Best hope of seeing general relativity break down
"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.
"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 dense star together. The Strong Equivalence Principle says that this binding energy will still react 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.
The system was first spotted by West Virginia University graduate student Jason Boyles, who found it as part of a large-scale search for pulsars using the National Science Foundation's Green Bank Telescope (GBT).
Pulsars are neutron stars that emit beams of radio waves that rapidly sweep through space as the object spins on its axis, much like a lighthouse beam sweeps across the sea. Some of these stars are called millisecond pulsars, because they spin multiple times per second, allowing astronomers to study phenomena like rare gravitational waves.
"The gravitational perturbations imposed on each member of this system by the others are incredibly pure and strong," Ransom said. "The millisecond pulsar serves as an extremely powerful tool for measuring those perturbations incredibly well."
Boffins are now extensively studying the system using the GBT, the Arecibo radio telescope in Puerto Rico and the Westerbork Synthesis Radio Telescope in the Netherlands, as well as pulling in data from the Sloan Digital Sky Survey, the GALEX satellite, the WIYN telescope on Kitt Peak, Arizona, and the Spitzer Space Telescope.
By accurately timing the pulsar's pulses, scientists were able to calculate the geometry of the system and the masses of the stars incredibly accurately.
"We have made some of the most accurate measurements of masses in astrophysics," said Anne Archibald, of the Netherlands Institute for Radio Astronomy. "Some of our measurements of the relative positions of the stars in the system are accurate to hundreds of metres."
Using the system to investigate the Strong Equivalence Principle, the boffins will be hoping to show that the gravitational effect of the outer white dwarf would be identical for both the inner white dwarf and the neutron star. But if the principle is invalid, the effect would be slightly different, a result the high-precision pulsar timing should easily show.
"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."
Ransom, Archibald and Stairs were part of an international team of researchers who contributed to the paper "A millisecond pulsar in a stellar triple system" published in Nature. ®