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.
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 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.
Since a number of the sites are arrays, the initial data obtained for the Event Horizon observations was, in digital terms, enormous. So the people behind the Event Horizon telescope built set of data recorders capable of gathering information at a 16 Gigabits/second rate, and spreading it across 32 hard drives. Each site in the telescope received four of these data recording units; at the end of the observations, they were shipped to one of two data processing centers—in total, half a ton of hard drives were shipped around.
The process of reconstructing an image is then a colossal computing task. Which is part of the reason that there’s been a significant lag from the observations made in April of 2017. (Waiting for spring in the Southern Hemisphere so the data from the South Pole Telescope could be transported out was another source of delay.)
What are we looking for?
The telescope did its imaging while pointed at two different targets: the supermassive black hole at the center of our galaxy, and one in the large galaxy M87. Part of that was simply geometry, in that having hardware scattered across one side of the Earth limits the locations that can be imaged. But part of the reason these two were chosen is because they are very different examples of supermassive black holes.
Our galaxy’s central black hole is a relatively quiet one. While there is some matter in its vicinity, it hasn’t built up the features typical of active central black holes: a large disk of brightly glowing material and huge jets sent out of both poles at nearly the speed of light. Those, however, seem to be present on the Event Horizon Telescope’s second target, the central black hole of galaxy M87. That’s a larger, active black hole, and it has at least one jet (it’s oriented so we can’t see the second) that extends for thousands of light years.
Obviously, given their nature, we won’t be looking at these black holes themselves. But we can learn a great deal about the environment around them. For example, there’s no consensus about how, precisely, jets of material get accelerated to nearly the speed of light. The Event Horizon Telescope was designed to give us the best images yet of the base of the jets, showing how they’re related to the other structures near thee black hole.
Another feature that scientists were interested in is what’s called the “shadow” of the black hole. The intense gravity near black holes warps the space around it, pulling some of it around for multiple orbits of the object. This has strange effects on the light originating from the material, creating a pattern that’s referred to as the black hole’s shadow. Since that shadow depends on the paths that light can travel around the black hole, it provides a sensitive test of relativity, and could rule out some alternative theories of gravity.
The shadow also depends on the mass and spin of the black hole, and so provides information regarding its physical properties.
