The Event Horizon Telescope project isn't resting on its laurels.
In April 2019, the EHT collaboration revealed the first-ever image of a black hole, which captured the behemoth at the heart of the galaxy M87. The project had been working for that moment for two decades, but the unveiling did not mark a climax or a culmination. Rather, it was a turning point, the opening of a window onto even more exciting discoveries to come, team members said.
"We'll be looking back on this 20 years from now — you know, we'll all be drinking umbrella drinks somewhere — and I think we'll recognize the M87 image as just the beginning," EHT founding director Shep Doeleman, an astronomer at the Harvard-Smithsonian Center for Astrophysics (CfA) in Massachusetts, told reporters on April 10 during a webcast event celebrating the one-year anniversary of the photo's unveiling.
"It really will be just the beginning," Doeleman added. "I think that discovery is going to keep rolling on."
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Big black hole ambitions
The next big moment could involve our own Milky Way galaxy's supermassive black hole, which is known as Sagittarius A*, or Sgr A* for short. The EHT team has been observing Sgr A* but has not yet been able to generate an image from the data.
Photographing a black hole is much more involved than just pointing and shooting. The EHT combines data from eight radio telescopes around the world, which are linked to form a virtual megascope the size of Earth. The team devises algorithms to make sense of all this information, integrating it to generate an image depicting the black hole's event horizon — the "point of no return" beyond which nothing, not even light, can escape. (Photographing a black hole's interior is impossible, unless you're in there yourself, which is not advised.)
Such work is painstaking and time-consuming; for example, the M87 data that enabled last year's image were gathered in 2017. And Sgr A* is a tougher target still, even though it's much closer to us (26,000 light-years versus 55 million light-years for M87). Sgr A* is a relatively lightweight supermassive black hole, harboring the mass of "only" 4.3 million suns, and therefore operates on shorter timescales than the 6.5-billion-solar-mass M87 monster.
"One of the challenges for Sagittarius A* is that it does evolve so quickly during the course of an evening," Doeleman said. "So, we're developing new algorithms that can specifically address that, to add a time variability to the modeling. That just takes more time."
Adding Sgr A* to the photo album is a high priority for EHT team members, in part because they're keen to get a look at an object so different than M87's supermassive black hole. But the project's ambitions don't end there. The collaboration plans to double the number of constituent EHT telescopes, creating a "next-generation EHT" that will push discovery to the next level.
The modified megascope "will give us even sharper views and let us make the first movies of black holes," Doeleman said.
The next-gen EHT will bring even more black holes into range for the researchers. And it could even enable the team to zoom in on a black hole's "photon ring," a bizarre structure that appears to contain an infinite, information-packed sequence of subrings. Such in-depth observations would "allow ultraprecise tests of Einstein's theories, and even extraction of spin," Doeleman said.
That would be a big deal indeed. "Supermassive black holes are elemental objects described only by their size and rotation," EHT team member Michael Johnson, also of the CfA, said during the April 10 event. EHT observations already allow scientists to nail down mass, so getting spin as well would allow scientists to really get the goods on these light-gobbling monsters.
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The next-gen EHT will provide other benefits as well, Doeleman and Johnson said. For example, the upgraded partnership should allow the collaboration to gain a much better understanding of black hole jets, beams of particles that the behemoths blast out at nearly the speed of light.
The collaboration has already secured funding from the U.S. National Science Foundation to get the ball rolling on the next-gen EHT. The goal is to get a final design in place within three or four years and have everything up and running before the end of the decade, Doeleman said. (New dishes aren't the only way to boost the EHT, by the way. Resolution can also be improved by observing targets in multiple electromagnetic frequencies, Doeleman and Johnson said.)
The EHT could get yet another boost after that. Expanding the megascope's reach out into space would greatly increase its resolving power, and the team is looking into ways to do just that. The collaboration is investigating the possibility of incorporating already-existing space assets as well as mounting new, dedicated missions, Johnson said.
Adding even a single instrument in geosynchronous orbit, more than 22,000 miles (35,400 kilometers) above our planet, would make a difference. "And then, certainly, once you get out to the moon — that's where I think we would really be looking at entirely new science," Johnson told Space.com last month, when the EHT team announced its photon-ring results. (That distance would be about 10 times farther; the moon is an average of 238,855 miles, or 384,400 km, from Earth.)
A bump in the road
The coronavirus pandemic has made the path toward future EHT discoveries bumpier than it would have been ordinarily: The project's 2020 observing campaign was called off because several of the participating scopes shut down for health and safety reasons. (The project gathers data just once per year, during a window in March and April when weather tends to be good at all of its widespread observing sites.)
The cancellation was the right move, Doeleman said. But it was "heartbreaking," he added, especially because this year's observations would have included first-time contributions from instruments in Greenland, Arizona and the French Alps. So, the 2020 campaign would have featured an expanded, more-powerful 11-scope EHT.
But the team can still do a lot of data analysis and start laying the foundation for next year's observations, Doeleman stressed. So, spirits among the researchers are far from broken.
"Across the collaboration now, there isn't a tone of despair; I do think we're sort of united by the suffering," Johnson said.
"The black hole solution was derived by Karl Schwarzschild in the trenches during World War I," he said, referring to the German physicist who came up with the first exact solution to Einstein's general relativity equations. "So, science continues, and I think it has an important role even amid these challenges."
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Mike Wall is the author of "Out There" (Grand Central Publishing, 2018; illustrated by Karl Tate), a book about the search for alien life. Follow him on Twitter @michaeldwall. Follow us on Twitter @Spacedotcom or Facebook.
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