Black holes, by definition, emit no light. They are unseeable.
But astronomers would like to get as close as they can by zooming in on the region immediately surrounding a black hole. That is the objective of the Event Horizon Telescope (EHT), a network of linked radio telescopes around the globe.
An actual event horizon—the point beyond which light and matter alike become hopelessly lost to a black hole's pull—remains out of sight, but the telescope has now succeeded in piercing the veil of a nearby supermassive black hole to peer into unprecedented depths of its turbulent surroundings.
Researchers trained EHT radio dishes in Hawaii, Arizona and California on the giant elliptical galaxy M87, some 54.5 million light-years away. The galaxy features a dramatic jet, thousands of light-years long, emanating from its center and thus, presumably, from the galaxy's black hole. In a study published online September 27 in Science, Sheperd Doeleman of the Massachusetts Institute of Technology's Haystack Observatory in Westford, Mass., and his colleagues report that the dish network has resolved the base of M87's jet. The size of the jet at that position, close to its origin, in turn allowed the researchers to deduce some of the most fundamental attributes of both the galaxy's behemoth black hole, which weighs in at a mass of 6.6 billion suns, and the swirling disk of matter surrounding it.
Outside the event horizon of a black hole orbits a disk of material pulled in but not yet consumed by the gravitational pull of the black hole. That accretion disk grows quite dense and hot as infalling material collides and compresses, emitting copious amounts of radiation in the process. Accretion disks can also accelerate particles into a jet of plasma that propagates outward at a substantial fraction of light speed.
Doeleman and his colleagues measured the base of the jet in M87 to ascertain the inner edge of stability within the black hole's accretion disk, beyond which matter quickly falls inward to its doom. That edge, the densest and fastest-moving part of the accretion disk, can fling particles outward with ease. "The jets that we see from M87 are likely launched from right around this region," Doeleman says.
The EHT, with its superior resolving power from the long baselines separating its individual sites, allowed the researchers to measure a size for the jet's footprint of just 5.5 times the black hole's Schwarzschild radius. (The Schwarzschild radius is the size below which a given mass cannot be compressed without collapsing into a black hole.) "We saw something that was just impossibly small, startlingly small," Doeleman says.
The size of the jet—and, by inference, the size of the innermost stable orbit within the accretion disk—implies that the black hole is spinning, and that the accretion disk is rotating in the same direction. A nonrotating black hole would feature a much wider jet, and an accretion disk spinning counter to the black hole's rotation would launch a fountain that was broader still.
In measuring the jet's footprint the researchers had to account for distortions, caused by the warped spacetime of Einstein's general theory of relativity, inherent to observations of such massive objects. Because of the distortion, a particle jet measured by Earth-based tools can appear larger than it actually is. "The black hole acts as its own lens," Doeleman says. "That's just because the black hole is bending the light rays like taffy."
Simply visualizing the event horizon is not the only goal of the project. A central aim is to peer into an astrophysical environment dominated by supermassive objects to see if gravity works as predicted. "If Einstein's theory is going to break down, it's probably going to be near a black hole," Doeleman says, before acknowledging that the reigning theory of gravity has survived countless challenges before. "It is never wise to bet against Einstein," he adds. "I think the bookies in Vegas give you very long odds. But you have to try."