![]() General relativity has passed every time.īut it takes a strong gravitational field, like the one around M87’s black hole, to kick the tests up a notch. Those aspects of gravity have been tested with the way stars’ light is deflected during a solar eclipse for example, and the way laser light sent to spacecraft flying away from the sun takes longer than expected to return to Earth ( SN: 5/29/19). These additions are related to things like how light and mass travel in a warped spacetime, or how gravity makes time flow more slowly. In weak gravitational fields, like within the solar system, physicists can test whether “first-order” additions to Newton’s equations are consistent with general relativity or not. The more add-ons or factors added to a test, the more confidence there is in a result. ![]() If measurements of how gravity works in the universe deviate from those predictions, then physicists know general relativity is not the full story. General relativity predicts what those add-ons should be. Generally, physicists think of general relativity as a set of corrections or add-ons to Isaac Newton’s theory of gravity. But the new study is interesting because “it’s the first attempt at constraining a effect through a black hole observation,” says physicist Emanuele Berti of Johns Hopkins University, who was not involved in the new work. The results are on par with those from gravitational wave experiments like the Advanced Laser Interferometer Gravitational-Wave Observatory, which has detected ripples in spacetime from the merger of black holes smaller than M87’s ( SN: 9/16/19). So far so good for relativity, the researchers found when they performed this second-order test. ![]() That “can’t really be done in the solar system” because the gravitational field is too weak, says EHT team member Lia Medeiros of the Institute for Advanced Study in Princeton, N.J. Specifically, the researchers used the size of the black hole to perform what’s known as a “second-order” test of general relativity geared toward boosting confidence in the result. In a study published October 1 in Physical Review Letters, Psaltis and colleagues have used the shadow of M87’s black hole to take a major step toward ruling out those alternative theories. ![]() That question is key because it’s still possible that some other theory of gravity could describe the universe, but masquerade as general relativity near a black hole. That result, reported by the Event Horizon Telescope Collaboration, answered one question: Is the size of M87’s black hole consistent with general relativity?īut “it is very difficult to answer the opposite question: How much can I tweak general relativity, and still be consistent with the measurement?” says EHT team member Dimitrios Psaltis of the University of Arizona in Tucson. In other words, Einstein was right - again. That iconic image, of the supermassive black hole at the center of the galaxy M87 about 55 million light-years away, showed that the shadow closely matched general relativity’s predictions of its size ( SN: 4/10/19). The theory not only describes the way matter warps spacetime, but it also predicts the very existence of black holes, including the size of the shadow cast by a black hole on the bright disk of material that swirls around some of the dense objects. When the first-ever image of a black hole was released in April 2019, it marked a powerful confirmation of Albert Einstein’s theory of gravity, or general relativity.
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