They are the heirs of the celestial police, that society of six astronomers formed in 1800 at the Johann Schröter observatory, in Lilienthal, in northern Germany, to find a supposed planet lost between the orbits of Mars and Jupiter. Two centuries later, another society of astronomers – this time more than two hundred, spread over all corners of the planet – gathered around a network of twenty radio telescopes with a historic mission: to point to the sky to photograph a black hole for the first time –The one of the Messier 87 galaxy– and record the first videos of the one that lives in the center of our galaxy, the Milky Way.
In the first phase of this project, they used eight radio telescopes located on different continents –among them, one in Sierra Nevada, in Granada–, with which they carried out two observation missions in March and April 2017 and 2018.
The result is already the history of astronomy: on April 10, 2019 they published a photograph of the supermassive black hole of the galaxy Messier 87 – also known as M87 -, which belongs to the so-called Virgo cluster. It was the first time that humanity saw with its eyes one of the most enigmatic and extreme elements of the cosmos; a point at which the laws of the physics of the universe are put in check.
Shortly after this event, the project received a notable boost: the National Science Foundation of the United States , an agency of the federal government, granted it $ 12.7 million to take it to its second phase, the so-called Next Generation of the Telescope. Event Horizon (ngEHT). In 2021, the network, already made up of twenty radio telescopes, has dedicated itself to scanning the sky with a fundamental objective: the black hole of the Milky Way. “It’s not just about photographing him. We aspire to go further and obtain videos of these mysterious cosmic phenomena. We want humanity to see the first movies of black holes, ”explains Sheperd Doeleman, project coordinator and astrophysicist at Harvard University, to VERY.
The history of black holes begins in November 1915 , when Albert Einstein publishes a series of articles in which he defines his theory of general relativity. From it, physicists deduced the probable existence of black holes. The first to do it was the German Karl Schwarzschild, a year later, although neither he nor anyone at that time called them that way. The name of black hole took half a century to appear. It was offered by the American physicist John Archibald Wheeler at a conference in New York in 1967. Four years later, British astronomers Martin Rees and Donald Lynden-Bell hypothesized that our galaxy, the Milky Way, would have a kind of center. in which there would be a supermassive black hole. In 1974, Bruce Balick and Robert Brown confirmed its existence and named it Sagittarius A *.
“You would think that black holes are just that, black; But is not so exactly; in fact, they can be one of the brightest elements in the universe, “says Doeleman. “Of course, in the region that surrounds them, the so-called event horizon, the force of gravity is so powerful that not even light can escape. That’s where the nickname of blacks comes from ”.
The event horizon is the limit of the hole, the point from which the laws of physics cease to exist and the space-time map of this cosmos disappears; from that frontier, physicists have no way of knowing what happens. In Christopher Nolan’s film Interstellar (2014), when the pilot Joseph Cooper (Matthew McConaughey) enters the Gargantua black hole, he enters a five-dimensional zone: the four known dimensions -the three dimensions plus space-time- and one extra, also space-time. This allows him to interact with the space-time of his daughter Murph, in the past, to change his future.
“If black holes drop matter into them, a ring of cosmic dust and gases accumulates around them,” explains Doeleman. “That is the visible and shiny part of the hole. In addition, according to Einstein’s theory, all that surrounding matter, when reaching extremely high speeds, reaches very hot temperatures, of billions of degrees Celsius. Finally, the holes rotate on themselves around two magnetic poles, and this leads them to launch a kind of high-speed jet between one pole and the other . So the holes, in addition to being black, are also bright and dynamically very active ”. That’s the activity Doeleman wants to capture.
In the observation missions of 2017 and 2018, the radio telescopes pointed towards other black holes more distant than that of M87 and Sagittarius A *. In those cases, their silhouette could not be seen, but the astronomers did manage to obtain detailed images of their jets, specifically, in the quasar 3C 279. Quasars are very bright sources of electromagnetic energy, so much so that, when they were discovered, they were they took for stars, although they did not appear to be. So what were they? Well, that’s precisely where its name comes from: quasi-star; in English, quasi star, or quasar, in Spanish. 3C 279 is related to the presence of a black hole approximately one billion times more massive than our sun.
In the 1970s the question of how to be able to contemplate black holes began to arise, and that is where radio telescopes play a key role. They obtain an image of the universe, but instead of obtaining it by capturing light waves, they produce it from radio waves; that is to say, of the radio frequency emissions that come to us from the cosmos. To capture those signals and generate an image of them, astronomers have developed a technique called very long-base interferometry (VLBI).
“The way a radio telescope works is similar to any other telescope,” says Doeleman. “It is the same as a camera: it captures the waves through a lens, projects it to a focal point and from there the image is built.” The human eye can see light because it is able to see at that wavelength, which is not the case with radio waves. But radio telescopes can. Once these radiofrequency emissions have been captured by these instruments, an image of the cosmos can be obtained through the technique of interferometry . This technology was, in fact, the one used in 1974 by Balick and Brown to confirm the existence of Sagittarius A *.
Among the objects and phenomena in the universe that emit radio frequency radiation are black holes. “In order for a conventional telescope located on Earth to be able to capture the photons found in the region flooded with dust and gases that exist around black holes, before such photons would have to pass through all that cloud, something that most do not achieve. of them, “says Doeleman. “However, radio waves can not only travel through all of that, but also through the interstellar medium – and thereby bypass the gas between the stars that can blur and scatter light. Furthermore, they are capable of passing through the Earth’s atmosphere, something that not all wavelengths can do ”.
As this expert explains, if the interferometry technique has to do with the reproduction of radio frequency waves, the name “very long base” is related to the fact that if one wants to see a black hole, the telescope has to be as big as Earth to get adequate resolution. “You need a resolution similar to that of a normal telescope that could see an orange on the surface of the Moon, and that can only be obtained by means of a network of telescopes distributed on several continents, so that they generate between them a lens of a size almost similar to that of Earth ”. This is the network with a very long base that makes up the twenty telescopes of the ngEHT project, the Earth’s eye from which the team led by Doeleman wants to see and record black holes, including that of the Milky Way.
“With these telescopes scattered around the Earth, we aim at the source from which these radio frequency wave emissions come, in this case the black hole, and with the data received we use a supercomputer. Then you have to apply interferometry and push it to its absolute limits to debug the small variations that exist due to the travel of radio waves through the universe, “says Doeleman. “In this way it is possible to reproduce the image of a specific point. It looks like magic. The first time I knew that this was possible, I was amazed, because I had no idea that you could do something like that, that all of this worked. “
Doeleman has been working with a team of astronomers for years to refine this technique. They carried out the first experiments in 2006 and 2007. “We managed to carry out for the first time in history a measurement of the emissions around the black hole of the Milky Way,” he recalls.
The data collected around Sagittarius A * described it as a black hole the size of the orbit of Mercury, with a mass of four million times our sun and so far away from us that it takes time for light from its surroundings to reach Earth. 25,000 years. With that measurement, they confirmed one extreme of Einstein’s theory of general relativity , and they did so not on paper full of calculations, but by looking at it. “Those are the great moments in the history of any astronomer, when you verify that an object that you observe is exactly the same size that the theory predicted,” says Doeleman.
According to Einstein’s postulates, if a black hole has a given mass, its action is so strong that the emission ring around it would look five times larger than it actually is; And since the size of the hole is deduced from the ring, that would make the black hole appear five times larger. “It’s like a deforming crystal: the black hole’s mass alters the light and makes the surrounding ring appear larger,” explains the Harvard astrophysicist. “In 2007, when we made measurements of the Milky Way’s black hole, we could see for the first time that it was exactly five times larger than it should be.” Doeleman and his team were on the right track.
However, they could not go beyond detecting the size of the hole. They only had three telescopes and they couldn’t get any images. “However, it was at that moment when we knew that we could have it, using interferometry and adding more radio telescopes. That experiment proved it to us ”. And this is how the photo published on April 10, 2019 arrived, obtained thanks to the work of eight radio telescopes and after two observation sessions, in March and April 2017 and 2018.
Doeleman recalls that to obtain this snapshot it took many hours of observation and many more to study the vast data captured by radio telescopes. “In the 2017 session we collected seven petabytes of data, which is equivalent to spending five thousand years in a row listening to music in MP3 format files. By increasing the number of telescopes to twenty, we collected vastly more. The only way to share that information is to save it on hard drives, put them on a plane and send them to the recipient, “he says.
Doeleman’s team is eager to re-target Sagittarius A * black hole, and others as well. These campaigns will come after the jug of cold water that meant being two years in a row without having been able to look at the center of the galaxies, since the observations planned in 2019 and in March and April 2020 had to be canceled. In the first case, because two of the radio telescopes were not ready; and in the second, due to the covid-19 pandemic. “All of this was very demoralizing,” admits the scientist.
That nailed thorn was alleviated in part with the publication of the aforementioned first photo in the history of a black hole and with that of another of the same structure, this time in polarized light, in March 2021.
However, when the astronomer Martin Rees contemplated the first of them, he claimed to feel somewhat disappointed, because there was nowhere to be seen the stream of energy and particles that should be ejected from the very center of the hole. “We reliably know that a jet emerges from them, and it is thought that the hole is very likely the engine of the same,” insists Doeleman. “The problem is that when the aforementioned jet tends to be very large, it is blurred and attenuated; and when it becomes brighter it becomes very small and is confused with the black hole itself. In 2017 we only had eight telescopes and we could not see the emission when it is larger. With more telescopes we will gain in resolution, we will improve the technique and we will try to see it ”.
For similar reasons, in the photo of M87’s black hole, one part of the bright ring is seen in front of a dimmer one. “It is exactly what we expected, because the gas moves very fast around the black hole: if it moves towards you, it is brighter; if you move away, it becomes dimmer and more blurred. This was also foreseen in Einstein’s theory ”, explains Doeleman. “So, with that photo, suddenly, we had before us another demonstration of it, that of the characteristics of the dynamics of light around a black hole, just as they had been predicted,” he says.
However, the ngEHT project , as announced by doeleman, will be specifically dedicated to capturing these jets from black holes. “By having twenty radio telescopes we hope to reach the necessary definition to see them,” he adds. And who knows if future images will offer clues to another of astronomy’s deeper questions: “Although the black hole is involved in the formation of that jet, it is not yet known exactly how it happens,” says Doeleman.
“ There is no way to test what happens in black holes on Earth, so the best way we have to do it is to observe them ; and if Einstein’s theory suffers some inconsistency at some point in the universe, the best place to test it is in them ”, he comments. For this reason, the astronomer is enthusiastic about the project of being able to obtain videos of the black hole in our galaxy. The network of twenty radio telescopes also anticipates shots with much better definition than Messier 87. “To do this, we will capture the hole with images that we will then superimpose as if they were still frames; thus, we will generate the sensation of movement, the same as the cinema does. We will need many high-quality shots, because we want to take photos of the black hole every few minutes, ”he explains.
Such an initiative is not a whim. “It is transcendental, as it will show the black hole evolving before our eyes. The goal is to test Einstein’s theory of gravity on the light around black holes; and if we can contemplate in a film the movement of matter and how it moves around the black hole and at what speed it rotates, we will be able to put the aforementioned theory to the test in a completely new way ”.
The coordinator of the ngEHT project feels that “when you see the image of a black hole it is like having made a mental trip to it. Taking that trip is very important, but so is returning, telling and explaining to the world what has been seen. Years ago I made a prediction in a research article and stated that by 2020 we would have the first photo of a black hole. Now I foresee that by 2030 we will be able to have the first film ”.
