The answer: one records not the invisible object itself, but its immediate surroundings - and thus makes it visible as a dark center in a luminous ring. This coup has now been achieved for the second time by an international team of researchers - this time with the black hole at the center of our home galaxy, the Milky Way.

"What could be cooler than seeing the black hole at the center of our own Milky Way?" said Katie Bouman, a computer scientist from the California Institute of Technology who was involved in the research, at a press conference in the United States.

"We have reached the next level," said Anton Zensus of the Max Planck Institute for Radio Astronomy (MPIfR) in Bonn, one of the main initiators of the EHT project. "I am proud of our entire worldwide team." For the image, he said, the world's largest radio telescopes were combined into a single camera the size of Earth. Vincent Fish of the MIT Haystack Observatory in the U.S. said the telescopes had collected about three and a half petabytes of data - the equivalent of about 100 million TikTok videos. "It's way too much data to stream over the internet. We actually have to send hard drives around.”

A comparison with computer models shows, among other things, that the black hole is rotating, the scientists report in a special issue of the journal “Astrophysical Journal Letters”. Eight radio telescopes on four continents were linked together to make the image. Together, they form the Event Horizon Telescope (EHT). Scientists call the boundary around a black hole the event horizon, because nothing, not even light, can escape from the area behind it.

The data from the telescopes are combined with special supercomputers to create a gigantic virtual telescope the diameter of the Earth. As researchers involved in the project once explained, it has a level of detail that would allow an orange to be identified on Earth from the moon. Or read a newspaper in New York from Berlin.

The international ALMA observatory in Chile, consisting of 66 individual antennas, was involved in the measurements. Also involved was the Institute for Radio Astronomy in the Millimeter Range (IRAM), operated by a German-French-Spanish collaboration, which works with the 30-meter telescope in Spain and the NOEMA interferometer in France.

After years of preliminary work, the EHT researchers - a total of about 80 institutes with 300 scientists involved - had been able to make the first observations with the telescope network in 2017. After the complicated analysis of the data, the team presented the first photo of a black hole - or more precisely: its immediate surroundings - in 2019. The image shows a glowing ring around the supermassive black hole at the center of the galaxy M87, which is about 55 million light-years away. The mass of the black hole is enormous: It is equivalent to the mass of 6.5 billion suns.

But in April 2017, the researchers had pointed the EHT's many radio antennas not only at this distant galaxy, but also at the Milky Way's center, which is much closer at 27,000 light-years and also contains a massive black hole. But even though this object, called Sagittarius A*, is much closer to Earth, analyzing the observational data proved far more difficult. "The radiation from the black hole of M87 is constant over hours," explained Anton Zensus of the MPIfR in Bonn. "The object in the galactic center, on the other hand, changes over the course of just a few minutes. We therefore had to develop completely new methods for the evaluation."

The gas near the black holes moves almost as fast as light in both cases, explained EHT scientist Chi-kwan Chan of the Steward Observatory in the United States. To circle the much larger black hole in M87, however, it takes days to weeks - for the much smaller black hole in the Milky Way, on the other hand, only a few minutes. Brightness and patterns in the surrounding area change accordingly fast. It is "a bit like trying to take a sharp picture of a puppy chasing its tail fast.”

Five years after the observations, astronomers can finally present the result - the first photo of the black hole in the center of the Milky Way. As with M87, it shows a luminous ring around a dark core. The researchers refer to this dark area as the "shadow" of the black hole - it is about twice the size of the actual event horizon, because the light is deflected around the black hole by its strong gravitational pull, and thus both the front and back sides of the object can be seen.

The luminous ring is heated gas swirling around the black hole, the so-called accretion disk. Gravity also forces the radiation emanating from this gas onto curved paths, providing a distorted view of the black hole's surroundings.

Using computer models, the scientists compared their observations with the predictions of Albert Einstein's theory of general relativity about black holes: according to the results, the photo obtained is very consistent with the expected distortion for a black hole with four million times the mass of the Sun. "We were amazed at how well the size of the ring matched the predictions of Einstein's general theory of relativity," said EHT scientist Geoffrey Bower of the Institute of Astronomy and Astrophysics at Academia Sinica in Taipei.

This value, in turn, is very consistent with previous measurements based on the motion of stars around the black hole. And close comparison with different models allows even further conclusions. "The models that fit best are those that assume rotation of the black hole," says Karl Schuster of the Institute of Millimeter-Wave Radio Astronomy in France. "Moreover, the black hole's rotation axis seems to be more or less tilted toward Earth," he adds. This is unusual, he said, because it does not coincide with the axis of rotation of the Milky Way.

In the Milky Way, our Sun orbits Sagittarius A* together with 200 or 300 billion other stars and clouds of gas and dust. For the EHT researchers, the photo of the galactic center is a great success, but nonetheless only a first step. "We have images of two black holes - one at the top and one at the bottom of the supermassive black holes in the universe - so we can make much more progress than ever before in studying the behavior of gravity in these extreme environments," said EHT scientist Keiichi Asada of Academia Sinica in Taipei.

For the future, Zensus, a researcher from Bonn, hopes to expand the EHT network, if possible by adding antennas in space. This would make it possible to obtain images with considerably higher resolution and, it is hoped, to gain completely new insights into the physical processes in the immediate vicinity of supermassive black holes. Eleven observatories were already involved in a major EHT campaign in March 2022. The results are eagerly awaited.