More than 100 years later, Einstein's theory of relativity continues to stand the test of time. It accurately predicted the appearance of the supermassive black hole in galaxy M87, which astronomers with the Event Horizon Telescope project imaged for the first time several years ago. Now, further analysis of M87 shows the shadowy monster is spinning, again, just as relativity predicts.
The black hole at the center of M87 is enormous, anchoring the entire galaxy with 6.5 million times the mass of the sun. When black holes like M87 are consuming other celestial objects, they often produce jets of energized plasma that can travel thousands of light years at nearly the speed of light. The research team, led by post-doctoral researcher Cui Yuzhu of the Chinese Academy of Sciences, focused on the relativistic jets in M87 in order to test one of Einstein's predictions.
Scientists believe that black holes begin spinning faster as the progenitor star collapses, like a figure skater whirling faster as they pull their arms in. Over time, matter falling into the accretion disk can accelerate the spin. According to general relativity, a spinning black hole should exert intense gravitational pressure on nearby space, causing a phenomenon called frame-dragging. Astronomers have speculated that this effect could be the driving force behind relativistic jets, but no one has ever been able to confirm through observations that black holes spin.
M87 is about 55 million light-years away, which is reasonably close in intergalactic terms. It still took 170 observational campaigns between 2020 and 2022 to get enough data to make a determination. The team used radio telescopes around the globe to observe M87, including the Very Long Baseline Array (VLBA), East Asian VLBI Network (EAVN), and China's Tianma 65-meter radio telescope.
Analysis of the radio data shows that the jet extending from M87 is misaligned with the spin axis. More importantly, it moves over time, a property called precession. This wobble, which scientists calculate at an 11-year cycle, proves that the black hole is spinning. This, once again, confirms General Relativity was right.
"We are thrilled by this significant finding," says Cui Yuzhu. "Since the misalignment between the black hole and the disk is relatively small and the precession period is around 11 years, accumulating high-resolution data tracing M87's structure over two decades and thorough analysis are essential to obtain this achievement."
The data so far does prove that M87 spins, but the rate of spin and the precise effects of frame-dragging on the accretion disk are still unclear. It will take more work to make those determinations, and scientists will no doubt go looking for the same phenomenon in other galaxies.
