German astrophysicist Reinhard Genzel was just awarded the Nobel Prize in physics. In an interview, he talks about how he discovered the black hole at the center of the Milky Way.
DER SPIEGEL: Professor Genzel, congratulations on receiving the Nobel Prize in physics. What were you doing when you received the call from Stockholm?
Genzel: The same thing that we scientists have been doing all day everyday for the last six months: Zoom, Zoom, Zoom. I was sitting with 25 other people in the Zoom conference of a virtual committee belonging to the Max Planck Society.
DER SPIEGEL: And your telephone rang right in the middle of the conference?
Genzel: It was almost funny. I was sitting in front of the screen, knowing that I would have to spend the next six hours doing the same. Then the phone rang, and somebody said: “This is Stockholm.” Then, the call started breaking up. It took some time before I could hear the “sekreterare” again. During that time, I went over to the window and started thinking: “This damned pandemic. Now I’ve started hallucinating.”
DER SPIEGEL: It’s a Tuesday in early October, shortly after 11 a.m. – time for the Nobel Prize in physics to be announced. And you want us to believe that you didn’t know who was on the line?
Genzel: No. You have to believe me. This year, I wasn’t thinking about it at all. In previous years I was, like in 2011. We had come a long way with our measurements that year and I thought to myself: It could really be our turn. I’m pretty sure that we were close that time.
DER SPIEGEL: And after the call, you returned jubilantly to your Zoom call to tell everyone?
Genzel: No, no, everything goes according to a strict protocol. Twenty minutes pass between informing the winner and announcing the prize. During that time, you’re not allowed to tell anyone. They really insist on that.
DER SPIEGEL: So you acted as if nothing had happened?
Genzel: Well, I didn’t exactly do that either. I said to the committee chairman, a vice president of the Max Planck Society: “Mr. Blaum, I have to take care of something. Maybe you should turn on the television in about a quarter of an hour.”
DER SPIEGEL: Why is it that you were hoping for a Nobel Prize in 2011 but weren’t expecting one in 2020?
Genzel: There are several reasons for that. For one, I was out of the running in a sense since I had received the Crafoord Prize eight years ago. For Sweden, that is the equivalent to the Nobel Prize for fields of research that don’t fit into the Nobel categories – things like mathematics, earth sciences and my field, astronomy. Accordingly, I didn’t think I had a chance any longer, and certainly not this year.
DER SPIEGEL: What is different about this year?
Genzel: If you look at the Nobel Prize for physics over the last five years, you’ll see that they have been awarded for neutrinos, gravitational waves, cosmology and exoplanets. Was it to be astrophysics again? You can perhaps imagine that people in other fields of physics might start grumbling.
DER SPIEGEL: We are apparently living in the golden age of astronomy.
Genzel: Absolutely. It is unbelievable all that is going on at the moment. And it will continue. Take just exoplanets, for example. We are currently experiencing a downright explosion in knowledge. And GRAVITY, our interferometer at the Very Large Telescope (VLT) in Chile is part of this explosion. We have just recently measured the atmospheres of some exoplanets. We are on the brink of being able to practice astrochemistry on planets outside our Solar System.
DER SPIEGEL: There is another reason why people may have been surprised that you were awarded the Nobel Prize. Your discovery of the black hole in the middle of the Milky Way was overshadowed by the spectacular image of a black hole published by the Event Horizon Telescope team last year. Why were they not rewarded?
Genzel: It was good that their image received a lot of attention. It is important to get people excited about research. And astronomy has a special role to play.
DER SPIEGEL: Are you trying to say that the image was good for attracting an audience, but wasn’t all that important from a scientific point of view?
Genzel: No, I wouldn’t say that. It is true, though, that such a beautiful, orange picture is enticing, even if it can’t be clearly interpreted. An open discussion is still ongoing among experts: Are we really sure of what we are looking at in this picture?
DER SPIEGEL: Wherever black holes are discussed, that picture is shown. And you are now telling us that we don’t really even know what it is?
Genzel: Exactly. It could be that we are looking at the shadow of a black hole, as it is commonly portrayed. But it could also be the outer wall of a jet that is coming directly at us at the speed of light. To know for sure, we need additional measurements. But we have a problem at the moment: the corona pandemic. Most Earth-based telescopes have been switched off.
DER SPIEGEL: Tell us a little bit about your research. What is the importance of a black hole at the center of the Milky Way?
Genzel: We are talking here about a supermassive black hole around which gravity is particularly strong. Most interesting, of course, would be to take measurements from inside of it. That, though, isn’t possible. There is a natural limit: the event horizon. Our goal is thus to creep as close as we can to this limit in an extreme environment where everything is moving at around half the speed of light, a place where the tidal forces of gravitation are so strong that they tear everything apart. By studying such black holes increasingly precisely, we are gaining a better and better understanding as to why our Milky Way is a rotating spiral galaxy while other galaxies have the form of an ellipse. Because black holes play a decisive role in that difference.
DER SPIEGEL: When did you become interested in supermassive black holes?
Genzel: At U.C. Berkeley back in the 1980s. At the time, everyone was trying to figure out what quasars are, though curious objects in space that emit enormous amounts of energy. The theoreticians said: It can’t be explained with nuclear fusion like in stars. But it could be explained with massive black holes into which a lot of material is falling. According to that explanation, quasars were extremely well-fed black holes. The question, though, was: How can we prove it? The idea quickly emerged to study the effect of gravitation on objects orbiting the black hole. But it was also clear: quasars are much too far away to be able to calculate the individual orbits of stars. So we had to get much closer to our objects of desire. And everybody, of course, thought about the center of our own galaxy.
DER SPIEGEL: When you began taking your measurements, it wasn’t even clear yet that there was a black hole in the center of the galaxy. How possible did people think it was back then?
Genzel: I would say that a third of practicing astronomers thought it was plausible. A second third would have said: I’m not interested. And the final third vehemently rejected the idea.
DER SPIEGEL: You yourself seemed to have little doubt. We reported on your research for the first time in 1992. You said: “It is difficult to interpret the measurements in any other way than through the existence of a black hole.” That means that in the almost 30 years since then, you have simply continually reaffirmed what you already considered likely?
Genzel: Exactly, just that our measurements today are a million times better than they were back then.
DER SPIEGEL: Nobody is as familiar with the center of our galaxy as you are. What does it look like?
Genzel: Bright. If you were to travel to the center of our galaxy, you would be surprised by how bright it is there. The concentration of stars, relative to our neighborhood, a million times higher. And there aren’t just a lot of them, the stars are gigantic. In short, the stars in the sky would be dramatic.
DER SPIEGEL: Could one even live in such a region? Would we have the possibility of finding shelter on our journey?
“I embarked on a pilgrimage to her in California, where she berated me for a full day.”
Genzel: It’s a good question, and I don’t have an answer for you. I would guess that your chances would be rather poor because the gravity from the giant stars and the black hole are sufficient to pull apart all planets in alternating fly-bys. But we don’t know for sure.
DER SPIEGEL: There is another person on Earth who might make for a good guide on such a trip: Andrea Ghez, with whom you are sharing the Nobel Prize. Your relationship doesn’t appear to be completely free of friction.
Genzel: We have consistently been in competition with each other over the years. And early on, this competition was immensely profitable for all of us. We started our measurements in the early 1990s and Andrea joined us in 1995. But she enjoyed the advantage of having access to the Keck in Hawaii, a telescope 10 meters in diameter. At the time, we were measuring the speed of stars near the center of the galaxy and we both arrived at the same result. When two groups reach the same results independent of each other, the scientific community says: We believe it. The competition was incredibly helpful in earning respect.
DER SPIEGEL: It became more difficult later on?
Genzel: Yes. The goal was measuring the path of a star in the center of the galaxy for the first time. We were both lucky that there was this one star that needed just 16 years to race around the galactic center at a speed of 7,000 kilometers per second. It was dramatic and unexpected: On one day, it would go in one direction, two months later a completely different one and another two months later it would be different again.
DER SPIEGEL: And where was the problem?
Genzel: As chance would have it, we moved that year to the VLT in Chile, the large European telescope. We had a lot of observation time and were thus able to achieve early results. We published immediately, as fast as possible, which upset Andrea. At a conference at the time, she went so far as to say that we had fabricated our results. She said we couldn’t have collected such data because we didn’t have the telescope to do so – until someone whispered to her: “They are no longer working with a three-and-a-half-meter telescope.” “I didn’t know that,” she responded, adding that it was unfair that nobody had informed her previously. She was extremely angry with me as a result. Even if I didn’t feel I had done anything wrong, I had to accept that such was her view of the situation. So I told myself: Time to swallow my pride. I embarked on a pilgrimage to her in California, where she berated me for a full day.
DER SPIEGEL: And? Did you make peace?
Genzel: Well, I promised her from then on, I would tell her ahead of time what we were doing. It was simply the case that we were always ahead. We were technically further along, we had more telescope time and we had a larger group. As such, we always had the results first. Then, at some point, the aforementioned star was again approaching the center of the galaxy and we both knew: Things are about to get exciting. Just that we were in a much better position. Because we now had GRAVITY. It was clear that if we were able from a technical point of view to get our instruments up and running, we would have 20-times better resolution. So I wrote her: “You know that we have GRAVITY. Wouldn’t it be better for all of us if we would work together?” But she declined.
DER SPIEGEL: You claim that you were ahead all these years. Why are you both getting the Nobel Prize now?
Genzel: Interestingly, I received the 2008 Shaw Prize alone. In the first phase, we were ahead. The fact that we both received the Crafoord Prize was completely justified. We were dead even. I think that if the current prize had come a bit later, the effect of GRAVITY would have been even larger. It is really an unbelievable instrument that will continue to play a huge role in astronomy down the road. On the other hand, I am personally convinced that she is the reason why we have been awarded this prize at all.
DER SPIEGEL: You are referring to the share of women among Nobel Prize recipients?
Genzel: Perhaps you recall the criticism on Monday when the Nobel Prize in medicine was announced: Again three old, white men, people said. I understand. I know it myself as a member of the Shaw Prize committee. We are under immense pressure. And in some cases, the problem is that in order to respond to the gender imbalance, we choose women for the selection committee. But by doing so, we lose half of the best women candidates.
DER SPIEGEL: The prizes have been announced, but they haven’t yet been presented. What is the situation in this year marked by COVID-19. Are you planning on traveling to Stockholm?
Genzel: No, there won’t be a presentation. They told me they are planning to make up for it next year. I have also heard rumors that we will be able to pick up our diplomas and plaques from the ambassador.
DER SPIEGEL: Professor Genzel, we thank you for this interview