In the century since Albert Einstein proposed his theory of general relativity physicists have put it through the wringer with extensive experimental tests—and it has withstood them all. But these experiments were conducted in environments of relatively weak gravity. Scientists have thus been left to wonder how well the theory describes the universe under more extreme conditions, like those found in the regions around black holes. To that end a new study suggests a means of testing the limits of the theory: Researchers have determined that if general relativity does break down near black holes, the effects may be detectable in x-rays blazing off the infalling matter. The study was published in Physical Review D on July 22.
According to general relativity, the phenomenon we experience as gravitational force is a result of spacetime (the combination of the three spatial dimensions and fourth dimension of time) curving around mass. The denser an object, the more severely it warps the fabric of spacetime and the stronger its gravitational field. Around objects like black holes—the remnants of exploded massive stars that are so compact not even light can escape their gravitational tug—spacetime is severely distorted. Physicists have used relativity, along with the so-called no-hair theorem, which states that black holes only have two defining characteristics (mass and rotation), to predict how spacetime curves around black holes. They call that curvature the Kerr solution.
Proving that the Kerr solution provides an accurate description of spacetime near a black hole would show that general relativity holds up even in environments of extreme gravity. But so far no one has been able to prove the Kerr solution’s correctness. Ideally, astrophysicists would record the motions of an object as it traveled through the region around a black hole to characterize the spacetime curvature. But some black holes are mere kilometers across, which is tiny on the cosmic scale. Scientists are not currently capable of tracking a single object moving in such a confined space from light-years away.
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Even though no observational evidence yet suggests that relativity breaks down inside black holes, “that hasn’t stopped theorists from coming up with all sorts of alternative [theories of gravity],” says astrophysicist Jonathan Gair of the Institute of Astronomy at the University of Cambridge in England. These alternative theories produce their own descriptions of spacetime curvature that differ from the Kerr solution.
To help determine whether relativity or one of its competitors actually describes the warping of spacetime around black holes, Gair and fellow institute astrophysicist Christopher Moore used computer modeling to simulate several black holes with spacetime curves that somehow deviate from the topography described in the Kerr solution. They call these deviations “bumps.” The team wanted to know if Earthbound observers could detect the weird curvature around these bumpy black holes, so they looked for bump signatures in the radiation from the simulated black holes. As a black hole gobbles up surrounding matter, that material emits x-rays, which have slightly different characteristics depending on the black hole’s gravitational field.
Moore and Gair simulated black holes both with bumps described in specific alternative theories of gravity as well as a more general group of variously deformed black holes. “We wanted to do something more generic, because chances are, if general relativity is not right, we haven’t actually thought of the correct deviation,” Gair says. The researchers split their bumpy black holes into three categories, each with progressively smaller bumps: In the first category the spacetime curvature around the black hole is significantly different than the Kerr solution, even at long distances from the hole. In the second and third categories the bumps shrink more and more quickly, Moore explains.
By examining the x-rays produced by their simulated bumpy black holes, Moore and Gair determined which deformations scientists would be able to detect in the real thing, and which would blend in with the rest of the relativity-abiding universe. According to Moore, the team identified four bumps, all of which diminish in the slowest possible way, that scientists could feasibly detect. Knowing the x-ray signatures that correspond to particular black hole “bumps” provides a valuable reference for scientists taking x-ray observations of black holes in the future. According to Dimitrios Psaltis, an astrophysicist at the University of Arizona who was not involved with this study, the European Space Agency’s ATHENA x-ray observatory will conduct these observations when it launches in 2028.
Mike Kesden, an astrophysicist at The University of Texas at Dallas who was not involved with this study, notes that because only a few of Moore and Gair’s black hole bumps raise red flags in x-ray emission, this study provides further support for building gravitational wave detectors. Gravitational waves—ripples in spacetime—could encode information about the shape of spacetime around black holes. In fact, Moore and Gair plan to publish a follow-up study about the possibility of using gravitational waves to measure their simulated black hole bumps. So even if x-ray emission does not reveal the breakdown of general relativity near bumpy black holes, gravitational waves just might.