The 2017 observations from the Event Horizon Telescope (EHT) of M87*, a 6.5-billion-solar-mass black hole in the center of the giant elliptical galaxy Messier 87, have led to the first measurement of the size of a black-hole shadow. Based on an analysis of M87*’s shadow, the EHT researchers have now conducted a unique test of general relativity, deepening understanding about the unusual properties of black holes and ruling out many alternatives.
Despite its success, Albert Einstein’s theory remains mathematically irreconcilable with quantum mechanics, the scientific understanding of the subatomic world.
Testing general relativity is important because the ultimate theory of the Universe must encompass both gravity and quantum mechanics.
“We expect a complete theory of gravity to be different from general relativity, but there are many ways one can modify it,” said Professor Dimitrios Psaltis, an astrophysicist in the Steward Observatory and Department of Astronomy at the University of Arizona.
“We found that whatever the correct theory is, it can’t be significantly different from general relativity when it comes to black holes.”
“We really squeezed down the space of possible modifications.”
“This is a brand-new way to test general relativity using supermassive black holes,” added Dr. Keiichi Asada, a researcher in the Academia Sinica Institute of Astronomy and Astrophysics.
To perform the test, the EHT team used the first image ever taken of the supermassive black hole.
The first results had shown that the size of the black-hole shadow was consistent with the size predicted by general relativity.
“At that time, we were not able to ask the opposite question: How different can a gravity theory be from general relativity and still be consistent with the shadow size?” said Dr. Pierre Christian, also from the Steward Observatory and Department of Astronomy at the University of Arizona.
“We wondered if there was anything we could do with these observations in order to cull some of the alternatives.”
Gravitational tests have been conducted in a variety of cosmic settings. During the 1919 solar eclipse, the first evidence of general relativity was seen based on the displacement of starlight, traveling along the curvature of spacetime caused by the Sun’s gravity.
More recently, tests have been conducted to probe gravity outside the Solar System and on a cosmological scale. Examples include the detection of gravitational waves at the LIGO observatory.
“Using the gauge we developed, we showed that the measured size of the black hole shadow in M87 tightens the wiggle room for modifications to Einstein’s theory of general relativity by almost a factor of 500, compared to previous tests in the Solar System,” said Professor Feryal Özel, also from the Steward Observatory and Department of Astronomy at the University of Arizona.
“Many ways to modify general relativity fail at this new and tighter black hole shadow test.”
“Black hole images provide a completely new angle for testing Einstein’s theory of general relativity,” said Dr. Michael Kramer, director of the Max Planck Institute for Radio Astronomy.
“Together with gravitational wave observations, this marks the beginning of a new era in black hole astrophysics,” Professor Psaltis said.
“This is really just the beginning. We have now shown that it is possible to use an image of a black hole to test the theory of gravity,” said Dr. Lia Medeiros, a researcher in the School of Natural Sciences at the Institute for Advanced Study.
“This test will be even more powerful once we image the black hole in the center of our own Milky Way Galaxy and in future EHT observations with additional telescopes that are being added to the array.”
The research was published in the journal Physical Review Letters.
Dimitrios Psaltis et al. (EHT Collaboration). 2020. Gravitational Test beyond the First Post-Newtonian Order with the Shadow of the M87 Black Hole. Phys. Rev. Lett 125 (14): 141104; doi: 10.1103/PhysRevLett.125.141104