On Thursday (12), scientists from an international consortium released for the first time a photo of Sagittarius A*a supermassive black hole that lives at the center of our Milky Way.
The researchers involved in the feat participate in the ‘Event Horizon Telescope’ (EHT), a collaborative network of astronomers, astrophysicists and scientists from around the world who work with nine radio telescopes spread across the globe to capture images of black holes.
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But how does it work and what are the current goals of the project that revealed, in 2019, the first photograph of a black hole ever recorded, Pōwehi? THE g1 spoke with experts and with Brazilian researcher Lia Medeiros, who coordinated one of the EHT studies released this week, to answer the following questions:
- What is the Event Horizon Telescope project and what is its purpose?
- Where are the project telescopes?
- How many researchers participate in the collaboration?
- How are observations made?
- What were the main findings revealed by the EHT so far?
The Event Horizon Telescope is the name of an international collaboration that aims to obtain the first direct images of black holes.
The main focus of the project is two supermassive black holes: one at the center of the Messier 87 galaxyfrom the 2019 photo, and another at the center of our Milky Way, Sagittarius A*.
See location of the black hole — Photo: Art:g1
The Brazilian researcher Lia Medeiros, from the Institute for Advanced Studies of the National Science Foundation of the United States, says that, after several others had been observed, these two black holes in particular were chosen because they are the largest from our point of view here on Earth, which makes them easier to observe.
“Therefore, we can reach the best resolution for these two in particular. There are other black holes that we have also observed. But we will not be able to see that ‘little ring’ of the black hole itself for these others because they are very further away”, he explains.
What is a black hole?
Where are the project telescopes?
Are 11 radio observatoriesdistributed in 8 different locationsgenerally remote regions, at high altitude and with a dry atmosphere, to avoid the effect of water vapor, which makes observations difficult.
France, Spain, Greenland, Chile, United States (Arizona and Hawaii), Mexico and Antarctica are the places where the instruments are installed.
Location of the telescopes that make up the EHT network. — Photo: ESO/M. Kornmesser
To make the photo of Sagittarius A*, astronomers used only eight observatories of this network. That’s because the measurements were made in 2017, when the Greenland, France and Arizona telescopes were not yet added to the network.
Therefore, Lia explains that we can expect even more detailed images of the collaboration in the coming years. The researcher told the g1 who is even working on a specific project about it.
“We have three years of observations that we’re working on. There’s a lot to come. But it won’t be tomorrow. [risos]”.
South Pole Telescope, illuminated by the aurora australis and the Milky Way. The outside temperature at the site reaches -60°C. — Photo: Daniel Michalik/South Pole Telescope
How many researchers participate in the collaboration?
There are different groups of scientists specialized in antenna optimization, receiver development, signal correlation, calibration, imaging methods, black hole theory, jets and general relativity, numerical simulations, among many other techniques.
In total, more than 300 individual researchers from 80 institutions around the world work collaboratively.
“A long time ago we had that idea of a genius who created a theory and changed Science. We are already past that stage. Nowadays, most research is done with collaboration”, highlights Medeiros.
Lia Medeiros participated in the project that revealed the first image of a black hole — Photo: Reproduction/TV Globo
“This [o projeto de observação] it’s not a result that depended on five people. There are a lot of people who do very different things that have contributed in different ways, from different states. People who contribute to the instrument itself or who do what I do: simulations that we analyze to test the Theory of General Relativity”
How are observations made?
As black holes do not emit light, the program’s main mission is to capture images of their “shadow” or “silhouette”, matter that circulates around the object.
There were more than a thousand hours of radio observation. And capturing those images is not easy. Not even Hubble or the newly launched and super modern James Webb can do this feat.
“The hardest part was not making one image, but actually verifying that the ring structure was real,” says Katie Bouman, professor of Computer Science and Mathematics at Caltech and a member of the EHT network.
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She explains that scientists had to go through all the gas and dust in our galaxy to get close enough to Sagittarius A* to see this ring of light, and that’s why the network’s observatories are all radio telescopes. “Visible light would be blocked by this dust, while the short radio waves we observe are unaffected by dust and gas near the black hole.”
“After literally years of exhaustive testing using real and simulated data, we were finally able to conclude that there was compelling evidence that the structure is ring-shaped.”
Infrastructure at Mauna Kea Observatories in Hawaii, which is also part of the Event Horizon Telescope. — Photo: J. Weintroub/ESO
“But the Event Horizon Telescope manages to overcome this barrier and has the observation quality necessary to be able to image the event horizons [o ponto de não-retorno dos buracos negros]”, says Rodrigo Nemmen, astrophysicist and professor at the Institute of Astronomy, Geophysics and Atmospheric Sciences at USP, who was not related to the work of the EHT.
To focus on a black hole, explains Nemmen, we need imaging power that is only possible when we virtually combine telescopes scattered across the Earth into a single telescope that simulates the size of our globe. Thus, by combining the power of each of these observatories, we have an impressive resolution capacity that allows the formation of these images.
“To give you an idea, the EHT would be able to image, in radio waves, an apple on the surface of the Moon”, says the researcher.
Process resembles a GPS system
The Brazilian Lia Medeiros, who participated in the collaboration, says that the operation of a GPS is an analogy that helps to explain how the EHT operates.
A GPS receiver, like the one in our cell phones, works together with the help of satellites orbiting the Earth to pinpoint its exact location. It receives signals from different satellites and calculates how long that signal took to reach it. Since this receiver knows where these GPS satellites are, by calculating the time each signal arrives, it has the information it needs: where it is located.
“We use a very similar technique, because what we are recording is actually the time that each radio wave takes to reach each of our telescopes”, says the researcher. “We use the time each telescope detects the wave to calculate where it came from in the image.”
“This technique is called interferometry“, says Lia Medeiros. “We use these telescopes and they act as a team. They all point to the same place in the sky at the same.”
How the Event Horizon Telescope works — Photo: Art: g1
These radio waves are emitted by matter in the plasma that forms a “swirl” of hyper-heated gas that surrounds some black holes, explains to g1 Andre Landulfo, professor of Physics at UFABC, specialist in the subject.
Thus, the signal from each telescope records the radio waves that a black hole emits and combines this information with the help of atomic clocks, which guarantee the accuracy of the data.
“Each telescope station on Earth observes these signals, takes a ‘partial image’ of their limited perspective. Then, using sophisticated interferometry and imaging techniques, these signals are combined to form a single image (‘uniting’ the different perspectives)”.
But this is not instantaneous. On the contrary. Transporting this gigantic computational information takes time and processing it much more.
In the 2019 photo alone, 5 petabytes of images were produced (1 petabyte = 1 million gigabytes).
“It’s the equivalent of 5,000 years of mp3 files or the entire collection of 40,000 people’s lifetime selfies,” compared the project’s director, Sheperd Doeleman, at the time.
What were the key findings revealed by the EHT besides Sagittarius A*?
The 2019 image was the first photographic record of a black hole, but since then, it hasn’t been the only photo released by the project.
Comparison of the sizes of the two black holes: M87* and Sagittarius A*. White dot in the center of the image highlighted on the right shows the diameter of the Sun. — Photo: EHT collaboration (acknowledgment: Lia Medeiros, xkcd)
In 2021, EHT astronomers revealed a new view of the same black hole at the center of Messier 87, showing a sharp image of its magnetic field. (see photo below).
With this image, researchers were able to understand a little more about one of the most mysterious and energetic structures in the galaxy: the jets that a black hole manages to launch from its center and that extend beyond its galaxy.
First sharp image of a black hole’s magnetic field. — Photo: EHT Collaboration
The phenomenon intrigues scientists because, as we saw before, the black hole attracts everything that passes its event horizon. However, some particles escape moments before being captured and are launched into space in the form of jets. And science still doesn’t know exactly why this happens.
“Observations suggest that the magnetic fields at the edge of the black hole are strong enough to push the hot gas and help it resist the pull of gravity. Only the gas that escapes the magnetic field can spiral toward the black hole’s event horizon,” explained Jason Dexter, coordinator of the EHT Theoretical Working Group at the time.