Enlarge / The first image of the environment around a black hole. As a matter of fact, it's not all dark.National Science Foundation
Two years ago, telescopes around the world turned their attention to two supermassive black holes. Now, after a massive computational effort, their data has been combined in a way that allowed them to function as a single, Earth-sized telescope. The results are an unprecedented glimpse of the environment around supermassive black holes, and confirm that relativity still works under the most extreme gravitational forces.
Enlarge / The environment near the black hole appears to change on very short time scales, though we're not sure about the significance of this. White circles reflect the resolving power of the Event Horizon Telescope.Astrophysical Journal
The black hole in question is a supermassive one at the center of the galaxy M87, 55 million light years away, an active galaxy where the black hole is feeding on matter and ejecting jets of material. The image is made from photons that were temporarily trapped in orbit around the black hole. Here, the intense gravity causes matter—and even space itself—to move at approximately the speed of light. The eventual escape of these photons causes a bright ring to appear around the black hole itself, with the details of the ring reflecting the physics of the black hole itself.
A monster
At a press conference this morning, Avery Broderick of the Perimeter Institute described what the images tell us about the black hole itself. One key finding is that the object is a black hole, at least as we've understood black holes using relativity. It does not have any visible surface, and the "shadow" of light it creates is circular within the limits of our observations. We can also tell that it spins clockwise. All of the properties we can infer from these images are consistent with relativity. "I was a little stunned that it matched the predictions we made so well," said Broderick.
The University of Amsterdam's Sera Markoff said that the size of the black hole provided a new estimate of its mass; she called it "really a monster, even by black hole standards." It's roughly the size of the Solar System, but has a mass that's 6.5 billion times that of our Sun. This actually resolved a conflict between two other measures of its mass, one from the motion of gas clouds nearby, the other from tracking the stars orbiting it. This may help us refine estimates of mass for black holes elsewhere.
Missing so far is any discussion of the jets launched by black holes that are ingesting mass. Some process causes a portion of the material falling towards the black hole to get ejected at roughly light speed in two jets. It was hoped that the Event Horizon Telescope would help clarify how these jets start, but there was no mention of the topic in the press conference. Details may reside in one of the six papers released today.
Project lead Shep Doeleman, when asked how he reacted to the first images, said it was intensely satisfying. "We could have seen blobs, and we have seen blobs," he said, talking about past results. "We saw something that was so true."
The telescope
The Event Horizon Telescope isn't a telescope in the traditional sense. Instead, it's a collection of telescopes scattered around the globe. In its current iteration, it includes hardware from Hawaii to Europe, and from the South Pole to Greenland, though not all of these were active during the initial observations. The telescope is created by a process called interferometry, which uses light captured at different locations to build an image with a resolution similar to that of a telescope the size of the most distant locations.
Interferometry has been used for facilities like ALMA, the Atacama Large Millimeter/submillimeter Array, where telescopes can be spread across 16km of desert. But in theory, there's no upper limit on the size of the array. Practically, however, there are several challenges. To know which photons originated at the same time at the source, you need very precise location and timing information on each of the sites. And you still have to gather sufficient photons in order to see anything. In general, that means atomic clocks (which had to be installed at many of the locations) and extremely precise GPS measurements built up over time. For the Event Horizon Telescope, the large collecting area of ALMA, combined with choosing a wavelength where supermassive black holes are very bright, ensured sufficient photons.
The net result is a telescope that can do the equivalent of reading the year stamped on a coin in Los Angeles from New York City—assuming the coin was glowing at radio wavelengths. There's no way we can do better without relying on hardware that's not located on Earth.
The locations of the individual telescopes that make up the Event Horizon Telescope. Event Horizon Telescope
