On May 12, an image of the supermassive black hole at the center of the Milky Way was released by the Event Horizon Telescope (EHT) project, an international collaboration involving more than 300 scientists. It was the second direct image of one of these extremely dense and mysterious celestial objects, which have such a strong gravitational pull that they absorb all matter, including light, from within a certain radius. The first was released in April 2019 when the same EHT team captured a snapshot of the black hole at the center of Messier 87 (M87), a giant galaxy 55 million light-years from Earth.
Brazilian astrophysicist Lia Medeiros, 31, who is a member of the EHT team and is currently doing a postdoctoral fellowship at the Institute for Advanced Study in Princeton, USA, participated in both projects. Since early 2020, she has been one of two coordinators of a group at EHT studying the gravitational physics of black holes, based essentially on Albert Einstein’s theory of general relativity (1879–1955). The image of Sagittarius A*, the name of the black hole at the center of our galaxy, was published alongside six scientific articles in a special issue of Astrophysical Journal Letters. Medeiros was one of the leaders of the group that wrote one of these six papers. “Black holes are a laboratory for testing Einstein’s theories on gravity,” says the Rio de Janeiro–born researcher who has lived in the USA since she was a child.
In both cases, the cosmic portraits were created by processing and interpreting observational data captured by a network of ground-based radio telescopes located around the world. By definition, black holes do not emit light, but their surroundings “glow” in certain radio wave lengths. The so-called direct images of the objects located at the hearts of the two galaxies actually show the black holes’ shadows: a dark central region surrounded by a ring of matter and hot gas artificially colored in orange tones.
So far, the images of these two black hole shadows confirm the ideas proposed by the German physicist. Medeiros explained it all in an interview given by video conference while she was being driven from Washington DC to Princeton on May 13. The day before, she and her EHT colleagues had been invited by the National Science Foundation (NSF) to participate in an online public event, broadcast from the US capital, during which they answered questions about the image of Sagittarius A*.
What does the process of imaging a black hole involve?
We use a technique called interferometry, which combines observational data obtained by radio telescopes, making it possible to create images with a higher resolution. The greater the distance between the telescopes in the network, the higher the resolution of the resulting image. But we didn’t build this network of telescopes. We used existing ones, made some changes to them, and installed new cameras so they could work together. We have thus created a virtual radio telescope nearly the size of Earth.
How many telescopes were used in the observations?
In 2017, the EHT observed the black hole in galaxy M87 and Sagittarius A* with eight radio telescopes: two in Chile, two in Hawaii, one in Arizona [USA], one in Mexico, one in Spain, and one in Antarctica. We divided the telescopes into pairs that operate synchronously, with each pair observing a specific area in space and obtaining certain information. We then put all this information together using special algorithms and software, and the result is an image. Our observations were made at a very short radio wavelength of 1.3 millimeters (mm), although that is longer than visible light waves. Based on the difference in time that these waves arrive at each pair of telescopes, we were able to very precisely calculate where they came from. It’s a lot like a GPS system, which records how long it takes a signal emitted by multiple satellites in space to reach Earth and uses the results to calculate the location of a cell phone, for example.
Images of black holes in galaxy M87 and the Milky Way confirm the theory of general relativity
But how is the mountain of data obtained from these observations used to produce an image?
It’s much more complicated than simply pointing a camera into space and taking a picture of an object. We have to carry out lots of calculations to construct an image from the data collected by the telescopes, which are located high in the mountains to make sure they are not affected by water vapor in the atmosphere—1.3mm radio waves interact with water. We recorded all the data for each observed radio wave. Not just where the wave came from, but also the exact time it was emitted, all with high precision. The telescopes have extremely accurate atomic clocks. Each pair of telescopes records a tiny bit of information about the image.
For how long were the two black holes observed?
Each black hole was observed for about 12 hours over the course of a week in April 2017. We made several observations during the week. Some data were obtained at the beginning of the observation period, others at the end. Each point in the image was confirmed by two telescopes simultaneously. What each telescope can observe is determined by its geographic location. It’s not possible to see all areas of space all the time from everywhere on Earth. For the image of Sagittarius A*, the telescope at the South Pole was really important. It can see this black hole almost all the time.
Why did the images take years to create after the observations were made?
Data access and processing is highly complex. The telescopes store the information about every radio wave they have been able to detect on hard drives. This amounts to something like 3.5 petabytes of data—equivalent to about 100 million videos on TikTok. There is no way to transmit that amount of data over the internet. The hard drives from each of the eight telescopes have to be transported by plane to computing and analysis centers in two locations: Bonn, Germany, and Boston, USA. At these centers, the data are correlated and observations from each telescope are synchronized with the rest. Then, the work of processing the data to generate the images begins. Another obstacle is that we don’t have data on all the points in space that we would like in order to build a picture of the black holes.
In many places on Earth, there are no telescopes, so there are some areas of the black holes that can’t be observed. Our information is incomplete. Based on our computer simulations and the data we have, we try to predict what these areas with missing information might be like. It’s like trying to play a song on a piano with some of the keys missing. It’s impossible to know exactly what the song should sound like and how the missing keys should be used to play it, but with the information available, you can try to guess. So at the end of the project, we had generated thousands of images of the same black hole that are technically compatible and consistent with our observational data. For the images of both the M87 black hole and Sagittarius A*, what we released was a sort of average of all our reconstructions, the version most compatible with our data and theory.
The Sagittarius A* image was published three years after the M87 image. Does that mean it was harder to create?
Yes, it was more complicated. Sagittarius A* is at the center of the Milky Way and Earth is located in one of the galaxy’s arms, a position that obstructs our field of vision. The radio waves we observe have to cross the galaxy and are affected by magnetic fields generated by ionized [electrically charged] matter in the Milky Way. Also, because it is much smaller than the black hole at the center of galaxy M87, the contours of Sagittarius A* appear more unstable to us. The brightness and distribution of the gas around Sagittarius A* changes much more rapidly. In M87, the gas in the ring takes days or weeks to complete an orbit around the black hole. In the Milky Way, this occurs in just a few minutes. It’s like trying to take a clear picture of an object in constant motion.
There are three brighter spots in the ring around Sagittarius A*. What are they?
We don’t believe these bright points actually exist, so we don’t want to waste time trying to understand them. They are most likely caused by the instruments used in the observations and the computational algorithms used to generate the image. We have produced other versions of the black hole image that are also consistent with our data but the bright points are in different places in the ring. In other words, their existence is uncertain. We are confident that the black hole at the center of the Milky Way has a ring—we were able to measure its diameter and width—and a dark region in the center of that ring. We were able to compare the intensity of the dark region with that of the ring itself. These are the black hole structures our theories have described. The image of Sagittarius A* is very similar to the image of the black hole in galaxy M87. The only difference is that the glow in the south of the latter’s ring is more intense than in the north. This distinction is real.
What could this difference in brightness mean?
When a ring of matter surrounds a black hole, a relativistic effect called Doppler beaming occurs. Matter moving towards the observer at close to the speed of light appears brighter, while matter on the opposite side, moving away from us, appears less intense. This effect changes the length and intensity of the light, which explains what we see in the M87 black hole image.
What is your role at the EHT?
The collaboration is divided into study groups that focus on different areas, such as producing images of black holes or performing theoretical simulations of them. I’ve participated in most of these research groups, especially the simulation ones. During my PhD, I ran simulations of how the matter around a black hole falls into it, then used them to predict what Sagittarius A* might look like. In 2019, when we released the M87 image, there was no group dedicated to testing Einstein’s theory of gravity, general relativity. In January 2020, I started coordinating this group together with a colleague from the project. Each group has two or three coordinators.
In what way do the images of the black holes support the theory of general relativity?
Einstein posited that gravity is a curvature of spacetime caused by the uneven distribution of matter. His theory predicted the existence and characteristics of black holes, in particular their geometry, which would be highly specific. The geometry of a black hole can be calculated using the Kerr metric, which is a solution based on the field equations of general relativity. The rings in the black hole images contain a lot of information about the black hole itself. By measuring the size of the ring, we can determine whether the black hole has Kerr geometry. In both the M87 and the Milky Way black holes, the images confirm the theory. It was very important to test relativity on black holes of different masses and with extremely strong gravitational forces near the event horizon [the radius around a black hole from within which matter is pulled]. The mass of Sagittarius A*, which is 27,000 light-years away, is equivalent to about 4 million Suns. M87’s black hole is 2,000 times farther away, but its mass is 1,500 times greater: equivalent to 6.5 billion Suns.
What sparked your interest in astrophysics?
I was born in Rio de Janeiro and spent most of my early childhood living in various cities around Brazil, plus a few years in Cambridge, UK. I spoke both Portuguese and English and kept switching between the two languages. Then we moved to the US while I was still a child. From a young age I realized that no matter what country you live in, mathematics stays the same, so I decided to focus on that. I knew it would never change. It was something fundamental and universal.
And why did you decide to study black holes?
At the age of 16 or 17 I was studying calculus, physics, and astronomy at high school in California and I realized that mathematics was the language used not only to describe but also to predict the Universe. For me, black holes are the best example of this. I was fascinated when I discovered that they can affect time, dilating it, and I wanted to learn more. I studied physics and astrophysics at the University of California, Berkeley, and then did a master’s and PhD in theoretical physics at the University of California, Santa Barbara. During my PhD, after I finished my classes in Santa Barbara, I spent three years at the University of Arizona’s Steward Observatory and one at Harvard’s Black Hole Initiative. Then, thanks to a grant from the NSF, I went on to do a postdoctoral fellowship at the Institute for Advanced Study [the last place Einstein worked].
Are there any other Brazilians working on the EHT Collaboration?
Not as far as I know. I know many Brazilian astronomers—friends and colleagues. I gave a series of lectures in Brazil in 2019 and I always spend vacation there with my family. But I have never worked with the Brazilian scientific community. I’m very interested in doing that someday.