How did astronomers photograph a black hole?

First photo of a black hole

Because of the great attraction of black holes, even light cannot escape them. That makes direct photography impossible. But with the Event Horizon Telescope - an amalgamation of eight radio telescopes - photography of a black hole has now been successful for the first time. As the scientists involved reported today at six press conferences at the same time, they have now succeeded in capturing the shadow of a black hole for the first time. This shadow is created by the radiation of the distorted light as it disappears irrevocably in the black hole.

The black hole shown lies in the center of the galaxy Messier 87, which is around 55 million light years from Earth and is located in the Virgo Galaxy Cluster. As the historical recordings show, this black hole has a radius of almost 20 billion kilometers. That is roughly three times the distance from the sun to Neptune, the outermost planet in our solar system. From the galaxy core of Messier 87 a strongly bundled beam of matter shoots at high speed, which reaches out at least 5000 light years into space.

The image now taken shows hot matter that has gathered in a ring around the supermassive black hole. In fact, from our point of view, much of the luminous matter is actually behind the black hole. Because these super-massive objects bend space-time strongly and thus also deflect light. Using the curvature of light, it was also possible to estimate the mass of the black hole: At 6.5 billion solar masses, it is more than a thousand times heavier than the black hole in the center of the Milky Way.

Only the merger of eight radio telescopes such as ALMA and APEX in Chile as well as observatories in Europe, Hawaii and even at the South Pole made the recording possible. The more than 200 participating astronomers from 13 institutes combined the signals from all eight telescopes and precisely coordinated them with one another using atomic clocks. In this way, a virtual telescope was created, the resolution of which corresponds to a magnification of two million times - sufficient to locate a tennis ball on the moon.

In order to filter out the signals from the investigated black hole from the background noise, the researchers had to analyze enormous amounts of data. This data was not transmitted via the Internet, but by post: entire stacks of hard drives reached the data centers of the Max Planck Institute for Radio Astronomy in Bonn and the Haystack Observatory in Westford, Massachusetts. The researchers at the radio telescope at the South Pole even had to wait months until winter was over and planes could land again.

The observations do not show any unexpected behavior: the results so far agree with the predictions of Albert Einstein's general theory of relativity. In the coming years, however, further tests will take place when additional telescopes are added to the Event Horizon Telescope and longer observation phases become possible. In addition, the researchers want to investigate Sagittarius A * in more detail - the black hole in the center of the Milky Way. Due to its smaller size, the matter rotates much faster there, so that recordings are much more demanding.