“An astounding coincidence with theory”

Max Planck Director Anton Zensus on the first observation of the shadow of a black hole

What sounds paradoxical is reality: Black holes have a shadow! In the Messier 87 galaxy, astronomers were able to observe such a phenomenon for the first time with the Event Horizon Telescope (EHT). The Max Planck Institute for Radio Astronomy in Bonn had a first row seat. Anton Zensus heads the “Very-Long-Baseline Interferometry” department. The Department deals with a technique that enabled the discovery in the first place. We talked to Anton Zensus, Chairman of the EHT Collaboration Council, about how the successful observation came about and what the results mean.

"We have now reached the peak of a long development."  Anton Zensus, Director at the Max Planck Institute for Radio Astronomy and Chairman of the EHT Collaboration Council.

Prof. Zensus, how long has the Event Horizon Telescope project been running?

Officially, it started two years ago. But the preparations have been going on for over a decade. And if you factor in the preparatory and pioneering work, it has been about 20 years. During this time, we have fundamentally improved the quality of our measurements and we have already investigated important questions about active galaxies like M87 – such as the nature of the gigantic matter jets from their central regions. In this sense, we have now reached the peak of a long development.

You say that EHT had only been conducting observations for two years. Were you surprised by your success after this relatively short time?

Yes indeed! It was also astonishing that so many things worked right away. After all, the Event Horizon Telescope consisted in 2017 of a combination of eight different telescopes. One of these telescopes – which is called ALMA – is located at an altitude of 5000 m in the Chilean Atacama Desert and comprises 66 individual antennas. In order to be able to integrate this system into the EHT, we had to interconnect all individual antennas via software. This "phasing" was an enormous technical challenge for us and essential for the EHT. The weather conditions also played into our hands – they were quite good right from the start.

What does an observation with eight telescopes look like?

The key word is very long baseline interferometry (VLBI). We aim several radio telescopes, which are far away from each other, at the same celestial object at the same time. The signals collected are combined in a special computer – the correlator. In this way, a virtual telescope is created. This provides an image sharpness corresponding to that of a single antenna with the diameter of the distance between the most distant antennas – in the case of the Event Horizon Telescope, this is about 8000 km. Just imagine: if your eyes were as sharp as the EHT and ignoring for a moment that the Earth is curved, you could theoretically read a newspaper in New York from Bonn. However, the EHT does not see any optical light but rather radio radiation with wavelengths of just over one millimetre.

How is your Institute involved in the Event Horizon Telescope?

At the supercomputer: scientists use the so-called correlator at the Max Planck Institute for Radio Astronomy to evaluate the data of the Event Horizon Telescope. Laura La Porta and Hermann Sturm are sitting in front of the screens.

The EHT includes the 12-metre APEX telescope, which is operated by our Max Planck Institute for Radio Astronomy together with the European Southern Observatory and the Swedish Onsala Space Observatory. It is located close to the ALMA site. The Max-Planck-Gesellschaft is also involved with the IRAM 30-metre antenna on the Pico Veleta in the Spanish Sierra Nevada and with the NOEMA telescope near Grenoble, which will participate in future observations. A total of thirteen partner organizations from all over the world work for EHT. Our Institute operates a supercomputer that calibrates and evaluates the data. In fact, enormous amounts of data are generated. Each of the EHT’s individual telescopes deliver up to 350 terabytes per day.

You knew what you were looking for: the shadow of a black hole. I suppose theoretical considerations played an important role?

Yes, Einstein’s general theory of relativity of 1915 provides the theoretical background. Also about a hundred years ago, astronomers observed jets for the first time. These are gas flows that shoot out of the centre of active galaxies and must be generated at enormously high energies. Since the 1970s, we have suspected that there are super-massive black holes behind it. The theory of relativity predicts that a massive object can deflect light. The English astronomer Arthur Eddington measured this phenomenon during a total solar eclipse when he observed a small shift of the star positions near the solar disk. By the way, that was on 29 May 1919 – exactly one hundred years ago. This completes the circle.

The black hole in the centre of the Milky Way is much closer than that in Messier 87. Why was the EHT still successful with M87?

Our Milky Way is a bit hesitant to disclose its last secrets (laughs). But joking aside, there are, of course, sound reasons. On one hand, the heart of our Milky Way is hidden in a dense fog of charged particles. This leads to a flickering of the radio radiation and thus to blurred images of the centre of the Milky Way, which makes the measurements more difficult. But I am confident that we will ultimately overcome this difficulty. On the other hand, M87 is about 2000 times further away. However, the black hole in its centre is also about 1000 times more massive than the one in our Milky Way. The greater mass makes up for the greater distance. The shadow of the black hole in M87 therefore appears to us to be about half the size of the one from the gravity trap in our Milky Way.

How can one picture the shadow of a black hole?

A black hole deflects the light even more than our sun. The theory of relativity predicts that a radiation ring should be observed around a dark spot where the black hole is located. Some casually refer to this dark spot as the shadow of the black hole.

But where does the light come from? Black holes are black, aren’t they?

Behind the horizon: This illustration shows a rotating black hole. The ergosphere is the area in which any object must rotate. The event horizon is something like the surface of the black hole; what disappears behind it is literally out of the world.

According to the general theory of relativity, black holes have an “event horizon”. It describes the region within which nothing can escape from the black hole. The event horizon – but also the area within – should therefore appear black to us. According to theory, outside the event horizon, attracted by the enormous mass, there is a huge amount of gas swirling around in a vortex-like disk structure at tremendous speeds. The gas heats up and begins to glow. Relativistic particles – those that move at almost the speed of light in a magnetic field – also release synchrotron radiation. So around a black hole, it “shines”, while the hole itself – as the name suggests – appears black. We’ve observed this blackness.

What did you discern from the shadow?

To be honest, we were amazed at how well the dark spot we observed matches the structure predicted by our computer simulations. From the shadow itself, the mass, the rotation, and the magnetic field of the black hole can be derived. For this purpose, 60,000 different simulations of black holes were carried out on the computer and compared with the EHT results.

How will this successful observation advance astronomy?

We are at the beginning of a phase in which many new insights await us. We will soon be able to confidently exclude alternative explanations for black holes – such as boson stars or gravastars. We will better understand the galactic centres and obtain a complete picture of the formation and evolution of active galaxies. We will also be able to observe pulsars in the vicinity of the black hole in our Milky Way and thus thoroughly check the general theory of relativity. Because black holes are an ideal laboratory for measurements under strong gravity.

The interview was conducted by Helmut Hornung

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