Sheperd S. Doeleman

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ORAL HISTORIES

Credit: Brigitte Lacombe

Interviewed by

David Zierler

Interview date

May 21, 2021

Location

Video conference

See catalog record for this interview.

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Interview of Sheperd S. Doeleman by David Zierler on May 21, 2021,
Niels Bohr Library & Archives, American Institute of Physics,
College Park, MD USA,
www.aip.org/history-programs/niels-bohr-library/oral-histories/46822

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Abstract

Interview with Sheperd Doeleman, an astronomer at the Harvard Smithsonian Center for Astrophysics, founding member of the Black Hole Initiative, and founding director of the Event Horizon Telescope. He surveys his global initiatives and his interest in fostering black hole research in Africa and he describes how the pandemic has slowed down his work. Doeleman affirms that he is of the generation for which black holes were always “real” and not theoretical abstractions, and he provides a history of the discovery that supermassive black holes were at the center of galaxies. He reflects on the applied science that was achieved in the course of creating EHT, and he describes the unique values that land and space-based telescopes offer. Doeleman recounts his childhood in Oregon and his admission to Reed College when he was fifteen. He explains his motivations in completing a solo research mission in Antarctica and he describes the opportunities that led to his graduate research at MIT, where he worked with Alan Rogers at the Haystack Observatory on the 3mm VLBI. Doeleman narrates the technical advances that allowed his team to achieve an eight-fold increase in bandwidth, and he describes the EHT’s administrative origins and the events leading to the measurement of the Sagittarius A* black hole. He describes what it meant to image the black hole, and he conveys the deep care and caution that went into the analysis before EHT was ready to publicize its findings. Doeleman discusses winning the Breakthrough Prize as the public face of a large collaboration, and at the end of the interview, he considers the ways that EHT’s achievement can serve as a launchpad to future discovery.

Well, I think it's fair to say that the Event Horizon Telescope is a hard act to follow. We imaged a black hole, and that was really spectacular, not just for the science that came out of it, but also there was a human dimension to it. We're coming to grips with its impact across multiple fields, not just mathematics, astronomy, physics, but even in the areas of philosophy, for example.

But it turns out that with all great discoveries, you want to ask, what's next? And in this case, what's next is something that we called the Next Generation Event Horizon Telescope, which is going to take us from still images of black holes to movies of black holes. The driving motivation there is that you can say a lot from the still image, which shows you the strong signature of light-bending around this ultra-massive, compact object, but you can say even more if you can understand the dynamics. This will give us some sense of how gas and particles with mass move around the black hole, as opposed to photons that are lensed around the black hole. This is hugely exciting. This is going to take up my attention and the attention of many people over the next decade, I think. We hope to come with a full, built out, next generation array by 2030.

Zierler:

Shep, your day has already been full. What was your presentation this morning for Africa at 5 a.m.? What were you talking about?

Yeah, it turns out that it's pretty clear how stellar mass black holes form. These are black holes that have a mass of a few times, maybe five or six times what our sun weighs, up to maybe 100 times what our sun weighs. The mass of those black holes is determined by the size of the star that exploded to give rise to the black hole in the end stage of the star's life. So, the star explodes, the whole core collapses, you wind up getting a black hole. And then there are supermassive black holes that weigh millions or billions of times what our sun does. And the big question is, how do those form? And the answer is, we don't really know perfectly how they form. It's likely that you wind up with mergers of these stellar mass black holes, and we've seen these events with the LIGO interferometer, so we know for a fact that black holes that weigh many tens of what our sun does merge together to make even larger black holes. That part of the merging process is pretty clear. And then we know that there are supermassive black holes that are eating voraciously at higher redshift.

So, farther away from us or earlier in time in the history of universe, there are periods when a lot of the black holes we see at the centers of galaxies are accreting almost maximally. As fast as they can accrete matter, they're accreting matter. So, they're growing rapidly. And somewhere in between, you wind up with this hierarchical merging that likely happens where kind of big black holes, like hundreds of solar masses, start to merge. And then, supermassive black holes in the centers of galaxies merge when the galaxies collide, and ultimately you wind up with this range of masses from the small, few solar masses, up to the billion solar masses. There's been a search for the mid-range. Where are the black holes that weigh 1,000 solar masses? Where are the black holes that weigh 10,000 solar masses? The problem is that they're likely not accreting as fast as other black holes. They may not be as luminous, and they're difficult to detect. There have been some detections that I think are pretty creditable recently that begin to open a window on this mid-range of black holes. But predominantly, the ones that we can see clearly are the ultra-large supermassive black holes, and the stellar mass black holes.

Zierler:

Shep, as you well know, all fundamental discovery requires an interplay of theory and observation or experimentation. Sometimes the theorists provide guidance to the experimenters, and sometimes vice versa. So, both in historical perspective and where you are right now, who is leading whom? Where is the guidance coming from for the questions that are most important to you now, and how has that developed over the course of your career?

Doeleman:

That's a great question. At the current time with the Event Horizon Telescope effort and the Next Generation EHT that's forming, I would say there is a real 50/50 contribution here, especially with the Next Generation instrument. I mean, this is a wonderful opportunity. Here we're asking ourselves, where do we put telescopes on the surface of the planet in an optimal way to make movies of black holes? When we ask what is optimal, it's not just where we put these telescopes. Sure, we could put them in a number of different places, and it would probably give us pretty good images, allow us to make pretty good movies.

But we'd like to tailor it. We'd like to ask ourselves, where do simulations point us? What are the open questions in the plasma physics around the black hole? What are the open questions in jet launching from the north and south poles of these spinning black holes? Can we fine-tune the instrument to be directly responsive to these kinds of questions? And we also don't want to leave anything on the table. God forbid, we wind up building this instrument and someone says, "You know, if you had just expanded your bandwidth a little bit, we could do this." Or, "If you had just been able to be responsive to this polarization of light, we could do that." So, we're at the point now where we want to make sure that we have the most traction on the most pressing problems. That happens, truly, when you have a mixture of the theory and experiment.

The other part of your question that you asked is also very interesting. How do things get started? What's the spark to get things going? And in the Event Horizon Telescope, really it was for a long time instrumental. The real developments came from an instrumental point of view. So, in the 1960s it started to become apparent that there were supermassive black holes in the centers of galaxies. In 1974, we discovered the radio emission from Sagittarius A*, the compact radio source in the center of our Milky Way Galaxy. And all of a sudden, we had our own private active galactic nucleus. We had our own private supermassive black hole in our backyard. And there was a race at that time to resolve it. Was it really a point source, or did it have some dimension to it?

So, all the way dating back to the 1970s, people were focused on this one source, and building ever-better interferometers in the radio to try to resolve it, to see if it had extent. And this happened in two ways: one, people started to move the telescopes farther apart which gives you higher angular resolution. But you quickly run into a problem. The problem you run into is the interstellar medium has a lot of free electrons in it, and these free electrons scatter radiation. So, even if you had a point source at the center of the Milky Way Galaxy, and you looked at it with an interferometer that had infinite resolution, and you could see with crystal clarity, it would still look fuzzy because the light that comes from that point source is scattered as though you were looking through shower glass, through this haze of electrons between us and the galactic center.

So, the first thing you realize is you have to go higher in frequency because only at higher frequencies does that haze begin to become transparent, and you can see through that haze all the way through to the center of the galaxy. And then, another challenge arises. Sagittarius A* is accreting all this gas. Magnetized plasma is madly scrambling to get to the event horizon. You can imagine the chaos that ensues because a huge amount of gas is trying to get into an impossibly small volume. And of course, it heats up just as when you rub your hands together and they get hot. And in this case, the plasma heats to a 100 billion degrees. Now, plasma that's that hot is opaque to its own radiation at longer wavelengths.

For example, if you try to observe that very, very hot gas at long wavelengths, you will only see the outer boundary of the accreting gas. You have to go higher in observing frequency (to shorter wavelengths) to see all the way through to the event horizon. So, you've got to get through the interstellar medium, you've got to get through this hot gas, and then once you've got the right wavelength of radiation, which is around 1mm wavelength in the radio, or 230 gigahertz if you like frequency, then you have to build a telescope that has the native angular resolution to resolve what you think you're going to see at the core of this black hole. And there, you need to build something that has an angular resolution of just a few tens of micro arc seconds, equivalently the size of an orange on the moon, or something like that. Or if you like money, reading the date on a quarter in Los Angeles if you're standing in New York. So, the technical challenge was to observe at high frequencies, and to create an interferometer that could really see what was going on where the action was.

Now, where does the theory come in? Well, the theory was driving us because there were a lot of models that told us how the emission would be layered as we got closer and closer to the black hole. So, we had some predictions about that. But mostly, we were just driving forward instrumentally, and we knew that we would see something interesting if we could keep pushing higher in frequency and higher angular resolution. When I was a graduate student, it was all about just building the best interferometer that you could. It was understood this that would be the key development that would push us past this barrier of not knowing what was happening at the black hole.

Now, there were a few other things going on at the same time. In the late 1970s, a true visionary by the name of Jean-Pierre Luminet created the first visualization of what a black hole would look like if it were surrounded by an accretion disc. This was phenomenal. I've talked with Jean-Pierre before. I visited with him here in Cambridge, Massachusetts, and I recently sent him an image, like the one that's hanging behind me here, of the black hole, and I know it was very well received. He was the first person who ray traced a real simulation, and he also made a dynamical simulation of it. But I think that work for a long time was under appreciated and kind of lost. It wasn't until the early 2000s when there were some more simulations done, and people like Heino Falcke, Avery Broderick, Rohta Takahashi, and others, started to ask the question anew, kind of rediscovering Luminet's work - what would we see if we could resolve this black hole?

But at that point, the underpinnings of the event horizon telescope were underway. So, we'd already started to push to 1mm. We'd already made successful observations at 3mm. Those were heroic days. Everybody's driven my something different, and for me, it's getting new equipment and going to a telescope to make it work. The exhilaration of working with cherished colleagues in those early days, going to tops of mountains, making reel to reel tape recorders work in low humidity conditions, it was really all about the instrumentation. Until we made the first detections of compact structure in Sagittarius A*, it was all academic.

Zierler:

On the question of instrument building, if you can reflect on the divide between basic science and applied research. In other words, black hole research as a global initiative obviously needs to be understood in the framework of basic science. There's nothing more fundamental in terms of discovering how nature works than learning about black holes. But -- and I'll share with you a really important point that Nergis Mavalvala made to me about LIGO, which is in the course of building all of this instrumentation -- so much stuff was created in terms of detectors, in terms of dampening, that have value and applications that have nothing to do with LIGO. So, I wonder if you might reflect on the extent that's true for the research that you're involved with.

Doeleman:

What a wonderful question. What I would say is that the instrumental component of the Event Horizon Telescope really drove things, but it was not in the area of fundamental device physics. We weren't creating new kinds of radio receivers. We weren't creating new optics as LIGO does. We weren't pushing the boundaries of the very receptors that turn the photons into 1s and 0s. The secret sauce, the kernel of the EHT, was linking to an explosion in bandwidth. Just as our lives have been completely changed by the fact that you and I can have a Zoom conversation that requires bandwidth that we couldn't have dreamed of 20 years ago, the Event Horizon Telescope took advantage of that burst of capability, of high-speed commodity electronics and computers and protocols, and harnessed it in the service of fundamental physics. I think it's a wonderful story. It's really a model for how you can change completely the paradigm of how instrumentation is built.

So, the example is that the interferometric systems that we used to use took decades to design, test, build, and commission. It would take a decade for one of these systems to come into being. And they had names like the Mark IV, or the Mark III. It was as though we were building tanks, like we were going to war. That's the effort that was required to make these systems. Teams of engineers would get together and build the Mark IV. And it was a monumental and heroic effort. Something really changed. There was a transition in the early 2000s. I got a call from someone at Berkeley, a collaborator of mine, Geoff Bower, who at the time was junior faculty at Berkeley. And I had been talking with Geoff, and I'd been telling him we have to break through this sensitivity barrier. The only way to make the Event Horizon Telescope work, the only way to detect Sagittarius A* on long baselines, where you're only receiving part of the energy of the black hole -- the longer the interferometer baseline, the less energy from the black hole it can detect – was to increase the bandwidth of our instruments. And he said, "You should talk to some people that are here that are specializing in using field-programmable gate arrays." These are small chips that can be nimbly reconfigured to do what floor to ceiling racks filled with analog electronics were being used to do in VLBI.

The mind races with analogies, right? You're absolutely right. I'm not sure if we can do that again, but I'm not sure the EHT has to do that again. We've already proven what we can do. So, now it's fair to say, okay, where do we put the next ten telescopes? And we think the sweet spot is adding ten new telescopes at optimal locations around the globe, which can work with one or two large telescopes, like the ALMA Array or the Large Millimeter Telescope, so that even a number of small modest-sized telescope can fill in the gaps in the Earth-size virtual lens. That will take us to making the first movies of black holes, to studying the dynamics in real time around Sagittarius A*, for example. That will open whole new areas of inquiry, including how energy is extracted from black holes, the dynamical interplay of magnetic fields that launch jets from the north and south pole of these black holes, and testing Einstein's theory in new ways. We're still limited though, and one dares to dream. The next step would be to go into space, because once you've built an Earth-size virtual telescope, there's nowhere to go but beyond the Earth.

Zierler:

When you were a kid, were you a tinkerer? Was that sort of part of your world?

Doeleman:

Kind of. I was pretty good at taking things apart. Not so good at putting them back together. I did some of the things you would associate with kids who were curious cats. I used to build rockets and launch them. I have distinct memories of doing this with my brother and running out of the materials to build a real rocket. So, we just started gluing wings onto the engines, which is horribly dangerous. There's no stabilization involved whatsoever. Normally, you build a big tube, and there's a housing for the motor, and there's a nose cone made of balsa wood, you glue on fins, and you let it dry. We were just so impatient. We didn’t have the time to go buy another kit, and we had all these engines. I remember thinking, I'm not sure this is a good idea, but gamely just gluing fins directly onto this little, tiny high-powered engine filled with explosive material. And we lit them off, and they'd just go in crazy directions. No forward propulsion at all, just random directions. And one of them imbedded itself in the tiles of our neighbor's house. The thing about these engines is that they have an explosive charge at the very end, normally which is used to blow a parachute out the top of the rocket. So, we watched this thing burrow and burrow and burrow, slowly getting into the tiles of this roof. And then there was this pause, which is typical, because you want to wait for the charge to detonate until you reach the apex of the rocket’s flight, and then after about a second, there was this big bang, and the whole rocket flew out from this tile and it started the roof on fire. Thankfully, we had a hose, and we were able to douse any potential fire there. I assume the statute of limitations has expired on things like this. But we were curious. We had a lot of fun. But I wasn't a boy astronomer, grinding lenses and building telescopes and things like that.

Zierler:

As an undergraduate, to the extent that you appreciated the binary in physics between experimentation/observation and theory, were you specifically looking for experimentation type work? Did you dabble in the idea that you would pursue a career in theory at all?

Doeleman:

You know, I was pretty undecided at that point. I was still looking for direction. I think what decided me was the experience in Antarctica. We're all shaped by some experience. How does passion flower in people's minds? It's a very personal journey, and a lot of it is due to happenstance: where you happen to be, who your mentor is. I think two identical people with different mentors will take different directions.

What were some of the feedback that you were getting that the scientific requirement to work at 1mm was worth the ask?

Doeleman:

There was a lot of faith. There was some feedback in that we got some new results at 3mm. The VLBA then extended its operation to that new frequency. This is what always happens. You're in the vanguard, and you do some first measurements at a frontier frequency, and then a national facility will move into that space. So, in the mid '90s, the VLBA started to become available at 3mm wavelength, but still working with a very narrow bandwidth. There was a limit to what even the VLBA could do. There was a limit to the number of targets it could look at, and especially for Sagittarius A*, it was difficult even for the VLBA, to make much progress there because it was limited in bandwidth. So, then, moving to 1mm in the late 1990s, I began some experiments in Arizona, linking to telescopes in Europe. We had a very good and strong relationship with the Max Planck Institute in Bonn, and we were working with their team. They had some success detecting Sagittarius A* at 1mm. In 1998, they published a paper that was really also pioneering, in which they detected Sagittarius A* on a baseline between France and Spain.

Why was this interesting? This was interesting because all of a sudden there was this proof of concept that you could detect Sagittarius A*, a pretty faint supermassive black hole, at that wavelength. I believe it was 1.4mm. The problem with that detection is that the baseline was not long enough to get the angular resolution to clearly show what the size was. They derived a size limit and indeed the size that they felt was the best fit was much larger than the lensed photon ring of Sagittarius A*. If you believed that result, you would have naively thought there's no point in trying to image a black hole, because we're only going the see the surface of last scattering. The hot gas surrounding the event horizon would prevent us from seeing the shadow. So, this was a case where the first result was interesting, but it kind of led us in the wrong direction. And I think it actually served to diminish people's interest in SgrA* for a while, because they said, look, there's no point. We've reached the end of the road. We can't do any better than this size. But I always felt that the calibration of this result was uncertain. I thought, maybe this result, as interesting a demonstration as it is, is not the final word. I really felt as though the final word had not been written on Sagittarius A*, and the real key was to go from a shorter, inter-European baseline, to a transatlantic, or a transpacific baseline from Hawaii to the continental United States.

So, we launched a number of experiments between Europe and the U.S., and they all failed because we just didn't have the bandwidth. You have to understand that with those long baselines, the angular resolution is so fine that you're only going to be looking at a small fraction of the energy emitted from Sagittarius A*. If you had a short baseline, your effective resolution would be so coarse that you'd see all the energy from the black hole. That's what they were seeing with the baseline in Europe. When you go to much longer baselines, you start to receive exponentially less flux density, or exponentially less energy from the black hole, and you need to be very sensitive to do that. So, we retrenched. We said, let's back up, and let's start doing 2mm, or 150 GHz VLBI, and then we were finally able to make an intercontinental detection between Europe and Arizona at that wavelength.

It's hard to know. I think at this level it's all personalities. It was all convincing the director of CARMA, who I think at the time was Leo Blitz. I told him we had all this other telescope time lined up, and it's really important that CARMA is part of it. And I think that the topic, the enthusiasm, the fact that he saw that people were willing to go to the mat for it, convinced him that, you know, I think we should give some time to this project. I think they're glad they did, and I was certainly immensely grateful that they did. But it's moments like that that drive home the fact that you need champions for this kind of work. You need people that are just going to go and get it done and convince people that really it should happen.

That's a good question. I'm not sure if there was any one person at the National Science Foundation who championed this. It's not as though there was a person like there was for LIGO, who championed that particular project. I think, even though we saw the finish line, I was always seen as high risk, and it was not identified as a key project. I'll give you an example. After the SgrA* detection, well-received results which we published in the journal Nature, I wrote a white paper outlining the whole way forward to the US Astronomy Decadal Review. Every ten years, all of U.S. astronomy gazes at its naval and says, what are the big projects to do? And I wrote a paper with colleagues describing the clear path to imaging a black hole, and I went and gave in-person testimony to the panel. Then, it turned out that the Event Horizon Telescope as I described it was not mentioned in the entire decadal report. It just didn't appear. Even though some people realized this was a possibility, it was just not even worth mentioning, according to the gurus of astronomy in the U.S. And so, that's perhaps why we didn't have a champion at the National Science Foundation. There was nobody who really was going to take up the torch.

If the cognoscenti of astronomy did not see fit to put it into the decadal review, who are we to step into this? It didn't diminish our enthusiasm at all, and we just kept writing grant proposals that were successful, and ultimately, I was able to raise millions for this project, but we didn't have a champion. In fact, I do remember talking to one National Science Foundation official, and I told them "Look, this is really a fantastic project, and we really need to get more funding for it." And they said, "Look, the most I can really see this getting is maybe half a million dollars. It looks like it's all ‘blobology’ to me. You're going to look at a blob. Maybe you'll see something, maybe you won't." Now, that was one interaction I had with the NSF, but generally, I have to say that they really came through, because we submitted our proposals through peer review process, they were highly rated. So, the NSF had a system in which, even though we were not mentioned in the decadal review, there was still room for reviewers to say, "This is good stuff. We need to invest in this." And all my proposals got consistently very high marks, and we were able to push this forward.

There was another key development which really needs to be discussed if you're going to talk about why the EHT succeeded. We had already solved the bandwidth problem. So, now we were able to multiply by orders of magnitude the bandwidth we were recording, but even that was likely not enough. The last key development that we thought was going to make the EHT a success was phasing up the ALMA Array. Now, ALMA was this site that was coming online in the late 2000s and the early 2010s, that was roughly 60 dishes, each 12-meters diameter in one of the best observing locations for millimeter and submillimeter astronomy on the planet. But in order to make it work for the Event Horizon Telescope, we had to phase up all of those dishes to make them look like one large dish. The hooks to do that had been built into the system, but nobody had ever completed it.

So, recognizing that that was the key, that was the next big hurdle to jump over, I started working with some engineers at the National Radio Astronomy Observatory. Rich Lacasse, Joe Greenberg, Ray Escoffier -- these were the people who had built the electronics for the ALMA Array. Also, I was working with Jonathan Weintroub at the Center for Astrophysics, and Rurik Primiani on this, and over the course of a couple years, starting in 2009, we came up with a really interesting design that would allow all the dishes in ALMA to phase up together, to act as essentially a 75-meter effective dish. Now, bandwidth, as you can imagine, increases our sensitivity, but only as the square root of bandwidth. The diameter of the dish, though, that goes in linearly. So, you wind up getting a huge increase in sensitivity by adding a very, very large dish to your array. ALMA was going to increase the sensitivity of the Event Horizon Telescope by a factor of 10. So, we really wanted to add that to the array.

We went to the ALMA board, who was very reluctant to do anything with us, because they were busy just getting their facility online, and they didn't want to see us coming. We were an unnecessary complication. And I wrote to them, and I said, "Here's a way we can phase up ALMA for VLBI." They wrote back and said, "This is very interesting, but it's too complicated." And it was complicated because we were going to use electronics at each individual telescope to adjust the phase to bring everything into alignment. And they said, "We don't want you to do that. You have to come up with a scheme and a system that will only use the central electronics in the central building. We don't want you changing anything at the telescopes themselves." So, we went back to the drawing board, and we found a way, using some of the electronics in the central location, to make those minor adjustments for each incoming stream of data from the individual telescopes. And then we went again to the board.

Meanwhile, I wrote a proposal, one of the biggest proposals I had ever written, to the National Science Foundation saying we want to phase up ALMA. But I couldn't submit that proposal until we had a letter from the board of ALMA that said, "We grant you permission to do this." That was really a race against time. So, we wrote to ALMA, and they were deliberating and deliberating, and that's when again I called in favors. I called in French collaborators, and I said, "Look, you've got to call the chair of the ALMA board and tell him to write this letter." The chair of the board at that time was Laurent Vigroux who was a French astronomer. We called in all the favors we possibly could, and ultimately, with about 2 or 3 days to spare, I got the letter from the ALMA board saying, "We agree to go forward with this plan." And I was able to put that into the proposal, and then I sent that to the National Science Foundation. That proposal received the highest marks I've ever gotten from any proposal ever submitted, because I think all the reviewers could see this is the key, the EHT was possible, and this could be potentially transformative.

Zierler:

You're also building a sequence of near-misses here, between CARMA and then ALMA, where the whole thing could have fallen apart quite easily.

Doeleman:

You know, you're exactly right. Imagine that we had not gotten that letter. That might have delayed the phasing up of ALMA for years. Right? It's just little things like this. If you don't push it, if you don't devote yourself heart and soul to something like this, there are many different points where it can go off the rails. That's the human dimension to some of these efforts.

Zierler:

Shep, to go back to this idea that you've made the measurement, and then the race is on, the excitement is on. To go back further to where you said you fully embraced Moore's law as a matter of faith that you'll keep pace with the instrumentation, and the instrumentation will keep pace with you. Was there every any concern that you were moving so fast that the things you wanted to do would not be available with off the shelf technology from 2009 to 2019?

Doeleman:

Yeah, that's a good question. It was a little bit the opposite. We had to make sure that we were using with optimal efficiency what was available to us. So, we went through a number of generations of this equipment. The first piece of equipment we built used one generation of field-programmable gate array, that allowed us to collect a lot of bandwidth. That was, let's say, 4 gigabits per second. That, in and of itself, was a factor of 8 faster than any other VLBI network on the planet. And then we built another version called the RDBE. That was about the same bandwidth, but it was more flexible. And then, after we got an influx of funding from the Gordon and Betty Moore Foundation, which was absolutely critical, we developed in very short order a device that allowed us to go to 32 gigabits per second. And then, ultimately, to 64 gigabits per second. So, the same devices, the final generation, the third generation of these backends, we started building in 2012. A wonderful postdoc, Laura Vertatschitsch was the driving force behind those, as well as Jonathan Weintroub and a lot of engineers and scientists at the Smithsonian as well as well collaborators at Haystack Observatory that used commodity, off the shelf equipment, to get us to 32 gigabits per second. So, we were keeping pace with Moore's law. We were not out in front of it at all. Partly, the reason for that is you don't want to be on the bleeding edge.

You want to be on the cutting edge, but not on the bleeding edge, because if you're too far, then you can build equipment that may not work right off the bat, and we couldn't afford that. We wanted to use tried and true commodity electronics so there was no doubt that it would work. Then we could build robust systems that would operate at high altitude, which means they couldn't be running at the limits of their cooling capacity. We couldn't be running these components super hot, because it’s difficult to cool electronics at these rarified altitudes to keep them in their normal operating ranges. We needed to be just shy of bleeding edge, and ultimately, we were able to get to 32 gigabits per second in 2017, which was about a factor of 16 faster than any other network on the planet at that time. We were already an order of magnitude wider bandwidth than other networks, and with ALMA phased we achieved breakthrough sensitivity. All this came together in 2017.

Zierler:

Shep, on the administrative side, what were some of the considerations leading to you being founding director of EHT? In other words, do you have people specifically that report to you within that context, or is this sort of extracurricular from Haystack?

It turned out, what had happened is that the PR people at the Max Planck Institute had used an old internet link for their new announcement. So, the old address was still floating around out there, and somebody had accidentally typed it in, and seen the full black hole image press release. It was a journalist, but thankfully, that journalist was someone of great integrity, and they reported it to us first. They said, "Are you sure you want this to be public yet? This seems really irregular, and I want to check before I write a story about this." And we said, "Thank you. Please do not write the story." The head of the AAS, the American Astronomical Society press corps was informed about this, and he got in touch with me. But we were able to diffuse this, and we were able to get people in the very early morning hours in Europe to close off this site. I was able to convince the National Science Foundation not to release anything, and we wound up making a very nice announcement that morning.

Zierler:

Obviously, this is going to shake the physics and astronomy, but did you have a sense, and were you prepared with the communications team, that this was going to be all over the newspapers and cable news?

Doeleman:

We were not prepared. I mean, we knew it was going to be big. We were calibrating ourselves with LIGO, for example. I mean, LIGO was a big story. But somehow, the visualization of this black hole wound up being much larger.

Zierler:

And for whatever reason, the public has some grasp of a black hole much more than gravitational waves. Like, the man on the street has heard of a black hole. Probably not gravitational waves.

Doeleman:

Well, you know, it's more than that. It's that the people on the street have some idea of what a black hole might look like because it's in literature, because it's in the movies, because it's part of science fiction. It's part of our culture, and it represents the deepest mysteries. If you don't know something, you throw it into a black hole. If you want to time travel, you go into a black hole. I mean, Interstellar showed us that you can put anything into a black hole, even bookcases, and you can communicate with the power of love from inside a black hole. There appears to be no limit to what can happen in a black hole. It's the tabula rasa. It's this empty notepad you can write your heart and dreams into. Really capturing the essence of the black hole, showing people what it looked like resonated deeply across cultures, across borders. Everyone was able to appreciate the significance of it. And going even further, it was because this was seen as being really impossible.

Maybe hearing a black hole with gravitational waves, you can kind of think, maybe that can happen, but the very nature of a black hole is that it forms an event horizon, that light can't escape from. And just simply stated like that, the average person, even if they're well-informed, thinks that imaging one is an impossible problem. At the very least, they say, that's a really hard problem. And then when you say, we had to build an Earth-size telescope to do it, and we had to draw on expertise from around the globe to do it, and we had to use the determination of early career astronomers, and the experience of veteran astronomers, and we had to organize this politically across the globe, people, I think, see it as a human endeavor in which we were able to tackle a hard problem and solve it. With the pandemic that we're experiencing now, with climate change that we're experiencing now, even healthcare, food distribution, these are all seen as being impossible problems. Almost intractable problems. The only answer will be a global response. The only answer will be a solution motivated through coherent vision and drawing on the world's resources. The Event Horizon Telescope showed everyone that we could rise to these types of challenges. I think that was the uplifting part of it. There was a science part, there was a wonder part, there was an against all odds success part. But then there was the uplifting part that we had done it as an endeavor of humankind, not just as science. And I think that's what spoke to people.

The primary way they aim to study general relativity and the progenitors of black holes is to analyze many, many events. Now, imagine if the EHT could do the same thing. Right now, we're complementary to LIGO because we have only a couple of sources, but we can observe them over decades. We can observe the dynamical changes. We can observe the turbulence and the accretion flow as a function of time. We can observe the changes in magnetic field topology as a function of time. When M87 and SgrA* exhibit bursts in the radio spectrum, we can localize those radio bursts and see exactly what drives those. So, it's a completely different view of black holes. If we could further extend that by launching a satellite into space, for example, and creating a telescope larger than the surface of our planet, then we would have access to tens or hundreds of black holes. Then we can start to look at the spin and the mass of these black holes over cosmic time, using radio interferometers that link the Earth to a space platform. That's something to dream about. So, there's a whole new dimension that we can explore into. This is just the beginning. It's just the beginning.

Zierler:

Shep, last question. For you personally, and for EHT generally, to what extent is the imaging of a black hole the launchpad for everything else to do, and to what extent do you not want to be specifically bound to one discovery at a particular place and time, because the universe is big and there's lots to learn?

Doeleman:

Yeah, that's a great question. So, with all great discoveries -- same thing goes for LIGO -- this is the start of something. It's easy to look this image and say, okay, let's put a bow on that and it's done. But really, this is just the start. We'll get ever better observations, ever better resolution on the event horizon. We can start to ask daring questions about whether the internal quantum states of the black hole might be manifest on horizon scales. We can only do that with much better precision than we have now. So, there's a whole new avenue of questions one can ask as we refine these measurements that bear on the information paradox, for example.

Similarly, understanding the true nature of jets and how black holes launch them is still a very open question. We can make the most detailed observations of that process in the future. So, there are multiple directions that we could go in, even on tests of Einstein's relativity. Right now, we're looking at light bending around the black hole and making some statement about how well it agrees with general relativity. But imagine that we could time the orbital periods of matter as it circularizes around the black hole. That will give us an entirely new way to test GR, and also to measure the spin of the black hole.

One of the things I like to point out is that the size of the shadow is relatively insensitive to spin. If you look at a non-spinning black hole, what we call a Schwarzschild black hole, then the size is about 5.2 times the Schwarzschild radius. If you spin the black hole maximally, so it's the most rapidly spinning black hole you can have, it shrinks only to about 4.5 times the Schwarzschild radius. So, a very slight change, but the spacetime of course is much different around a spinning black hole than it is around a non-spinning black hole. That's because even though the photon orbit is shrinking, it gets magnified more strongly by the spacetime of the spinning black hole. So, these cancel each other out, and you wind up seeing the same approximate shadow size. But the period of orbital matter around the black hole, at the innermost stable circular orbit, changes by an order of magnitude. For Sagittarius A*, it ranges from 4 minutes if the matter is co-rotating with the black hole, to an hour if it's counter rotating. That's a huge difference. So, if we see a signature that's 4 minutes in length due to the orbital period of matter around the black hole, that will nail the spin. And if it's an hour, then we'll know that it's spinning in the opposite direction, and we’ll be able to make a very good measurement of the spin of the black hole. So, these are all new ways of using time domain radio astronomy and high precision imaging that will lead us in entirely new and unexpected directions. So, I'm as excited as ever by what's to come.

Zierler:

Shep, this has been a fantastic conversation. I want to thank you so much for spending this time with me. I really appreciate it.