Published: Fri, July 06, 2018
Science | By Michele Flores

Yet another test for general relativity; yet another "A" for Einstein

Yet another test for general relativity; yet another

Massive objects, such as galaxies, warp space-time, according to Einstein's theory of general relativity.

However, it is hard to determine whether the equivalence principle applies in every situations-when the objects which are used are supremely heavy or giant, just like This wiggle room has given some expectations to supports of many other gravity theories, although these kinds of folks remain in the minority. An worldwide team of astronomers tested the equivalence principle under extreme conditions: a system composed of two superdense stellar corpses known as white dwarfs and an even denser neutron star. Of course, on Earth air gets in the way, but Dave Scott demonstrated on the airless moon that it worked even with a feather. This was a re-creation of a supposed test by Galileo where he dropped two balls made of differing materials off the Leaning Tower of Pisa, and observed them reaching the ground at the same time.

"This research shows how routine and careful observation of distant stars can give us a high-precision test of one of the fundamental theories of physics", said Ingrid Stairs, professor in the department of physics and astronomy at UBC and a co-author of the study. "In fact, once you have an object with strong gravity, Einstein's theory is nearly the only one where objects with strong gravity fall the same way as normal objects".

To once again prove the validity of the arguments of the great physicist, Einstein was able by observation of the unusual star system J0337+1715 with the constellation Taurus.

A violation of the equivalence principle would manifest as a distortion in the pulsar's orbit - a difference between the neutron star's path and that of its interior white-dwarf companion.

According to study co-author Ryan Lynch, from the Green Bank Observatory in West Virginia, these singular conditions offer the ideal staging area for the ultimate gravity experiment.

Through meticulous observations and careful calculations, the researchers were able to test the system's gravity using the pulses of the neutron star alone. Neutron stars are the remains of massive stars that have exploded as supernovae. Neutron stars, on the other hand are even smaller and denser. This particular neutron star was found to "pulse" or rotate at a rate of 366 times every second. "That is a really precise track of where the neutron star has been and where it is going".

The team has kept their eyes on the pulsar for about six years now, observing it with three giant telescopes: the National Science Foundation's (NSF) Green Bank Telescope (GBT), which incidentally discovered the triple star system, the Westerbork Synthesis Radio Telescope in the Netherlands, and the NSF's Arecibo Observatory in Puerto Rico.

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At the center of the system is an extremely dense neutron star 1.4 times the mass of the Earth's sun.

"We can tell its location to within a few hundred metres".

This principle has now been tested on neutron stars.

PSR J0337+1715 consists of two white dwarfs and a neutron star. New research published in Nature confirms that, even in an extreme gravity system, the theory of relativity still applies.

"If there is a difference, it is no more than three parts in a million", says Nina Gusinskaia, PhD student at the University of Amsterdam and a co-author of the study.

"This groundbreaking result limits the room for any alternative theories of gravity and has improved upon the best previous tests by a factor of about ten". On the other hand, according to Einstein's theory of general relativity, pulsar's behavior under an external gravitational field should not be different from that of any other body within that field, be it a feather or a star.

"We still don't know if general relativity is correct, but now we know that the right gravitational theory must look even more similar to general relativity that we previously thought".

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