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The 2017 Event Horizon Telescope (EHT) observations of M87 *, a 6.5 billion solar mass black hole at 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 the shadow of M87 *, the EHT researchers have now performed a unique test of general relativity, deepening understanding about the unusual properties of black holes and ruling out many alternatives.
Visualization of the new meter developed to test the predictions of modified gravitational theories against the measurement of the shadow size of M87 *. Image credit: D. Psaltis, University of Arizona / EHT Collaboration.
Despite its success, Albert Einstein’s theory remains mathematically irreconcilable with quantum mechanics, the scientific understanding of the subatomic world.
Proving 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 it can be modified,” said Professor Dimitrios Psaltis, an astrophysicist at the Steward Observatory and Department of Astronomy at the University of Arizona.
“We found that whatever the theory is correct, it cannot be significantly different from general relativity when it comes to black holes.”
“We really reduced the space for possible modifications.”
“This is a completely new way to test general relativity using supermassive black holes,” added Dr. Keiichi Asada, a researcher at the Academia Sinica Institute of Astronomy and Astrophysics.
To conduct 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’s shadow was consistent with the size predicted by general relativity.
“At the time, we couldn’t ask the opposite question: How different can a theory of gravity be from general relativity and still be consistent with the size of the shadow?” said Dr. Pierre Christian, also of the Steward Observatory and the Department of Astronomy at the University of Arizona.
“We wondered if there was anything we could do with these observations to rule out some of the alternatives.”
Gravitational tests have been conducted in a variety of cosmic settings. During the solar eclipse of 1919, the first evidence of general relativity based on the displacement of starlight was observed, traveling along the curvature of space-time 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 meter we developed, we show that the measured size of the black hole shadow in M87 reduces the leeway 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 the Department of Astronomy at the University of Arizona.
“Many ways to modify general relativity fail this new and more stringent black hole shadow test.”
“The black hole images provide a completely new angle to test Einstein’s theory of general relativity,” said Dr. Michael Kramer, director of the Max Planck Institute for Radio Astronomy.
“Together with observations of gravitational waves, this marks the beginning of a new era in black hole astrophysics,” said Professor Psaltis.
“This is really just the beginning. Now we have 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 at the Faculty of Natural Sciences at the Institute for Advanced Study.
“This test will be even more powerful once we image the black hole at the center of our own galaxy, the Milky Way, and in future EHT observations with additional telescopes that will be added to the array.”
The research was published in the journal Physical Review Letters.
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Dimitrios Psaltis et al. (EHT collaboration). 2020. Gravitational proof beyond the first post-Newtonian order with the shadow of the black hole M87. Phys. Rev. Lett 125 (14): 141104; doi: 10.1103 / PhysRevLett.125.141104
