How Do You Detect A Black Hole? Interview with Reinhard Genzel (2008 Shaw Prize & 2020 Nobel Prize)

Hello maths fans. I'm here in Hong Kong at the Hong Kong laurate forum and I'm delighted today to be joined by Professor Reinhardt Gendzel who is a physicist and was awarded the Shaw Prize and the Nobel Prize plus many other prizes. Um, and I'd like to kick us off by asking what to me I think is uh just a crazy question we could even ask. How do you measure and detect a black hole? Yeah. Well, the key characteristic of a black hole is gravity. Okay. Uh it's certainly at some level at its surface or outside can generate energy in electromagnetic form. But that's not certain. It's gravity. How do you detect gravity? How did humanity detect gravity? Do you know that? observing things moving. That's right. Dynamics. The solar system. That's it. So, by mapping out how the the planets move around the sun and then finding, you know, that there's water to it, not just random. Uh that's the Kepler's laws and so forth. And that's how Newton then formed what we call a real mathematical theory. There it is. Same thing we do with uh black holes. So we can use particles that is uh yeah planets is not what we do is yeah too far away they would be too faint. We are using stars okay um we're using gas hot gas and we're using photons. Mhm. So because uh in contrast to you know uh Newton's physics if you uh shoot you know a light beam from the backside against the black hole it gets bent and that bending you can see. So there are three ways of doing it. The fourth way uh which is very new is that the theory of Einstein uh put the context so to speak what used to be the ather yes into the word spacetime. Okay. And spacetime has a structure and that structure is space and time and the the cement of all of this is is the curvature of it. Mhm. And a mass when it moves, you know, changes the curvature of spaceime. So this change then actually moves through spaceime at the speed of light. We didn't know that actually until relatively recently. Now we know that theoretical prediction is right. So and that we call gravitational waves. So that's another one. And how do we do all of this from Earth then in So that's the theory. So would that be would it be fair to say the down the downside of course therefore is um you would say for gravity theory uh and black holes and the likes uh well we have the trouble there but that that's hopeless. just like you know we now know about 5,000 extra solar planets and I always say uh when the question comes and are there some humans there or you know people and I say well I'm afraid that the telephone conversation will be very boring they will be hello this is Reinhardt and then I wait 40 years until the answer come So no we have to use so to speak communication or measurement techniques which we can do from afar. Mhm. And uh that involves uh you know gravitational waves uh that involves photons uh long photons radio photons but also uh optical photons and so forth. So that's as our measurement way. Uh it's it's in a way indirect way uh but you know it works actually. And with your work was it sort of you worked on the theory, heard about the theory and then thought okay how can I build instruments to measure this? Or did you build interesting instruments and then think what can we measure with them? No. my way of getting in there is a little complicated but has

to do with mentors. Okay. And I I mean that's always forgotten nowadays. I mean this sort of the lone genius who thinks about something and then makes a breakthrough. Right. I had fantastic mentors. My father was a mentor. He was a physicist. And uh then later on I had Charles Towns uh Nobel laureate in the invention of the laser and the maser as a mentor and he had an experiment uh already going to use infrared waves to look at the galactic center. The speculation that there might be black holes out there had been sort of getting more firm 10 years before that. But 70 years after Einstein. Yeah. That takes that tells you that sometimes this sort of uh you know somebody says oh maybe the sky is blue because and then you make a next daily experiment and then you know everything is not true. It takes sometimes a long time. In this case, the whole thing from beginning to end will last longer than the hundred years. As we've been hearing from Reinhardt, the majority of learning doesn't happen alone. You need to work with others to share ideas and to receive feedback on your thoughts, which works great if you're at school or university, but can often be difficult when studying alone, particularly in the lead up to an exam. But don't worry, today's sponsor, Medly, is here to help. Medly is the world's first AI tutor where you specify not only your subject but also your exam board. Your personal AI tutor teaches you how to answer every question in your exams to get full marks. The platform has three modes to choose from. Textbook mode gives you the key piece of information you need to know with plenty of worked examples to bring the material to life, whilst practice mode generates mock exam style questions so you can test yourself on what you've been learning. But my favorite is the new learn mode, which brings together both practice and textbook to create the perfect learning package. As well as being able to work through a topic step by step, just as with textbook mode, practice questions will also pop up to check your understanding, giving you the opportunity to put your new knowledge to the test as quickly as possible. And of course, if you ever get stuck or would like further explanation on a topic or exam question, you can always ask Medly for expert guidance designed to replicate a conversation with a teacher or personal tutor. Rather than just giving you the answers, it guides you through step-by-step reasoning, which is the only way to truly understand a topic, not just for your exams, but perhaps one day for a future Nobel Prize winning piece of work. Speaking of which, let's head back to the Hong Kong laureate forum and hear more from Reinhardt on how he ended up working on the detection of black holes. I was of course doing some research before we were speaking and I saw that there was evidence that there could be black holes at the center of the universe but it wasn't strongly believed let's say when you began sort of in was it around 1980 when you switched fields in a sense was how I think you describe it yourself. Yeah. So what was it that drew you in? like how come you were specifically felt convinced by this versus other people who were like it was this joining of Charles Towns true I mean now why did I uh joined Charlie's group um towns I told you the invented the ma and the laser and he had one motto which was he would say oh well let's see science is a wonderful thing and you have to look at it Like, you know, you walk into a forest which you don't know and you look at the forest and you learn about it and the flowers and the trees and the mushrooms and all of this. And so over time you make some discoveries as you write down or publish. Then on some day you will hear noise behind you. That's your colleagues. That's when you leave the forest. So towns had done that. He had left Mazos and Le and in his later career moved to California and then thought oh astronomy might be good and he in particular he already had before because he came from molecules. I mean the maser works with ammonia molecules ammonia. Okay. And so he knew everything about molecules and he was speculating there might be a molecules in

interstellar space which every astronomy at the time astronomer at the time would say ah I mean Charlie I mean you're a famous physicist but you have you oh no I'm sorry and that actually happened they sent delegations to him but he already had a bad price but he would just go ahead and do it And he indeed found water and then ammonia and then I in my studies found read these papers these first period I was fascinating with so I knew him's name from my father and so it was my ultimate goal somehow to get to this great man and I did he became sort of my second father I must say from him I learned so many things and so I mean when you say uh who did the who had the idea so to speak and who do the I just learned along I just you know I was the little boy on the from the countryside who had so that's fascinating so you very early on knew you wanted to work with this person okay so how did that opportunity come about was it you seeking every opportunity to like be in the room No, they don't do that. No, no. There was a there was an opportunity after I done my first post talk in at Harvard um to uh give a talk in in Yans's group. Mhm. And then uh he I went to him and I asked whether there were post-doal fellows and he said, "Oh, let's see. " Yeah. very non-committal. Yeah. Classic academic response. Okay. So, and did you know then beforehand when you were giving that talk, did you feel like the pressure in a sense of be like I need to make sure this is if there's a day to do a good talk and impress somebody, this is the day. Absolutely. An even better story was uh I was only about six months a postal fellow in town school. Then he came up to me and said, "Oh, let's see, Reard, um UNICE, this university from time to time makes uh appointments in uh in faculty, uh you know, physics faculty. Uh maybe you should give it a try. " I said, "What mean? " Oh, no. You could do that. And then he basically had told me to go to the chair of the department and then uh three weeks later I had an offer. Wow. Okay. That's Yeah, that's incredible. Yeah. No, I in that sense I was super lucky. I mean I didn't have to go through I never in my life applied for a job. Actually, I have to admit I was I was super lucky and meeting with Towns was a great great boon to my entire life. Uh to see how he did it, how he went at things, the technique of of how to go at a problem, how to not go at a problem. which prompts to leave aside that was just fantastic. So, do you think that I'm sort of if I'm reading between the lines on the way you've been talking about this, were you drawn into then specifically his approach that story you told about the forest? That idea of let's go for the, you know, let's wander there by ourselves, see what's going on, and as soon as anyone else is coming, let's go somewhere else. Like, did you think that also appealed to you? Yeah. And I did that later on. Maybe not so I would say so literally uh because the forest to be explored later on got bigger so it took longer and uh and more effort was needed but yes I changed fields. Now you asked initially about the the black holes as such and uh you know in initially yes towns obviously had heard that there were these discussions that there are black holes in the centers of galaxies and if one would you know pick a region which is close enough by what there's a chance to see one then the galactics and the is a good

one but then it's bad in the sense that you can't look there in the optical because there's so much stuff in front of the galactic center but uh yeah I know he just um did an experiment with a spectrometer and then uh and saw uh some things which oh looks like there might be a mass here and then improved on that and the experiment was improved and then it was sort of an iterative thing of getting better and better measurements. Yeah. So, was there a and I know this is a tricky question, but was there a specific moment when you were like you yourself were absolutely certain that there was a black hole or did you kind of believe it immediately from the beginning? No, no. Uh I would say by the mid80s Mhm. We already thought we had pretty good evidence, but nobody would believe us, right? But you were sure by the mist. We were sure. Why? Because we were using a technique having to do with uh gases. Mhm. Gases moving according I mean, you know, dynamics, right? Movement. Yeah. And others said, well, but these gases could be pushed around by forces other than gravity. Okay. Right. So then the next step uh to go to use stars instead of uh gases actually then went on without towns. I then actually left uh University of California although I came back later part-time uh but uh I then became expert director and there I you know I started with building instruments and so forth which could then also do these uh stars but not only that we also use this these measurements to look at galaxies and so understanding the galaxies and their evolution and finding the black holes and being sure about them turned out in the end to be sort of uh you know uh in the two sides of the same story if you look at for it from instrumentation point of view. Mhm. Okay. And again when I was sort of reading through your um autobiography around the Shaw prize, you mentioned that uh well to me it seemed at least that it was a difficult decision to leave Berkeley to go back to Germany, but sort of I think you said it was ultimately worth it for the academic freedom. So did that feel like kind of stepping away and going off into the forest on your own as it were? Oh, that's a very good question because uh when I got the offer from Maxplank, I sat in my room in Berkeley for 10 days and every day I would make a table of pros and cons, you know, blue skies and Yeah. Yes. the weather in California. Uhhuh. And for 10 days the answer was always undecided. Yeah. And I guess in the end what I'm didn't like most in the US system nearly the daily hunting after money. Okay. Was what pushed me in the direction of Max Bang. And it was the right decision. Okay. It definitely was started to see. Yeah, it was only that it this was 1987 around there. Then in the end 90s, I don't know whether you recall this was after the reunification and all of these wonderful things which got in Europe. Yeah. uh Europe went into sort of a sleep type st. Mhm. actually in Germany in particular all over what had kept Germany alive and pushing on somehow people wanted the African very much like null and and and so I began to feel oh Americans would do much better and so all of a sudden I got offers from the US and I was tempted to go to Caltech to uh Princeton. I got a number of offers but uh in the end I took a part-time job in Berkeley which I kept until uh Mr. Trump took it away from me. Was that you would split your time roughly equally between not equally but but a quarter. Right. Okay. And 100. Mhm. Yeah. If you're going to get some sun, warm up from the cold of

your Okay. um sort of changing tack a little bit. Um so it's interesting you talked about um your father being a physicist and sort of again I read that lovely story about you learning how to build a spectrometer from its component pieces when you were a child and different things. But then to me that side of that contrasts with you went to a humanist gymnasium. So to me if I'm imagining you know the typical image of a child playing with building experimental instruments then would you not naturally have gone into a science sort of program? So how did that happen? because the uh the school I went to in a way was uh the best uh gimnazium in that uh wonderful little black forest town fry book. Mhm. Uh not that the others are much worse, but I mean this was really the best. And to tell you the truth, I was good for me and I still cherish that because I have other hobbies. uh history is one of them and I actually when I was 14 years old I was sure I want to become an archaeologist. Okay. Wow. Yeah. And I would still do that now. I mean there's so many things to do, you know, just if one would have another life that could do this then could do that and that's just absolutely fantastic and learning Latin and Greek um was okay was I fortunately I didn't have the problems many of the students uh had but it was wonderful and also keen sportsman as I want I want to know more about the the Olympics. So you were training as part of the team. Can we hear more about this story? It's just fascinating that you know do all this amazing physics work also then interest in history archaeology nearly went to the Olympics. Like it's just brilliant to hear. So as you said made you more wellrounded. Of course nowadays I look at this more as a failure cuz I could have no it's like winning a silver medal and looking at a failure, right? I was only second. No, I mean uh the sports stuff started early on when I was maybe 14. I started playing handball in a in a club and uh so from that I got quite a bit of strength and condition and uh but only the team playing was not enough. So I got into track and field. Mhm. And there was a university club in in Fbook. And so I started shoput then discus. Mhm. Then pendl pendlon. Mhm. Later, but pendlon and then javelin and uh There was one year I was 16 years old when I broke my leg while doing a hike in the Black Forest. And so I couldn't run much. But once the cast was off, I could at least stand and do something. And so I could, you know, take a javelin and throw it javelin and throw it. So I from standing still, no run up. Okay. Yeah. And I was throwing javelin for about uh 2 months. And I got so good at this that I then actually threw the best uh distance for that age uh in Germany. And then the and Germany at that time, you know, the Olympic games uh were about to come and everyone was very excited about Munich and so I got into the B team if you like. I was too young. I mean I was about to say and when you said you was 16. Yeah. However, I did get to know all the key people. In fact, uh the winner uh Claus Wolerman uh I had him at my house two years ago when he ged about the old days and it was wonderful. No, that gave me a lot I must say. You see what you get from that is grinding. Mhm. And you think that fed into your academic career? Yeah, absolutely. Mhm. When things get hard, you just have to, you know

I have a German word for this which you won't understand. Unhusten meaning you you like I want this. Yeah. And yeah, sports was very important I must say. But then you know uh archaeology I could have done history as a field not so much but archaeology yeah definitely there still time music is another thing of mine. Yeah. Wow. I have a wonderful piano and if I had more time I would get into piano as well. Yeah. No, I was say it's fantastic cuz I talk to my students about this a lot and it's great to hear you sort of saying the lessons that you learned from these other music, sports, other interests beyond just physics and science cuz I try and as much as I can instill this into my life. Let's be sure about it. I had to pay for this. Yep. I was in the school second best student. Mhm. the best sports person and all of this elite stuff. You know what what you become in Germany if you do this at age 15, you become the outsider nobody likes. There is a word in Germany called someone who Yeah. And that's the worst you can be. You're sort of to speak you're unacceptable to the entire class. And you know, I want to, you know, say this now, now that I can, when I was 16, the only way I got out of jail, so to speak, was that I would write exams, say in math, not only for myself, but for about four other students se seated around me who were about to fail the entire class. That's how I got out of jail. Mhm. Or very nice of you. Yeah. Um it's amazing to hear so much about your life story and I can already feel myself to I there's so many things I want to take away or go share with my students. Um, but if we can sort of just end on a final if you can sort of piece of advice or sort of life motto, let's say, having gone and had this incredible career and life that you've clearly had that you would share with anyone watching this who's currently at university and thinking what do I do? Maybe they want to go in to do research to go into academia. Is there anything that you think is something that you found very helpful or something you've always adhered towards in your I don't have any generic advice when you clearly the first one you who would say it just as me as well as me is try to do what you really like. Now, if then the answer in reality is I'd like best to sit at the beach. Don't we all? Yeah. Then that's not quite the right answer. Yeah. But other than that, I would say if there's really something you are excited about, although people tell you, "Oh, don't do this. is you can't earn any money or earning money. Yeah. Okay. You have to earn money but uh getting rich I I'm not rich. I never care to be rich. Yeah. I'm earning enough and but having these experiences and thinking about them and being a little proud about them from time Yeah. No, absolutely. Very very warranted. Yeah. Well, it's been absolute pleasure, Reinhardt. Thank you so much. Um, and thank you as always everyone for watching. Um, if you would like to see more amazing interviews with incredible people like Reinhardt, then please do subscribe to the channel. Uh, and I'll see you all very soon. Take care.