Reinhard Genzel

Interview

Telephone interview, October 2020

“We can sense the motions of stars which are orbiting the black hole with exquisite precision”

Telephone interview with Reinhard Genzel following the announcement of the 2020 Nobel Prize in Physics on 6 October 2020. The interviewer is Adam Smith, Chief Scientific Officer of Nobel Media.

In keeping with the times, Reinhard Genzel was in the middle of a virtual conference when he was surprised by the call announcing, “This is Stockholm!” In this interview, he briefly summarises his 40 year effort to image the galactic centre, which began with his joining his “second father”, 1964 Nobel Laureate in Physics Charles Townes, as a postdoc in Berkeley. Genzel describes the new technological developments that allow him to sense the gravity of the supermassive black hole by observing the motion of the stars orbiting it with exquisite precision, and looks ahead to possible future tests of the theory of general relativity in the decades to come.

Interview transcript

Adam Smith: Hello, am I speaking with Reinhard Genzel?

Reinhard Genzel: Yes, speaking.

AS: First of all, many congratulations on the award of the Nobel Prize.

RG: Well, thank you. It was completely unexpected and I’m … wow … I’m on cloud 17.

AS: How lovely. How did the news reach you? Where were you?

RG: I was in my office, in fact, working, as we do these days, in a virtual conference on hiring of another Max Planck Institute when I was called, and … and the almost stereotypic telephone call took place which I never would have thought I ever get, which is ‘this is Stockholm’.

AS: So they actually say ‘this is Stockholm calling’?

RG: Yes, yes, yes.

AS: This prize is a lovely marriage of experiment and theory, which is so much the way that physics proceeds, isn’t it?

RG: Oh yes. No, I think that’s a very strong theme in the black hole research, and if you think about more broadly in terms of gravity, I mean, here you know we celebrated a few years ago the theory of general relativity. In fact I was in Berlin for the ceremony. A hundred years later, research on gravity and black holes is at the very core … forefront of physics, in several domains, gravitational waves and so forth. It’s extremely exciting.

AS: And it took an enormous amount of effort to discover this black hole at the centre of our galaxy.

RG: Yeah, well you see this goes back in my case to another Nobel Laureate whose second father, or second son I feel I am, that’s Charles Townes, who got the prize in ‘64 for the laser and the maser, and then turned astronomer. And he in fact, after the discovery of the quasars in the ‘60s, felt that, okay, well maybe one should look and the galactic centre is so close but you can’t look at it in the visible. So Charlie then developed the first instrumentation to look at basically the doppler shift of gas, and I joined him as a postdoc and then a colleague in Berkley and so we started working on this problem back in the 1980s, so that’s 40 years ago. And yes it took a you know, a lot, a lot of patience, luck and always trying hard to get better and better and better.

AS: It’s a lovely illustration of the way that research hands from person to person, that it grows and it’s such a personal thing really.

RG: Yep, yeah. No, absolutely.

AS: When you image the black hole at the centre of the galaxy, what do you see?

RG: Well, I mean, we are sensing … I mean, what we’re doing, we’re using electromagnetic waves and mostly infrared range with the telescopes of the European Southern Observatory in Chile, and initially we used one of the big 8m telescopes, then you have to combat the Earth’s atmosphere, and make sure that the blurring of the images is removed. That’s called adaptive optics. That was the first phase of innovation in the late ‘90s. But that’s not sufficient to come very close to the galactic centre, so our most recent innovation has been to combine four of these telescopes, two of them cause an interferometer, so there’s four 8m telescopes. And with that, we can then sense the motions of stars which are orbiting the black hole with exquisite precision. We also see actually gas in the … very close in the accretion zone around the black hole. That’s about as close as you can get because any closer all material has to disappear in the black hole. So in that sense we are seeing not it, but we are seeing so to speak, we are sensing its gravity, and we are seeing, you know, gas and stars moving around it, and just by how the gas moves, how the stars move we can then infer with high precision what it is and that there must be a black hole. And also in fact that general relativity holds even in this super strong regime of curvature.

AS: Yes, sensing its impact on the surroundings. And it’s extremely active just at the moment, isn’t it?

RG: It’s a little more active than it was, but on the scale of active black holes it is a dwarf, it doesn’t do much. I mean it’s under-luminous compared to what it could be. Let’s be glad that this is so because indeed if super-massive black holes get fed at the maximum rate they can destroy their own galaxy systems, and they do. They have, in particular in the past. So that’s why massive black holes turn out to be such an important regulator, if you like, in the ecosystems of galaxies.

AS: And I suppose one point to make is that this takes us back to Einstein and how extraordinary it was that general relativity so precisely defines so much around us.

RG: Absolutely, absolutely, it’s incredible. But, you know … All of physics of course suspects it must be wrong somewhere, you know certainly on the smaller scales – now those scales we cannot reach. But at least another option would have been that it is mass dependent, and that we have now tested, and so you know we can be sure that theory is right. Now to really be sure that what is called a parametric is correct to the innermost region of waves and trace matter we still don’t know that. And gravitational waves haven’t done that either. So the story will go on experimentally, I would say, for at least another one or two decades. I predict that the most likely final triumph will be a space mission, with gravitational waves seeing an in spiral a solar mass star into a black hole. That probably would clinch the case. Unfortunately I’m afraid I won’t be there. The mission is being studied by the European Space Agency for some time now and is very expensive; very, very difficult mission. There will be collaboration with NASA too, but I assume that the Europeans will lead, and the most likely slot for launch will be in the late ‘30s I would expect.

AS: It’s been a huge pleasure speaking to you. Thank you very much indeed. I guess you’re about to have a total onslaught of the press and well-wishers. How do you feel about that?

RG: Well, okay, we’ll see how we do on that. I know, I guess I’ll try to do my best, but it’s a great distinction. It’s just wonderful.

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To cite this section
MLA style: Reinhard Genzel – Interview. NobelPrize.org. Nobel Media AB 2020. Wed. 28 Oct 2020. <https://www.nobelprize.org/prizes/physics/2020/genzel/interview/>

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