Transcript

• “The Discovery of Black Holes”

  Alexander Blum: Black holes have become this kind of frontier.

  Juan-Andres Leon Gomez: It’s always been this object that starts to infringe on what you can ask as a physicist.

  Stephanie Hood: Science Social, a podcast series about how science, history and society connect with and add to the big questions that we all have today. This show is created by the Max Planck Institute for the History of Science. My name is Stephanie Hood and in each episode, I’m joined by guests from our institute to talk about their research, their big questions and some of the weird and wonderful experiences they’ve had along the way.

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  Stephanie Hood: Very pleased today to welcome Juan-Andres León, a historian of science and visiting scholar in Department I at our institute, Structural Changes in Systems of Knowledge and visiting scholar in our research program History of the Max Planck Society. And Alexander Blum, historian of science and Max Planck research group leader in our research group Historical Epistemology of the Final Theory Program. Welcome, Juan-Andres, welcome, Alex. So, we’re here today with our masks on and with the windows open because of the pandemic. I hope that doesn’t affect the sound too much. If you hear any birds singing then that’s why. We’re here today to talk about black holes. Big topic. So, I’m looking at the moment, at the first image of a black hole, which was shot by the Event Horizon Telescope in 2019. So, this is the supermassive black hole in the center of Galaxy M87. You might remember it, I mean just to give you a description of what I’m looking at right now is it’s kind of this, looks like an orange and yellow blob with a black background. So how was this for you as historians of physics to hear about this image being produced and seeing the picture for yourself for the first time?

  Juan-Andres Leon Gomez: For me, it was a moment of a bit of closure and relief because we had been following this adventure a bit. You know, the Event Horizon Telescope (EHT) is not one instrument, but it’s like a dozen telescopes around the world that are combined to produce this picture. And we had followed when the measurements had been taken in 2017 and then there was this long, long silence and it took like a year longer than people were expecting. But it was the question, well, is this going to be good enough as a picture that comes out of it? But actually, it has become more iconic than other discoveries that were done in the, say, in the past five years, there has been a lot of research, of results of research about black holes and, in the end, this is what people will remember the most. So, it’s a good moment of closure.

  Alexander Blum: I mean, the picture is really cool too, right? I mean, it came out really nicely. We invited Heino Falke, one of the main scientists on the EHT, and I asked him about how they chose the color scheme, right? Because I mean, it’s not really a color photograph, right? It’s not like we can actually see colors there. And he was like “Yeah, yeah, that was me who chose the color scheme to make it look like a heat map.” And he was really proud of that. And I think rightly so. Because I mean, that really does… That’s an important element in making it so iconic.

  Stephanie Hood: That’s an interesting nugget of information actually. I was going to ask you specifically, I mean, you obviously know a lot about black holes and also about the story behind this. How does this image or the production of this image actually connect to your research as historians?

  Alexander Blum: We have, of course, images of black holes that are around before. Like drawings by people, somehow based on theoretical ideas, trying to map out the physics of black holes.

  Juan-Andres Leon Gomez: Yeah, I think from my point of view, black holes are an example of an entity that goes from a theoretical hypothesis to something that can be slowly ascertained indirectly by its effects on other entities that you can see. So, it becomes more and more something that should exist, but for a long time you cannot see the thing directly. We just have more and more clues.

  Alexander Blum: Yeah, and in a way, I mean, black holes are the ultimate object that defy taking images of them because they swallow all light, reflect nothing. So, the image is not just taking an image of something that’s very rare, but it’s taking an image of something that is really fundamentally impossible to photograph in a way. So, it’s interesting to see how this kind of image changes the way people think about black holes, kind of the intuition. So, the contrast there, I think, is very interesting for an historian looking forward.

  Stephanie Hood: This is something that was really fascinating to me, I think. I mean only having a very basic background in physics. I mean, I’m also more of a visual person, I think. Being able to see the image was something that to me was really inspiring and got me really interested in looking, like learning more about this topic full start. So, before we talk a bit more about black holes, there’s one theory that was essential to discovering them, which was this theory of general relativity: As I understand it, general relativity is this theory of gravitation that was published by Albert Einstein in 1915 and is also the current description of gravitation in modern physics. So, general relativity sort of refines Newton’s law of universal gravitation to try and provide a unified description of gravity as a property of space and time. And then also under Einstein’s theory of general relativity, gravity can actually bend time. And so, something that I found really fascinating was that clocks on the International Space Station (ISS) also go very, very slightly slower than they do on Earth. And that actually has to be compensated for on the International Space Station so that they’re using the same time as on Earth. Is that right? Not making that up. Okay, good. Just to kind of summarize general relativity. I’m going to use this quote by the theoretical physics John Wheeler, that space-time tells matter how to move, matter tells space-time how to curve. Is that right? Do I have this right? Or is there anything else that you need to add?

  Alexander Blum: There is a lot to add of course… But yeah, I didn't see anything. It seems a fine introduction to me.

  Stephanie Hood: Okay. I mean, this has important astrophysical implications. And this is then when we come to black holes, right? What this does is: it implies the existence of black holes. That there are regions of outer space in which space and time are distorted in such a way that nothing, not even light, can escape. So today, general relativity is one of the cornerstones of modern physics. But when Einstein published this in 1915, the reactions were quite different. Can you tell me a bit more about the first reactions to Einstein’s general relativity?

  Alexander Blum: Well, I think within the specialized physics community, it was pretty well received. But it also didn’t make this big splash right away outside the community. That all changed a few years later. When one of the major predictions of the theory, the bending of light from distant stars in the gravitational field of the sun, was confirmed. That was a big media hype at the time, right? This is right after World War I and people were very eager for good news, especially good news, you know, about scientists peacefully collaborating, you know, a German making a prediction and an Englishman confirming it. And this really elevates Einstein to international superstardom. So, that’s like in 1919, a few years after he proposed the theory for the first time. That, of course, then also brings some resistance, right? I mean, there’s a lot of pushback from proto-Nazi physicists who are like, “This is not real science,” you know, “This is crazy stuff.” But still, I mean, by 1919 or in 1919, it’s pretty much established, as a stroke of genius and probably true theory. But it’s just not quite clear what to do with it, right? It’s just like, okay, this is cool. Now, we kind of understand gravity better. It kind of goes hand in hand with a new picture of what space and time are. But there’s just a handful of people who can actually do some research with it. It’s not quite clear what kind of significant implications it has. But so, it kind of enters a lull. And people are more interested in what’s going on at the time, in kind of microscopic physics. Because this is also the time then, in the 1920s, when quantum mechanics is discovered. And that’s kind of where all of the action is. I mean, the main reason why people didn’t pay that much attention to it maybe then, especially within science, is not because they didn’t believe in it or think it was silly, but just that there was not enough to do with it in a way.

  Juan-Andres Leon Gomez: There was a weak connection between the theory that is formulated and what kind of observations and experiments you can do with it; and how to establish a continuous interaction and feedback between theory and observation is not easily possible with general relativity in the first part of the century.

  Alexander Blum: It’s like this very major philosophical theory about how space and time are different than we ever imagined them. But what it ultimately produces is just like small numerical corrections that you can actually calculate from it originally, right? Say, the clocks running differently on the ISS that you mentioned. It’s a minute effect that you have to take into account. But still it’s a minute effect because there’s no such thing like a black hole, right? We’re like, okay, this is something we could never have imagined. It’s out there. We can only understand it through general relativity, like qualitatively. There’s no such thing like that.

  Stephanie Hood: This was something actually. I mean talking of the philosophical implications in your research group itself, you’re trying to bring historians and physicists and philosophers together. And can you sort of say something about the kind of wider philosophical implications that this theory of general relativity might have had for Einstein himself and then also more generally?

  Alexander Blum: It really, like I said, gives us a new idea of what space and time is, right? If you look at the paradigmatic thing is like what is Kant in the 18th century. And it’s basically the idea of space and time that they’re nothing we can really critically evaluate. They’re just kind of there… They’re there before we even think anything. We already have to have space and time. And then suddenly, with Einstein, space and time become like these malleable things where the presence of the sun or the earth or some other big object is actually going to distort them. That’s really a major shift. It turns space and time from foundations to an actual thing that scientists can investigate and study.

  Stephanie Hood: Talking of black holes actually, and maybe extending a little bit from this, I mean, from this kind of big shift that happened: It sort of seemed like the theory disappeared a bit and then came back in the 50s. Is that right?

  Alexander Blum: Like I said, there just didn’t seem to be too much to do with general relativity in the meantime. And then this is something that happens in the late 1920s. And this was maybe the first major thing to happen–that the expansion of the universe is discovered. And that’s something that you can only understand in the framework of general relativity. So as far as, you know, what we call cosmology, so the evolution of the universe is concerned, there certainly was something going on at that time. But still, a lot of that was mainly speculative again to many. We knew that the universe expanded, but that was kind of all that one knew. And then we could think about, you know, has it been expanding forever? and will it keep on expanding? or was there a big bang or something? But it was only in the 1950s that we really see people coming together. So, people who were working on these kind of speculative, unified field theories. People working on cosmology; also mathematicians who were very interested in general relativity. Post-docs doing their PhD here and then going to a different center. And like, kind of a real cohesion, they start having conferences, they start having journals devoted specifically to general relativity and all of that kind of picks up in the 1950s.

  Juan-Andres Leon Gomez: Yeah, well, something also very interesting on the socio-political side is that up to 1929 was a moment of economic expansion and the roaring 20s, the large telescopes of the previous generation were really showing their big results. These first indications of the expansion of the universe had been riding on that. And then comes the Great Depression. One of the obvious victims of this was astronomy. And then, basically, there's almost a generation from 1930 to until after the war where there's a whole institutional and disciplinary crisis that only picks up after the war.

  Alexander Blum: Money starts pouring in again after the war, right? And more money than ever starts pouring into physics because, I mean, the physicists had just won the war, right? They built the bomb, they built radar. And this small general relativity community kind of piggybacks on that. Suddenly you have not just the government but even like the military funding basic research.

  Stephanie Hood: Okay, cool. So, this actually connects to what I wanted to, well, it possibly connects to what I wanted to discuss next, which is going a little bit more into black holes and how this became also a thing in the history of physics. Really starting from the beginning: when was there the first indication of black holes existing?

  Alexander Blum: You can write down something that looks like a black hole in general relativity rather easily, right? And this was done essentially right after Einstein proposed this theory in 1915. And the first solution was then given by Karl Schwarzschild in 1916. As a theoretical idealization, you can just take it to be a point. Like you said, like the black hole is a point. But if it were literally a point, then the gravitational field closer to it would indeed be so strong that no light comes out. But people took this to be an idealization and were never going to have any object that’s so compact that you could put all of its mass in a point. So, even though the formula for a black hole was there, this was not something that was considered to be physical in any way. People really start thinking about black holes really only once they start understanding how the sun works. People start working out that these are nuclear fusion processes which keep the sun burning. And as soon as that’s understood, it’s also clear that stars like our sun and other stars can use up their fuel at some point. And as soon as its fuel is burnt out, it’s going to collapse from its own gravitational pull. It’s going to collapse into itself. The question is: how far is it going to collapse? Like, you know, what is it going to collapse to? These are questions that people start debating in the 1930s. And they come up with some stable configurations that it can collapse to. These are white dwarves–that’s what’s going to happen to our sun. For bigger suns, they would be so heavy that they could turn into what are called neutron stars, which is basically a huge atomic nucleus. But it was slowly becoming clear that for even bigger stars, the question was whether you could have stars that were so heavy that they would basically collapse all the way into a point and actually become the kind of black hole that one had thought was just an idealization. But Wheeler, whom we mentioned earlier, did not believe that at first. He was like, there has to be something else, either some other stable configuration, which is even denser than an atomic nucleus than a neutron star would be. Or the whole thing is just going to blow up. But over the course of time Wheeler got convinced both by theoretical calculations in general relativity and especially also through computer simulations. So, people had been working on computer simulations of these kinds of implosions because they were interested in bombs, right? And so, by the early 1960s, even Wheeler, who was one of the main skeptics, is convinced that if you have these big heavy stars and they’ve burned out their fuel, they are going to collapse into something like a black hole.

  Juan-Andres Leon Gomez: It is an interesting coincidence that simultaneously as you have this renaissance from the theoretical side, this is exactly the right moment when the major observations of the post-war era really kick in. Radio astronomy had been born basically during the war when the first generation of post-war radio telescopes were German radars repurposed by the occupied countries for this purpose. They start to discover a vastly different universe that was always there but was not accessible with the traditional optical telescopes–a much more violent universe, more dynamic. And then exactly in 1962, there is this incredible observational development what were called quasars. And it’s these observations from radioastronomy that originally didn’t easily match to what was already known from the optical view of the universe. And it turns out that it’s because the objects that are so far away that also the amount of energy and violence of the processes that lead to these emissions is just something that is a different kind of thing. And this also makes it thinkable more and more that general relativity is necessary to explain what is going on. And through radio astronomy you also start to see very good evidence of the Big Bang having actually occurred. So, I think through the 1960s, there are very extreme phenomena and entities that suddenly become observable that were very speculative before. So, in a way, black holes are not alone in this. It’s easier to think about something like a black hole because you start to also see, well, maybe you can see the origins of the universe. Or, for example, as Alex mentioned earlier, neutron stars. So, suddenly all these hypotheses become observable.

  Alexander Blum: So, you can see we’ve got this nice division with Juan-Andres talking about the observation while I’m talking more about the theoretical aspect. And I think maybe there’s one more thing for me to mention on that theory side as we go into the 60s and 70s. It’s not really essential for a black hole to collapse all the way to a point. Essentially, what you need to have to get a black hole is for your star to collapse far enough. So, that it’s a dense enough object for the gravitational pull to be strong enough that it doesn’t even let out light anymore. And a collapsing star is going to do that already before it’s shrunken all the way to a point, right? Basically, at some point it’s just so heavy that it forms around itself what’s called a horizon. But the question was then–and it’s kind of a theoretical question because it’s a question about what happens within the black hole, which we can’t see anyway–was whether it would actually contract all the way to become a single point. And “point” doesn’t just mean like a speck of dust. It really means a point in the geometrical sense. So, there’s no extension. It’s just like one idealized point, right? It has no height, no width, no breadth, but it’s just a point point. And if you take the entire mass of the sun or something even bigger and contract it to a single point, the gravitational field is not just going to be strong enough to keep in the light, it’s going to be infinite. And basically then, you know, all hell breaks loose and your equations collapse and everything. So, it was like a big deal theoretically. And yeah, so that’s where Penrose comes in. This is at 1960/65. He sets up the so-called singularity theorem, which says, no matter what, if we have something that collapses to a black hole, it’s going to collapse all the way to a point, to what physicists call a singularity. Which basically means, you know, everything we know breaks down at this one point in space. And that’s kind of the moment people realize black holes are not just these extreme astronomical objects, but they’re really a point where also our theoretical understanding is going to break down, right? Where we need some new physical concepts to understand what’s happening within the black hole. And that’s kind of when black holes have become one of the essential objects at the frontiers of fundamental physics.

  Stephanie Hood: This is fascinating, I’m learning so much. Actually, I even don’t know if we have one other last thing I want to go to before we talk about the very small matter of the center of our galaxy… How did this research then proceed then into the 70s and 80s from there?

  Alexander Blum: There, we really have this disconnect between the theoretical investigation and the observation. You really see this divide between black holes as an extreme physical object where you can test your foundational theories–your theories of everything. But all of this has nothing to do anymore with finding the black hole observationally.

  Juan-Andres Leon Gomez: It’s still the case today, as we have mentioned earlier, that you can sort of compartmentalize out, you can be very positivistic as a scientist and say, well, whatever goes on in there, I can’t know, so I won’t get into. And a lot of the observational programs to find and see and say, asserting how black holes behave astrophysically, can afford not to look too much into it.

  Stephanie Hood: Going into one very specific black hole: Can you tell me how this idea came about that there could be a black hole in the center of our galaxy?

  Juan-Andres Leon Gomez: Say, the black holes that led to a lot of the theoretical conceptualization about it were the ones Alex talked about earlier–those end products of the life of stars. By the 1960s, this is, say, firmly established. At the same time, stellar black holes are already becoming established as an observable entity–indirectly observable. Because there’s this other form of astronomy that becomes possible: X-ray astronomy. And then by the early 1970s, they are discovering sources of X-rays that necessarily come from a black hole. At the same time, this happens at relatively low masses. I mean, we’re talking about less than 10 times the mass of the sun already produces a sort of black hole. The center of the galaxy, however, cannot be the end product of stellar evolution as we know it. Because if we’re talking about masses in the millions and billions of the mass of our sun, so... The universe that we see out there has much more extreme scenarios. Like we have entities that are millions, billions of times heavier out there (…quasars in 1962 already…). It’s a good candidate place to have a black hole at the center because these are like galaxies from a long time ago, very large, where a lot of matter is being swallowed up to the center producing all these massive amounts of radiation.

  Stephanie Hood: So, this was all then sort of leading up to the discovery of Sagittarius A, this black hole?

  Juan-Andres Leon Gomez: Even if it's necessary to ask whether what you have there is a black hole, well, it's interesting to ask, is there something at the center? And when radioastronomy starts to develop, one of the important sources that is discovered very early is the center of the galaxy. So, you know, there is something interesting going on there. The problem is that radioastronomy doesn't have a great resolution. So, you can sort of know from which direction things come from, but it's not like you can really pinpoint sources very accurately. And it's in the early 1970s when you start to have interferometry. So, what you do is you combine telescopes that are at a distance from each other. And then, in some ways, they act as though you had a telescope of the size spanning the distance between the two telescopes. And people start to be able to use interferometry to pinpoint an actual point source in the region of the center of the galaxy that seems to have very interesting emissions. And this is why they name this Sagittarius A star.

  Stephanie Hood: Can I ask you a question? Actually, I mean on the topic of proving this and just also connecting to something that you both wrote in this feature story for us last year about Reinhard Genzel: So, he’s one of the directors of the Max Planck Institute for Extraterrestrial Physics, and his work had something to do with this, didn’t it?

  Juan-Andres Leon Gomez: So, this is basically the Nobel Prize, very well deserved. It kind of spans this last generation where you go from more indirect to more direct ways of observing things. So, one of the main problems with the center of the galaxy in particular is that it’s a very dusty environment and light doesn’t really reach us from there. But this other form of astronomy called infrared astronomy, it’s the same wavelength that is used in night vision, crosses more easily through this kind of dusty, opaque environment. And it’s making huge progress. There’s a lot of these technologies that will have an impact in how accurately you can look at the center of the galaxy. And people of the generation of Reinhard Genzel recognize that this may make possible to find out what’s going on there. And you need to observe over several years what kind of orbits the objects trace. And over the course of many years, you can ascertain that the visible stars are orbiting a very heavy object that is in one specific point. And everything hints at more and more that it has to be what we think of as a black hole. And then you can start to propose observational experiments to see if the behavior in the vicinity of this entity starts to be in agreement with the more extreme implications of general relativity. So, it’s no longer just tracing orbits and all that, but for instance doing spectrometry: see how the color of light shifts in the vicinity of this entity. And the first good orbits of this region were already traced, say, around the year 2000. And over the next 10, 15, 20 years, people are starting to go even further and apply all possible astronomical methods to see how this thing behaves there. But totally in parallel, there’s also the development of gravitational wave detectors. And then you have this picture of the black hole coming out of interferometric observations in another wavelength, which is what we talked about earlier. These parallel research programs also end up supporting each other from what people in entirely different types of doing astronomy and astrophysics are also finding. And in the end, I think the Nobel prizes of the past half decade that have gone to things that involve black holes also reinforced each other. It would have been much more difficult if it was only one strand of research leading into this.

  Stephanie Hood: It’s fascinating. It also makes me feel like we’re living history in the making somehow with all of this. I imagine how people will be talking about it now in 50 years and the history of physics and the history of black holes and where we’re going. I actually just want to finish off honestly that you’re both really good storytellers and I could probably listen to a lot more of this. I just wanted to finish off by asking a little bit about both of the projects that you’re involved in. What in your eyes, for both of you, were the findings that you had that most surprised or fascinated you?

  Alexander Blum: For me, maybe, and I think this is also an important element of my thinking about the history of science in general, is how a historical investigation can lead to taking apart a concept like a black hole and looking at the independent components that go into it. Because like I said, if you look at it from the perspective  of a contemporary physicist, looking at this paper in 1916 when Schwarzschild writes down something that looks like a black hole… oh, it’s all there, right? They should have known everything. But if you look at the history you can see how all of these individual components come together and get agglomerated into this notion of a black hole. You have the idea that they’re the end product of the collapse of a star, which is not at all present there. You have the idea that the point is really an actual point and not just an idealization. All of these things I mentioned are all gathered together in like one big idea of a black hole that fits this work with today. But if you look at it historically, you can see how many different ideas, which are to some extent independent, such a conscious black hole is made of. And while I find that just, you know, fascinating from an intellectual perspective…

  Stephanie Hood: … I feel like we could go in so many different directions. I really want to learn more.… Well, Andres?

  Juan-Andres Leon Gomez: Yeah, I’ll go in an entirely different direction because I entirely agree with Alex, but in the kind of research that I do, what is most fascinating about black holes is that how the scale of research has shifted through the 20th century. Because you start with an individually crafted theoretical entity at the beginning of the century whose existence in the world is almost unprovable. And through the decades, and especially after World War II, you have all these changes in the way scientific research is done that create the conditions of possibility for observing such entities out there. And how many of the scientists that work on the theory of black holes were tied to the weapons programs of the war and the post-war era. The way that it’s no longer the work of individuals but of entire communities of hundreds, thousands of researchers that can lead to a scientific answer. And then, on top of it, we get these incredible proofs of what are otherwise very esoteric, say, concepts.

  Stephanie Hood: That seems to me like a really good place to finish. So, thank you very much Juan-Andres, thank you very much Alex, this has been really fascinating learning about black holes and I really want to learn more, I have so many more questions.

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