In a new test from the Event Horizon Telescope using the first image ever taken of the supermassive black hole at the center of massive galaxy M87 the size of our solar system imaged for the first time ever by the Event Horizon Telescope (EHT) on April 10, 2019 and described by the EHT team as — “paradoxical, intriguing, frightening” and “the end of spacetime– has shown that the size of the black-hole’s shadow was consistent with the size predicted by Einstein’s theory of general relativity—the idea that gravity is matter warping spacetime. Einstein’s theory remains mathematically irreconcilable with quantum mechanics, the scientific understanding of the subatomic world.
A Supermassive Test
“This is a brand-new way to test general relativity using supermassive black holes,” said Keiichi Asada, an EHT science council member and an expert on radio observations of black holes for Academia Sinica Institute of Astronomy and Astrophysics, referring to the eerie orange glow imaged using sophisticated signal processing to combine data from radio telescopes from around the world into one image of M87’s core. The image marked the culmination of years of work undertaken by a team of 200 scientists in 59 institutes across 18 countries, drawing on data collected by eight telescopes whose locations range from Hawaii to the South Pole, creating the equivalent of a lens the size of planet Earth that’s 4,000 times more powerful than the Hubble Space Telescope.
Visualization of the new gauge developed to test the predictions of modified gravity theories against the measurement of the size of the M87 shadow. Credit: D. Psaltis, UArizona; EHT Collaboration
“We expect a complete theory of gravity to be different from general relativity, but there are many ways one can modify it. 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,” said lead author and University of Arizona astrophysicist Dimitrios Psaltis, who until recently was the project scientist of the Event Horizon Telescope collaboration.
M87 black hole showing the motion of plasma as it swirls around the black hole. The bright thin ring that can be seen in blue is the edge of what we call the black hole shadow. (L. Medeiros; C. Chan; D. Psaltis; F. Özel; UArizona; IAS).
The Shadow Knows?
“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 University of Arizona Steward Theory Fellow Pierre Christian. “We wondered if there was anything we could do with these observations in order to cull some of the alternatives.”
The team did a very broad analysis of many modifications to the theory of general relativity to identify the unique characteristic of a theory of gravity that determines the size of a black hole shadow.
The team focused on the range of alternatives that had passed all the previous tests in the solar system.
“Tightens the Wiggle Room”
“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 University of Arizona astrophysics professor Feryal Özel, a senior member of the EHT collaboration. “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 Michael Kramer, director of the Max Planck Institute for Radio Astronomy and EHT collaboration member.
New Era in Black Hole Astrophysics
“Together with gravitational wave observations, this marks the beginning of a new era in black hole astrophysics,” Psaltis said. Testing the theory of gravity is an ongoing quest: Are the general relativity predictions for various astrophysical objects good enough for astrophysicists to not worry about any potential differences or modifications to general relativity?
“We always say general relativity passed all tests with flying colors—if I had a dime for every time I heard that,” Özel observed. “But it is true, when you do certain tests, you don’t see that the results deviate from what general relativity predicts. What we’re saying is that while all of that is correct, for the first time we have a different gauge by which we can do a test that’s 500 times better, and that gauge is the shadow size of a black hole.”
Next, the EHT team expects higher fidelity images that will be captured by the expanded array of telescopes, which includes the Greenland Telescope, the 12-meter Telescope on Kitt Peak near Tucson, and the Northern Extended Millimeter Array Observatory in France.
“When we obtain an image of the black hole at the center of our own galaxy, then we can constrain deviations from general relativity even further,” Özel said.
The Daily Galaxy, Sam Cabot, via University of Arizona
Image at top of page” M87 Black Hole EHT Victor Tangerman