Pulsar detection furthers testing of general relativity with extreme physics

May 10, 2021

Scientists are using pulsars to test Einstein's most famous theory. (Jurik Peter)

Scientists are looking to new discoveries of some of the most extreme objects in the universe, pulsars, to test Einstein's theory of general relativity, figuring that if a theory stands strong even through the wildest of circumstances, one can safely say it's probably accurate.

The Murchison Widefield Array, Australia's groundbreaking radio telescope, located the first low-luminosity pulsar in an area of space uncharted by a certain frequency band, according to results published April 21 in The Astrophysical Journal Letters. Notably, the object was found within less than 1% of the obtained data, the totality of which accounts for roughly 80% of the sky.

The object's luminosity isn't the point: The discovery of any new pulsar, a spinning neutron star, in such limited space means there are likely scores of them out there, waiting to be discovered and employed in examinations of fundamental theories.

Collaborating with universities around the world, the researchers behind the new study have been working on the Murchison Widefield Array, which they call a major pathfinder telescope. It locates previously unseen objects in the universe, and they say it will eventually lead to one of the most ambitious telescopes ever built, called the Square Kilometre Array telescope. 

Discovery of the new pulsar is evidence of these tools' utility.

"This particular [pulsar] was in a place where other telescopes can also see, but we are looking at a new part of the frequency band, so that gives us some advantages," study author Ramesh Bhat, a senior research fellow at Curtin University in Australia, told The Academic Times.

"We are essentially into uncharted territory of the discovery space, so when you venture into that sort of an area," Bhat said, "you increase the probability" of finding new pulsars.

Increasing the probability of finding pulsars means heightening the likelihood of finding the ideal one to research the objects' dynamics in space. That could lead to understanding how such conditions play into theories that were developed based on Earth's restricted materials.

Considered the most dense bodies in the universe, they have features that are inconceivable to humans, let alone duplicable in a laboratory.

"Pulsars have a gravity which is almost something like a thousand billion times stronger than what is there on Earth," Bhat said. "It has magnetic fields which are a thousand billion times stronger than what you would find on Earth, and it is spinning at a rate which is like a million times faster than any other star that we can imagine."

A single tablespoon of a pulsar is equivalent to the mass of Mount Everest. But with such an otherworldly resume, the bodies are perfect candidates to test otherworldly theories — in particular, Albert Einstein's theory of general relativity.

"One of the most beautiful things about Einstein's theory is that it explains gravity in a way that's much, much deeper than we can understand from any of the previous theories," Bhat said. 

Developed more than 100 years ago, the theory builds on Issac Newton's. The English physicist and mathematician figured out that gravity exists, and put a formula to the force that explains its strength: the Law of Universal Gravitation.

Einstein was more interested in how gravity is exerting its force in the first place. With general relativity theory, the German-born physicist says it comes from the fact that space-time is a fabric that is subject to distortion.

One can imagine the concept by thinking of a trampoline. If there isn't anything on the trampoline, it sits flat, analogous to the universe having absolutely nothing in it. But such a notion is incompatible with the real universe. When something heavy is placed on the trampoline, parallel to the sun, it pulls the center inward, distorting the fabric.

If one were to drop a marble into the trampoline, the marble would begin rotating around the heavier item in the center — like the Earth orbiting the sun. If an even tinier ball were placed on the trampoline, it wouldn't rotate for very long; it would fall along the distortion that one of the other items created — like humans standing on Earth. In accordance with the Law of Universal Gravitation, the heavier an object, the greater the gravitational impact. 

That's general relativity in a nutshell — and it has never been disproven. 

"Normally, in any area of physics," Bhat explained, "a theory can be proven incorrect by showing that one thing is [wrong], even though many things are right."

He continued, "Einstein's theory, on the other hand — it has been more than 100 years, and every single time we have tried to test it, it has proven correct, passing with flying colors."

Given the theory's airtight premises, Bhat and his team decided to take its principles to the extreme regions that surround pulsars, noting that Einstein wouldn't have imagined these tests in his "wildest possible dreams." It would be like adding a neutron star proxy item to the trampoline, and seeing what the intense force would do to the fabric and surrounding "planets."

"It has what we call extreme physical conditions," Bhat said, "which means it is a place to do what we call extreme physics beyond what you can actually do using our own resources."

In addition to the telescopes' potential for tracking down these celestial bodies, Bhat explained that the technology used to create them is unprecedented. 

"The real hope is that building and making that telescope function in the next 10 years is going to have some technological spin-off," he said. "In the late '70s or early '80s, scientists and engineers tried to use one of our Australian telescopes to detect what they thought will be radiation that is coming from a very tiny black hole."

They didn't find radiation, but the technology they came up with formed the foundation for today's Wi-Fi.

The study, "Discovery of a steep-spectrum low-luminosity pulsar with the Murchison Widefield Array," published April 21 in The Astrophysical Journal Letters, was authored by N. A. Swainston, N. D. R. Bhat, M. Sokolowski, S. J. McSweeney, K. R. Smith, I. S. Morrison, S. E. Tremblay, A. Williams, G. Sleap, M. Johnston-Hollitt, S. J. Tingay and R. B. Wayth, Curtin University; S. Kudale, Tata Institute of Fundamental Research; S. Dai, CSIRO Astronomy and Space Science and Western Sydney University; R. M. Shannon, Swinburne University of Technology; W. van Straten, Auckland University of Technology; S. M. Ord, CSIRO Astronomy and Space Science, M. Xue, Chinese Academy of Sciences; B. W. Meyers, University of British Columbia; and D. L. Kaplan, University of Wisconsin–Milwaukee.

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