Richard Ellis

Richard Ellis

Professor of Astrophysics, University College London

By David Zierler, Director of the Caltech Heritage Project

August 17, 2023

DAVID ZIERLER: This is David Zierler, Director of the Caltech Heritage Project. It's Thursday, August 17, 2023. I am delighted to be with Professor Richard Ellis. Richard, thank you so much for joining me today.

RICHARD ELLIS: It's a pleasure. Nice to be back at Caltech.

ZIERLER: That's right. Richard, just why you're here, tell me what brought you back to Pasadena. What's the event that has you here?

ELLIS: Well, I reached 70 years of age in 2020, and I was here at Caltech for 16 years, so I had a whole bevy of great students and post-docs. And it was suggested there be a 70th birthday conference in my honor. And of course, I spend a lot of my time in the UK, so in addition we planned a parallel meeting there. And then, something called the pandemic hit. [Laugh]

ZIERLER: Yes.

ELLIS: And so, here in 2023, both conferences were held and called RSE, which are my initials, at 70-plus. Plus being three years.

ZIERLER: You're still 70. [Laugh]

ELLIS: I'm still 70 at heart, yeah. It's been great. We just had a two-day meeting and a conference dinner, and I'm on an all-time high because it was great to see all my former students, who are all doing extremely well. And that's what Caltech does well, it trains these students to be leaders in the field.

ZIERLER: That's right.

ELLIS: And they've all done very well.

ZIERLER: Seeing your students in this venue where they're all talking about their research, is there an academic lineage? Did you sense a commonality in some of the themes of their presentations? Which is also probably a window as to cutting-edge observational cosmology today.

ELLIS: Yeah, well, there are many answers to that. Firstly, they're all doing different science, but the common theme is the enthusiasm that they picked up here at Caltech which is with them still, and they've learned how to have fun doing science, and they've transferred that to their own students. Now, of course, most of them are in their 40s. The students presenting at the London meeting are now in their 50s because I left the UK in 1999. Nonetheless, the common theme is curiosity, enthusiasm for doing science, and working with young people. And you only find that spirit in universities. I've worked in observatories and government institutions, and they have important roles in science, of course. But there isn't the vibrancy of working in a university department.

ZIERLER: Mentorship is unbeatable.

ELLIS: And working with young people. They put you on the spot. You can't be complacent with a bright young person. He or she says, "Well, why does it work this way?" And if you don't know the answer, you look stupid, so you'd better get it right. [Laugh]

ZIERLER: Just to orient researchers who will be reading or listening to this interview, oral historians can't get enough of you. There's one on file at the AIP, as we discussed.

ELLIS: Yeah.

ZIERLER: And you did an epic eight-part series with Heidi Aspaturian at Caltech–

ELLIS: That's right.

ZIERLER: –in, I believe, 2013. Unbeknownst to you at the time, you were on your way out. But you didn't know that during the interview.

ELLIS: That's right. I'm very proud of that oral history. She did a great job. And I would recommend people to go back because it covers my childhood, family background and early education. (see https://oralhistories.library.caltech.edu/234/). For example I went to London for my undergraduate studies. Amazingly, that's where I am now. I've done full circle. I went to Durham University in the north of England as a postdoc eventually getting tenure as a professor. I went to Cambridge as the director of the Institute of Astronomy following many eminent people like Arthur Eddington, Fred Hoyle and now Martin Rees, who's still alive as Astronomer Royal. That was a really exciting place. And then, one day, I got a phone call from California asking if I'd ever thought of moving to the United States? And I replied "Frequently." So in 1999, we jumped ship.

ZIERLER: Who made that call? Who got in touch with you?

ELLIS: Chuck Steidel, who's still here as a professor. Of course, I'd visited Caltech. Who hasn't? It's the mecca of world astronomy. And of course, I often wondered what it would be like to be here. Although I was happy at the Institute of Astronomy, I did have itchy feet. I didn't want to stay in Cambridge indefinitely. I've always been an adventurer and I wanted to work in the United States. Eventually I received two job offers, one from Caltech, and one from the University of Hawaii. Ultimately, there was no choice. Caltech was such a great place – how could I refuse?

ZIERLER: Plus, you'd get as much Keck time from Caltech as you would from Hawaii.

ELLIS: Yeah, and that's one reason why I came. Britain did not have abundant access to 8-10-meter class optical ground-based telescopes.

ZIERLER: Or blue skies, for that matter. [Laugh]

ELLIS: Or blue skies, yeah. And I was doing faint-object astronomy. Not everybody does what I do. Some astronomers study stars, some study nearby galaxies. But in the 90s my research was increasingly focused on studying distant, high redshift objects seen when the universe was young.

ZIERLER: Was that prompted by technological advances?

ELLIS: Yes. And if I can advertise my book, When Galaxies Were Born: The Quest for Cosmic Dawn (Princeton University Press, 2022), it describes how technology enabled us to make progress in studying faint, distant objects. Examples include CCD detectors that were first developed at Bell Labs, robotic positioners that enable us to study hundreds of objects simultaneously, bigger telescopes, larger mirrors, all of that happened during my career. I exploited these advances to extend our knowledge of distant galaxies.I felt I'd done everything I could in Britain. The biggest telescope we had was the William Herschel in the Canary Islands. It's a 4.2-meter telescope built in the 1980s. And I used that very extensively in Cambridge in the 1990s. But then, suddenly, there were two monster 10-meter Keck telescopes in Hawaii, and we were just being completely outclassed. Caltech was just racing away into the sunset with all this wonderful data.

ZIERLER: Now, circa 1999, was anyone talking about ELTs? Was the TMT on the agenda?

ELLIS: It was apparently, but I didn't find out until a little bit later.

ZIERLER: So that was not part of your decision-making?

ELLIS: It was part of Caltech's decision to hire me I think. There were many people they could have hired, and they tried to hire a very famous guy, now a Nobel laureate, Reinhard Genzel. He's German, and he came out here. And the story is, and I don't think it's a secret, that his demands were quite formidable. And in the end, Caltech couldn't meet them. And so, there was a guy here at the time called Roger Blandford, a very famous astrophysicist.

ZIERLER: At Stanford now.

ELLIS: Who's now at Stanford. And he said, "Who else could we try –why don't we ask Richard Ellis if he's interested?"

ZIERLER: Where do you know Roger?

ELLIS: Well, he's British and got his PhD at Cambridge. He was offered the Cambridge professorship that I eventually got in 1993, but he turned it down because his wife didn't want to move back to England. We're close and crossed paths many times. And the head of Physics, Math, and Astronomy at the time was Tom Tombrello. You've got to read his oral history, it's fascinating.

ZIERLER: One of the legends of Caltech.

ELLIS: One of the legends of Caltech. And he was amazingly effective. I'll give you just one anecdote. When I was offered the job as Director of the Institute of Astronomy in Honolulu, Hawaii, I said, "Well, I'd like my wife to visit Hawaii." And they said, "Of course, yes, it's a big decision for you. You really need your wife to come and visit." We were preparing to visit from Cambridge, England to Honolulu. And then, I got an email from the people at the University of Hawaii saying that, as a state university, they wouldn't be able to pay Barbara's airfare. And I said, well, of course, I understood that. They knew I was being courted by Caltech and would visit Pasadena en route. So I said, I'll just use airmiles for my wife's air ticket for the sector from Los Angeles to Honolulu. But then I got an email from Tombrello saying that Caltech would pay for Barbara to go to Honolulu to see that it wasn't the right place for us to go. [Laugh]

ZIERLER: [Laugh] That's great.

ELLIS: And I suddenly thought to myself, "Goodness, it seems you can achieve things at Caltech you can't achieve anywhere else." Barbara said, "I think you'd better go to Caltech." And remember, we were leaving our grown-up children back in London. From Honolulu to London is two overnight flights so not great for visiting family back home. So Barbara was on board and we finally came to Caltech. And clearly Tombrello already had a role in TMT for me in his mind.

ZIERLER: But the concept of ELTs was already circulating.

ELLIS: Definitely. I'll tell you where the TMT story began – it's in Heidi's oral history of Tombrello. The second of the twin Keck Telescopes had just been built in '97. There was some anniversary at Owens Valley, and Steve Koonin (Provost) went up to Tombrello and said, "When are you going to give me the proposal for the next big thing?" They were courting me from '98 to '99. In '99, although I was still in Cambridge, I got invited to a meeting at the Keck headquarters where we met our University of California partners. And in a little room there, the California Extremely Large Telescope (CELT) partnership was established. Jerry Nelson was there as the project scientist for Keck and later TMT. But I was still employed in England. I didn't arrive here until September '99. So I think Tombrello saw my appointment as a catalyst for starting TMT. Of course, many played a part. The late Wal Sargent was another, he was in the room in Hawaii and a major figure in getting TMT started. A fellow Brit, actually.

ZIERLER: And we saw Anneila at the dinner the other night.

ELLIS: Yeah, we saw Anneila at the dinner. Poor Wal. I overlapped with him here for 12 years. He was a great mentor to me.

ZIERLER: A gentleman astronomer, he's been referred to as.

ELLIS: Well, he could be grumpy! Anneila probably won't mind my saying that. But he was a giant in the subject. And died prematurely, of course. Very sad.

ZIERLER: I want to pick up the story after you did the oral history with Heidi in 2013, before you chose to leave. We'll get into your decision-making about why you left. But once you had made the decision to move on from Caltech, I wonder if it was an opportunity to reflect about the big questions, why you had come here, what you had accomplished, what's unique about Caltech, and as we move closer to the present, as you visit here now, what you miss about Caltech.

ELLIS: Yeah, all good questions. Firstly, it was probably the best decision I ever made in my career to come here. It was like a breath of fresh air. Suddenly, anything seemed possible. Remember, the financial situation in the US was at an all-time high in the early 2000s. Tombrello thought a 30-meter telescope was entirely feasible, that we would find donors, so we got started on that right away. I was originally appointed as Wal's deputy director of Palomar Observatory with a responsibility also for Caltech's share of the twin Keck telescopes. But more or less, as soon as I arrived, Wal resigned. He didn't get on well with Tombrello. Tombrello could be quite a pushy guy.

ZIERLER: Great but imperious, I've heard him described.

ELLIS: Yeah, that's right. Wal did not like being pushed around. Six months after I arrived Wal quit and the baton was handed to me. I became director of Palomar in, I think, May 2000. Making the case for TMT was a major activity. I had to consider questions such as, "What's the future of Palomar?" In the UK there was an unwritten rule in astronomy that when a big new telescope comes online, the oldest one in the system has to be shut or repurposed to save operational costs.

ZIERLER: But the sky surveys were already well underway at that point.

ELLIS: No, they'd more or less finished, actually. I decided initially to try and rejuvenate the 200-inch. We developed new instruments during my directorship and I tried very hard to get partnerships with detector companies. At the time we were moving from using CCD detectors, which are used for optical imaging and spectrographs, to infrared detectors. At Caltech the infrared was always seen as the frontier following the pioneering work done by Gerry Neugebauer and Tom Soifer, who's still here.

ZIERLER: Infrared is not solid state?

ELLIS: Infrared detectors are solid state devices, but not based on silicon. The relevant materials are either a compound of mercury, cadmium, and telluride, or indium antimonide. These detectors are more difficult to manufacture, much more expensive, and they weren't available in large formats. Under Tombrello's enthusiastic push, as director I was trying to get access to large state-of-the-art infrared detectors by investing in companies that were testing them. And the company in the vanguard was Rockwell Corporation in Thousand Oaks, which is now called Teledyne. This was a fascinating time during my five years as director. Of course, I was also involved in planning future instruments with Keck.

Back to your big-picture question, at Caltech I felt I was at the center of the universe. I was doing great science with Keck, I was guiding the astronomy department in the exploitation of Keck and Palomar, I was overseeing new detectors and instruments on the telescopes. And at the end of my five year term, I handed the directorship over to Shri Kulkarni who had a very different vision. He realized that transient astronomy, the study of celestial objects that change, explode, or vary, was the frontier. And he very carefully reengineered Palomar to exploit that area. He remained as director for, oh, gosh, 10 years or more. So I focused my research on high redshift galaxies. Of course, I enjoyed teaching at Caltech, too. And I had these great students. My wife was working in public relations. She was a writer for Engineering & Science Magazine for alumni, which was a very high-class product. Probably a little too highbrow for the modern world.

And then, in 2006-7, came the turndown. And Caltech had to lay off a lot of people, including Barbara. Barbara started doing part-time work. So we contemplated going back to Europe. I could see Barbara wanted to go home. And so I applied for what is called a Royal Society Research Professorship. These are very competitive. Across the whole of science, there are only maybe three or four a year. And they are purposely research professorships. You're not allowed to be doing any administration or teaching. Although they're five year appointments, one is guaranteed tenure at the end. I applied to hold one of these at Oxford. I'd been at Oxford before as a PhD student. Anyway, I got the professorship so off we go in 2008, and Tombrello was very smart [Laugh]. He said, "Well, look. Richard has to go back to England to show Barbara that he's not happy there, and that he'd rather be back at Caltech."

ZIERLER: [Laugh]

ELLIS: He said to me, "Richard, why don't you go for six months? We'll keep your job here. You can have six months here, six months in Oxford for a couple of years to try it out." Of course, we chose winters in Pasadena, summers in Oxford. [Laugh] So off we went, and we hated it. Oxford was incredibly stuffy. We weren't that welcome. Arriving as a senior professor I had sort of "parachuted into" the department and several Oxford faculty clearly indicated: "Well, I didn't vote for this person." That never happened at Caltech. I also arrived here as a senior person, but was immediately welcomed. Nobody said, "Why did we hire this guy? Why didn't we promote one of our own?" But in Oxford, there was a lot of cold-shouldering.

ZIERLER: I wonder if the Cambridge affiliation had anything to do with that.

ELLIS: Yeah, maybe. But I think the attitude in Oxford was, "Here he is. He's been in California, comes back with a sun tan, and he wants to reorganize the department." Actually I did not wish to advocate any changes and in fact kept my head down . I negotiated a junior position, and we interviewed for that, and made a good appointment. When I came back to Caltech after the first six months, Tombrello had been replaced by Andrew Lang as the PMA division chair. Poor guy who committed suicide. And Andrew said, "We miss you." And I said, "God, well I miss Caltech." He said, "Well, come back." So I did. And Barbara said, "I could see you were unhappy." Hubble was doing great, Keck was doing great. And in 2011 I got an enormous amount of time on Hubble to observe what is called the ultra-deep field. We took the deepest image ever with Hubble–it still is the deepest image–in several infrared filters. And we found galaxies as distant as a redshift of 12.

ZIERLER: For a sense of perspective, how far out is that?

ELLIS: Well, in terms of what we call "look-back time" it's going back to when the university is about 3% of its present age. Even with James Webb now, we've only looked a tad further back.

ZIERLER: This is, like, 380,000 years after the Big Bang?

ELLIS: No, you're confusing this with the microwave background. Let's just get some terminology straight here. The universe began 13.8 billion years ago with the Big Bang. It expanded and cooled, and then the hydrogen atom formed. And we see the glow of what we call the microwave background from that period, 380,000 years after the Big Bang. But there are no stars, no galaxies. The universe then goes through a period we call the dark ages, where there's no starlight. And then, eventually, the first galaxies form about 200 million years after the Big Bang. The galaxies we found in 2012 were being seen about 400 to 500 million years after the Big Bang.

ZIERLER: This is to say that theoretically, you can go back another 200 million years to the very first galaxies?

ELLIS: No, the first galaxies took 200 million years to form. This moment when galaxies first emerged out of darkness is euphemistically called cosmic dawn, which is the theme of my book. It hasn't yet been pinpointed. We're getting very close with James Webb, and let's talk about that later. If anything, I did more exciting research after the Oxford about-turn than I did earlier. When I came back from Oxford in 2008, I focused almost entirely on this very high redshift stuff, whereas before I was doing a broader portfolio of astronomy. So no regrets about coming back.

So why did I eventually leave Caltech in 2015? I was 65, and there were several domestic issues. If we stayed in the United States indefinitely, it was necessary to become a citizen. I wasn't a citizen, I had a green card. For example, if we wanted our children to inherit our property in the US, we more or less had to become citizens. It would've been a nightmare if we weren't. On the other hand if we did become US citizens and then went back to England, it would actually be quite complicated. We'd have to file taxes every year, we'd be taxed in both Britain and the US and have to claim on a US-UK treaty. Very complicated.

ZIERLER: Oof.

ELLIS: Yeah. America is one of the few countries that taxes you on your worldwide income. And Barbara was pushing very hard to go home now. We had our first grandchild in 2009.

ZIERLER: Oh, game over. That's it.

ELLIS: Yeah. We had the second one in 2012. And so, she was itching to go home. So I applied for two jobs. The first was to secure a European Advanced Grant at a UK university. These are generous grants indeed; 2.5 million euros for five years, which enables you to build a big group, and you can choose which university to hold it.

ZIERLER: Did you make it known you were looking? Did Caltech have the option to try to retain you?

ELLIS: No, I didn't broadcast my intentions and kept the options under wraps.

ZIERLER: Because your mind was made up, it didn't matter.

ELLIS: Well, let's step back a bit. When Chameau was president and I was planning to go to Oxford in 2008, various people tried to stop me and said, "You're nuts going to Oxford. Why do you want to do that? Everything is here." Chameau came to my office, actually. It's not often the president comes to your office unannounced. He sat down with me and pointed out of the window. He said, "Look at the weather. You're returning to England. What are you doing?" And of course I did return. But second time around in 2015, my mind was pretty well made up. Barbara had convinced me we ought to go back. We had decided the end game was to be in England.

ZIERLER: But ironically, a lot of people from Europe come here when they reach retirement age in Europe because they have to go emeritus.

ELLIS: Yes, that's right.

ZIERLER: That was not a problem for you?

ELLIS: No, Britain doesn't have retirement anymore in academia. So I'm still teaching and still fully employed at 73. Anyway, I applied for this European Advanced Grant. The second position I applied for was at the European Southern Observatory HQ near Munich, Germany. I should probably describe this organization. It is really astronomy big-time. It operates the European large telescopes in Chile.

ZIERLER: And you will be competing against TMT.

ELLIS: Well, initially competing with Keck but ultimately yes, ESO was beginning to construct the 39-meter Extremely Large Telescope (ELT). ESO is the equivalent, in astronomy, of CERN. The head quarters based outside Munich, Germany employs 600 people. They offered me an appointment as their senior scientist. They wanted me to get involved in the ELT, which is Europe's version of TMT.

ZIERLER: Let me just pause right here. In 2014, when the protests started among the native communities of Hawaii, was that also sort of a get-out-of-Dodge moment for you? Did you see that this might gum up the TMT?

ELLIS: That's not why I left Caltech. I want to make it clear, I didn't cowardly run away when things got troublesome with TMT.

ZIERLER: No, but what I'm asking is, in South America, absent those problems, did it seem like an ELT project there might be more buildable, more feasible?

ELLIS: Oh, that was obvious. And for the record, when I was on the TMT board of

directors, we had to decide where TMT was going. From 2002 to 2008, the project did

comprehensive tests of the qualities of several sites in Chile as well as Mexico and Mauna

Kea, Hawaii. I was convinced that Chile was the best choice.

ZIERLER: Wouldn't the NSF have objected on non-American soil?

ELLIS: Exactly right. The TMT board of directors voted to select the site based on the site-testing report as well as other factors. One argument, which I find now amusing, was that Chile wasn't a stable country and Mauna Kea would be more secure. Secondly, Gordon Moore, obviously a major supporter, was living in Hawaii, and it would be an insult to put the telescope somewhere else.

ZIERLER: He would've loved to see it built while he was still there.

ELLIS: Yeah, out of his bedroom window. And thirdly, it was argued that the NSF, who we knew at some point would be requested to contribute operational costs, would actually insist TMT had to be on US soil.

ZIERLER: No one was talking Canary Islands at that point.

ELLIS: No, not at all. It was not even considered by the site-testing team.

ZIERLER: This is a contingency later on.

ELLIS: The record is very clear, and incidentally, I have all the TMT papers. And at one point, I was going to donate them to the Caltech archive. They're all in my office in London.

ZIERLER: Let's hope when you do, it's because it's getting built, not as an obituary.

ELLIS: Yeah, I wholeheartily agree. Anyway, it was a very interesting TMT board discussion; I was the only person who voted for Chile. But I voted largely because the Chilean statistics of clear weather was by far the best. Mauna Kea had slightly better seeing and was a colder site which is good for infrared astronomy. But in terms of clear weather, all the Chilean sites were way ahead. And later we tried to establish a partnership with the Europeans so that we could trade time, share instruments and so forth, but the Europeans didn't want to play ball. The director general, the same guy who hired me later, Tim de Zeeuw, didn't really want to enter any form of partnership with TMT.

ZIERLER: One obvious concern if you were advocating Chile, now this is two ELTs in the southern hemisphere.

ELLIS: That's another argument.

ZIERLER: What's the response to that?

ELLIS: For most of the work that we're planning with TMT, the southern hemisphere is actually preferred. We knew at the time that the Large Synoptic Survey Telescope (now the Vera Rubin Telescope), which was the top-ranked decadal instrument in ground-based astronomy in 2010, was going to be a major US investment in the south. I thought having TMT alongside it was logical. The southern hemisphere has more nearby galaxies, the center of the Milky Way is there, most Galactic globular clusters are there. And for cosmological studies the sky is uniform, so there's no benefit being in one hemisphere over the other.

ZIERLER: But what about the issue of redundancy, having two ELTs in the south?

ELLIS: Well, at the time of the vote, you've got to remember that we were ahead of Europe so we'd make all the discoveries first. All of these counter-arguments you're making (Gordon Moore, NSF, two hemispheres) persuaded people like Ed Stone but not me. Synergy with Keck was another argument. It would save operational costs if we operated TMT within the same organization that's operating Keck. You'd save money on people, logistics, and so forth. I appreciated all those arguments, but on balance, I went for Chile. Of course, I lost. The majority vote was Mauna Kea. And then I was placed in the awkward position of being the spokesman at a press conference to announce our decision!

ZIERLER: Do I have it right that you were responsible for bringing the Japanese into the collaboration?

ELLIS: Yeah.

ZIERLER: Ironically, they must've been instrumental in the push for Hawaii.

ELLIS: Yeah, indeed they were dead against going to Chile as they were already operating the Subaru 8-meter telescope on Mauna Kea. I also brought the Canadians into TMT. At the press conference I had to answer questions from journalists, about the merits of Mauna Kea, even though I'd actually voted the other way. [Laugh] At the time my fellow TMT members and astronomy colleagues at Caltech were incredulous that I had voted for Chile, but in retrospect don't you agree things would have been much easier had we decided to go there?

So, back to my leaving Caltech, I secured this senior scientist position from ESO, and was also successfully awarded an European Advance Grant, which I decided to hold at University College, London. So, in 2015 we moved straight from Pasadena to Munich for two years. I could only be at ESO for two years because I would then be 67, the mandatory retirement age in that organization. And then, I moved, finally, back to London in 2017.

In spring 2015 when all this got clarified, Tom Soifer was division chair, and Ed Stolper was provost. I wrote to them and announced my departure in August. Tom Rosenbaum was fantastic. He said, "Oh, Richard, this makes a lot of sense. You've had a great time here. We'll miss you, but it makes sense that you should go back for family reasons." When I now look back on the 16 years I was here, it was fantastic. I worked hard on TMT. I was director of Palomar. I did a lot of excellent science. I got some amazing prizes. In 2008 I was awarded the title of Commander of the British Empire by Queen Elizabeth. It's one step below knighthood. I recall George Djorgovski burst into my office, and he said, "Congratulations on being Commander of the British Empire. What British empire?" [Laugh] And it's true. The empire has shrunk to a few dotted islands in the Caribbean.

ZIERLER: You can tell George at least it's bigger than the Serbian empire. [Laugh]

ELLIS: [Laugh] Yes, that's right. In 2011 I was awarded the Gold Medal of the Royal Astronomical Society, its highest honour. Chameau organized a dinner to celebrate my Gold Medal. Caltech is such a wonderful place. It really respects its faculty. When I look back on Caltech, there's nothing like being a Caltech professor. You have freedom, people respect you. You'd phone somebody in administration, and they'd respond, "How can I help you?" Let me tell you, at UCL an administrator is usually telling you bluntly what you can't do. [Laugh]

ZIERLER: Were you reasonably confident, given how crestfallen you were about your reception at Oxford. Is UCL a less stuffy place than Oxford?

ELLIS: To be frank I'm a bit disappointed and got a mixed reception at UCL too. Well you see, UCL is a gigantic university with over 40,000 students. Contrast that with Caltech. There are so many professors that you can't have the influence you have here. You can't walk to the Red Door and bump into the president or provost. You have to adjust your mindset. But I've done a lot of good science at UCL, and of course, now, we've got James Webb.

ZIERLER: What was the game plan when you arrived at UCL? What did you take with you?

ELLIS: Well, I had this large Advanced Grant. And, to be honest, I couldn't get that kind of long term support at Caltech. When I secured observing time on Hubble to take the ultra-deep field, I got $500,000 of grant money at Caltech. So I could pay for students, post-docs, and so forth. I could travel pretty well anywhere I wanted. But that kind of money doesn't come in every year. As you get older, NSF money gets harder to get. I was on various NSF panels, and there was an unwritten rule, which you might argue is justified, that grant money should be prioritized for young people building their careers. Even somebody as respected as Chuck Steidel was having difficulty getting NSF money. He couldn't fly to Hawaii regularly anymore. And I remember realizing that the amount of funding that I would have at Caltech was inevitably going to diminish, and I'd have to shrink my empire and set more modest goals. In contrast, when I landed in London I could immediately form a big group - three post-docs, two or three students and had ample travel funds.

ZIERLER: Did you take anyone with you from Caltech?

ELLIS: No. There were students that were still finishing but generally-speaking I have found Americans are reluctant to move to the UK. They had all kinds of personal reasons for staying here. So I had to set up shop anew. I had to advertise and go through UCL's annoyingly formal processes for hiring people. Once again I had to learn a new system. I have had to adapt every time I moved. When I was in Oxford, I had to adapt to Oxford. When I was in Cambridge, likewise. When I came here also, then again when I returned to UCL.

ZIERLER: Now, you stopped working in affiliation with TMT?

ELLIS: Yes. And there's a story there, which is a little unpleasant, and it's in Heidi's oral history. Because I was director of Palomar, or what became Caltech Optical Observatories, I was naturally a member of the TMT board. When Shri took over as director, he became a board member, and that was 2006, I think. But I had put so much effort internationally into bringing Canada and Japan into TMT that I was regarded as special individual in the project. I tried to get it formalized as TMT spokesperson when I returned from Oxford but it never happened. However, Andrew Lang convinced Ed Stone that I should remain a full member of the board. That was unusual because Caltech would then have more board members than the other partners, University of California, Japan, Canada, and so forth. Then one day in 2014, I was simply told that I wasn't a board member anymore. There was no, "Well, thank you for your fourteen years of service to the project." There was a board meeting coming up, and I just didn't get the papers.

ZIERLER: You were ghosted.

ELLIS: Yeah. There was a secretary, Holly Novack, who distributed the papers, and I emailed her and asked why I hadn't received any papers. And she replied with a brutal one-liner, " Your name was not on the Board Roster, so I did not send any announcements/meeting materials to you. " This was the only time Caltech failed in respecting my contributions but I decided it was not worth complaining.

ZIERLER: Was there concern that you were starting to ramp up with ESO, that you'd get involved with the ELT?

ELLIS: No, this was several months before I decided to apply to ESO. Indeed, this disappointing event contributed in part to convincing me to leave Caltech. I think that Ed Stone was uncomfortable with the special role I had and decided to terminate it. Representing Caltech on the TMT board included Shri, Tom Soifer and Ed Stone (in the chair). Shri was never a very big fan of TMT, I have to say, even from the beginning. So I think at some point Ed was embarrassed that I had this special role and considered it unfair to other partners. Nonetheless, I would've accepted his argument if he only had spared time to tell me and thank me for my contributions for all those years in person, effectively from the time I arrived to 2014. But, apart from that one blemish, everything at Caltech was wonderful. I'm still, of course, devoted to helping TMT.

Anyway, returning to 2015 I was setting up my new life in Munich discussing the ELT, a rival facility. Whatever you say about ESO, it's a well-honed organization. The funds are guaranteed by international treaties, so it's a completely different ball game to Caltech and UC.

ZIERLER: Are you surprised, then, that the Giant Magellan Telescope isn't further along than it is at this point?

ELLIS: The Carnegie Telescope? While I was here, we had various conversations with Carnegie about merging TMT and GMT, because these two were rival facilities. GMT has suffered equally. Not by political objections to the site, because Carnegie owns Las Campanas, but largely funding. If you consider the situation today, the real reason TMT and GMT aren't operating is a combination of politics in the case of TMT and funds in the case of both of them. Their costs have both spiraled. It's astonishing how expensive they are now. Now, we're relying on the 2020 decadal report, which gave high priority to both and getting NSF funds to complete them. Well, we're already in '23, and there's no obvious action, is there? TMT still doesn't have permission to build on Mauna Kea. It's a very sad situation given how far ahead of Europe we were in the mid-2000s.

ZIERLER: When you arrived at UCL, what were the big astronomy projects that were most relevant and exciting for you at that point? Was the James Webb already becoming a reality?

ELLIS: I wrote my Advanced Grant proposal in August 2014 when I was still at Caltech.. And in that proposal, entitled "First Light", I proposed to locate cosmic dawn, to look back far enough to see the first objects. James Webb, at that time, was going to be ready in 2017. It featured prominently in my proposal. When I arrived at UCL, disappointingly James Webb was delayed, eventually until 2022. But I've been very fortunate that my grant has been extended by three years. We've now had a full year of exciting James Webb data. Also when I arrived I began using the Very Large Telescope in Chile since Britain is a member of the ESO family. The VLT comprises four 8-meter telescopes so it is just as impressive as Keck and we've done some really great work with it pushing out to galaxies at redshift nine and studying them in detail.

One thing I'm proud of is that at UCL we estimated the ages of these high redshift galaxies. Say you don't look far enough back in time to witness the birth of galaxies directly, if you can measure the ages of some of the galaxies that are nearly at that distance, then you can estimate when they were born. We estimated the ages for six galaxies using the Chilean telescopes and, in 2021 before its launch, we predicted that James Webb would see galaxies beyond the horizon achievable with Hubble. That prediction has now been verified by the early Webb results. So that period at UCL, 2017 - 2021, has been very exciting. But turning to the present, James Webb has outperformed everything that we imagined.

ZIERLER: Spectacular.

ELLIS: It's unbelievably exciting. Its efficiency is better than expected and there's lots of publicly available data. Just since July of last year, when the first science data emerged, there have been over 120 papers on early galaxies. It's hard work to keep up, truly! And so, in my conference earlier this week, every talk presented new results. I've lived in an amazing time, to see all the way from photographic plates, through CCD detectors, to this monster JWST telescope 1.5 million kilometers away!

ZIERLER: Let's step back because as an observational cosmologist, you have employed land-based and space-based telescopes.

ELLIS: That's right.

ZIERLER: Broadly conceived, are you after the same questions with two different kinds of telescopes, or are you pursuing different questions based on the unique capabilities of land-based and space-based telescopes?

ELLIS: It depends on the science topic. But generally speaking, yes, they're complementary. Let's start with Hubble and Keck. From 1990 to 2015 or so, the partnership between Keck and Hubble was very important. Above the Earth's atmosphere, Hubble produced unique high-quality imaging. But it didn't have efficient spectrographs. In contrast, the larger aperture of the Keck telescopes made them more powerful. So we could match imaging in space with spectroscopy (chemistry, measuring redshifts) on the ground - a perfect partnership. We haven't talked about gravitational lensing, the bending of light by massive objects. I worked very hard to exploit clusters of galaxies as giant telephoto lenses in space that would magnify background objects that would otherwise not be visible - using Hubble to find such lensed objects and then determining their redshifts with Keck.

ZIERLER: These are basically galactic magnifying glasses?

ELLIS: Yes. That's right. It's like having a free telescope in space. You have to look in certain directions to utilize this property but massive foreground clusters act as free telephoto lenses in space magnifying background galaxies.

ZIERLER: What is the phenomenon that allows that to happen?

ELLIS: Einstein showed that light is bent by massive objects because it distorts the space around them. Think of space as a fabric, which is one way understand General Relativity. When you dump a massive object in space, like a cluster of galaxies, it distorts the adjacent space, and light travels on contours in that curved space. Space dips down like a wash basin and light then takes a contour around the basin a bit like water spiraling down the plug hole. The light is focused and, in certain configurations, you can see a background object in more than one position, which we call multiple images. The same object produces an image in one part of the sky and again in another part of the sky. Moreover, via the magnification the size and brightness of the object can be increased. Objects that would be too faint to see are miraculously brought into view. What kind of magnifications? Well, in certain configurations, the magnifications can be 20 to 50 times, so you can detect objects that would be impossible to see any other way. This began when I was in Cambridge with Hubble, but really, I put it into full flight when I got here and we started using Keck.

We would use Keck to scan foreground clusters blindly, in areas of the sky where we thought the magnification would be high, without knowing in advance what we would find. We'd take a Hubble image and say, "In this area of this cluster, the magnification will be 50. we don't know what we'll find." So we would go to this area of the sky with Keck and just move the telescope across this area, and magnified objects would pop up. We didn't know they were there beforehand, but we'd detect their magnified signal with Keck. We found feeble galaxies at redshifts 6-7 that were magnified 30 times. This was a huge success in exploring faint objects at enormous distances. This is a good example of space-ground complementarity. And that continued with Hubble when I was in Germany and at UCL. Now, with James Webb, the situation is different. Webb is a 6.5-meter telescope, whereas Hubble is only a 2.5-meter telescope, so Webb is much more powerful. It has its own spectrographs. That's the game-changer. It's a self-contained observatory.

ZIERLER: Meaning that you don't have to go back to land-based astronomy.

ELLIS: That's right. People are now a little worried, and this was mentioned briefly at my meeting, that Keck is no longer competitive in this particular area. We struggled with Keck to measure the redshifts of early galaxies, but you can often achieve an impressive result in only an hour with James Webb. The young people today don't realize how hard it was for us prior to Webb. So many astronomers now don't see any benefit in using the ground-based telescopes for this particular research area. Radically, some believe James Webb is so powerful we need to rethink what we should do with the ground-based telescopes, at least in studies of distant galaxies.

So here's an example. I'm building a multi-object spectrograph for the Japanese Subaru telescope which will do a survey of several million galaxies over a wide area of sky. That can't be done with James Webb because it has a very small field of view, whereas the Subaru telescope has a field of view of several square degrees. It's a panoramic telescope. With a panoramic telescope, of course, you can chart the sky and measure the three-dimensional distribution of galaxies, which is important for cosmology. We haven't talked much about cosmology. I was involved in the supernova studies that discovered that the universe is accelerating in the 90s, and I continued work on supernova when I was at Caltech.

ZIERLER: Did you have partnership with Harvard when they were doing this?

ELLIS: No, I was on the other team led by Saul Perlmutter at LBL, Berkeley. There were two rival teams. Kirshner was on the Harvard team with Adam Riess, who got the Nobel Prize, and I was on the team from Berkeley. And, in Cambridge, I was the UK lead of the Perlmutter team.

ZIERLER: Who was your connector there, Saul?

ELLIS: Yes. Saul and I were in it from the beginning. I did a pilot program with a colleague Warrick Couch and a Danish team in the late 80s where we found the first distant supernovae at a majestic red shift of 0.3. [Laugh] And that was regarded as a demonstration of the method. And then, I teamed up with Saul. Only three people can get the Nobel Prize. I was slightly annoyed that the other team got two, Brian Schmidt and Adam Riess, whereas our team only got one, Saul. But we all went to Stockholm, we had a great time. The story is amazing. Neither team ever imagined that they would discover that the universe is accelerating.

ZIERLER: Why was that beyond theory?

ELLIS: In the 90s, what kind of universe did we think we lived in? We thought we lived in a universe that contained matter – dark and baryonic. There were arguments about whether there was enough matter to halt the expansion. Most of us thought there wasn't. But the universe would still be slowing down because, like throwing a ball in the air, gravity slows it down, no matter how much gravity there is. It was simply a question of whether it was slowing down sufficiently to stop expanding in the future. Most astronomers believed otherwise and that the universe would expand forever, slowing down all the way but never stopping. Mathematically we needed to measure a "deceleration parameter". Both teams set out to make this measurement using distant supernovae as "standard candles," comparing their redshifts (the speed of the expansion at a given time in the past) with their distances (inferred from their brightnesses). Initially, Saul's team was ahead of the game. He was very smart and had the entire armory of the Lawrence Berkeley Lab behind him. Originally a particle physicist, some astronomers were skeptical of Saul. One famous astronomer said, "Saul Perlmutter is a particle physicist operating as an astronomer without a license." [Laugh] But Saul was very organized and incredibly persistent.

ZIERLER: Where did you first meet Saul?

ELLIS: Berkeley. There was an earlier team leader at LBL named Carl Pennypacker; a very likable guy. As soon as we published, in 1988, this distant supernova, Pennypacker was so excited and said, "Oh my God, let's do this project." He came to Durham, where I was at the time, and through this partnership I started visiting Berkeley and also met Saul. And then, in a management decision taken by Berkeley, Pennypacker was demoted and Saul was placed in charge. It was probably the sensible thing to do as Carl was enthusiastic but not as well-organized. I had a soft spot for Carl and felt a bit sorry for him.

There's an interesting aspect of the eventual discovery of the cosmic acceleration. I was visiting Caltech, maybe in '97, and so was Bob Kirshner. And we were sharing an office. I was in the room when Adam Riess, Kirshner's student at Harvard, phoned him. There we were, Kirshner and I, in the same office in Robinson at Caltech. And Bob was on the phone to Riess, and Bob said, "It can't be right. that doesn't make sense." And I was sitting there quietly because I knew already that Saul had found an equally surprising result. I thought, "I'd better leave the room so the two of them could chat." So I stood up, and I turned to Bob, and I said, "I'm going to leave you to it." As I left I said, "It can't be the cosmological constant." This story is faithfully recorded in Kirshner's book The Extravagant Universe.

ZIERLER: Oh, that's great.

ELLIS: Both teams were shocked. And I think the fact that both teams discovered it independently made it believable. I think if there'd only been one team, there'd have been huge skepticism. But because two teams found the same result, people took it seriously. That was a very exciting period of my career. But most of that was when I was in Cambridge. When I moved to Caltech, Saul announced to his team "Richard Ellis is going to have an enormous amount of Keck time. We'd better be nice to him." As soon as I arrived, he invited me up there. We started doing spectroscopy of more distant supernovae. We'd already published the prize-winning paper that showed the universe was accelerating. But now, the goal was to improve the result quantitatively. However, in the end, I drifted away from Saul and teamed up with the Canadians who were doing a big supernova search with the Canada-France-Hawaii Telescope on Mauna Kea, and so I became the Canadian's team spectroscopist using Keck. I continued to work on supernovae until about 2009 and then, when I returned from Oxford, I focused primarily on studying high-redshift galaxies.

ZIERLER: Were you following LIGO? Were you part of LIGO at all?

ELLIS: No, not at all.

ZIERLER: Just too different of a field?

ELLIS: Yeah, too different. Prior to the detection of gravity waves, I considered LIGO to be mostly a physics experiment. There was nobody in Robinson really involved. Sterl Phinney has gotten involved in LISA – a space facility. Gary Sanders, I knew because of TMT. I remember Ronald Drever who was here when I arrived. He was a sad character who'd been treated badly according to Wal Sargent. And of course, I knew Kip. But LIGO was not prominent in the astronomical department at all. Obviously, it was a high-profile program at Caltech and MIT. But the astronomers were mostly concerned with using Keck, Palomar and Owens Valley's radio facilities.

ZIERLER: To go back to the complementarity, when James Webb launched, what did that mean for you? First of all, how much time did you expect to have on it?

ELLIS: We did extremely well. Oh my God, we were so lucky. Let's talk about how time is allocated on space telescopes. There are two kinds of astronomers. There are the astronomers that built the instruments for James Webb. They get what's called guaranteed time - gobs of time. Well, I didn't build any instrument for James Webb so I never got that kind of time.

ZIERLER: They get time above what should be credited for them is what you're saying?

ELLIS: They get time guaranteed and select targets from the literature that the riff-raff can't touch; it's insulting! Furthermore, they can also apply for regular time. I was on the original committee that proposed James Webb in 1996. Alan Dressler was charged in 1994 with looking at the future of space astronomy after Hubble. And so, he chaired a committee called the HST and Beyond Committee.

ZIERLER: This was at Carnegie?

ELLIS: It was an AURA-sponsored activity. This is a measure of the ambition of US astronomy. Hubble had only just been launched in 1990. It had only just been repaired in 1993 after a lot of embarrassment. Remember the blurred images, the spherical aberration and the bad jokes about Hubble? One year later, AURA said, "Let's have a committee that considers the next big thing." It reminds me of the ambition of Tombrello and TMT.

ZIERLER: Well, it's Hollywood. It's the sequel.

ELLIS: It is, yeah. [Laugh] At Cambridge, I was the only Europe-based astronomer on the committee. That was a great experience. We developed the science case based on two big themes. One was looking for cosmic dawn, and the other was finding exoplanets. Alan did a fantastic job. It was a very visionary report.

ZIERLER: Was IPAC involved in this at all?

ELLIS: No, not at all. It was all academics. Shri was on the committee, Wendy Freedman was on it, Kirshner was on it. In many ways, this got me enthused about moving to America, actually. Here I was, a Cambridge professor in England, going to meetings in Illinois or Baltimore, and seeing 15 senior American astronomers all thinking about the future.

ZIERLER: This was dynamism that was not happening in England.

ELLIS: This was dynamism that was not happening in England. And I thought, "Wow, it would just be great to work in America and be part of this huge, vibrant community. You might say, "Well, it's easy to predict that in 1994 high-redshift galaxies would be considered important in 2022." But regarding exoplanets, there were hardly any.

ZIERLER: Yeah. There were just a few.

ELLIS: There were just a few. And the idea that we'd be observing their atmospheres to hunt for signs of biomarkers was extraordinarily visionary in 1994. You asked about my emotions when, on Christmas day 2021, James Webb was launched. Firstly, of course, I was very sorry it was delayed because I'd written my Advanced Grant in 2014, thinking it would be ready in 2017. And there we were in Cambridge, England, having Christmas lunch. It was very emotional. There's a little side story here. My former student Guido Roberts-Borsani, who was at this week's meeting, decided to go to French Guyana as a tourist to see the launch. He didn't have VIP status from NASA or anything. He simply bought his own air ticket there and found a hotel.

Then, on the morning of the launch, all the VIPs were going on an official bus to the launch site and Guido just turned up there on the off-chance. He said he was an astronomer and his research was based on James Webb and wondered if, by any chance, there was a spare seat on the bus. And of course, the guy in charge said, "You've got to be joking. Have you any idea how selective we've had to be to decide who are the VIPs?" Then, astonishingly, a senior astronomer, overheard this conversation and said to Guido, "You can have my pass. I've decided I want to watch the launch from somewhere else." So at the very last moment Guido gets on the official bus and even gets into the control room. So while I'm eating my Christmas turkey, he is texting me with all these photos from a prime viewpoint! I responded, "How on earth did you sweet talk your way into the control room?" It was so moving, yeah!

ZIERLER: You had time guaranteed from launch?

ELLIS: No, let's go back to that. There were the guaranteed time people, and there were the plebs, like us, who write normal proposals that are peer-reviewed. We wrote 11 proposals.

ZIERLER: What was the theme of those proposals?

ELLIS: Quite varied, but nearly all high redshift. Let's talk about chemistry. When a galaxy is born for the first time, it's made of hydrogen and helium only because are the only elements made in the Big Bang. Everything else, iron, carbon, calcium, is synthesized in stars. Everything in this room is material that's synthesized in stars.

ZIERLER: From hydrogen and helium?

ELLIS: Hydrogen fuses to helium, helium fuses to carbon, carbon is burned to make magnesium, silicon, iron, oxygen. All of those elements are made in stars. Accordingly, as we go back in time, we would expect the universe to become more and more chemically pristine, with fewer and fewer heavy elements. There was no way we could test this chemical evolution with Hubble. It isn't powerful enough. It's also very hard to make progress from the ground because of the brightness of the Earth's atmosphere.

ZIERLER: I wonder if you could narrate visually, what does it mean to see elements?

ELLIS: When you take a spectrum of a galaxy, you can see the fingerprints of the composition of the gas. We're not talking about stars, we're talking about gas in the galaxy. A galaxy has lots of stars obviously, but there's gas, which collapses and forms stars. There's always gas in galaxies, especially at these early times. The gas is heated by the stars and it glows according to atomic processes that occur in the various chemical species. We can recognise bright emission lines in the spectrum which gives us the chemical composition of the gas. But it's very demanding on observing time. One of the huge advances with James Webb is that it has these spectrographs, and the telescope is powerful enough that we can, for the first time, measure the chemistry of these early galaxies as we go back to the beginning.

And the telltale signature of cosmic dawn would be a galaxy that doesn't have any of these heavy elements. It's forming stars for the very first time so no chemical enrichment. Observing time on James Webb is allocated in annual cycles. The first year was so-called Cycle one, and we submitted 11 proposals. Six of them were approved, which was a huge success rate. The telescope is oversubscribed five or six times. Many people have the same idea, so the quality of the proposal has to be perfect. You have to justify the exposure times and the science impact. It's brutal salesmanship, really. You have to sell yourself. Over the years, I've spent a huge amount of effort tailoring how to write good proposals for observing time. We did really well in Cycle two as well. We've got a lot of observing time, thankfully, although nowhere near as much as the instrumental builders who were awarded "guaranteed time" – sigh!

ZIERLER: I asked about themes of these proposals. We've covered chemistry. What else?

ELLIS: James Webb has both imaging cameras and spectrographs. It can image deeper than Hubble, so one of the big programs is imaging a large area to do a robust census of galaxies at early times. Another is to follow some of these objects up in detail with the spectrographs and study their chemistry. So there's a two-tiered approach, counting galaxies as a function of look-back time is one theme, which has been very successful, and we've published a couple of papers on that. And then, the next is tracing the chemical composition of the gas over cosmic time. Ultimately, we'd like to study the chemical composition of maybe 50 early galaxies but it's very demanding. That's something that we may have to do in the future. Fortunately, James Webb is going to be up for 20 years. It's going to be amazing.

ZIERLER: 20 years is the expectation, but obviously if it keeps going, this is gravy.

ELLIS: Well, it will eventually run out of fuel. It's not stationary. It's beyond the moon in a position called the second Lagrangian point, L2, as it's known, which is in gravitational balance. The pull of the earth and the sun on James Webb equals the pull that the earth gets from the sun. This means that James Webb and the Earth go around the sun at the same angular speed so we can easily communicate with it. Normally, if James Webb was further away from the sun than the Earth, then the Earth would go around the sun faster than James Webb, and the two would get out of sync and eventually be very far apart.

ZIERLER: So an Earth year and a Webb year are the same.

ELLIS: Exactly. But to keep James Webb at this point is tricky because it's an unstable location. If the telescope wandered a little bit away, then it would drift away. It's actually in a sort of figure-eight orbit around this neutral point.

ZIERLER: It's expending energy to stay like that?

ELLIS: Every now and again, NASA has to do a little bit of an engine burn to keep it precisely where it has to remain, so it will consume fuel. The original lifetime of James Webb was predicted to be between 5 and 10 years, and that's because it was thought that on the way from French Guyana to L2, there would have to be many mid-course corrections. But the launch trajectory was so perfect that much of the fuel set aside for these corrections going from French Guyana to L2 is still available. That means that there's much more fuel left for these little corrections in keeping it where it should be at L2. And that has increased the lifetime from 5 to 10 years to 15 to 20 years. It's yet more fantastic good news. Amazing.

ZIERLER: Did you get involved in exoplanets in the proposals?

ELLIS: No, not at all.

ZIERLER: What are you excited about in terms of Webb's capabilities?

ELLIS: Well, witnessing this moment of cosmic dawn. Let's talk about that. As I said, in 2021, before James Webb, we did the best job we could with Hubble, Keck, and the Very Large Telescope to take a population of galaxies at redshift nine and see how old they are. How long have they been there? An analogy may be helpful: you walk down the street and meet a little boy. You weren't present at his birth, but if you ask him how old he is, you know when he was born. Likewise we go to these galaxies that are seen 500 million years after the Big Bang, measure the ages of their stars to be 300 million years, so we can estimate they were born 200 million years after the Big Bang. And that's beyond what Hubble could see. We made a prediction in 2021 that James Webb would see galaxies beyond Hubble's horizon. And already in the space of six months, that's been verified and we now see galaxies out to redshifts of 15, when the universe was only 250 million years old.

ZIERLER: Is that the theoretical limit? Are there older galaxies than that?

ELLIS: No, we could in principle see further back. At the moment, there are candidates that we and others have found beyond, but we need to get more data to prove whether they are at these great distances. There's a lot of work still to do. But so far, as is mentioned in this week's meeting and the one I had in London, we're seeing back to when the universe was only 250 million years old. What we now need to do is measure the chemistry of these objects to see how immature they are, and that's part of the proposals that we're writing for next cycle, Cycle three, the deadline for which is in October. That's what we're working on right now. It's an intense period.

ZIERLER: Is it possible that the dark ages, the cosmic dark ages, are not as long as we think?

ELLIS: Yes.

ZIERLER: Is it possible we can find galaxies older than what the theory suggests?

ELLIS: Absolutely. As always, theorists are very confident that their computer simulations approach reality. But ultimately, the great adventure is exploration and observation. I'm not a theorist and many of us relish in making a challenging discovery!

ZIERLER: Well, that's science.

ELLIS: That's science. Let's just get out there and see what's there!

ZIERLER: Ultimately, is the big picture here to get as close as possible to the singularity, to T = 0?

ELLIS: No, not really. The idea of just looking back to the beginning of the universe itself is separate from our discussion of when galaxies formed, of course, because galaxies are big things that take time to develop. Let's talk about cosmology. We discussed earlier the microwave background, the glow from the Big Bang, which is seen when the universe cooled sufficiently that hydrogen atoms form. Why is that? When the universe expands, it cools. When it's very hot, the hydrogen atom is broken into its proton and electron. And the electron is a real nuisance particle. It scatters light. If you have a gas with lots of electrons, light is constantly scattered by the electrons. Light doesn't travel in a straight line, and you have little information from light when the associated gas is full of electrons. But as the universe expands, it cools, and eventually, the electrons slow down. And because they're negatively charged and the proton is positively charged, the two eventually bind together and make the hydrogen atom. Then, suddenly, the universe becomes transparent, and light can now travel freely. Jim Peebles, Nobel laureate at Princeton, predicted this "last scattering surface" would one day be detected. And then, in 1965, it was discovered by accident in New Jersey of all places.

ZIERLER: Penzias and Wilson.

ELLIS: Yes. And I often joke when I give cosmology talks, "Thank God something useful came out of New Jersey." [Laugh]

ZIERLER: [Laugh]

ELLIS: How do you go further back? There is dark matter and neutrinos, and they detach from the expansion of the universe even earlier. And then, there's inflation. We think that the universe accelerated in its expansion at early times.

ZIERLER: And you're an adherent to the inflationary model?

ELLIS: Well, I don't observe anything in this area myself. I follow it, and I'm fascinated.

ZIERLER: But it's more compelling than string-gas cosmology, for example.

ELLIS: Oh, yeah. There's a hope that we might see the glow from neutrinos with neutrino telescopes from very early periods of history, but that's probably decades away. We don't have the technology to do that at the moment.

ZIERLER: There's no such thing as a neutrino telescope now?

ELLIS: Well, there are neutrino detectors in Antarctica, but there's not an all-sky neutrino facility.

ZIERLER: This is like the IceCube project.

ELLIS: Yes, that's right.

ZIERLER: Is there a breakthrough with IceCube that would lead to a technological…

ELLIS: No, what IceCube is detecting at the moment is neutrinos largely from supernovae in nearby galaxies.

ZIERLER: What would a neutrino-detecting telescope look like?

ELLIS: God knows. It's decades away in terms of technology. But it would be something like the Planck Satellite is to microwave radiation. It'd have to be hugely sensitive to detect the background from cosmological neutrinos. It would have to be an all-sky signal, same as Penzias and Wilson. As an analogy, the present neutrino capability is akin to the naked eye. We see individual stars. That's where we are with IceCube. We see individual neutrinos from nearby supernovae. Imagine now moving forward to seeing a background of neutrinos from the early universe. That's a long way off in technology.

ZIERLER: What are Webb's capabilities in terms of learning about the size of the universe? We're still pretty agnostic about whether the universe ends.

ELLIS: I think most people would believe it's infinite. James Webb is not really a cosmology facility. It's very good for looking back in time. It's very good for studying exoplanets It's a good all-purpose infrared telescope. Remember, it's mostly infrared, unlike Hubble. That means that most of our understanding of cosmology, dark matter, dark energy, acceleration of the universe, is coming from survey telescopes that chart large areas of the sky, both on the ground and in space. I mentioned the Japanese instrument I'm building which will measure the redshifts of millions of galaxies and look at large-scale structure and how the three-dimensional topology of galaxies is changing as the universe ages. And that tells us the competition between gravity, which is holding the structures together, and whatever dark energy is that accelerates the expansion. Euclid, the European satellite that's just been launched, is going to look at weak lensing, which is the distortion in the shapes of galaxies by foreground material, which traces where the dark matter is and how it clusters with time. These cosmological experiments are separate from James Webb. It may seem surprising, but James Webb is not a cosmology machine, really. The big questions in cosmology was the topic of a talk at my meeting by Mike Turner. Do you know him?

ZIERLER: Of course.

ELLIS: He's now back in LA.

ZIERLER: Where did you get to know Mike?

ELLIS: Largely through cosmology conferences. I went to Chicago a few years ago to give a named lecture, the Brinson Lecture. Mike gave a cosmology update in my meeting earlier this week. It's a headache, "What's dark matter?" For God's sake, it was discovered, really, by Zwicky, a hero here, in the 1930s. Vera Rubin also, of course. And then, we've made maps of the dark matter from lensing. This was something that I did with Nick Scoville when I was at Caltech in the early 2000s using Hubble data. And dark energy, a big surprise - Nobel Prize. We've discovered the acceleration of the universe. Well, what is it? Why is the universe accelerating? Gravity is an attractive force. What's going on? We have a theory called the cold dark matter model. The theorists stand by it rigidly and think it's wonderful.

ZIERLER: This holds that dark matter is a particle.

ELLIS: That's right. And one of the questions at my meeting earlier this week was, when you look at baryons, the stuff that makes up you and me, there's a mix of things. There's protons, electrons, muons, all kinds of particles. Why should the dark matter be one particle? Consider for a moment you were a dark matter person. You'd look at the baryon universe and say, "God, what a mess. There are all these things." Well then why do we, as baryons, look at the dark matter universe and assume it's just one mysterious particle? Could it be many things? [Laugh] The cold dark matter model posits it is a cold WIMP, a weakly interacting massive particle. The particle physicists have spent decades trying to find it, and they can't. That's a big headache. Then, the acceleration of the universe is a big headache. What causes it?

ZIERLER: Are you disappointed that there aren't particle accelerator facilities that are farther along than they might be? If there was an SSC, if we had an ILC…

ELLIS: Whenever I go to cosmology conferences where particle physicists are present, to be frank, I get lost in their terminology. I try to keep up, but it's very hard. The Standard Model seems to need something extra, and I don't know what it is. Arguably, it would be addressed by more powerful facilities. The supercollider, the SSC, I remember talking to Gary Sanders pre-LIGO, that's where he was. Those huge underground tunnels were built, and then Congress pulled the plug. That was pretty devastating. Particle physics is bewildering to me, actually. If somebody said, "Richard, summarize the Standard Model, I think I'd have to go away for a week and do a lot of background research. [Laugh]

ZIERLER: [Laugh] The James Webb is going to keep you busy as long as you want to be active.

ELLIS: As long as I'm healthy, yeah.

ZIERLER: Do you envision successor projects?

ELLIS: Yeah. We are planning now a telescope in Europe, which goes by the romantic name, SpecTel. It is a very wide-field telescope, it's a 12-meter aperture, so bigger than Keck, but with a field of view of seven degrees. And we're planning to put 20,000 fibers on this facility and undertake both cosmology and galaxy studies. That is not likely to happen until '33 or something like that, when I'll be in my 80s.

ZIERLER: RSE at 80, we'll have you back.

ELLIS: RSE at 80, yeah. I've pushed big telescopes all my career.

ZIERLER: When you say that the Webb is not really a cosmology machine…

ELLIS: Mostly because its field of view is so small.

ZIERLER: Does that mean that you're looking for the next great space telescope that is a cosmology machine? Or is that really the domain of land-based astronomy?

ELLIS: No, what's in the 2020 Decadal Report in space is a replacement for Hubble, really. And there's a feeling that the ultraviolet community has been forgotten. Why is the ultraviolet community interesting? There's a lot of spectroscopy in the ultraviolet that's interesting chemistry and so forth, but not at early times.

ZIERLER: What kinds of questions can you field?

ELLIS: Phil Hopkins here at Caltech is very interested in what we call feedback. That is, what regulates a galaxy? Galaxies are like gigantic ecosystems. Gas flows in, it forms stars, the stars explode, they go supernova, the pressure from the supernova drives the gas out. This cycle is a little like adjusting the thermostat to keep the temperature nice at this time of year with air conditioning. This little feedback loop seems to be very important in governing how galaxies slowly grow. We don't want them to grow too quickly, according to the observations. We want them to grow slowly. Then, what's the role of black holes? We haven't talked about black holes. How do black holes grow, and what do they do? The black hole in our Milky Way is, thankfully, quiet. It's not doing much energetically. It's there, and it's gobbling up stuff, but it's not active. It's not shining in any way. It's just dark and hidden.

But quasars contain active black holes. That is, the material that's going into them is glowing, and there are jets coming out and all this interesting astrophysics. These are questions that are best studied nearby, redshifts less than one. And at ultraviolet wavelengths, there are many tools and many interesting phenomena which would benefit from a space telescope. You can't observe the ultraviolet from the ground. The Earth's atmosphere absorbs in the ultraviolet thankfully, otherwise we'd get roasted. On the ground, however, there's a lot of push for cosmology programs like the wide-field telescope I mentioned in Europe. That's the way the subject is going. There are X-ray telescopes being proposed. They've had a hard time. They've ballooned in price, so they're being downsized at the moment. There's a big project called ATHENA, which is a very ambitious X-ray telescope that was sort of semi-approved in Europe, but it's run over budget, and I don't know what its fate is going to be. Space is expensive. That's the problem.

ZIERLER: Of all the different astronomical techniques, where are you most bullish that we'll detect bio-signatures or techno-signatures? What's the kind of telescope that would do that?

ELLIS: Well, James Webb is very good at that, then some of these future space telescopes in the decadal report aim to look at exo-planetary atmospheres and study absorption signatures.

ZIERLER: What does that mean, absorption signatures?

ELLIS: A planet goes around a star and, if the plane of its orbit is aligned with the direction to earth, then as the planet goes in front of the star, there's a moment during the orbit when the atmosphere of the planet lies in front of the star. So the light rays from the star pass through the atmosphere. If we can examine the spectrum at that moment, we can study the properties of the planet's gaseous atmosphere, just like when we look at the sun and see the dark lines in the sun spectrum. People are now trying to predict the likely bio-signatures including oxygen, water vapor, that kind of thing. Next up can we imagine taking a detailed picture one of these planets? There are very ambitious ideas, very futuristic, based on combining telescopes as what we call interferometers over the baseline of the Earth. The Event Horizon Telescope enabled us to almost resolve the black hole in a nearby galaxy, Messier 87. We might ultimately connect and bring together the light from separate telescopes around the Earth.

ZIERLER: Is this like the adaptive optics for Keck 1 and Keck 2?

ELLIS: No, this is much more powerful. Let's separate the two. Adaptive optics is the correcting of the blurring of the earth's atmosphere for one telescope. And the way that works is by monitoring atmospheric turbulence with a laser-induced artificial star. We shine a laser in the atmosphere, it reflects off sodium atoms in the atmosphere and creates an artificial star. We monitor its twinkling with a mirror and create a distorting signal that drives a flexible mirror that we use to correct the image that's coming in from a celestial object, like a galaxy. The movement of this flexible mirror is a reflex of the time-dependent turbulent pattern that's being detected by the laser star. In this way we correct for the blurring in the earth's atmosphere and produce a sharp image of one object. And that's all done on one telescope. What I'm talking about is now using the baseline of many telescopes across the face of the earth and somehow syncing them all together as if it's a giant telescope whose diameter is the entire earth. Far-fetched, you might say. Some people think it's possible. And if that's the case, then we'd be able to image the surface of these planets. Now, is that 50 years away, 100 years away?

ZIERLER: That's not just atmospheric clues, that's seeing what's going on.

ELLIS: That's seeing what an exo-planet surface looks like. My God, could you imagine that? Now, I was on a committee recently, talking about Chicago again, where a donor, who I'd better not mention, was interested in getting together maybe 10 astronomers and asking them to think that far ahead, and that was one of the most interesting meetings I've ever been to. And this idea was one of them. Wow. Sometimes, it's fun to dream, isn't it? [Laugh]

ZIERLER: I'm thinking not just the technological hurdles, but the administrative and bureaucratic hurdles because these are coming from all different kinds of countries, all different kinds of funding agencies.

ELLIS: Yeah.

ZIERLER: This would be a truly global collaboration.

ELLIS: Well, we're heading that way. There's only one James Webb, there's only one LSST or Vera Rubin Telescope, there's only one Roman Space Telescope, there's only one Hubble Telescope. Everybody in the world is using Hubble and James Webb, so we are already in a global era. And it was a shame to me that TMT and the Europeans didn't connect. Here we were in Pasadena, the ESO Director-General came, and we said, "Let's see if we can somehow connect TMT and ELT. Maybe swap observing time, maybe share instruments." But the Europeans didn't want to bite at that time. But that time will come.

ZIERLER: A political question, if you followed the recent controversy over naming the telescope after James Webb. Did you follow that?

ELLIS: Of course, yeah.

ZIERLER: What are your thoughts there?

ELLIS: In my opinion it should have been named after a scientist, like Einstein or Hubble, who has passed away. That was the tradition. So, firstly I was a bit surprised that it was named after a NASA administrator, not a scientist. We have the Spitzer Space Telescope. I met Lyman Spitzer. What a wonderful guy. There's the Hubble. Of course, I never met Hubble. There's the Einstein Telescope and so forth. There are plenty of distinguished physicists and astronomers we could have considered.

ZIERLER: And we have ZTF after Zwicky.

ELLIS: And we have ZTF. Even irascible Fritz got a telescope named after him! I was a bit surprised that something so dramatic as the Next Generation Space Telescope that Dressler's HST and Beyond Committee proposed got named after a NASA administrator. I'd never heard of him, to be frank. But then, when all this controversy emerged, there was a study by NASA of the accusations against him, and, although I didn't follow the discussion in enormous detail, I was satisfied that the study exonerated him. I was willing to believe that NASA had done a fair job in this respect because they had to take all the accusations seriously. But, as I understand it, the American Astronomical Society was not particularly happy with the report from NASA that exonerated James Webb. Personally, I trusted that NASA took the accusations seriously and commissioned a study and we should respect its verdict.

To be honest, I find this constant reassessing of history a little unnerving.

ZIERLER: We've gone through it here at Caltech.

ELLIS: We've gone through it at UCL, too.

ZIERLER: Oh, really?

ELLIS: The building I'm in at UCL used to be the Pearson Building. Well, it turned out that Pearson was involved in eugenics at some level. And so, the building had to be renamed the Northwest Wing. And now, one possibility is, we name it after Margaret Burbidge, who was an astronomer at UCL for a while. I think that's probably what'll happen, which will be fine. But I thought renaming the Millikan building a little bit over the top and that a less draconian solution balancing his pluses and minuses might have been found. After all, Millikan founded Caltech and got a Nobel Prize. But I know many people fiercely disagree and of course I respect their views.

ZIERLER: I want to go back to some institutional questions. One obvious challenge leaving Caltech, how do you keep up with the quality of the research when you're no longer at Caltech? How did you try to square that circle?

ELLIS: Well, I travel. I have a visiting position here. I come twice a year, and that's great. I really look forward to the trips, of course.

ZIERLER: Do you treat yourself and stay at the Langham?

ELLIS: No, no. Sometimes, I'm very fortunate to get a contribution financially, but usually, I'm paying my own way, so I'm in an economical Airbnb. But yeah, I love coming to Caltech. I regularly visit. And I visit other places. I have a visiting position in Cambridge, I go to Edinburgh regularly, I like going to Tokyo. By traveling I keep up to date. But the main way we all keep appraised of what's happening is with the publication archive. Every day, like this morning, there's yet another James Webb paper. I had to download it, and I'll read it later today. Since July 22, I have read 120 papers from James Webb on distant galaxies. If you don't read the literature, you lose your way very soon. People complimented me at the meeting, saying that I have a voracious appetite for reading the literature. I find if I don't read the literature, I won't keep up. The pace is so fast now compared to when I was a post-doc.

ZIERLER: Some high-level questions about what these papers say. First, what's been expected, and what's been surprising, now that Webb is up there, and all these papers are coming?

ELLIS: Well, the big news is two things. One, as we predicted, there are galaxies further out to earlier cosmic times than Hubble could see. But secondly, they're very bright. They're brighter than the theorists predicted. Their chemical composition is peculiar. There's a lot of nitrogen. Nobody knows why. Nitrogen and oxygen are produced by the same nuclear cycle in stars, which is called carbon burning. Carbon burns with hydrogen to make nitrogen and oxygen. And there's a lot of nitrogen in some of these galaxies, which doesn't make sense. Nobody knows why. So there are puzzles. One suggestion is that galaxies are having strong bursts of star formation. These early galaxies seem to be unlike the Milky Way, which is forming stars at a smooth rate. In contrast, early galaxies are erratic. Some of them are in an "on" mode, maybe some of them are in an "off" mode.

There seems to be something qualitatively different about the earliest galaxies compared to those that are seen later. Which is exciting. In my opinion, it may be telling us we're close to the birth of all galaxies. We're seeing them in some immature, initial phase, a bit like when you start a car. It does something special until it settles down. It's exciting because it tells us we're close to the beginning. But we haven't put it all together in a physical story. As always, there are several theoretical papers that are attempting to do this. But sometimes, I like to try to figure it out myself rather than reading the theory papers. [Laugh] That's just in this area of astronomy. I try to keep up with other areas of astronomy as well, but it's getting harder and harder to maintain the broad perspective. Obviously, I still follow cosmology, supernovae, Euclid, this big telescope that we're planning. I have to write the science case for that.

ZIERLER: Which will be what? What's the case to be made?

ELLIS: Mostly cosmology and large-scale structure. Studying the evolution of the universe . There's this feature of large-scale structure called the cosmic web. The web is a sort of filamentary structure of dark matter that is predicted by the cold dark matter model. And we'd like to image this web. And there are various tools for doing that. And we'd like to know whether the properties of a galaxy depends on its position in the cosmic web. If it's on a filament, does it form stars more slowly? Where these filaments connect is what we call a node. If a galaxy is on a node, is it more likely to have a black hole? How do we investigate this? We need to survey a huge area of sky, huge volume, and trace this cosmic web, then measure the properties of the galaxies separately, then statistically correlate them. That's the kind of thing that a future telescope will do.

ZIERLER: Are you surprised at all of the black holes that seem to be popping up in the universe? Or has this been expected?

ELLIS: Well, we've known for a long time that black holes are common in nearby galaxies. We know that we see black holes out to redshifts of six and seven because of the discovery of quasars. But the quasars are relatively rare, and that's because the black holes within them are active. We know that there are probably many more black holes that are inactive because we can do a census nearby. The real challenge now is, how do you form these massive black holes? Let's put some numbers on it. The black hole in the center of the Milky Way is a few million times the mass of the sun. It's clearly an impressive black hole, but compared to the black holes that are in quasars, it's a tiddly little thing.

The black hole in a quasar can be as much as several billion times the mass of the sun. Quasars can be seen to redshift seven, when the universe was only 800 million years old. How do you create such a monster black hole in the center of a galaxy so quickly? One suggestion is that sometimes when a hydrogen cloud collapses when the universe is young, it doesn't form stars or galaxies, it just collapses straight into a monster black hole. And this is what's called a direct-collapse black hole. This is a hypothesis. But if it's true that there are direct-collapse black holes of a few million, or even tens or hundreds of millions times the mass of the sun, sitting out in space, that's pretty exciting. This is something where James Webb will have an impact, and that's by tracing how far back in time we can find quasars, measuring the masses of their black holes, and trying to understand where they come from?

ZIERLER: Are there primordial black holes that predate galaxies?

ELLIS: There could be. That's another hypothesis, yeah. But these are suggestions. LIGO doesn't have the capability to detect merging black holes from the early universe, but LISA might. That won't be observing until the mid 2030s.

ZIERLER: And that gets us closer to the singularity.

ELLIS: Well, that's not really the goal. The goal is to look largely at massive black holes that are merging in the early universe. A separate question is the detection of gravity waves from the very early universe.

ZIERLER: It's almost a philosophical question, but why is dark matter so difficult to understand? Is it possible that we simply lack the scientific language to even categorize it?

ELLIS: We know some things about it. We know where it is, we know how it's distributed and it forms the backbone around which normal matter assembles. We also know it's more abundant than normal stuff. But the question of what is it is beyond astronomy, unfortunately, unless we can determine whether it's completely non-interacting. What does "cold" in cold dark matter mean? It means that it doesn't interact, other than gravitationally, with the rest of the universe. But there are suggestions that it could interact with itself. And when we talk about interaction, we're talking about scattering. It could be slightly warm, which would mean the growth of structure forms at a different rate. All of the questions that these big survey telescopes are trying to address: how the cosmic web evolves as the universe expands, and how fast structure comes together, will constrain some of these variations on the dark matter model. But ultimately, why is it so difficult to find? Well, maybe there's some problem in particle physics. There are still areas in parameter space that haven't been completely explored. But I agree, it's depressing that it's such a hurdle to understand what is it!

ZIERLER: Would it have to be a fifth force? Is it possible it exists as a subset of one of the four?

ELLIS: No, I don't think so. Like I said, it's maybe that we're just being too naive and that it's not simply one thing. That would be frightening, of course. It would lead to puzzles about what else it could be. It's the same with dark energy. There's no shortage of ideas about what dark energy is, but nobody's making any progress in finding out what it is.

ZIERLER: Do you see discoveries sequentially? Will figuring out dark matter lead to dark energy?

ELLIS: Yes, it could be. One explanation–nobody likes it–for dark energy is, there's something wrong with relativity, Einstein's theory. When we calculate how the universe should evolve, we follow Einstein's equations. And it may be on large scales that there's a tweak. It may sound scandalous to challenge Einstein, but of course, when Einstein first came along, people were skeptical because he was challenging Newton. We should be prepared for a failure of General Relativity. And we can test that with Euclid. Euclid will measure the rate at which the universe is expanding with time, how fast it's accelerating, and so forth. And it will also measure how the clustering of matter grows. If Einstein's correct, the two should give the same answer as to the properties of dark energy. But if there's a difference, it could hint that there's something wrong with Einstein's equations on large scales.

ZIERLER: A cultural question, the use of the term cosmology in historical perspective. For example, I was very lucky to talk to Steve Weinberg, and he told me when he was an undergraduate at Cornell in the 50s, cosmology was, like, mystical or even kooky. No serious physicist or astronomer would've called themselves a cosmologist. Are you of a generation or a place in time when for as far back as you would've wanted to, cosmology was a respectable discipline?

ELLIS: Yes, I think that's true and it's because we now have good observations. It was a mystery and even kooky when it was just theoretical speculation.

ZIERLER: Or did you experience that yourself?

ELLIS: Indeed, I lived at a very interesting time. This goes back to after I finished my PhD, which, incidentally, was not on anything we've talked about. I was a laboratory physicist, measuring oscillator strengths. The oscillator strength is a transition probability of a spectrum line. I was studying neutral titanium. And for my thesis, I was in a laboratory operating a furnace containing a bar of titanium. By shining a beam of light through a furnace of known temperature into a high resolution spectrograph I could measure the absorption spectrum of titanium and measure many oscillator strengths. And then, I would apply those in model stellar atmospheres to get the abundance of titanium in the sun and other stars. And after three years of doing that, I thought, "God, this is so boring," and I decided to become an extragalactic astronomer. And in 1974, when I got my PhD, I moved to Durham, and Jim Peebles was writing these papers on the clustering of galaxies. And we started getting very excited about using the clustering of galaxies to determine how much material there is in the universe. How dense is the universe? What's the abundance of matter? I was only in my 20s. I was already doing cosmology at that time.

ZIERLER: Clustering relative to what? Would Andromeda and the Milky Way be considered a cluster?

ELLIS: Sort of. Jim Peebles developed a formalism for what we call correlation functions. Say you have the positions of 100 galaxies. You can use correlation functions to determine statistically whether they're randomly distributed or not. You measure the pair-wise distances between them all, count them as a function of separation, and then you compare the number of pairs you see at a given separation with that expected for a random distribution. And any excess signal that you find is a measure of how clustered they are on a particular scale. Peebles developed the necessary elegant mathematics. We started doing this at Durham, measuring the clustering of galaxies using their 2-D positions on deep photographic plates. Then, eventually, we realized that we needed to get the redshifts of galaxies to do this properly in 3-D. And that 3-D measurement of the clustering indicated that the gravity between nearby galaxies was consistent with a low-density universe.

We found that the total amount of material, dark plus baryonic, was only 30% of what was needed to fill the universe and make it spatially flat. And that was in the early 80s. I was already doing cosmology in the 80s. And then, later in the 80s, I discovered the first distant supernova. And then, in the 90s, of course, I was working with Perlmutter. Then, we discovered in '98, '99, that the universe was accelerating, and we hadn't a clue what it meant. Then, we started doing these big surveys to measure the cosmic web and so forth. I've been a cosmologist off and on all my life, and it's been great fun. I would say I've spent 30 to 40% of my time doing cosmology, and the rest studying the evolution of galaxies. Not for cosmological purposes, but studying their evolution, how they assemble with time, when they were born etc. Sometimes people regard this as cosmology but I've kept them separate in my own career.

ZIERLER: Being an astrophysicist versus being a cosmologist, are those meaningful distinctions for you?

ELLIS: Yeah, I think so. I don't mind being introduced as an astrophysicist or a cosmologist.

ZIERLER: And then where would astronomer fit in?

ELLIS: An astrophysicist sounds more respectable, doesn't it? An astronomer could be an amateur, somebody who just looks through a telescope eyepiece. However, if I'm sitting on a plane and somebody says, "What do you do?" I say, "I'm an astronomer."

ZIERLER: It's more accessible.

ELLIS: It's more accessible. An astrophysicist sounds too pompous. [Laugh]

ZIERLER: [Laugh] Are you glad that you're back at Caltech? Is this a good time for you?

ELLIS: Yeah, I miss it so much. And Barbara can see that. I hope to keep returning.

ZIERLER: Have you jumped on the artificial intelligence bandwagon?

ELLIS: No, not really.

ZIERLER: Are you following what George is doing, for example?

ELLIS: The colleague in the office next to me at UCL, his name's Ofer Lahav, and he's always full of excitement about AI. And I pay attention to it, of course, but life's too short. I can't do everything.

ZIERLER: But isn't there undeniably a data problem?

ELLIS: Oh yes, of course.

ZIERLER: Isn't it necessary, whether you like it or not?

ELLIS: Yeah, I'm happy to listen to other people getting excited about it. It's a bit like whether I'm ready to learn another computer language. At this stage of my life, life's too short for me to learn this as well.

ZIERLER: But as a field, if you want to talk about you personally, this is sort of beyond…

ELLIS: Oh, it's big business, yeah.

ZIERLER: But is it compelling, the idea that you need AI to find the signals amid the noise? Are we drowning in data?

ELLIS: We are, yeah.

ZIERLER: So it's bigger than whether or not you want to learn it, it's a necessary tool.

ELLIS: It's a necessary tool, yeah. I would say for cosmology, for the big surveys and this telescope we're planning, the wide-field telescope, SpecTel, so yes you're right. We're going to be swamped with data. And these techniques for finding the approximate redshifts, what we call photometric redshifts, which are determined not by finding atomic lines using a spectrograph but simply by analysing the colors of the galaxy is an example of one area where I've dabbled in AI. You have a measurement of a galaxy in the blue filter, the red filter, the green filter, the ultraviolet filter, the infrared filter. Collectively these provide the energy spectrum of the galaxy that contains clues as to whether it's been redshifted or not. You have a series of templates that are generated either artificially or by looking at examples whose redshifts are accurately known from spectroscopy, and then you match the data for maybe a million galaxies with these templates and see how well it works. I've done that with my students. That's an illustration of how you can apply such techniques to large amounts of data to get approximate redshifts.

Next, consider morphological information. What kind of galaxies are we looking at with Hubble or Webb? Are they spiral galaxies, elliptical galaxies or irregular galaxies? In the olden days, I would sit with my computer screen, and I would bring up each galaxy one-by-one, and I would mark the morphology on a piece of paper, "That galaxy's a spiral. That galaxy's elliptical." And we don't do that anymore. We use tools that determine whether it's symmetrical and/or concentrated alongside a catalog, which is either artificially generated, or based on a training set where someone has carefully categorized a subset. The technique is then applied to maybe a million galaxies, something that we couldn't possibly do manually. I've gone along the machine learning curve that far. And there's clearly many other aspects of AI where the young people are getting very excited. And of course, I welcome it. But I'm not an expert. I think there are two kinds of astronomers here.

Returning to my colleague at UCL Ofer Lahav; he and I are very different. He's fascinated more by the statistical methods and the innovations that are possible, including bringing in ideas from other disciplines like statistics, even biology. You could say I'm an instrumentalist at heart. I build and exploit new instruments. And that's driven by technology; robotic fiber positioners, new infrared detectors, new telescopes etc. Ofer's fascinated by the application of these new statistical tools to all kinds of areas in physics and astronomy. And we complement each other very well. Ultimately, it's the science that matters, isn't it? To just be fascinated by the techniques or the technology is not enough. It's whether you're getting good scientific results. I've witnessed people who are obsessed with the statistical methods and instrumentalists who love building instruments but let others use them. Whichever way you go, ultimately, you've got to be enthused by the science. When I listen to these AI talks, it's often just about the method. "Oh, I'm amazed." some student will say, "I've been applying this," and the whole talk will be the method.

ZIERLER: Where's the punchline?

ELLIS: Where's the punchline? Exactly. There's a little bit too much of that going on, I think, in AI at the moment. Fascination with machine learning, AI. Yes, I'm sure it can be important. Demonstrate it with a scientific result.

ZIERLER: Have you talked with Joe Silk about building a telescope on the moon?

ELLIS: I've heard him give the talk. We were together recently in Oxford about a month ago, and it was the same old Joe. And he's written a book about it, Back to the Moon.

ZIERLER: Just the technological hurdles of getting the instrument to the moon. Once it was that theoretically, would that be a game-changer?

ELLIS: Well, as I understand it, the argument is that on the dark side of the moon, it will be a very, very radio-quiet area. Let's talk about the dark ages. How do we explore the dark ages? There are no galaxies, no stars, just hydrogen gas. Hydrogen, especially cold hydrogen, does emit radiation at radio wavelengths, the so-called 21-centimeter line. Joe's idea is we can learn about the dark ages with a radio telescope. But we've got to get away from all the interference of radio transmission on the Earth. People are trying to do this in Western Australia, in South Africa, somewhere miles away from anywhere else. But ultimately, we would either do this in space–but space is very expensive, so the next best thing would be the dark side of the moon. I've heard him give the sales pitch and it's fascinating. Well, James Webb is in a very quiet place too. Obviously, there's no light pollution at L2, and the telescope is kept very cold with a sun shield so that we can see infrared signals that are very, very faint. The name of the game is to get away to a place where the background is very, very low. I have no idea how much Joe's idea would cost. And how do you maintain and operate that facility?

ZIERLER: Would it be bigger than the Webb because it's land-based?

ELLIS: Oh, yeah. Well, it would be an interferometer. This square kilometer array that's being built in Australia and South Africa is a forerunner of what Joe has in mind. It would be hundreds of radio antennae spread over an area of a square mile or more on the surface of the moon. And you'd need a maintenance crew; they'd have to have water and food, and they'd have to operate it. And maybe they'd have to visit every now and again. I don't know whether they'd have to be permanently there. But yeah. Would you go? [Laugh]

ZIERLER: [Laugh] It's a big idea.

ELLIS: It's a big idea, yeah. Joe's great. He's in his early 80s now. He got the prize that I got in July but a few years earlier; the Gruber Prize in Cosmology. He shared it with a guy called Nick Kaiser who was a pioneer in cosmology and gravitational lensing. Very sadly, Nick just died aged 68 from a heart attack.

ZIERLER: One thing we haven't talked about yet, breakthroughs with dark energy. What about the effort to develop a quantum theory of gravity and all of the excitement around quantum information, quantum computers? Are you following that at all, at least from an outsider's perspective?

ELLIS: From an outsider's perspective for sure, and it's bewildering. I get two views. Let's talk about quantum computing. I hear that it's not going to be useful any time soon - the pessimist's view. And the other is the huge attention it's getting in magazines. It's difficult to know how soon we're going to really see the benefits of quantum computing. Some of these concepts, like entanglement, are so counterintuitive that if somebody came up and said, "Richard, could you give a lecture on all of this next week?" I would have to work really hard to prepare something useful. I follow it, but I find it difficult to know how real it is, and I get confused by the terminology. Astronomy is so engaging that it's difficult to keep up often with developments in physics. Obviously, I need physics for a lot of the work I do, but not quantum information per se. I'm following it in magazine articles, press releases, things on Caltech's webpages.

ZIERLER: What about string theory? When that was really exciting in the 80s and 90s, were you following that?

ELLIS: Not really. I'm hearing that it's past it. That's the message I get. That it didn't deliver. When I arrived, there was an attempt to get Ed Witten to Caltech. They were giving him every opportunity to build a big group here. I remember the enthusiasm at the time. But the person I've interacted most is my namesake, John Ellis, the particle physicist. I talk to him from time to time. I spoke a little bit to Mike Turner as well this week about the fifth force and all this, just to try to keep up. But it's hard. There's too much happening in astronomy to find time to keep up to date with all of physics.

ZIERLER: Did you ever interact with John Schwarz when you were here?

ELLIS: No. I knew him of course. The great thing about Caltech is the gym; you would meet people from all disciplines in the gym. I would go home at the end of the day and I said, "I had three Nobel laureates all naked in the gym with me today!"

ZIERLER: [Laugh]

ELLIS: I used to meet Tombrello in the gym. I would go every day. I would do a bit on the rowing machines, and then I'd go for a swim. I have a Caltech card, but for some reason it doesn't get me into the gym anymore. [Laugh] Can I please have honorary gym status? [Laugh]

ZIERLER: I want to ask some questions about Caltech people, current and past. When you were growing up academically, in England, were you aware of Caltech institutionally? Were names like Feynman, Gell-Mann, Zwicky, Wilson aware to you?

ELLIS: Not at high school. As an undergraduate, I did a final-year research project on quasar absorption lines, and that's when I first realized the huge impact of the 200-inch at Palomar. I read papers by John Bahcall, who was here at the time, Wal Sargent, Bev Oke, Jim Gunn, all of these people. Then, when I started counting galaxies on photographic plates in the late 70s, I first met Jim Gunn. And then, I realized what a powerhouse Caltech was. Feynman, Gell-Mann, not so much until later. The first time I visited Caltech, I realized the heritage of the place in physics as well, Willi Fowler and nucleosynthesis – it turns out we lived in his former house on West California Blvd! Most of the immediate impression of Caltech was the mighty 200-inch telescope and these pioneering observers like Jim Gunn, Wal Sargent, Maarten Schmidt and so forth. Also Jesse Greenstein, who I did briefly meet when I arrived before he died.

ZIERLER: When you got here, who were some of the individuals you were really excited either to collaborate with or simply to interact with?

ELLIS: I was very impressed with Chuck Steidel. He's the guy who phoned me up. He was very welcoming, and he offered to take me to Keck to see how it worked. George Djorgovski was helpful, I asked him for advice on how to get observing time. And Wal Sargent. Those are the three that I recall making an immediate impression on me. Nick Scoville and Anneila Sargent were welcoming. I found the whole place abuzz. Shri was the astronomy executive officer and created an enormous office for my group which we called "Ellis Island." The excitement in Robinson at the time was what we call gamma ray bursts. I didn't even know about them until I got to Caltech. And everybody here was working on them. Djorgovski was working with Shri at the time, Chuck was pounding away at Keck on redshift three galaxies. I'd never seen anything like it in the UK. It was just wonderful.

ZIERLER: What niche did you see you would fill?

ELLIS: I was a little worried that I shouldn't tread on the toes of what the other people were doing. The message I got is each Caltech professor should pioneer their own research rather than team up and collaborate. I started out by working on gravitational lensing. Nobody at Caltech was active in studies of gravitational lensing.

ZIERLER: George wasn't involved with that at that point?

ELLIS: Not really, no. There's strong lensing, for example by foreground clusters which leads to large magnifications and multiple images – we discussed this earlier. And then there's weak lensing, which is the tiny distortion of light rays by large-scale structure. And I started doing work on weak lensing when I got here because that was an area which I'd already got excited about in Cambridge and nobody else here was working on it. That was great fun. Chuck Steidel seemed pre-occupied by studying galaxies at redshift three. This is a time we call cosmic noon, where galaxies are, for the first time, mature and forming stars at a prodigious rate. And I decided that I could go beyond this, so I started pushing out to much higher redshifts in about 2006. I was also working on distant supernovae, which nobody else was. It was fairly easy to find niches. I didn't feel that I was avoiding an area I wanted to work on. It's not as if I was dodging people - "Oh, gosh, I'd like to work on that, but Chuck's already booked it," kind of thing. I never felt that. I felt there was enough to do, and I was able to find the territory that I liked.

ZIERLER: What about the quality of graduate students and post-docs you had here?

ELLIS: Absolutely brilliant. Particularly the students. Think how many places I've worked. Oxford, Durham, Cambridge, Munich, London. I've seen grad students everywhere, and the Caltech grad students are well ahead in quality. There's an air of camaraderie amongst the graduate students at Caltech. They help one another, and this raises the level because student A sees that student B is working really hard and doing amazing work, so student A feels that if they don't pull their weight, they're letting the team down. Student A is lifted up by watching what student B does. And this is not some tense form of competition. Student B actually helps student A improve. I thought this was just wonderful. Post-docs, obviously, you choose them yourself, so you naturally select very good ones. Occasionally, some of them are not so good. But generally speaking, the fact that Caltech allowed post-docs to have their own observing time on Keck (which, by the way, University of California does not) meant that we attracted very good applicants. I never had problems getting a good post-doc.

ZIERLER: Now that it's been 10 years, but you get to come back to Caltech with some frequency that allows you a comparative perspective. How has Caltech changed over these past 10 years?

ELLIS: Well, I think it's gotten more bureaucratic, of necessity, due to rules set by the federal government and funding agencies. There's much more paperwork. When I was here, especially in the late 90s, there was more flexibility in spending money, for instance. You could vector money sideways into various activities. It's still a more flexible place for doing science and hiring people than anywhere I know, but I think it has gotten more tedious for the professors compared to, say, 20 years ago. In other respects it's the same place. The demographics of the campus are unchanged –obviously, a few buildings have gone up here and there, but it's still a delightful place. The Red Door Cafe, the canteen, and meeting important people, that's not changed. People may be a bit busier. Obviously, as a visitor, you're in a more leisurely mode than people who are working here. I meet a professor in the corridor, and they say, "Oh, Richard, great to see you." And after a few minutes, "Richard, I've got to dash because I've got to be somewhere else." There's a bit more of that than perhaps there was 20 years ago, people are more busy.

ZIERLER: If, heaven forbid, the TMT sort of goes kaput, what will that mean reputationally for Caltech astronomy?

ELLIS: Oh, it's already affected the reputation of Caltech. There's a general view outside Pasadena that Caltech bit off more than it could chew, that these 30-meter telescopes are international partnerships, and Caltech and UC entered this thinking it was something similar to Keck, but it's too big for them.

ZIERLER: Is that really the case, though, or is that just an effective stand-in for the protests? In other words, had the protests never existed, wouldn't the telescope have been built at this point, and it would be seen that Caltech and UC did not bite off more than they could chew?

ELLIS: You may be right, that's true. But these telescopes take 20 years or more to build from inception, and I think that many didn't have the patience for the long haul. I think people got exhausted. I hate to say this, but wandering around, I think many professors in Cahill have lost interest in TMT. When I was here, certainly in the first five years, it was a topic of everyday conversation. "Which instruments should we have?" "Richard, do you think the Canadians are going to be good partners?" I don't get that anymore when I visit. There's almost nobody talking about it. That's not good. I think people have already lost interest and are exhausted by it.

ZIERLER: What's the path forward?

ELLIS: Well, let's hope that the NSF coughs up the bucks. I worry that the NSF will only fund one US large telescope. Why should the US have two telescopes? Europe doesn't. ESO is huge. Australia is likely to join ESO soon. I think something like 40% of all professional astronomers worldwide are now members of ESO. They will only have one ELT.

ZIERLER: By the decadal giving equal weight to both American ELTs, do you think they're basically setting up a dog-eat-dog situation where one will emerge victorious?

ELLIS: Well, I think the decadal, and so far the NSF, chickened out of making a necessary decision, actually. This is unfortunate and going to hurt US astronomy. They asked me what I thought, and I said, "I think you've got to go for one. Even if it were GMT, it'd be better than what we have now. We're once again at a stalemate." Can the US really afford two of these? And if you're an X-ray astronomer, would you think that optical astronomy deserves this enormous investment?

ZIERLER: Tell me about your efforts to get Harvard into the TMT and what difference that would've made.

ELLIS: Well, in 2006, as we discussed, I was considering moving back to Europe. I was contemplating applying for a Royal Society research professorship which, eventually, would take me to Oxford in 2008. I was also invited to apply for the position of Director-General at ESO, an option which I didn't pursue. I suspect some of this leaked out and so in December 2006, Avi Loeb, who's now very famous… [Laugh]

ZIERLER: Infamous.

ELLIS: Infamous in some circles perhaps. Although, let me say, he's a great astrophysicist, and in high redshift galaxies, he's a theoretical pioneer. Avi Loeb phoned me on behalf of a search committee for a new professorship at Harvard. He said my name had come up and was I interested in applying? Our correspondence then involved discussions of how I couldn't function at Harvard without Keck access and how I was committed to completing TMT. Rememember, Caltech was the major lead in TMT and Harvard was, arguably, the wealthiest partner in GMT. These complexities were relayed back to the search committee and the response was quite surprising. Several members thought that, if appointed, Harvard would encourage me to accept by offering to purchase Keck observing time. This would naturally benefit other Harvard astronomers and so there was strong support. Moreover, several of the theorists on the search committee saw my possible desire to continue working for TMT as a way to get Harvard involved and to break the deadlock between TMT and GMT which they thought was damaging US astronomy. At least this is how it was reported to me by Avi, presumably to encourage me to take the matter seriously. So the key issue in moving forward was the following: if I moved to Harvard, would the university buy into Keck and TMT and thereby break the deadlock in funding US large telescopes?

And so one thing led to another. Charles Alcock, who was the director of Smithsonian until recently, came to see me in Pasadena. We entertained him at our house and he was very encouraging - although I'm not sure if he realized the bigger picture emerging in my mind of TMT vs GMT. And with strong encouragement, I flew to Harvard in February 2017, met with the faculty and higher-ups there, and everyone was very excited about my interest in coming!

Theorists Avi Loeb and Lars Hernquist and the exoplanet observer Dave Charbonneau thought my move would be a much-needed breath of fresh air on the large telescope front. Charbonneau thought I would be the catalyst to enable Harvard to buy into Keck and the theorists saw the advantages of Harvard entering the TMT partnership which was, arguably, ahead of GMT at the time. Regardless of which telescope was in the lead, they agreed that breaking the TMT-GMT deadlock was necessary for the benefits of US astronomy. It was all going swimmingly well, and I got to talk to the Dean who was supportive. I never got up to the president, but basically, I sensed an offer was coming. And then, they had some faculty meeting, and Charles Alcock phoned me up and said, "It's just not going to fly. It's going to disrupt the department to have you here because we value our relationship in GMT with the University of Arizona." I suspect the members of the search committee involved in GMT consulted their Arizona colleagues with predictable consequences.

So what might happened if it all worked out and Harvard joined TMT? NSF and AURA might have considered TMT to be significantly ahead of GMT. TMT would have had two of the wealthiest private institutions in the country behind it and Harvard would've gotten into Keck. Recall we were short of operational money for Keck. Shri was by now the Palomar director and he was running around the world trying to sell Keck time. We'd have brought in a very wealthy partner into Keck. Overall it would've been a major shift in the balance of US astronomy.

ELLIS: I've only told a few people this story. I told Shri, I told Tom Soifer. This is the first time I've explained it in detail. Of course, if Harvard had joined TMT and bought into Keck, it wouldn't have solved the protest problem. But it would have accelerated the financial situation. And it surely would have affected the perceived impasse between TMT and GMT.

ZIERLER: But there's no way out with the protest problem. There's never going to be, with all of the cultural understanding, and sensitivity training, and reflection on the part of the astronomy community, a point where there will be zero protestors. It's a noble goal, and we should work to get there, but it's not a matter of any one institution solving it. If TMT is going to be built on Hawaii, just to speak bluntly, there are going to be protestors to be pushed out of the way. It seems to be the unfortunate reality.

ELLIS: I agree. You could say that the Hawaii state government has not been firm enough. It's very sad. In 2005 and 2006, when Canada and later Japan came in, we were well ahead of the curve. We really were. And then, there was this stalemate with GMT. AURA, believe it or not, once did vote to join TMT, rejecting GMT. I remember the day clearly. I was skiing at Big Bear, I think. And I got a phone call from Matt Mountain saying, "It's good news. AURA has voted to join TMT, not GMT." In other words, it made a decision. And that evening, for some reason, there was an event at Wendy Freedman's house. And she came up to me and said, "Have you heard the news?" And I said, "Yes, I have. I'm sorry." And she said, "Well, best wishes and good luck." Then, what happened was, there was a backlash. The GMT people got together and complained formally to NSF and AURA and said, "This choice was unfair." And then, we reached this stalemate. My willingness to move to Harvard and my attempt to coax a new partnership between Harvard and Caltech was a sincere attempt to break the road block between TMT and GMT. Maybe I was naïve but I gave it my best shot. In the end, Harvard's longstanding partnership with Arizona and Roger Angel's Mirror Lab was just too painful a marriage to break up. And that was the end of that.

ZIERLER: And the ship has sailed, there's no chance that Harvard can join this late in the game.

ELLIS: No, I don't think so.

ZIERLER: I want to broaden to the question about the comparative perspective. Being in London but visiting Caltech, what about more generally, all of the places you've worked in Europe, all of the partnerships, all of the places you visit in the United States, how might you reflect on being a cosmologist, an astrophysicist, an astronomer in Europe versus working those trades in the United States?

ELLIS: The UK is small enough that obviously, everybody knows everybody else in the field that they're working, and it's reasonably harmonious, actually. Most people get on very well. There aren't institutional rivalries. The US is much larger and the departments are often self-contained empires with people collaborating within, perhaps less so at Caltech, however. There were a few things I did, obviously with Saul at Berkeley. When we went for the ultra-deep field and the supernova programs, I collaborated with Canada and Arizona. But I would say the US is so big that it's more compartmentalized. In terms of liveliness, there is this atmosphere of can-do in the US, everything can be done. And young people are inspired by this.

ZIERLER: This is baked into our culture.

ELLIS: It is, yeah. A post-doc can, in principle, get a large amount of money by simply coming up with a convincing Hubble proposal and getting a lot of observing time. The UK is much more hierarchical. The head of department would want to see the proposal, and judge it, and might say, "I don't think you have the stature to get this kind of money." In the US there's much more willingness to support young people, and there's far less hierarchy. I like that about the US. Young people get the opportunities. But it's getting tougher. As I said, the NSF doesn't have a lot of money for grants. Very few of the grants that are applied for are successful. That's unfortunately the same in the UK. But what's changed is Europe. The European Union has put a large amount of money into what we call the Horizon Program, which has these Advanced Grants for senior faculty, Consolidated Grants for mid-career folk, Starter Grants for young researchers – each is 5 years in duration and about 2 to 2.5 million Euros. And we really have done well out of these grants in Britain. Now, there are a few long-term fellowships in the US, but they're not as well-funded, I think, as the European programs. I think there are pluses and minuses on both sides. Better to be a young person in the US, more opportunities, less bureaucracy. You stand as good a chance of getting something going. Longer term funding for research, better in Europe, perhaps.

ZIERLER: Is that a contemporary observation? Would you have said the same at the beginning of your career?

ELLIS: Yeah, when I arrived in the US it was like that here, and it still is, so I don't think that's changed. The only thing that's changed is, getting NSF grants is harder, I think, and that's unfortunate.

ZIERLER: That's true across the board.

ELLIS: Yeah.

ZIERLER: Finally, I want to ask just a few fun questions that will put you in reflection mode. Scientific heroes, either that you've read about in history or that you've met in person. Who are your real heroes?

ELLIS: Allan Sandage is certainly one of my heroes.

ZIERLER: Even though he clung to an idea that didn't turn out to be true?

ELLIS: Sandage is my hero because he was a man of detail with a huge range of accomplishments – a giant in the subject. I first met him in Hawaii in 1986. I was doing a redshift survey to 17th magnitude, which is nothing much to crow about today. And he said, "Oh, gosh, I dream of doing a redshift survey to the 17th magnitude." I think he was humoring me. You know about the American Astronomical Society's annual meetings. In Britain, we never had such a national meeting until maybe 1990, I think. And I pushed for that very hard to be started with the Royal Astronomical Society. The first one we ever held was at Durham, where I was. It was called the National Astronomy Meeting. And I thought, "Well, we've got to get this annual series off to a grand start," so I invited Allan to come and give a talk. And he came and gave a fantastic talk, and then we had a party at our house for the astronomy graduate students at Durham. And he loved it. He loved talking to these young people. And I just realized, this guy is larger than life.

Later, in 1997, when I was at Cambridge, he commented on my Annual Review article "Faint Blue Galaxies." He was so diligent marking up every page with useful comments; I've kept the manuscript actually for posterity. Then I realized he did this every year for so many articles in the journal.

Finally, in order to get my job at Caltech, I had to give a colloquium. And I didn't notice at the time, but Allan attended. It was in the large lecture theater in physics. He came in through the back door and sat in the back row. Wal Sargent told me, "That's the first time Sandage has been on campus for over 20 years" Allan really hated Caltech, although he was a Caltech alum, because of the divorce settlement over the 200-inch between Carnegie and Caltech that Maarten Schmidt engineered. Over the years he was alive I'd regularly go up to Carnegie, and we'd get on very well together.

One thing that really impressed me was his history of the Carnegie observatories. If you read that huge book, you get a sense of the detail of this guy. His breadth of knowledge and depth of the subject of astronomy was unparalleled. But you're right, he could be stubborn. This is a guy that championed measuring the deceleration of the universe using galaxies that clearly evolved because they contain stars which evolve. He knew stars evolve because he's a pioneer of measuring the ages of globular clusters based on the fact. So how could he be so stupid to think that somehow, when you went back in time, these things canceled out and could be used as distance indicators?

ZIERLER: Just to clarify that, you're saying that this is evidence that he had that he refused to believe?

ELLIS: Yes.

ZIERLER: This is different than Einstein's deep misgivings about quantum mechanics.

ELLIS: He must have known. He was so besotted with the idea of following Hubble to measure more and more distant galaxies, and then plot what we call a Hubble diagram, which is the magnitude versus the redshift, to see whether it was curving one way or the other. Now, unlike Hubble, Sandage mastered the mathematics of the relativistic universe. He was trying to measure Hubble's constant and the deceleration parameter, the rate at which the universe is slowing down. As Kirshner and Turner mentioned this in their talks earlier this week, the universe was thought to be governed by these two numbers. This was Hubble's legacy. Hubble handed on the baton to young Sandage and said, "You are the future." Maybe he just didn't have the courage not to carry on. But it was so obvious that, as you go back in time, galaxies get brighter. And therefore, you can't use them to measure distance. And Jim Gunn realized this. Let's go to Jim, another hero. Jim was originally a competitor of mine. While we were using the telescopes in Australia, he and Dressler…

ZIERLER: He's a little older than you, though.

ELLIS: Oh, yeah, he's 12 years older. He and Dressler were on the 200-inch, and my colleagues and I were on the Anglo-Australian telescope, and we were looking at clusters of galaxies, considering the evolution of the morphologies and colors of the galaxies in the cluster. But I found Jim a wonderful guy, a brilliant theorist, a wizard at instrumentation, and a great observer. And it's very rare to get those three qualities in one person: theory, observation, and instrumentation. And of course, he's the brains behind the Sloan Digital Sky Survey. And the instrument we're now building for Japan, in which he's centrally involved, is a sort of giant eight-meter version of that. I got to know Jim very well over the years, and I just have a lot of respect for him, and he's a hero. In terms of theorists, I was always in awe of Martin Rees – the British Astronomer Royal. He gives amazing talks, he's very insightful, he's very smart. But we have had our disagreements over the years. He can be quite difficult if he doesn't get his way.

ZIERLER: Did you know him well?

ELLIS: Oh, yeah, I was appointed to his chair, the Plumian Professorship, at Cambridge in 1993. He moved sideways from this chair to one of these Royal Society reseach professorships which free you from teaching and administration in order that he would create a vacancy. And I don't know, I think he wanted Simon White, who's a very famous theorist who was at Cambridge at the time, to succeed him. In other words, he moved sideways so that Simon White could have his chair. Or perhaps Roger Blandford. Well, all three of us applied for this chair, Simon White, Roger Blandford, and I, and probably others, too. And Roger Blandford was offered Martin Rees's chair, but as we discussed, Roger's wife didn't want to go back to Cambridge, so he turned it down. I was ranked number two, and so I was offered the chair. I don't think Martin expected that, but that's the way it went. So we moved down to Cambridge. Simon White was still there. Simon realizes he's not going to get a chair at Cambridge, so he moves to Max Planck in Germany.

I think Martin was upset that Simon White didn't get his chair. Although that didn't affect our relationship too much, perhaps it wasn't a great start in our relationship. But what really upset Martin followed his achievement to get the Royal Greenwich Observatory moved from Hertmonceux, Sussex to Cambridge. This was a great coup for Cambridge and, as Astronomer Royal, he was very proud of it. As a research institution, the RGO goes back to 1676. However, as time moved on, it was obviously a mistake because Britain wanted to join ESO, and therefore, it needed money. And this famous Royal Observatory was going to be closed down. Martin is in print as stating that I didn't support him in fighting to keep the Royal Observatory open and even blames me for the demise of this historic institution, which is totally ridiculous and all my colleagues agree. And then, when I moved to America, Martin was very upset because he considered I'd "defected". He'd always had a love-hate relationship with the United States. But he's a brilliant man, and I've tried maintained a good friendship despite all of these ups and downs.

ZIERLER: What about Penrose?

ELLIS: No, I didn't interact with Roger. He was there when I was a student at Oxford, but he's too mathematical for me, and I didn't really interact with him in person. I interacted with Peebles a lot. I told you about our work on the clustering of galaxies. Peebles came to Durham several times. He wrote me a very nice note about my new book. He's a great guy, pioneer in the field of cosmology. In cosmology, I would say Martin Rees and Jim Peebles. In observations and instrumentation, Sandage and Gunn.

ZIERLER: Pretty good list!

ELLIS: That's a pretty good list, yeah. [Laugh] And three of those are still alive, which is nice.

ZIERLER: The notion of a multiverse, does it give you a headache? Is it fun to think about?

ELLIS: Oh, God. This is another bone of contention with Martin Rees. What's the point?

ZIERLER: Just because it's untestable.

ELLIS: It's untestable, and it's like science fiction. Oh, but people love to hear about it. Everybody loves to talk about it. And I talked to Mike Turner about this as well. Martin's view is that we're so young in the subject of what's out there, we're like ancient people standing on the coast, and all we can see is the horizon. If we had a boat, we'd be able to cross the Atlantic. I don't find that analogy particularly helpful. [Laugh] But Turner said, "Well, maybe in several centuries, we'll look back and say that we were naive." Maybe that's right, but I can't see it now. I agree it's a fascinating topic for a lecture. The public love to hear about multiverses. This business of throwing the dice, and all the constants change every time. "Imagine a universe where the mass of the electron is different. Would we still be here?" All this kind of thing. It's fascinating, but it's almost philosophy. It's not really science.

ZIERLER: Of everything that you've learned in studying the universe, are you comfortable with the idea that this is it, this is all that there is? Is it suggestive of that?

ELLIS: Well, this is all we can observe, so this is it, as far as I can see.

ZIERLER: And you're a strict empiricist.

ELLIS: Yes. And I'm not sure there's life elsewhere, either. My wife's a biologist, and the step from singular-cellular life to multicellular life is very difficult. It could be chance. And I know it's horrible to imagine we're alone, but it's a thought that occurs to me. Most people just use statistics. "There are so many stars, there are so many planets, there are so many galaxies. Isn't it ridiculous that we're alone?" Yes, but what is the probability of multicellular life? We don't know. It's an open question, I would say.

ZIERLER: Because you're still advising students, and there are five- and six-year commitments there, when are you going to get to the point that…

ELLIS: I have. I've stopped taking students. It wouldn't be fair on them. I'm a healthy guy - I cycle, I exercise, I go hiking in the Alps and all that. As long as I'm healthy, I'll keep going. But what if..?

ZIERLER: You'll still have post-docs?

ELLIS: I still work with post-docs, yeah. But students, I think it's a risk for them. UCL's pretty good, you can be a second supervisor. The bonus is, the first supervisor is funding the student or making the major judgments, but the student can come to you and work with projects. I'm a second supervisor to two students at the moment, and that's fine. I like that model.

ZIERLER: Do you have a plan to go emeritus but to stay active?

ELLIS: Yeah. I think I told you, my Advanced Grant runs out this year, and there are possibilities for smaller amounts of money, but this big European grant finally grinds to a halt. I'm due a sabbatical, so I'll take that. After that, UCL is very good, you can go down to three days a week, so I might try that and see how it goes.

ZIERLER: A soft landing.

ELLIS: A softer landing, yeah. But I fully intend to keep coming back to Caltech.

ZIERLER: Because it's just a treat for you.

ELLIS: It's a treat for me, yes.

ZIERLER: Finally, last question. For however long you want to remain active, however you define that–emeritus, non-emeritus, traveling, non-traveling–what are the big things that motivate you to stay active, to still read the literature, to still submit the proposals?

ELLIS: Curiosity. Surprises. There's something going on in the early universe. What is it? What's dark energy? I don't lose sleep over it, but I'm fascinated by it. And it is an observer's world now. It wasn't when I started. It is an observer's world, and we are blessed with these facilities, and there are going to be surprises. And I love surprises.

ZIERLER: And this is a statement just of the explosive growth of technology.

ELLIS: It is. Exactly right.

ZIERLER: When I originally emailed you, I thought we would have to do this over Zoom. It's been such a treat to have done it in person, and I want to thank you so much.

[End]