Away From The Theorist's Chalkboard

Charles Gammie is “one of the best of my generation of computationalists,” according to fellow  black hole researcher Adam Frank at the University of Rochester. He has a “great intuition for physics” and an important role in  “defining the next set of questions” for the field to answer, according to another colleague.

Despite the kind words and three decades modeling black holes on supercomputers, Charles Gammie wasn’t used to public praise or attention. That changed in 2019. Gammie is one of the lead scientists on the Event Horizon Telescope (EHT) project. A former Simons Fellow and PECASE winner, he is a physics professor at The Grainger College of Engineering. 

EHT is a global collaboration that brought the world its first image of a black hole last year. The image was the culmination of more than 10 years of work by hundreds of people. The project brought international attention, and, as one of about a dozen members of EHT’s Science Council, Gammie saw a lot of that attention.

The work earned him and his students a share of the Breakthrough Prize, which National Geographic calls “the Oscars of Science.” In December, it also landed him a spot on the Bloomberg 50, the magazine’s annual list of icons and innovators who have changed the global business landscape. The Bloomberg 50 also included Taylor Swift and Simone Biles. 

“Most astronomers, we’re used to pointing cameras at other places, not having cameras pointed at us,” Gammie said. “There’s something interesting about working on a project that’s seen by 4.5 billion pairs of eyes, but to get to that you have to focus on getting the work done. Day to day, this was heads down, really busy.” George Wong, a PhD candidate in Gammie’s lab, remembered the 2 a.m. meetings and 10-hour conference calls similarly. “When everyone was exhausted, they were also excited,” Wong said. “It was fun, but you don’t want to live that way forever.”

THE BEAUTY OF LOGICAL INFERENCE

Despite the intense effort, EHT’s results reminded Gammie of the “beauty” in theoretical physics. “It’s the power of logical inference. We can sit in Champaign-Urbana and…imagine things we have no experience of.”

What’s behind all that imagining? One of the most complex astronomy projects ever mounted.

Over the course of 10 days in 2017, the EHT project trained eight radio telescopes around the world on a black hole at the center of the Messier 87 galaxy. Though it’s a supermassive black hole, it’s very far away—about six billion times more massive than our sun and about 53 million light years away. That means the radio waves that the black hole throws off are extraordinarily weak.

Even after traveling 53 million years, the signal arrived at the telescopes—in Hawaii, Chile, Arizona, Mexico, Spain, and at the South Pole—at very slightly different times because the telescopes are slightly different distances from the source. Researchers used a technique called very-long-baseline interferometry to compare those readings and time differences and to create a much higher-resolution signal than any single telescope could capture.

Effectively, they stitched together the telescopes to create a single giant, and much more powerful, telescope. The resulting “images” are actually the amplitudes of Fourier transforms of the data, which are sometimes referred to as “visibilities.” In creating these visibilities, researchers collapsed five petabytes of data into thousands of 100-kilobyte images of the black hole from different moments in the telescopes’ 10-day runs.

“Once we have an image, we want to ask what that image means,” Wong explained. “What are the physical parameters that would produce that image?”

For the Messier 87 black hole, the EHT team looked at 7 physical parameters, including the black hole’s mass, angular momentum, and magnetic field strength.

To estimate those parameters, Gammie’s team ran more than 200,000 models of the Messier 87 black hole on a computing cluster at Illinois and a supercomputer in Texas. These models centered on a relatively narrow set of hypotheses of what ranges the 10 physical parameters would fall into, based on years of researchers’ theoretical and observational work. The team then ran statistical probabilistic comparisons of the visibilities and the 200,000 computational models in order to determine how the models and the observed data matched up.

The resemblance was striking, confirming that the numerical models that astrophysicists use to predict the behavior of unobservable black holes is sound. “Fewer than 20 percent of the models that we ran were rejected” because they were far off from the observed visibilities, Wong said. “Our reasonable guesses were close, so the theory is good.”

'A LEADER IN THIS BUSINESS'

EHT gave researchers a view of matter in gravitational fields that are much more extreme environments than those they can see with optical telescopes and lower-resolution radio telescopes. It allowed researchers to get a better understanding of several features of black holes. For example, astrophysical jets—blasts of ionized matter that streak through the universe at nearly the speed of light—have long been theorized and observed by optical telescopes. But astrophysicists still have questions about their source. 

“We were able to tie together the images and the supercomputer models to show that the jets are launched from very close to the black hole,” Gammie said. “The models say that the jets are powered by the black hole itself. Magnetic fields thread through the hole and brake its rotation, transforming the rotational energy of the hole into outgoing beams of electromagnetic energy. Black holes famously suck gas in from their surroundings. But they can also spew out energy. Now we have a better understanding of how that works.” 

The University of Illinois has been helping to provide those insights about black holes for decades. In the 1980s, its National Center for Supercomputing Applications was founded by an astrophysicist who knew that scientists and engineers needed supercomputing power to solve otherwise intractable problems and probe the universe using numerical models. The center has been supporting that work ever since. 

“U of I has long been a leader in this business,” he said. 

That leadership continues today as Illinois scientists help drive the work of the Event Horizon Telescope. Researchers will be delving into and drawing new discoveries from that work for years to come. 

“Black holes sit in messy environments at the centers of galaxies, where there are all kinds of weird stars and hot gas around.” Gammie said. Those messy environments—“out in nature, far away from the theorist’s chalkboard,” as Gammie described it—are coming into clearer focus every day.

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