Read out Astrophysicists prove: Black holes cast shadows. So far their existence is only indirectly proven - by the influence of their gravity on their environment. But new computer simulations show how the gravity traps can be visualized: Radio telescopes could already observe their shadow in the near future. Astrophysicists have photographed a shadow of the invisible: the border of a black hole. The picture is ghostly: in the midst of a gloomy glow, there is a circular zone of darkness. Mysteriously, the gullet bares into the unknown towards the viewer. The black hole weighs three million times our Sun and is located about 26, 000 light years away - in the center of the Milky Way.
Astronomers from the Max Planck Institute for Radio Astronomy in Bonn and the Max Planck Institute for Extraterrestrial Physics in Garching have already proven the mysterious gravitational trap a few years ago. But they succeeded only indirectly: The center of gravity betrayed itself by the movement of the stars, which revolve around it. The new photo, on the other hand, shows the outer boundary of the black hole itself - its event horizon. He is a place of no return. Anything that gets behind the event horizon can not escape the voracious maelstrom. Even light is too slow for that. But photons outside of the event horizon still manage to leave the gravitational pull and reach us in crooked ways. In the midst of this radiation cloud, the shadow of the black hole is circular in the photo. Because the black hole acts on itself as a gravitational lens and adventurously deforms the path of light, its event horizon appears to be magnified fivefold.
However, the photo is still a dream of the future - science fiction in the best sense. Fiction, because it exists so far only as a computer simulation. But also science, because this simulation is based on hard science - on Einstein's General Theory of Relativity. Heino Falcke of the Max Planck Institute for Radio Astronomy, Eric Agol of the Johns Hopkins University in Baltimore, Maryland, and Fulvio Melia of the University of Arizona in Tucson have used a "ray-tracing program" to calculate the confused paths of photons through the room curved by the black hole. "You follow the path of every photon that is emitted near the black hole back to the observer, " explains Fulvio Melia. "The program then calculates the effect that the black hole has on the trajectory and wavelength of the photons." The result: the event horizon is like a shadow. "This term describes very well what you see there, " explains Heino Falcke.
The astronomical observation technique could soon take a true picture of the shadow of a black hole. The trick: radio telescopes are connected all over the world to form a super telescope with many thousands of kilometers of base length. Astronomers speak of VLBI - Very Long Baseline Interferometry. Accordingly sharp are the pictures. "With today's resolution, we could already see a radio source the size of a mustard seed from Bonn in the distance from Los Angeles, " says Falcke. "Now we want to go a step further and discover a hole in this mustard seed." The computer calculations show, however, that with short radio waves, a shadow of the black hole may already be guessed. "At the current 1.3 millimeter wavelength of the VLBI, we probably have no luck. But at 0.8 millimeters, I see a real opportunity, "Falcke is convinced. If sufficient observation time is approved - which is by no means self-evident in the face of fierce competition in cutting-edge astronomical research - he hopes for a positive result as early as the decade. The astronomers at the Max Planck Institute for Radio Astronomy are already working on making VLBI possible at shorter wavelengths.

=== Rüdiger Vaas

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