A unique gravitational laboratory

Print edition : January 24, 2014

A 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). Photo: Bill Saxton/NRAO/AUI/NSF

AN international team of astronomers using the National Science Foundation’s Green Bank Telescope (GBT) in West Virginia have discovered a unique stellar system of two white dwarf stars and a superdense neutron star, all packed within a space smaller than the earth’s orbit around the sun. The closeness of the stars, combined with their nature, allowed the scientists to make the best measurements yet of the complex gravitational interactions in such a system. In addition, detailed studies of this system may provide a key clue for resolving one of the principal outstanding problems of fundamental physics: the true nature of gravity. The findings were published in the January 5 online edition of the journal Nature.

“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).

Pulsars are neutron stars that emit lighthouse-like beams of radio waves that rapidly sweep through space as the object spins on its axis. A large-scale search for pulsars using the GBT revealed a millisecond pulsar some 4,200 light years from the earth, spinning nearly 366 times per second. Such millisecond pulsars can be used as precision tools to study a variety of phenomena, including searches for the elusive 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.

“The gravitational perturbations imposed on each member of this system by the others are incredibly pure and strong,” Ransom said, adding, “The millisecond pulsar serves as an extremely powerful tool for measuring those perturbations incredibly well.”

By very 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. “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,” she said.

The system gives the scientists the best opportunity yet to discover a violation of 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. (The famous Apollo 15 experiment a la Galileo by Commander Dave Scott of dropping of a hammer and a falcon feather on the airless surface of the moon illustrates the principle.)

“While Einstein’s general theory of 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.

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 react gravitationally as if it were mass. Virtually all alternatives to general relativity hold that it will not.

“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,” said Ingrid Stairs of the University of British Columbia.

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