Gravity Clues From Stellar Triple System
January 6, 2014

Pulsar In Stellar Triple System Could Reveal True Nature Of Gravity

redOrbit Staff & Wire Reports - Your Universe Online

The discovery of a superdense neutron star in a stellar triple system has made it possible for researchers to collect the best measurements to date of the complex gravitational interactions present in these types of systems, according to a new study in Nature.

Using the National Science Foundation's Green Bank Telescope (GBT), the study authors were able to locate the pulsar along with two white dwarf stars in a compact area smaller than Earth’s orbital path around the sun.

Furthermore, thanks to the data they collected, as a result of the nature of the stars and their proximity, scientists may be able to learn more about the true nature of gravity, thus resolving one of the problems of fundamental physics.

“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,” explained Scott Ransom, an astronomer with the National Radio Astronomy Observatory (NRAO) and one of the study’s author.

The pulsar was first discovered by Jason Boyles, formerly a graduate student at West Virginia University and now a visiting assistant professor at Western Kentucky University, during a large-scale search for these radio wave-emitting neutron stars using the GBT.

“One of the search's discoveries was a pulsar some 4200 light-years from Earth, spinning nearly 366 times per second,” the NRAO said in a statement. These are known as millisecond pulsars, and according to the Observatory, astronomers can use them to study gravitational waves and other phenomena.

Further research revealed that this pulsar maintained a close orbit with a white dwarf star, and that the duo is also in orbit with a second, more distant white dwarf. This is the first time researchers have found a millisecond pulsar in a triple system, Ransom said, and he and his colleagues “immediately recognized that it provides us a tremendous opportunity” to conduct an in-depth analysis on the nature and impact of gravity.

Ransom and his colleagues analyzed the system using several different telescopes and satellites, and found that each member of the system imposed “incredibly pure and strong” gravitational perturbations on one-another.

The NRAO astronomer added that the millisecond pulsar “serves as an extremely powerful tool” for measuring those interactions, and that by accurately logging the arrival time of the neutron star’s pulses, he and his colleagues were able to precisely calculate the system’s geometry and the masses of all three starts.

According to the study authors, this system provides them with the best opportunity to date to locate a violation of a principle known as the Equivalence Principle, which states that the effect of gravity on an object is not reliant on that object’s nature or internal structure.

One specific form of this notion, the Strong Equivalence Principle, suggests the laws of gravitation are independent of velocity and location. When a superdense neutron star forms, some of its mass is converted into gravitational energy that binds it together, the researchers explained.

“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,” the Observatory said. Under this principle, “the gravitational effect of the outer white dwarf would be identical for both the inner white dwarf and the neutron star.”

However, if the Strong Equivalence Principle is shown to be invalid, based on the conditions found in this triple system, then the gravitational effect that the outer star has on the inner white dwarf and the pulsar will be slightly different. The high-precision pulsar timing observations will demonstrate whether or not this is the case.

“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,” explained University of British Columbia professor Ingrid Stairs. “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.”