https://phys.org/news/2020-10-einstein-description-gravity-harder.html Einstein's description of gravity just got much harder to beat by University of Arizona Please Register or Log in to view the hidden image! 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: Einstein's theory of general relativity—the idea that gravity is matter warping spacetime—has withstood over 100 years of scrutiny and testing, including the newest test from the Event Horizon Telescope collaboration, published today in the latest issue of Physical Review Letters. According to the findings, Einstein's theory just got 500 times harder to beat. Despite its successes, Einstein's robust 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. 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 UArizona astrophysics professor Dimitrios Psaltis, who until recently was the project scientist of the Event Horizon Telescope collaboration. Psaltis is lead author of a new paper that details the researchers' findings. "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. To perform the test, the team used the first image ever taken of the supermassive black hole at the center of nearby galaxy M87 obtained with the EHT last year. The first results had shown that the size of the black-hole shadow was consistent with the size predicted by general relativity. more at link: the paper: https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.125.141104 Gravitational Test beyond the First Post-Newtonian Order with the Shadow of the M87 Black Hole: ABSTRACT: The 2017 Event Horizon Telescope (EHT) observations of the central source in M87 have led to the first measurement of the size of a black-hole shadow. This observation offers a new and clean gravitational test of the black-hole metric in the strong-field regime. We show analytically that spacetimes that deviate from the Kerr metric but satisfy weak-field tests can lead to large deviations in the predicted black-hole shadows that are inconsistent with even the current EHT measurements. We use numerical calculations of regular, parametric, non-Kerr metrics to identify the common characteristic among these different parametrizations that control the predicted shadow size. We show that the shadow-size measurements place significant constraints on deviation parameters that control the second post-Newtonian and higher orders of each metric and are, therefore, inaccessible to weak-field tests. The new constraints are complementary to those imposed by observations of gravitational waves from stellar-mass sources. Published 1 October 2020
