Targeting two black holes
M87 gave Doeleman’s team a second target. Its supermassive black hole weighs about 6.5 billion Suns, as opposed to Sgr A*, which weighs about 4.5 million Suns. But based on M87’s distance of 55 million light-years, its silhouette is comparable in visual size to Sgr A*, which has a diameter akin to Mercury’s average distance from the Sun, but is over 2,000 times closer, about 26,000 light-years away.
“There are no other [black holes] we know of that present such a large silhouette on the sky,” Doeleman says. “Whenever we went out to observe, we’d observe both of them, because you never know which is going to be the easiest.”
For any successful EHT campaign, all observing locations must simultaneously experience perfect weather. As Earth rotates, consecutive sites pass off duties to maximize observing time. The rotation effectively sweeps the sites across different areas of the virtual dish, enhancing the quality of collected data.
In April 2017, five crystalline nights out of 10 at all six sites — Hawaii, Arizona, Mexico, the South Pole, Spain, and Chile — yielded pristine data on both M87 and Sgr A*. Petabytes of data stored on hard disks from all stations were physically shipped to supercomputers in Boston and Bonn for intense processing and correlation. M87’s black hole emerged the winner — along with Einstein. Predictions of what a black hole would look like, all based on his general theory of relativity, were remarkably similar to what the EHT produced.
By March 2020, there were murmurs at the BHI that a separate image of Sgr A*, based on the 2017 data, might be forthcoming. Although Sgr A* was Doeleman’s original target, it is a restless subject. And, like old-time cameras, the EHT needs its subjects to sit still for a sharp exposure. The plasma whirling around Sgr A* can change shape in a matter of minutes. That makes it hard to photograph. “The fact that Sgr A* moves means we will have to model its motion in order to image its structure — in effect, making a movie,” Doeleman says.
Also complicating matters is the 156 trillion miles (251 trillion kilometers) of material in the Milky Way’s disk that’s between the black hole and Earth. “The intervening gas that lies between us and Sgr A* has a blurring effect (even if Sgr A* were to remain still),” Doeleman explains, “so it’s a separate kind of problem. We get around the blurring largely by observing at higher frequencies where the effects of the blurring decrease.” The team is also developing specialized algorithms designed to further mitigate the smearing effects to create a clear picture.
M87’s black hole, however, is 1,000 times larger and more stable than Sgr A*— too huge to change profile in a single night. If all the stellar-mass black holes detected through gravitational waves were scaled to the size of gumdrops, M87’s black hole would loom next to them as a gaping mouth a mile wide. Next year’s observing campaign may reveal how much M87’s black hole has changed in four years. This year’s observing run was cancelled due to COVID-19.