No. 111/2020

26 HUMBOLDT KOSMOS 111/2020 CLOSE UP ON RESEARCH press. Since then, a poster with selected title stories has adorned the wall in Zensus’ office in Bonn. “We were surprised by this tsunami of public interest,” he admits. “Something like this can only happen to a scientist once in a lifetime.” Even for a sober physicist, black holes are a fascinating phenomenon because they are, literally, attractive: due to their enormous gravitational force, matter is unable to escape their vicinities. Even light and other electromag- netic waves fail to get away, which is precisely why they are “black”. The theoretical concept of black holes had been haunting the universe for more than a century, as they were believed to be a consequence of the General Theory of Relativity. But although the physicist Karl Schwarzschild derived them from Einstein’s equations back in 1916, Ein- stein himself doubted their existence. While there is very little doubt amongst astrophysicists today that the cosmos is full of such bizarre objects, until spring last year they had only indirect evidence. But despite this, it had to be possible to visually prove the existence of black holes – this had been Anton Zen- sus’ creed for much of his scientific career. He had dis- covered his interest in space while studying physics in Münster – and it never left him. But it was only perse- verance and unrelenting work on scientific and techni- cal innovations that finally led to success. A FASCINATING SCIENTIFIC PHENOMENON It was already well known that visible light is unsuitable for imaging black holes. Only shortwave radiation can travel the cosmic distances between the object of study and the observer on Earth unhindered. So, the required anten- nae first had to be prepared to receive the millimetre waves and computers had to learn to generate a photo-like image from the measurement data. Nor could the measurements themselves be taken by one single radio observatory. Only by linking eight individual telescopes in Chile, Mexico, Hawaii, Arizona, Spain and the Antarctic was it possible to create a sufficiently large virtual radio telescope with enough magnification to capture the relatively tiny black hole. In the technical jargon used by the experts, this is known as Very Long Baseline Inferrometry. In particular, the 66 ALMA antennae (Atacama Large Millimeter Array) in Chile that were integrated in the EHT network were a crucial success factor. “International cooperation is indispensable for our research,” explains Zensus who has spent 16 years of his career in the United States – at Caltech, California, in New Mexico, and later in Charlottesville, Virginia. Here, at the headquarters of the National Radio Astronomy Observatory, he honed both the technical and organisa- tional skills that were conducive to later becoming the head of the EHT Consortium, since the network of radio anten- “­ SOMETHING LIKE THIS CAN ONLY HAPPEN TO A SCIENTIST ONCE IN A LIFETIME.” COVER STAR The first image of a black hole went around the world in 2019.