A pale orange glow stands as proof of the efforts of a global network of scientists, including a few close to home. On Wednesday, April 10, researchers with the Event Horizon Telescope array revealed the first direct image of a black hole.
“We have seen what we thought was unseeable,” said Event Horizon Telescope Director Sheperd Doeleman, speaking from the National Press Club in Washington D.C.
While humanity’s understanding of physics has long hinted at their existence, before the EHT captured this data, black holes were only able to be visualized via simulation. But this recently released image of a supermassive black hole at the center of the Messier 87 galaxy is the first direct visual evidence of a black hole.
The EHT is an international collaboration formed in 2006 comprising eight radio telescopes across the world, including the Kitt Peak National Observatory southwest of Tucson. By combining the power of these telescopes in multiple countries, researchers essentially created a planet-sized telescope with enough power to penetrate deep space.
“The observations were a coordinated dance in which we simultaneously pointed our telescopes in a carefully planned sequence,” said Daniel Marrone, associate professor of astronomy at the University of Arizona and EHT scientist. “To make sure these observations were truly simultaneous, so that we could see the same wavefront of light as it landed on each telescope, we used extremely precise atomic clocks at each of the telescopes.”
The black hole at the center of Messier 87, a galaxy in the Virgo Cluster some 55 million light-years from Earth, is nearly the size of our solar system but has been difficult to photograph because of its extreme distance. Doeleman describes imaging across this distance as equal to standing in New York and reading the date on a quarter in Los Angeles.
Although recently released, the observations leading to the image were first captured in April 2017. However, each telescope of the EHT produced an extremely large amount of data that had to be parsed through and converted. According to Marrone, the EHT captured five petabytes of information—equal to 5,000 years of mp3 files in raw data.
UA researchers helped coordinate two of the radio telescopes in the EHT. Marrone, along with others from the university, traveled around the globe multiple times since EHT’s inception in order to synchronize all the participating telescopes. This even involved trips to Antarctica to deploy new software at the South Pole Telescope.
Due to the extreme difficulty of capturing such data, any additional obstacle must be removed if possible. This includes using telescopes located in very dry environments, such as the Sonoran Desert, to avoid water vapor in the atmosphere which could obstruct visuals.
“My experience with 10 years of doing these observations is that weather is usually the place where we fail,” Marrone said. “We have to have good weather in Hawaii and Spain at the same time, in Arizona and the South Pole. And that’s a lot to ask. But in 2017 we were very lucky. Our first three days of observations were some of the best weather we’ve ever seen.”
Black holes are known as intensely powerful forces that don’t let anything, even light, escape their clutches, hence the name “black” holes. So how did scientists capture an image of that which expels no light? According to Doeleman, capturing the image relied on a set of “very interesting cosmic coincidences.”
For light from the black hole to reach Earth, a photon had to leave the black hole from close to the event horizon, just on the lip of the point of no return. The photon then traveled through the hot gas being sucked into the black hole. Not every light wave can do this, but radio waves are small enough to. The light then traveled 65,000 years through its home galaxy, M87, then another 55 million years through intergalactic space before reaching Earth. Finally, the light hit the Earth, where according to Doeleman, “the greatest danger is that it will be absorbed by water vapor in our own atmosphere.” But the scientists were lucky enough to capture the data.
Scientists also built the EHT project to test Einstein’s Theory of General Relativity. Einstein’s theory implies the existence of black holes, and states spacetime undergoes curvature from gravity. These latest EHT discoveries are consistent with Einstein’s predictions.
“The Event Horizon Telescope allows us for the very first time to test the predictions of Einstein’s General Theory of Relativity around supermassive black holes in the centers of galaxies,” said EHT Project Scientist and UA professor of astronomy Dimitrios Psaltis. “The predicted size and shape of the shadow theory match our observations remarkably well, increasing our confidence in this century-old theory.”
Overall, 36 UA researchers are involved in the EHT. UA scientists, including Marrone, Psaltis, Feryal Ozel, Chi-kwan Chan, Lia Medeiros and several graduate and undergraduate students played instrumental roles to the EHT over the past few years. Using $6 million from a Partnerships for International Research and Education grant, these local scientists built and maintained computer infrastructure for the EHT. The team also used 140 graphical-processing units to power a supercomputer capable of modeling EHT observations.
Despite publication of the image, the EHT’s work is far from finished. The telescope array’s future goals involve understanding the accumulation of matter around a black hole and understanding plasma jets traveling out from the centers of galaxies.
“As with all great discoveries, this is just the beginning,” Doeleman said.