‘Astronomy is like a walk in the woods’ says Reinhard Genzel, winner of the Nobel Prize in Physics

“Every day you go into the jungle and admire it. You will discover new trees, shrubs and flowers in all kinds of colors. Record all discoveries neatly in your booklet. That is what astronomy is to a great extent: admiring and taking stock. It’s the reason I went into astronomy, the forest is so beautiful! I didn’t want to miss that,” said Reinhard Genzel.

The proverbial trees and flowers in Genzel’s life are interstellar gas clouds, stars and black holes. In the latter categories, there is one example that has defined its scientific career for some thirty years: the massive black hole at the center of the Milky Way, or Sagittarius A*.

Astronomer and Nobel Prize winner Reinhard Genzel of the Max Planck Institute for Extraterrestrial Physics near Munich.

NEMO Kennislink speaks to Genzel in his office at the Max Planck Institute for Extraterrestrial Physics near Munich. By video connection, because corona still dictates travel restrictions. When the connection is established, I can see Genzel sitting in his office from a distance, an unusual perspective for an online meeting. Fluttering blinds in the background, an empty conference table in the foreground and in between a desk filled with gems and fossils, behind which the brand new Nobel Prize winner sits. He proudly shows an ammonite, a fossil of a squid that died out tens of millions of years ago. Fossils on an astronomer’s desk? The work of a paleontologist is not fundamentally different from that of an astronomer, for the same reason mentioned earlier, according to Genzel.

He plays with the remote-controlled camera and slowly zooms in on himself. This isn’t the only camera he has control over. As an astronomer, he set one of the largest eyes in the world, the Very Large Telescope (VLT) in Chile, at the heart of the Milky Way. Since 1992, he and his colleagues have been monitoring the movements of the stars there, taking a few new photos every year. It turned out to be Nobel Prize material. Genzel received the prize last year in the Physics category together with American astronomer Andrea Ghez and British mathematician and physicist Roger Penrose.

The stars that kept Genzel and Ghez in the eye independently of each other did something extraordinary: they orbited a seemingly empty point in space at lightning speed. A video shows that a star named S2 is a remarkably complete circle in about sixteen years. What could make a star fly so ‘out of a corner’? The only plausible explanation was the presence of a black hole with a mass about four million times that of our sun. For many, this was proof that there is a black hole there: Sagittarius A*.

Movements of stars around the location of the black hole Sagittarius A*. The video covers a period of about twenty years.

As early as the 1930s, a strong radio source was found in the heart of the Milky Way. In the 1980s, gas clouds were found to be moving at tremendous speeds in the area. These were all pieces of evidence for the presence of a black hole. Evidence of massive black holes has also been found in other galaxies. Why wasn’t everyone convinced yet?

“In the 1990s, it was clear to many astronomers. My supervisor (Nobel laureate Charles Townes, who took measurements of Sagittarius A* – red.) was one of them. But the physicists wanted more evidence. A bright radio source is not yet a black hole and gas can theoretically also move by forces other than gravity, for example in a strong magnetic field or by stars that spew it out. Moreover, the region was too far away to make a convincing picture of it.”

One of the telescopes of the Very Large Telescope in Chile.

ESO / Y. Beletsky

Then you decided to set up the world’s most powerful telescope on it.

“We have built an instrument (NAOS-CONICA – red.) that combines the light from the four telescopes of the VLT in Chile, each with an eight-meter mirror. This instrument played a crucial role in the Nobel work. With that we also obtained so much observation time on a large telescope (about fifty nights from the start of the observations – red.). That was only possible because we donated our instrument to ESO, the organization that manages the telescope. Ultimately, other astronomers also benefit from it: they are allowed to use such an instrument. That’s the trick to getting so much time. “

“But nature also lent a hand. For example by ‘giving’ us stars that revolve so close to the black hole. That was in a way unexpected. I think every theoretical astronomer would have said beforehand that the experiment would fail. It was thought that there would be no stars in that area.”

Are all scientists now convinced that there is a black hole there?

“I’d say yes, now that we’ve been awarded the Nobel Prize in Physics. But look closely at the formulation of the Nobel Prize Committee: They say we win the prize for the discovery of a “super massive object.” So they don’t say it’s a black hole! When I spoke to the secretary of the committee, he did say that it is a black hole, and he also said this during public presentations. But it is not written.”

And why is that?

“There are still open questions. We have now proved that the spacetime around Sagittarius A* behaves as the general theory of relativity predicts for such a massive object. To really know for sure that it is a black hole, we have to determine the rotation of the thing. If we can do that, the physicists are also completely satisfied. We can then show that it is to the so-called no-hair theorem suffices: that it can be fully described with only three parameters: the mass, the charge and the rotation.”

“We are currently working on an experiment that can prove this: using a larger telescope to look at faint stars that are even closer to the black hole. Other research groups are trying to prove that by looking at gravitational waves. I think this evidence will be there in 10 to 20 years.”

An image of the black hole in the galaxy M87. It was made by radio telescopes from the Event Horizon Telescope.

But if those moving stars aren’t convincing enough, then that picture of the black hole in the neighboring galaxy M87 is, right?

“Here too you can say that a ring with a dark hole in the center can be many things in the astronomical forest. There are stars with a huge bright ring of matter around them where you can’t see the star itself. So it doesn’t necessarily have to be a black hole.”

“Okay, that’s a lame answer. A better answer is that the Event Horizon Telescope team has not yet shown that the size of the central ‘shadow’ exactly matches what the theory of relativity predicts. You can do that with the black hole in our own Milky Way, of which we have an accurate estimate of the mass. They are working hard on a picture of this black hole. I am hopeful that she will succeed, that would have great persuasiveness – also for the physics community.”

“Apart from that, it’s a great result. If you are not convinced of the usefulness of basic research, then this photo shows that it is at least very beautiful.”

The physicists in particular are therefore not yet completely convinced. How do they differ from astronomers?

“They have different roles to play in science. The astronomer notes everything he observes in the astronomical forest, the physicist then wonders why the blue flowers are always on the left side of the path.”

“I myself am half astronomer, half physicist. No wait, I’m sixty percent physicist and 40 percent astronomer. As a physicist I rolled into astronomy. In the Townes group I was a member of, a lot of experimental work was done. We really looked at the physics side of things.”

Do we understand black holes now?

“The concept of a black hole is not even that difficult. If you understand that light has a certain speed and is also influenced by gravity, you quickly come to the conclusion that there may be places from which no light can come, a black hole.”

“And yet: everything ends in the center of a black hole in a point that is infinitely small and has an infinitely high density, a so-called singularity. How is that possible!? Gravity at play here is still one of the most researched areas in science. We try to look beyond that event horizon in all sorts of ways. Can we perhaps catch a glimpse of that singularity? Could signals still escape from a black hole, such as the Hawking radiation? I cannot tell you whether this research will lead to satisfactory answers. It has taken us a hundred years to prove that black holes exist at all.”

You are now almost seventy years old. What scientific question would you like to see answered in your life?

“How life originated and whether that is also possible under other circumstances than on Earth. That’s a fundamental question that I find extremely interesting. It is also a question of where a breakthrough is possible in the coming years. There are more than five thousand known exoplanets, planets that orbit stars other than the sun. Scientists are now slowly starting to investigate the atmospheres of some of those planets, including with the machine we developed for our research on the black hole. Over the next twenty years, this research will accelerate with telescopes such as the James Webb Space Telescope and the Extremely Large Telescope. The question is whether we can identify life from the outside in this way. What conditions exist on such a planet? Is energy produced that comes from life?”

If you could start a new career right now, would you start in this field?

“No, I was going to be a neuroscientist! haha. That’s what my daughters do. I find the experiments they are doing in the field of memory and cognition fascinating. And I believe in what my mentor Townes said, you have to switch fields when too many people come in. That is certainly true for astronomy, the number of astronomers has grown considerably in recent decades. You can say that the field is successful, but it also means that you have to work in huge consortia of perhaps hundreds of people. That’s not really my thing. The path that I myself have had the pleasure and privilege of walking is not so easy anymore.”


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