General relativity passes yet another test, this time facing off with neutron stars

May 6, 2021
Space has given us another way to test Einstein. (NASA/JPL-Caltech/University of Wisconsin via AP)

Space has given us another way to test Einstein. (NASA/JPL-Caltech/University of Wisconsin via AP)

Despite distorting spacetime by degrees, neutron stars are difficult to study because their composition remains a mystery. However, physicists found a way to derive these stellar objects' properties irrespective of what they're made of — and even use them to confirm general relativity theory.

The researchers behind the derivations are credited with two novel discoveries, which were made by combining data from the Laser Interferometer Gravitational-Wave Observatory and its sister site Virgo with NASA's X-ray-based Neutron star Interior Composition Explorer telescope on the International Space Station. The findings were described in a paper published May 3 in Physical Review Letters.

"Given that we have these two facilities at the moment," study author Hector O. Silva told The Academic Times, "we were wondering whether we can combine this information that comes from two very different types of sources — X-rays and gravitational waves — to try to learn more about neutron stars and also to test general relativity."

The team is the first to identify neutron stars' moment of inertia, or how hard it is for the star to spin, its quadrupole moment along with surface eccentricity, which translate to how spherical the star is, and the Love number, which explains the stars' susceptibility to deformation in an external gravitational field. 

Figuring out these properties is notable because neutron stars are somewhat inconceivable to humans. The space-borne objects are only about 12 miles in diameter but have a gravitational pull 2 billion times that of Earth. If one were to somehow measure a teaspoon of such a star's matter, the utensil would would weigh 1 billion tons. 

Though this information is widely believed to be true, neutron stars' composition is still labeled with a huge question mark. 

"There's a lot of uncertainty about their interior composition," Silva explained. "But fortunately, there are relationships between different properties of neutron stars called quasi-universal — in the sense that they are independent of the interior composition."

Silva, a postdoctoral researcher at the Max Planck Institute for Gravitational Physics in Germany, explained that after putting those newfound connections together, the researchers also formed an unprecedented composition-independent way of using neutron stars as subjects to prove general relativity.

Since its initiation by Albert Einstein in the early 1900s, the theory has never been disproven. It is based on the idea that the universe is like a tangible piece of fabric, subject to distortion as objects are placed inside it. Each distortion, which theoretically resembles something like an indent in the fabric, is perceived by humans as gravity.

As things get caught in an indent, or distortion, they begin to either orbit the object that created the indent or fall into the object completely, the latter of which can be compared to humans being planted on Earth. When astronauts experience zero-gravity in space, it's because they're in between distortions.

Although classical Newtonian physics is sufficient for understanding less-intense objects in space, such as the sun, neutron stars' levels of gravity due to their mass makes them terrific candidates for applying the rules of general relativity. Such experimental conditions are impossible to mimic on Earth, as this would require a teaspoon that weighs a billion tons. 

This limitation is precisely why Silva's equations are a significant step forward for the scientific community's understanding of general relativity. The researchers call their test theory-agnostic, drawing on the idea that one can consider something to be neither true nor false. That's because it works irrespective of the star's composition.

"You can simply just take the relationship between the moment of inertia and the Love number as predicted by general relativity," he said, "and see if it is consistent with the observations that we have seen as well — and we do see that it is consistent."

So, general activity has passed yet another test — this time using neutron star observations. But the team took it one step further. 

"General relativity is based on certain foundational principles, and one of them is that the gravitational interaction preserves parity," Silva said.

When boiled down, parity is used to describe the symmetries found in the universe. In terms of gravity, general relativity proposes that if the universe were mirrored, the force's behavior would be preserved.

"One application of this theory-agnostic framework that we developed was to consider how this equation — these material composition-independent properties of interest — how they relate to one another in a theory in which this assumption that [parity] is preserved [exists]," Silva said.

They found that even if one were to relax the parity restriction in general relativity theory — which has been done over the years in an attempt to account for some phenomena — general relativity is still consistent. However, the researchers note that they created an upper boundary at which parity can be relaxed before the consistency starts to fall apart. 

The study, "Astrophysical and theoretical physics implications from multimessenger neutron star observations," published May 3 in Physical Review Letters, was authored by Hector O. Silva, Max-Planck-Institut für Gravitationsphysik and University of Illinois at Urbana-Champaign; A. Miguel Holgado, Carnegie Mellon University and University of Illinois at Urbana-Champaign; Alejandro Cárdenas-Avendaño, University of Illinois at Urbana-Champaign and Fundación Universitaria Konrad Lorenz; and Nicolás Yunes, University of Illinois at Urbana-Champaign. 

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