2017 observations by the Event Horizon Telescope (EHT) of M87 *, a 6.5 billion solar mass black hole at the center of the giant elliptical galaxy Messier 87, led to the first measurement of the size of a shadow black hole. Based on an analysis of the shadow of M87 *, the EHT researchers have now conducted a unique test of general relativity, deepening their understanding of 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 for general relativity is important because the ultimate theory of the Universe must include 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 changed,” said Professor Dimitrios Psaltis, an astrophysicist at the University of Arizona Steward Observatory and Department of Astronomy. .
“We found that whatever the correct theory is, it cannot be significantly different from general relativity when it comes to black holes.”
“We have really reduced 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 at the Sinica Institute of Astronomy and Astrophysics.
To perform the test, the EHT team used the first ever image of the supermassive black hole.
Early results showed that the size of the black hole’s shadow was consistent with the size predicted by general relativity.
“At that time, we weren’t able to 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 University of Arizona Department of Astronomy.
“We wondered if we could do something with these observations in order to select some of the alternatives.”
Gravity tests have been conducted in a variety of cosmic environments. During the solar eclipse of 1919, 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 meter we developed, we showed that the measured size of the black hole shadow in M87 narrows the room for maneuver for changes to Einstein’s general theory of relativity by nearly a factor of 500, compared to previous tests in the System. Solar, ”said Professor Feryal Özel, also of the Steward Observatory and the University of Arizona Department of Astronomy.
“Many ways to modify general relativity fail in this new, more rigorous black hole shadow test.”
“The images of black holes provide a whole 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 the 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. 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 at the School of Natural Sciences at the Institute for Advanced Study.
“This test will be even more powerful once we envision the black hole at the center of our Milky Way galaxy and in future EHT observations with additional telescopes to be 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 post-Newtonian first order with the shadow of the black hole M87. Phys. Rev. Lett 125 (14): 141104; doi: 10.1103 / PhysRevLett.125.141104