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Interview of Henry Tye by David Zierler on February 26, 2021,
Niels Bohr Library & Archives, American Institute of Physics,
College Park, MD USA,
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Interview with Henry Tye, professor emeritus of physics at Cornell, and subsequently professor emeritus of physics at Hong Kong University of Science and Technology (HKUST), and currently, Researcher at the Jockey Club Institute for Advanced Study at HKUST. Tye provides a brief history of HKUST, and he offers his views on China’s long-term goals in high energy physics. He recounts his childhood in Hong Kong where his family fled from mainland China during the Communist revolution, and he explains the opportunities that led to his undergraduate admission to Caltech. Tye describes how discussions of the Vietnam War permeated his college experience, and he describes the influence of Gerry Neugebauer on his interest in physics but that cosmology was far from his considerations at that point. He discusses his decision to study at MIT, where Francis Low became his advisor, and how he worked closely with Gabriele Veneziano on the relationship between the Thirring model and bosonic string theory. Tye explains the excitement surrounding the “November Revolution” which was unfolding just as he arrived at the SLAC Theory Group in 1974. He describes the origins of his interests in cosmology, and the source of his collaboration with Alan Guth during his postdoctoral work at Cornell, where he pursued matter-antimatter asymmetry. Tye explains how this collaboration ultimately created the field of inflation and why this addresses fundamental cosmological problems associated with flatness and the horizon. He explains how and why the original theory of inflation was revised by Andrei Linde and Paul Steinhardt, among others, and why he developed a subsequent interest in cosmic superstrings and branes which he recognized would give a perfect model for inflation. Tye describes why he is optimistic that technological advances will make cosmic superstrings a testable proposition, and that collaborations including the Sloan Digital Sky Survey and LIGO/Virgo are positive steps in that direction. He bemoans the dearth of string theorists focused on phenomenological work and why he thinks string theory will solve the quantum gravity problem. Tye describes his decision to join the Cornell faculty, why his notions of a “string landscape” suggest philosophical implications, why the cosmic landscape is central for understanding the wavefunction of the universe, and why both the universe and all multiverses can begin from truly nothing. At the end of the interview, Tye discusses his recent interests on the cosmological constant problem, the KLT relation, and the observations and experiments that are most likely to push cosmology into new and exciting areas of discovery.
You got it. Okay, this is David Zierler, oral historian for the American Institute of Physics. It is February 26th, 2021. I’m so happy to be here with Professor Henry Tye. Henry, it’s great to see you. Thank you so much for joining me.
It’s great to see you, David.
To start, would you please tell me your titles and institutional affiliations? And you’ll notice I pluralized both because I know you have more than one.
Right, I was a professor at Cornell, and I retired about six years ago, and then I had a professorship at Hong Kong University of Science and Technology (HKUST), from which I also retired last year. Right now, I’m a part-time researcher at the Jockey Club Institute for Advanced Study in the HKUST.
Henry, what was your motivations in retiring from Cornell and to transfer to Hong Kong University?
Yes, I grew up in Hong Kong, and I went to the U.S. for college, PhD, postdoc, and stayed on. My mother lived in Hong Kong, and we visited her almost every year, especially during Christmas/New Year time, to avoid the Ithaca winter. The Ithaca winter is a bit long. In 2008, my mom had a stroke, and that year, I came back from US to Hong Kong five times.
That’s a lot of flying.
And my wife said, maybe we should spend more time with our mothers since my wife’s mother also lived in Hong Kong. My wife (Bik Tye) was born and grew up in Hong Kong. So, at that time, we decided to move back, to spend more time with our parents.
Henry, of course, I know a lot about the physics department at Cornell but very little about the physics program at Hong Kong. Tell me a little bit about it.
Well, HKUST is a very young university founded in 1991. In the past, physics in Hong Kong was mostly semiconductors and condensed matter physics. There’s no fundamental physics. For example, at my university, there was no particle physics, no astrophysics, no cosmology, and, as a result, earlier, we never really thought of coming back because there’s no opportunity for me to come back to and, also, Cornell, you know, is a great place. So, after the year 2008, my wife tried to explore- she’s a life scientist, so she could come back relatively easily and found a job at HKUST.
The HKUST physics department had no interest in developing fundamental physics, particle physics, high-energy or cosmology, astrophysics. But the university had a job that could fit me. They started a new Institute for Advanced Study. Paul Chu (well known for his work on high temperature superconductivity) was the president of the university, and was also director of the IAS, and he needed somebody to take over the directorship. So, I got a job by being an administrator. The department had no interest in hiring a particle physicist but as administrator, they don’t care which area or which field you’re in. So, I got a job by being an administrator. Then the university promised to allow me to hire people, so I started to hire particle theorists. When I stepped down as director, Andrew Cohen (coincidently another particle theorist) from Boston University became the director here. We (the IAS) also recruited some other physicists, a mathematician and a chemist. In fact, George Smoot has been with us for quite some time, great interaction. Except, last February exactly a year ago, he thought the pandemic’s getting worse in Hong Kong, and the protest in Hong Kong was a problem, so he went back to Paris, right, to Europe. And then, of course, he got caught in the pandemic in Europe (laughter).
And so far, he has had difficulty coming back because of all the quarantine and things like that. But we hope to recruit one or two more people. And Hong Kong joined ATLAS at CERN LHC, so we started building up fundamental physics in Hong Kong together with two other universities. It took a while, but we sort of are on our feet right now, and we hope to grow more.
Mainland China wants to do big projects, and I figured that by coming back to Hong Kong, I can have Hong Kong play the role of a bridge between East and West, and to help develop the high-energy program in China. China has the idea to build a collider. At the moment, they have a proposal to build a 100-kilometer ring, which CERN in Europe also wants to do. Both projects have been studied extensively, and nobody knows what’s going to happen. Unfortunately, the relationship between East and West, especially China and U.S., have sort of deteriorated in the last few years, which made any international big science project much more difficult to move forward. But, you know, I’m retired so we have to wait for the next generation to push forward.
As we get farther away from the SSC in the United States, and hopes continue to fade about a high-energy physics project on that scale in the U.S., is your sense in the coming decades that it’s really going to be China that puts the resources into this level of research?
That’s a very good question. Hong Kong and myself have been involved as much as we can in the project in China. We have a meeting every year, January, to bring the team in China and the team in Europe together to discuss the design, what machine to build, and the science behind it. The first few years, we had a lot of U.S. physicists participating. But in the last two years, the U.S. participation has really gone down to zero because of the political situation.
Japan also wants to build an ILC, the international linear collider, and I think they have difficulty, though they are still pushing hard. The situation in China is not clear. It gets to the level that a decision will have to be made at the highest level, and that decision has to be made besides just science consideration. They have to worry about the international situation, the political situation, and the funding situation in China. I do feel the project has a much better chance if US government will allow the US physicists to participate. And Europe, of course, we don’t know. The difference between the CERN and the Chinese project – both involve a 100 kilometers ring - is that the CERN, you know, is still running LHC, which has a higher priority, and so they’re not going to start the 100-kilometer project in the next fifteen years probably. But China can start immediately. That’s the difference. But China cannot do it alone because they don’t have enough talent and manpower- it would certainly have to be an international project. So, how’s that going to work out? Nobody knows (laughter).
Henry, this is as much a cultural question as it is a scientific question, and, of course, I’m only asking for your views, and it’s a very complex question. In the United States, so much science is funded by the government simply because of basic research, simply because we want to understand how the world works. If China has the appetite to fund research in the coming decades at that level, to what extent are its motivations also basic science, and to what extent are they wrapped up in broader aspirations for achieving preeminence in the twenty-first century, both in a technological and in a national security sense?
That’s a very good question, and I certainly don’t know the answer. But my personal feeling is that China wants to emphasize basic science. The reason is because for the past decade, and also in the near future, all the emphasis has been on technology and developing a high-tech industry. And the government, again, I would say at the highest level, has been saying that we have to take the long view, and we have to emphasize basic science. So that’s the goal. How to achieve that goal, I’ve no idea.Certainly, to be eminent in basic science is one of the goals, but I think they also know that basic science sometimes drives technology as well. For example, the Chinese project called CEPC, at least a year ago, they already have seventy companies and research institutes participating in how to build the machine: every component, magnet, klystron etc.; in fact, many aspects of high tech are involved. And in pushing this project, many companies and research institutes in materials, in magnets, in other technical areas, all try to participate. So, they want to use that as a way to push other areas of science and technology to the frontier. In fact, in 1980, Deng Xiaoping actually wanted China to build the electron-positron collider, the Beijing EP machine. And at that time, China was substantially poorer than now, and Deng Xiaoping felt that by building such a machine, they had to push so many different aspects of technology in order to succeed, and that would help to drive technology.
And I think the current leadership has a similar thinking. But, you know, they have to balance many things, and, certainly, international politics seems to come to play a big role. I don’t know what’s going to happen now. We can only guess. There’s no question that Europe and China consider themselves to be economic rivals, not enemies, which is good (laughter). It would be even healthier if they position themselves as competitors, not rivals. Now, what will happen to this (U.S.) administration? I think rivalry is most likely. It’s not going to be enemy, but it will mean maybe competing in basic science, and real rivalry in technology. How to balance the two? I think the Biden administration has to make a decision. Then the physicists can know what to do.
Henry, one more question about the present before we develop the historical narrative of your life, and that is, as a theoretical physicist, I wonder if these past eleven months of the pandemic with all of the social isolation has perhaps given you more time to work on equations or other work in theory that you might not otherwise have, or, inversely, is your work style such that you really do benefit from in-person interpersonal interactions, like going all the way back to Alan Guth in the early 1980s?
(Laughter) The past year has been really actually wonderful for me, you know, because we have no visitors and no conference, and I still have students, postdoc and colleagues to interact with; we can work together, and I have no distraction. And, to me, the past year actually is ideal for the projects that I am trying to finish. We can’t go anywhere so the only activity I can have is go to office to get some work done, and a year is good enough to finish what I want to do. The U.S. election, the situation in Hong Kong are all very distracting, I would say. But since I don’t have to travel, it ends up allowing me to focus on my research. So, I’m actually very happy that I am able to finish the project I have in mind and finish it more or less now before I truly retire. So, in that sense, it’s worked out very nicely for me. Now, I’m eager to go to U.S.
Yeah, I hope that happens soon.
Yes, my younger daughter Lynne just gave birth to a baby girl earlier this month.
And I’m so eager to visit her, so, but-
Well, hopefully, Henry, soon we’re going to get enough vaccinations, we’ll-
-get there. We’ll get there.
Yes, the U.S. is doing very well on vaccinations.
So Hong Kong is a little bit slower.
Well, Henry, let’s take it all the way back to the beginning. Let’s start with your parents. Tell me a little bit about them and where they’re from.
Okay, my parents were from Shanghai. I was born in Shanghai. But in 1949, because my father was a merchant, in fact, the whole family including my grandparents were merchants, so they couldn’t stay in China. They had to rush out of Mainland China to Hong Kong as refugees, and so I grew up in Hong Kong. My father’s mother passed away very early, and his father passed away when he was twelve So, he only had primary school education. But he was a smart guy. He worked hard and he got recognized in the company he’s working at.
My mother was the daughter of the boss of the company. And, after junior high, she went to England for boarding school education, so her English was very good, better than mine, I would say (laughter). Her accent was, of course, a British accent. But then because of the war, World War II, she came back to China, and she got married with my father. Then they moved to Hong Kong as refugees. It was very tough for them. But, you know, as a kid, I didn’t know, or I couldn’t tell the difference. So, I felt I had a reasonably good childhood, and then I went to North America for education.
Was your sense, Henry, that because of your interests and talents in science that the United States would be the best place to pursue those interests?
Oh, yes, there’s no question on this. At the time when I was thinking I wanted to be a physicist, there’s no question. That was in the late sixties. No question the U.S. was by far the best place for physics, any aspects of physics, or, actually, any basic science. So, and I am very thankful for the U.S. university systems, which gave me a scholarship. I couldn’t afford it. My family couldn’t afford it otherwise, and so I was able to get paid by Caltech and MIT during my educations. And, of course, up to my postdoc, it was fine. And, in fact, when I was looking for postdoc, there was no chance of coming back to Hong Kong, as I mentioned. Nobody did fundamental physics, in those days. And in Europe in the seventies, there was nothing compared to Europe now. So, in the seventies, certainly, U.S. was preeminent.
Henry, how was your English before arriving in the United States?
I went to a Catholic school called La Salle College. I went to its primary school and high school. A lot of the classes were in English, although, for the kids, we always spoke Chinese among ourselves. So, we never really used it even though classes were in English. I had no difficulty with listening to lectures and coursework, although speaking English colloquially took a little bit longer to get used to.
What kind of schools did you apply to? Where did you think you’d be able to get in in the United States?
Oh, at that time? I didn’t know anything about schools. I was thinking I would just stay in Hong Kong. But my father felt that Hong Kong is not safe in a sense Hong Kong was so close to the Mainland that it could be taken over any time, so he wanted me to leave Hong Kong. He asked around among his friends, and his friends said, “Oh, McGill is the best university in the world” (laughter). So, he applied for me to universities in Canada, and so then I ended up in Toronto, and in engineering because my father thought engineering was the future. So, I spent half a year there, and didn’t like it. I had a high school classmate who went to Caltech, and he told me Caltech was great. I really started thinking about my own future only after I left Hong Kong, before that, I assumed that, as long as I studied, everything else will be fine. Everything else will be taken care of by somebody, my parents, my dad, for example. My dad, as I said, had only primary school education, but he self-learned English, so, but he felt he knew English. So, he took charge over my future while I was still in Hong Kong. I was disappointed with engineering school, not so much Toronto but engineering. I didn’t like the engineering program there. And my friend, high school classmate, Pui Kuan said, “Caltech’s great. You are free to choose whatever courses you like.” He encouraged me to apply to Caltech, so I applied to Caltech and I-
And, Henry, did you have any idea like what Caltech meant? Had you heard of people like Feynman or Gell-Mann? Did you have any idea of these things?
No, not really, not at that time.
I trusted my friend. I did know I had a very good chance to get in because, in high school, I’m the number one student, and my friend was the distant second. The fact he got in and did well there told me that I have a very good chance to get in. So that’s the only school I applied to in U.S. So, I got in, and then I read up. Ditch Day was the first thing I read up, and I told my parents about it. My father was very upset. He never heard of Caltech. He heard of McGill, Toronto, you know. Those were popular with his friends, because their kids went there. So he was upset about my intention to transfer. Fortunately, Reader’s Digest had an article on Ditch Day about Caltech. My mom showed him the article. He said, “Oh, okay, Caltech maybe okay” especially based on its costly tuition, which was above our means. But Caltech gave me a scholarship. So that’s how I got to Caltech. That’s the only place in the U.S. I applied.
Henry, was your plan to major in physics from the beginning, or that came later on?
I really didn’t know what I wanted to do. My parents set low expectations, as long as I finish high school, I would have satisfied their expectation of me, because that’s how far they went. So, I didn’t spend much time thinking about it. But I was pretty good at physics and math, I found it pretty easy compared with my classmates at high school, for example, and I just excelled in these subjects. So, I figured something along that direction would be good for me.
And how well-prepared, Henry, did you feel when you got to Caltech relative to your- the other students, mostly who came from American high schools? How well-prepared did you feel coming from Hong Kong with your secondary education there?
Actually, Caltech was fine. I didn’t find it very difficult. Actually, I didn’t find it difficult at all. But the hardest part for me at Caltech was the climate of Vietnam War hanging over the heads of most of my classmates. I lived in Dabney House- students were consumed by the worries of the draft to Vietnam. I must say, at that time, I didn’t really appreciate their worries as much as, you know, later on, when I looked back. And so, the discussion during dinner table was all about the draft, the Vietnam War, and many of them said they might have to give up their studies. So, from that point of view, I didn’t have a close relationship with the American students when I lived in the dorm, mostly because they were so much consumed by the draft. I understood the draft was obviously bad, not good for them, but, since I was not being drafted or had the possibility of being drafted, I did not appreciate the concern, the stress that they had. Looking back, I really feel I should have had a better understanding of what they were going through. For that reason, my education or study at Caltech really did not give me a full understand of the American systems. I did not get a full understanding of the American systems until I had kids, when they went to school, and I actually learned from them (laughter). So, it was a belated learning of the society.
In a way, Caltech was probably far from the norm. One day, a professor said, “We have no classes today. We should sit down and discuss the Vietnam War.” A good fraction of the students, probably the ones already knew they would not be drafted, did not want to participate. They wanted classes, which was very different from other places like Berkeley. So, there was a cultural clash between different groups of students and faculty at Caltech at that time. It was not the normal environment which I think students today have. It was a little bit regrettable. The time was tough, so, and many of my classmates’ sort of quit, they gave up one way or another.
Henry, was it a particular professor or a class that made you pursue physics specifically?
Well, when I decided to go there, I decided to do physics already. Yes, I had a high school teacher who was very good, and it’s the first time I realized physics is for me. He was doing nothing but Newton’s three laws, and I had difficulty memorizing things. So, for example, life science is terrible because, at that time, biology, you had to remember so many things. And chemistry, you have to remember a lot of things. Physics, you don’t have to remember much, three laws, everything can follow from that. It really fit into my way of trying to remember something. If I don’t remember, I can just deduce it. Of course, once I have done the deduction before, it’s very easy for me to deduce again as a way to remember what’s the outcome. So, it seemed that physics was just something I enjoy, and absolutely true still today. I feel I am in the right field.
Henry, who were some of the professors that you became close with or who exerted a strong intellectual influence over you?
The professors at Caltech- there were a few who I didn’t get close to but had influence on me. I worked with Gerry Neugebauer, who was an infrared astronomer at that time. His group had a few senior students like a senior and junior, as well as graduate students who I could interact with. And so, working in Neugebauer’s group, I learned what research was about.
And, I actually worked with Fritz Zwicky the astronomer for a few months before I went to Neugebauer. Astronomy seemed interesting to me, but the problem was that we had to observe. The Mount Wilson telescope was right there, but I always got sick, motion sick on the way there and back. And I felt that since all telescopes were high up in the mountains, it was probably too stressful for me. So, I decided theoretical physics might be better for me. The decision to be a theoretical physicist, more or less, was made by elimination of other topics. So that’s what happened.
Henry, did you come to Caltech with the understanding that you would be making a life for yourself in America, or did you hold out the possibility that you would return back home?
At the time, I must say (laughter), sorry, I didn’t think that far about my career. I just enjoyed it. My way of thinking is that when I get there, I will know what to do. I told myself just try to graduate, and if I graduate, I will go to graduate school. Then after that, god knows what’s going to happen. I still have that attitude now, and I feel that we cannot plan too far ahead. At the time, I was not thinking about career. The idea that I can graduate and get a job or go on to a PhD, and then get a job doing something other than what I was trained for, you know, was perfectly okay with me. In fact, it was that way of thinking kept me in the field because the job situation was very bad at that time (laughter). So, looking back, many of my contemporaries dropped out of particle theory because they saw no future.
These were very smart people, you know, and they worked up to a PhD or up to a postdoc, but couldn’t land a job, and they dropped out.
And, Henry, your intellectual environment at Caltech was strictly particle theory. No one was really talking about cosmology or astrophysics in your group at this point.
Oh, yeah, at that time, nobody talked about cosmology. But I worked for Neugebauer, and all I did was to go and do an infrared survey. He and Bob Leighton built sort of an epoxy telescope. You know, just put epoxy on the dish, and rotated it so that a parabola formed, and then they used that telescope to do surveys at 2.2 micron of infrared, to look for whatever infrared object they could find.
I was part of that team, to collect, study, and analyze data, whatever. But I really didn’t know much about what was going on. And then towards the end, they wrote a Scientific American article, and I read the article. Only after reading the article I realized what we were doing. According to the article, they explained that they were doing the infrared survey because they wanted to look for the missing mass in the universe.
There was some mass missing, that could be infrared or something, so they wanted to search for that. I said wow (laughter). That sounds really important! So, I told myself that I must study the missing mass theoretically, then I wouldn’t have to spend night after night at observations (laughter). So that “Eureka moment” decided my future to be a particle physicist. And cosmology only came later.
Henry, what kind of advice did you get about graduate programs to apply to? Did you think about particular professors to work with? Did you consider staying at Caltech?
At that time, Caltech didn’t encourage people to stay behind, and I had no intention of staying, so I applied to a number of places. But, again, I didn’t know too much. At that time, you might say that the best physics departments should be Harvard and Princeton. I didn’t apply to Harvard or Princeton (laughter)- but chose a few random places, and I ended up applying to MIT. Of course, I got accepted by all the places I applied to, but MIT gave me a fellowship, okay, more money than I have ever seen (laughter). Furthermore, Victor Weisskopf wrote me a letter saying, “You have to come.” I thought to myself, if Victor Weisskopf wrote me a letter asking me to go, I have to go.
So, I chose MIT partly because of that, and I didn’t know who I was going to work for. I had no idea at that time. Actually, I didn’t even know who were there. But I decided I wanted to do theoretical physics and went to MIT because Victor Weisskopf told me to go. So, it’s a little bit unusual how I chose my path.
How did you develop the relationship with Francis Low?
Okay, so I went there, and I remember—
And what year is this, Henry? What year do you start at MIT?
Okay, I went there in 1970.
So the first year, I took some courses. I took a particular course which Francis Low was teaching in weak interaction, but I learned nothing from it, okay (laughter). I didn’t have enough background to understand what he was talking about. But I walked around the Center for Theoretical Physics, and I was looking for a young person to give me a first project to work on, and I saw, you know, many offices had doors closed or were empty. But there was an office with an open door, and somebody was sitting there, so I walked in and asked for a project. And that was Veneziano, Gabriele Veneziano. So, he said, “Sure, I can give you a project.” So, he gave me a project to do, and I did it. Then, in the second half of the year, he said he and Francis Low and Jones and Jim Young, four or five of them, just finished a paper. He asked me to read it. I took the paper and read it. The next day I went to him, I said, “Interesting paper. You deduced this result, but I followed up and deduced another result.” And he said, “Oh, okay.” He said he would think about it. Two days later, Francis Low asked me to go to his office, to explain what I meant. Just like that, they put my name on the paper (laughter). Soon after that, Veneziano left MIT and went to CERN, and he said, “Why don’t you work with my good friend, Sergio Fubini?”
So, I went to see Sergio Fubini, and Sergio said, “Ah, you can work with me, but I’m leaving at the end of the year to be the head of the theory group at CERN.” I said, “Okay, that’s okay.” because I really just needed someone to sign my forms. So, I “worked” with Fubini for a year. When Sergio Fubini left, I was in my last year of graduate school. I still needed someone to sign my forms. So, I went and knocked on Steven Weinberg’s office door- that was 1973. His secretary whom I know said to me, “Why do you want to talk to Steve?” I said, “I need an advisor. Steve seems like a very good person to work with.”
And the secretary said, “Okay, come.” And when she opened Weinberg’s office door, everything was in boxes. She said, “Weinberg’s moving to Harvard.” So, I said to myself, “Okay, this is too much (laughter)- I need to actively find an advisor.” So, I went to the seminar room where the faculty usually had lunch, and asked, “Is there any faculty here who is not going to leave that can sign my thesis?” I happened to know most of them by then. Francis Low raised his hand and said, “Ah, I’m not going to leave. Even if I want to leave, my wife won’t allow me to leave.” So, I said, “Oh, can you sign my papers?” So that’s how I became Francis Low’s student.
Aha. Did you ever think about following Steve to Harvard?
No, no, no, he was packed already. I was an MIT student. If I was already working with him, he would have taken me with him. But I had not worked with him at all, so-
Henry, what was-
-and so that’s how-
-what was Francis Low working on when you first connected with him? What was his research at that time?
Ah, at that time, what they did is the Pomeron (laughter). Nobody talks about that now anymore. He worked on the decoupling of the Pomeron. They found a particular condition- using some inclusive reaction to show that the Pomeron decouples under those conditions. In the paper we showed that Pomeron will decouple under a more general condition (that was my contribution). That’s it. And it’s something that nobody works on nowadays. It was very phenomenological, in those days, inclusive cross-section was a big topic, and that’s what it was.
There is another memorable interaction with Francis that I recall. Francis was teaching weak interaction on something that he was working with Gell-Man, Goldberger and others. One day, Glashow came to MIT to give a talk on the GIM mechanism. I was impressed. I went to seek Francis’ opinion. I said, “This is really interesting!” And Francis said, “Yeah, ideas like this are a dime a dozen.” Clearly, he didn’t think much of it. So, I didn’t think further about it. Later on, I realized that Francis actually was commenting on Shelly Glashow that he had many ideas that didn’t pan out.
But that one panned out (laughter). The GIM mechanism turned out to be a breakthrough.
So it’s interesting how everybody’s style of doing research is very different, and the only way to be creative is to develop one’s own. I can see that you can be successful in very different ways. You can move forward by pushing your strength, while somebody else can be very successful by pushing their strength. Anyway, that was a very interesting lesson for me.
Henry, of course, your perspective is very different, being a graduate student at MIT, and an undergraduate at Caltech. But I wonder how you might compare the cultures of the two departments. Were they more similar? What were the big differences between them?
I interacted only with a limited number of people at Caltech and at MIT. So, you should not use my impression to generalize. At Caltech, I was mostly taking courses, both undergraduate courses, and some graduate courses later. I only worked with the professors whose courses I took, and with Neugebauer and his group. Gerry kind of also was my advisor, whom I would go to for advice, which was very useful. At MIT, I only interacted with the particle theory group in the Center of Theoretical Physics, so I don’t really have a perspective of MIT versus Caltech. But I do appreciate, looking back, the closeness of the Caltech community which was the very essence of the Caltech culture. At Caltech- faculty lived very close to the campus. So close that they had organized evening lectures at professors’ house where drinks and snacks were served. These gatherings provided an informal way for undergrads to interact socially and intellectually with professors.
I remember vividly, I think it was either Bob Leighton or Thomas Lauritsen’s house where Gell-Mann gave a talk, and he talked about quarks. That was 1967. So somehow ingrained in my mind, quarks exist, and I didn’t even know what a pion was at that time, so (laughter). It’s amazing how a few events like this you remember and can have a big impact on you. Sometimes you happen to go to a lecture like this by chance and it changes your thinking. And, of course, there was Feynman, everybody knows Feynman, our physics course was based on the Feynman Lecturers, the three-volume textbook. I still remember that there was this homework problem on quantum mechanics where my teaching assistant (Laura Jones), the professor (John Bahcall) and other TAs disagreed what the answer should be. So, we all went to Feynman for the correct answer. His answer was that the question was ambiguous (laughter). One had to be precise with a question in order to get a precise answer.
So, I feel that Caltech had this atmosphere because it was small, because the undergrad body was small, and undergrads were appreciated and treated as the trail blazers of science and technology of tomorrow. It’s quite different from MIT from this perspective. But both are great. I’m very happy that I got the good education from these two places.
Henry, how did you go about developing your thesis research? Did Francis Low essentially hand you a problem relevant to what he was working on?
No, no, no, my thesis, was really partly the work with Veneziano, and partly work I developed myself. Francis just told me that if I write up the thesis, it would be good enough for him. He was very generous about that, partly because we wrote a paper together, and he felt that I knew what I was doing. So, he was very supportive, and I could do whatever I wanted to do. So, I felt quite independent in the last two years of my PhD.
And what was your thesis research on? What did you study?
I studied inclusive reactions, and then I developed something of a relation between the Thirring model and the bosonic string theory, actually the Veneziano Model, maybe the precursor of the string theory. So once again, first impressions stick, because I worked with Veneziano earlier, somehow string theory seemed must be right to me (laughter). I was ten years ahead of most other people.
So when string theory came along in 1984, I felt, I would say, comfortable with it.
Who was on your thesis committee?
I don’t remember now. Good question, because people on my thesis are Veneziano and Fubini, but they left. I- sorry, I don’t remember. Nobody asked me that question (laughter). Ah, it could have been Ken Johnson? There was a plasma physicist on my thesis committee.
Aha. Henry, after you defended, what were your next opportunities? Were you thinking about postdocs, faculty positions?
Yeah, postdoc, postdoc. So, my wife also graduated from MIT in biology in 1974 (we were married in 1971), and she already got a fellowship to go to Stanford. The Stanford Biochem department at that time was top-notch, so she already accepted a job. So, I needed to find a postdoc at Stanford or, further away, Berkeley. So, I went to Stanford, SLAC. At that time, SLAC was the place. There was hardly anybody in the Stanford physics department, all the activities were at SLAC.
And what group did you join at SLAC?
The theory group. The group leader was Sidney Drell, there was Jim Bjorken and a whole bunch of faculty members there. So, I just joined the theory group of about twenty, thirty people, with half a dozen of them being faculty members. So, I just joined SLAC.
What was the research at the theory group at the time? What did you contribute to the group?
Oh, I arrived in September 1974, and then they discovered the psi, J/psi in November.
A very exciting time to join SLAC.
Very exciting. I still remember- I don’t remember which day, but I remember it was probably a Monday, we were told that a discovery would be announced. So, everybody got very excited, went to the auditorium, and then Roy Schwitters gave a talk about the psi particle, huge bump. In fact, they thought the machine broke down because the peak was so spiky. And what was very interesting was that - it so happened that Sam Ting just arrived at SLAC for some meeting, a committee meeting. And right in front of everyone, Burt Richter asked Sam, “Do you want to say something?” You would not be able to imagine anybody looking grimmer than Sam. He stood up, and then said his team also saw the particle- at Brookhaven. He wasn’t prepared. He had no slide, maybe a picture, I remember, so- and it was clear that he was been scooped (laughter). It was the whole room, everybody was watching him, and could feel the pain he was having.
You know, if he didn’t announce it at that moment, then SLAC would get all the credit (laughter). He had to announce it. But anyway, that was the announcement of the J/psi particle that day. And immediately for the next few months, that was all we worked on. It’s a little bit-should I say, discouraging for the young people because we all worked hard on it, we- everybody worked hard on it, and we got a lot of results. And then we saw papers coming out in Physical Review Letters, on things that we knew to be right or wrong, but we were not allowed to write papers on it because there were just too many of us (laughter). So, finally, the leadership decided we would write three reports, and each of us could join only one. In the end, they were never published. Despite no publication, it was a very, very exciting time.
What was so exciting from a discovery perspective? What was the big mystery that was unlocked at this moment?
It established that quarks really exist and are confined. Oh, first, we didn’t know what it could be, and then, because, in fact, right there, Julian Schwinger published a PRL paper saying that it was Z boson, a possibility also favored by many people, and then many people suggested that it was some other things, there were a lot of speculations. The experimentalists would walk up to us and ask, “What should we do next?” and everyone has a different suggestion. In the end, it was clear that if it was charmonium (charm quark anti-charm quark bund state), one should look for the excited states. Indeed, when they looked, they saw it, literally, within a week or so; or maybe not much longer than a week. And then what next? The experimentalists would come to the theorists’ floor to seek advice from everyone, including me as a young postdoc, because they wanted to survey what they should do next, before coming to a quick decision on what to do next. So even a young postdoc like me was consulted by some experimentalists, not directly by the leader but by some of the senior people. It was so very exciting. We knew very early on that it must be charmonium, although papers on other suggestions came out in PRL for the next few months or so. But-
Henry, you’re so-
-it was exciting.
-closely involved still in particle theory. What is the transition to cosmology? When do you start thinking about cosmology and the ideas that will lead to cosmic inflation?
This has to do with my time at Cornell, a postdoc at that time. I sort of read about the matter-antimatter asymmetry papers by a few people. And I said, oh, particle physics are being applied to cosmology, and cosmology informs particle theory. So, at that time, I began to study cosmology.
What was that initial connection intellectually? How did you see that particle physics would be relevant for the biggest questions in cosmology?
That had to do with the matter-antimatter asymmetry because that’s something-
Can you explain that? What does that mean? Just explain the science a little bit.
Okay, today, for example, we have a lot of baryons (protons and neutrons) but no antibaryons. So why? If you look at Dirac theory, particle-antiparticle is symmetric, so why do we have a lot more particle and no antiparticle? So that’s called the matter-antimatter asymmetry, and that’s an outstanding problem, which I learn, you know, anybody can learn this once one knows the Dirac’s equation. And at that time, there was a proposal that cosmology can address that issue, in particular, Sakharov’s idea, although I didn’t know about Sakharov’s idea then, but Grand Unified Theory (GUT) provides the essential ingredients (baryon number violation and CP violation). Essentially, cosmology can address fundamental particle physics problems, and I felt that that was a direction I could go into at that time. And, so, I looked into it, and monopole automatically came up. It was pretty clear, without doing much work, that there would be a lot of monopoles around, but why were there no monopole observed? Because I was studying the GUT at that time, and the baryon number in GUT is not conserved. So that people suggest that maybe that’s the reason why matter-antimatter asymmetry happen. So, I studied GUT, and since the idea applying GUT to the matter-antimatter asymmetry in cosmology was developed, I asked what else in GUT can be relevant to cosmology? Since monopole is automatically present in GUT, so I said that’s a direction to study.
Henry, was your work with Veneziano, was string theory relevant to this intellectual transition to you? Was it helpful for you thinking about cosmology coming from particle physics?
That has, at that time, nothing to do with string theory. At SLAC, I—after finishing all the excitement with J/psi particle, we moved to other topics. During my time at SLAC (1974-77), I also worked on a project with fellow postdoc Jack Ng and Tom DeGrand. We proposed that gluon jet can be the smoking gun for QCD (quantum chromodynamics). However, we were scooped by John Ellis, Mary Gaillard and Graham Ross (though we did discover a mistake in their paper). Steve Weinberg was spending a year at Stanford (that was also the time he interacted with Roberto Peccei and Helen Quinn there and discovered the axion) and he asked me some questions about gluon jets, but I did not know enough quantum field theory to appreciate his questions. Later Steve and George Sterman gave an elegant rigorous treatment of the jets in QCD. Only a couple of years later, in 1979, PETRA at DESY discovered the gluon jet, considered a breakthrough by many.
I did some string theory work with Rosco Giles. But it’s sort of hard to move forward. So, I studied GUT, and then tied it with cosmology, monopole- from GUT in cosmology become a natural problem that nobody has studied at that time. So, I decided to study that, and I worked on it for half a year, roughly, and then I realized that Alan have studied monopoles while he was at Columbia.
Alan, of course, being Alan Guth?
Yes, and so I went to talk to him, but he’s not too excited about it at first because-
Where did you first meet Alan? What was your first interaction with him?
Oh, first interaction, I don’t think he remembers. I’m not sure I have mentioned to him. But first interaction was when he graduated from MIT in 1972, going to be a postdoc at Princeton, at that time, I happened to meet him, and we talked a little bit. I asked him where he’s going. He says he’s going to Princeton, and he being Francis Low’s student, I ask how’s Francis? So, we have a conversation. That’s the first time I met him. And then at Cornell, we met again, so he’s- of all people, he’s the only one who really expressed some interest in monopoles. So, I talked to him, and he’s not interested my idea but he’s a very nice guy, so he sort of allowed me to keep talking (laughter). And then Steve Weinberg came and gave a talk- talk about matter-antimatter asymmetry from GUT linked to cosmology. I knew Weinberg’s paper, but Alan didn’t. But then he realized that doing cosmology for particle physicists actually is legitimate (laughter). It’s okay. So, he and I started working, and we- next door offices there, so, yeah, once we started, working and talking happened on a daily basis.
Henry, was your sense that Steve Weinberg was really the only more senior person at the time who was thinking along these lines? Was anybody else at Steve’s level in seniority, or was it mostly the younger generation?
I may have gotten a name wrong, but there’s a Japanese theorist who first proposed the idea, but his paper is wrong. Then Steve corrected it, and I think Tony Zee and others also realized that. So, there’s a few papers out on that subject. But most cosmologists were working on Big Bang nucleosynthesis.
What was it that Weinberg corrected? What was that important correction?
Oh, Weinberg essentially pointed out Sakharov’s point. The first paper didn’t realize that you cannot generate an asymmetry if you stay in thermal equilibrium. That person did not know that, wrote a paper which is wrong but still very influential. Weinberg corrected that.
So let’s go back to the interaction with Alan. So, you met Alan, you started talking with him, he’s interested in monopoles. What happens next?
Oh, yeah, then we have to work out the details because we have started from the SU(5) GUT with monopoles, and we study the monopoles, and see how we can get rid of the monopoles because they will be produced, and we have to get rid of it. And so, we developed a way which is called first-order phase transition or supercooling, and that can get rid of the monopoles by quickly decreasing its density. So, we were pretty happy about that.
Then I continued- by then Alan actually had moved to Stanford SLAC, so we communicated by phone. Then one day, I realized that if I continue supercooling and do not stop after the monopole problem is solved, then the universe is no longer power expanding. It’s expanding exponentially. And that really concerned me, so I told Alan on the phone.
It’s never clear that- yeah, never clear exactly what that was that I told him. But he didn’t really understand what I was saying. I was getting ready for my trip to China at that time, back to Hong Kong, so I told him, “okay, you just have to follow the equation we have, and don’t stop when all the radiation/matter/monopoles have been expanded out, then you’ll see that there’s a transition from power expansion to exponential expansion.” And so, I think that he followed that, and realized this exponential growth. He wasn’t sure that he remembers I told him that or not, because on the phone (the phone connection was a bit difficult) but he remembered that I told him to follow the equation because it’s very hard to explain how the behavior changes. We reached a point where the monopole density has been decreased enough and we just stopped (laughter). But you have to follow through, and then you find that something else happens. When I came back from the month-long China trip, Alan told me the beauty of the exponential growth.
Actually, before I left for the trip, I was bothered with the exponential growth because it was driven by a vacuum energy, and we didn’t understand the vacuum energy. I don’t remember which paper. I thought it was a paper by Veltman but later I couldn’t find it. Somebody said that, so I realized that it’s the vacuum energy that drives the exponential growth, and I just feel that our whole idea of supercooling hinges on something we do not understand. But Alan turned it round, said that’s good (laughter). What’s it good for? So, he deserves the credit, that I thought there’s a big problem when I left, and he realized that that’s the beauty of the exponential growth.
What is the beauty? What does that mean?
Oh, that solved the flatness and horizon problems, so it solves problems that people didn’t know how to solve before. He told me about those two problems. I certainly know the horizon problem because we studied cosmology by reading Weinberg’s First Three Minutes (laughter). There’s an explanation of the horizon problem, so I certainly know that, oh, yes, you can solve the horizon problem. But I don’t know the flatness problem. So that’s the biggest problem, the biggest- how we say - ignorance on my part. Dicke came to Cornell to give the Bethe Lecture on that topic.
This is Bob Dicke you’re referring to?
Right, and the talk was in the lecture hall in the chemistry department, and I was a little late. It was jam packed with people, so I found a seat at the back. It’s very hot, stuffy, and Dicke’s voice was very low. I got nothing out of the talk. Alan, to his credit, always tries to sit in the front, and take good notes. Not nowadays, okay. Now, he sits in the front row and falls asleep. But, in those days, he always sat in the front, and took good notes. So, he knows the flatness problem. Even when he told me the flatness problem, I didn’t know because I remembered- I don’t really know general relativity that well- the flatness- the curvature K equals zero or plus one or minus one in a textbook (laughter). One third chance that K=0, when the universe is flat. So, there’s no flatness problem, I thought. And only after Alan told me that I went back and realized the flatness problem. And Dicke, of course, spent a lot of time in his lecture explaining the flatness problem.
Henry, when did the term ‘inflation’ first come into use? Was that just during discussions between you and Alan?
Oh, Alan suggested that. My English is not as good as his, so he suggested that, and it sounded good to me. That’s entirely Alan, yes.
Henry, as a result of this breakthrough between you and Alan, what old questions were answered and what new questions were raised as a result?
So the questions that answered besides the flatness and the horizon problems, also the angular momentum, which means that the universe could have been rotating. Why our universe- I don’t mean galaxy with that. I mean whole universe is not rotating. And the other very interesting thing is that inflation removes the initial condition. Whatever you put in the universe, including curvature, matter, radiation or anything you put it, it’ll be inflated away. So, the universe starts with a clean slate. So, essentially, it gives you the beginning of the new universe with an initial condition that is determined.
Today, physics depends on initial conditions but the initial condition here is wiped clean by inflation, and then inflation will give you whatever follows that you can observe, like the temperature or density fluctuation. So immediately people realized that. Actually, I think Hawking did the first work on the fluctuation. But he made a mistake (laughter). So, today nobody gave him credit. But it’s a trivial mistake he made. He believed inflation so strongly that when the answer came out to be right, he took it. But it turned out that’s a mistake. But other people fixed it, and then immediately you just see the fluctuation is there, and so that’s the prediction. I mean, Alan and I talked about. We said, oh, yeah, yeah, good prediction, but we probably never going to see it or maybe one hundred years from now, we thought that that is something that would never ever be tested. So, it’s really surprising the cosmologists are able to discover the temperature fluctuation within a dozen years and then measure it with such precise data to put inflation on a concrete footing. So, that’s really amazing. We thought that we would never know whether the idea is correct or not. It would always be just an idea, a model. So that’s a shocking surprise to us both, a pleasant surprise.
Henry, when did you get this sense-
-in fifteen years-
When did you get this sense from your earliest interactions with Alan and the first development of the inflationary model that there would be all of this excitement and attention, and revision of the model in terms of people like Andrei Linde coming on to the scene, people like Paul Steinhardt? When did you get the sense that this was really the beginning of a broader theoretical discussion about the origins of the universe?
Ah, yes, I must say that I’m a little bit slow on that. Alan realized, day one, even though his original inflation of the universe doesn’t work; it clearly doesn’t work. I was stuck in a sense that if we have to use the vacuum energy to drive inflation, we have to understand it, why it dropped to an exceedingly small value later. If we don’t understand that, then whatever idea we build on that is questionable. And so even when the slow roll idea came along. Actually, Paul came to Cornell (’81) to explain to me the slow roll idea, I did not appreciate it then. Well, I should be- it’s a little bit distracting.
He came to explain to me, because Alan’s not there, his idea of slow roll. But he came with his family, and he was holding his baby when explaining to me, and baby cried (laughter). So, I was distracted, so I didn’t- I vaguely know what he’s talking about but I really never have a chance to think through that- why that’s good. So only when the paper came out later that slowly, and then you can use that to predict the fluctuation that cosmological background can see. I slowly, slowly appreciate it. It took me much longer than most other people to fully appreciate that. So maybe by ’82, I realized. But that’s two years late?
I probably should have immediately recognized that this is a breakthrough. It took me longer. And that’s one reason that when we wrote the paper, I said he can write the paper himself, partly because he’s looking for a job. I already had- I don’t have a faculty job but I already have a long-term senior research associate position at Cornell, which is doing phenomenology for the experimentalists.
Alan needed a job- needed a job at that time. If he doesn’t get a job, he will be screwed. So, I said, “Okay, you go ahead, write a paper. You need it. You need it for a job.” (laughter). So that’s roughly what happened in those days. Thinking back, many things we said is a little bit sort of- how we say- look at it the wrong way or not understanding fully what’s going on. So, I think that Linde and Hawking realized the true importance earlier than me. Of course, I know it’s important and very interesting, but I did not know this is a breakthrough.
Henry, you said that Alan needed a job. But, of course, you were a postdoc at Cornell at this point. Were you looking for faculty positions also?
We were both research associates at Cornell. I’m not eagerly looking for a job anywhere because my wife was a faculty at Cornell in life science. So, if Cornell can give me something, I will stay. So, they say I can be a senior research associate, so I don’t have to worry for a job in the next five years, or something. But Alan needed a job (laughter). If he doesn’t get a job soon, he may have to quit (laughter). He was postdoc at Princeton and then Columbia and then Cornell, and then Stanford, now it’s like, okay, it’s a very long postdoc.
You could stay as long as you wanted at Cornell? That was an open offer?
Well, no. Cornell has a e+/e- machine, and they needed theorists to help the experimental group to understand, help them to analyze the data. So, they said, if I wanted it, I could stay there on a five-year renewable appointment. Since my wife was faculty there, not easy to move because she had lab, I accepted that position. So, I didn’t have a job problem at that time-
And after this-
-at that moment.
-intensive collaboration with Alan, did you in some ways mostly go back to particle theory? Did you stay with cosmology?
Oh, yeah, I followed the literature, but I didn’t do anything because I felt that, well, what can I do in cosmology that can top that, right? So, I did not. And, furthermore, my job required me to spend time working with the experimentalists to help them to plan their strategic planning and analyze data. So that took some time. And then what do I do? I- after-
When did you get involved with cosmic superstrings?
Oh, cosmic superstring, that came much later.
That’s much later, okay.
Right, right, right.
So, I didn’t come back to cosmology because I’ve no good ideas. So, I did other things. Then string theory came along in 1984, and I worked on superstring theory. By year 2000, around that time, the branes have appeared in string theory, together with the brane world picture. Polchinski introduced the idea of D-branes, and then-
This is b-r-a-n-e. These are the branes in string theory, as in membranes.
B-r-a-n-e, yes, yes, not the b-r-a-i-n, right, yeah. In my family, I have to be very careful, because my daughter Kay is a brain scientist, the other brain (laughter). So, What I did then was with the brane. Immediately an idea came along, that branes will give a perfect model for inflation.
And where does the brane model come from? What are the sort of intellectual origins of the brane model?
Oh, the intellectual origin is Joe Polchinski. Joe Polchinski realized that string theory has not only strings but branes, so it’s really a theory of strings and branes, and that’s his, I think, most important contribution. And once you have branes, the picture totally changed string theory. So, in some sense, I would consider that as a second revolution of string theory.
But once you have branes, then we live in a brane, a three-dimensional brane that span the three-dimensional space. In the brane world scenario, the whole universe we see is in a three-dimensional brane, and then there’s outer space a little bit, but our Standard Model particles cannot go there. So then, naturally, the brane can play a role in inflation, and I started working on that. And so now in string theory, the brane inflation idea emerged. After our work, there’s a few other developments which is more specific. So, I think, at this moment, brane-anti-brane inflation is the mainstream idea from string theory on inflation.
Did you work with Polchinski directly at all?
No, I have never worked with Polchinski. But I have quite a few interactions with him. I’ve a couple of students who he took as postdocs. So, a total of four of my students he liked to take as postdoc. We have never worked together, but he’s a superb physicist who unfortunately passed away.
So once you have brane inflation, then cosmic superstrings automatically are produced, when brane and anti-brane collides and annihilates, so that’s how I work on the cosmic superstring. So, from brane inflation to cosmic superstring is how I got back into cosmology.
What were some of the advances with cosmic superstring cosmology that allowed us to understand even further the origins of the universe?
Oh, if it is discovered (laughter). All we need is for them to discover it.
Which tells you what? I mean, the big questions.
The big question is, first, they have to discover the superstrings, strings cosmically stretch across the sky. I should say these cosmic superstrings from string theory are totally in agreement with today’s observational bound. And, yet, in the near future, there’s a chance they can be observed. So that’s why it’s very interesting. And if they find it- it’s like the whole universe is a microscope. Electron, photon, etc are tiny, little strings, and then, somehow, the universe expansion will blow up a few strings to the size of the universe. So, if you see it, that’s as clear as anything. Now, people have proposed vortices as cosmic strings before. So, one has to look at their properties. The properties of the cosmic superstrings are quite different from the vortices people have proposed.
If it is discovered, then we can study its properties, and show that what they are like; in particular, cosmic superstring can have junctions. They can zip up and down, and a variety of properties that will distinguish them from ordinary vortices. So, people are looking for it. Not easy, but there’s a few ways one can look for it, and we just have to keep searching. I don’t know when. Maybe, hopefully it’s not another fifty or one hundred years.
(Laughter) Henry, is part of the big motivation looking for physics beyond the Standard Model?
Yeah, sure, definitely. In which sense this is beyond the Standard Model? So, right now, I’m working on ideas that maybe are too detailed, but people always wonder why the cosmological constant, vacuum energy is so small? Okay, for example, a few years ago, David Gross was saying, “Twenty problems in fundamental physics, and the cosmological constant is the number one problem.” So, we wonder why it can be naturally small and not fine-tuned, and only string theory can explain that. So, this problem I’ve been working on for ten years, and I have some concrete proposals, and now we will see how to test them. So, people always say string theory has no impact, and I don’t agree with that, and so I want to find string theory predictions that can be tested observationally or experimentally, which is quite different from many string theorists who worry about the mathematical structures.
Henry, on that point, of course, one of the historical criticisms of string theory is that it’s not testable, and, yet, you’re specifically motivated by finding ways to make it testable. Can you explain that a little more?
Well, cosmic superstring is one of them. That’s a very clear prediction and signature to test it. And then there’s other predictions. In fact, we just post a paper which can be tested, but in a model derived from string theory to a particle physics model where today it can be tested by atomic clock or by quasar spectral measurements. So, we just post that paper, and we’ll see if it can be tested or not. We believe that it’s close, it doesn’t take that much to test it. Maybe since data quality/quantity improves by a factor of order of ten every couple of years, we feel that in the near future, that can be tested. To me, that’s as direct as one can get. Not as direct as cosmic superstring, but pretty direct.
What might these tests look like, I mean, in terms of verifying the theories? What are the experiments or the advances in observational cosmology that might make these testable?
So the particular one is very clear. We- as we post the paper last week, earlier this week, yes, that you do atomic clock measurement. Does the electron mass is time dependent? Okay, and they can measure for example the electron mass impact or the rate of change of the Higgs vacuum expectation value. And the other one is using quasars spectra to see if the electron mass or mass of any of the particles, essentially vacuum expectation value of Higgs, is a little bit different from today’s value.
That means the expectation value of the electron mass or the W boson mass, all of them are fluctuating, oscillating at a very small amplitude, but still oscillating today. And the oscillation is so small that present observational bounds are completely satisfied, but the data improvement by a factor of ten, which I think will happen in a few years, then they can test it. So, it’s not a test far away in the future. It can be tested in pulsar timing, you know. So, there’s a series of tests that can be done in the next five to ten years. So, that I consider as a string theory test because the whole model is motivated, supported by string theory.
What are the advances that are allowing these tests as you envision them in the last- in the next five to ten years that haven’t been available in the last thirty years, essentially?
Oh, because of the rapid technological advances; and because the measurement- the data from cosmology-astrophysics is pouring in very fast. For example, earlier, we have quasars. But with the Sloan Digital Sky Survey, now we have around three-quarters of a million of quasars. Before that, we have relatively few. So, the number of quasars has increased by orders of magnitude. And the spectral measurement will come next, with so many quasars, they’re going to measure more spectra with better precision.
The spectrum will measure the electron mass, it deviates from today’s value by tiny, tiny fraction of a percent, and that’s what is being tested—will be tested. And atomic clock measurement also would test the change of the electron mass, for example. Again, they’re very, very small. People have been looking for it, and, yet, what we predict is a value smaller than what they have been able to constrain but not that much smaller.
Henry, as you know, there are many physicists who have lost patience with string theory, that say-
-it doesn’t explain nature. There’s no way to test it. So, what is the best-case scenario as you envision in the next five to ten years that will prove these doubters wrong?
Okay, so I would say discover cosmic superstrings, and that could be done- actually for LIGO/Virgo, they have a team of people searching for it. You can do that via lensing. So there are a few ways that people are searching. Every now and then, you see a paper which put a bound on cosmic string property, and certainly if they can improve the bound by two order of magnitude, I think it’s getting closer and closer.
But, certainly, I think it can be discovered any time, and it’s typically that once they discover it, in ten years’ time, they can do a precision measurement on it. So that’s what happened to cosmic microwave background and many things. Now the quasar data are pouring in, and many people are now analyzing the way we want them to analyze, so we hope that maybe they will do it. And atomic clock measurements, now, atomic clock measurements, hopefully atomic clocks can improve the precision by one or two orders of magnitude in the next few years. Either use atomic or maybe nuclear clock. There are other ways to do it.
So, I’m confident that, as atomic clock precision improves by one or two orders of magnitude, quasar measurements of the spectral lines come pouring in the next five, ten years, the chance of discovery is very good, or maybe they can- they cannot prove it wrong but they can squeeze it to a level that you think it’s unlikely. The unfortunate thing, from my perspective, is that many talented string theorists move in mathematical directions. That direction is very interesting, very exciting, but it’s not trying to address the criticism that some people have, which I think is legitimate. There are not enough talented young string theorists working on the phenomenological side, on searching for ways to test string theory.
Henry, as you know, Andrei Linde believes- he’s confident that the multiverse will eventually become a testable proposition. I wonder if that line of research is relevant at all in the way you see superstring theory, cosmic superstrings being testable.
Well, in inflation, we have branes colliding, but it’s possible there’s a set of branes separate from ours. So even today, the multiverse is quite natural in string theory. But I don’t know how to test it. The hard part is how to test it-
How might this revolutionize our understanding of general relativity or the way that gravity may or may not interact with other particles?
Oh, no, gravity (laughter)- I think gravity- Einstein- I would say Einsteinian gravity is correct, classically certainly, and if there is any deviation from Einstein’s classical general relativity, it will be minuscule at this level. Of course, quantum is where the problem is. That’s where string theory comes in to solve the quantum gravity problem. At this moment, it’s only conceptual. There’s no measurement to test that quantum radiative effects of gravity, certainly not that I know, so, but-
Now, back on Earth, you finally join the faculty in 1987 at Cornell.
Yes, yes, right.
Were you and were wife thinking about moving elsewhere, or you were happy in Ithaca?
Yes, I’m a- I don’t mind, you know, because when we have our girl in 1981, my wife, Bik, was very busy with her labs, being an assistant professor, but I had a flexible schedule. I could take care of my daughter. So, I babysat and took care of my daughter Kay more than she did during that time. But by the time ’86/’87, she felt that- she had tenure at Cornell by then, and we felt that I didn’t know how much longer I could stay at my position. So maybe we should go look for jobs.
And so, we were lucky that we had people like Ken Wilson who urged the department to hire me. That’s it. In Ken Wilson’s own case in the sixties, when his promotion came up, he had essentially no papers, you know, everybody knows that story. And as Hans Bethe said, “We have to give him tenure.” And Ken got tenure with almost no publications (laughter). So, when my case came up, Ken (and the particle theory group) strongly supported my appointment as a full professor. So, one needs someone in the department who is very well-respected and very prominent, and doesn’t push his weight around in general, and when he does, everybody listens. So, I’m very thankful for that. But, actually, earlier, I was exploring the job market. But the people in the theory group said, “You’re not going to leave because your wife is not going to move.” And that turned out to be true, so they felt I was not going to leave, so they didn’t have to do anything. Only when both I and my wife got offers at the same place, then they thought that there’s a chance we might leave, then they decided to move me from a senior research associate to a full professor. Not only that, they also asked me to help the department to hire two string theorists. So, we hired Brian Greene and Andre LeClair. But after five years, Brian left for Columbia. So not only they hired me, they felt that maybe I’m in a direction that is good for the group. Of course, I’m not sure I made the best decision and- but it’s a- how I’ll say, at a certain time, I felt a bit uneasy about the whole process. Later I realized that why should they give me more if I’m not going to leave? (laughter)
So, and only when I’m going to leave that they sat down and evaluate the future of the group, which direction they should go, and they- at that time, they had to think through the future of particle theory. There’s no reason why they should do something out of the ordinary. Soul searching on “string theory or not string theory” was happening in every physics department in USA. I liked the people at Cornell. They are nice, good, decent people.
Henry, when did you first meet Gia Dvali?
Oh, Gia Dvali, well, I don’t remember, may be at conferences. But we wrote the paper because he came to Cornell to give a talk. After his talk we decided to take a walk around Beebe Lake. And I told him the idea I had in string theory, but he didn’t know much string theory at that time. However, he thinks deep in quantum field theory. So, immediately, we put that together, and the brane inflation idea came. He’s very quick with field theory, so when I explained the brane idea for inflation to him, he was not familiar with string theory but he saw what I’m trying to do, and he immediately formulated in field theory how to realize that brane inflation picture. So that’s how we worked together. It’s a very quick, literally a couple of weeks’ work.
What is the string landscape? What does that mean and when did you start thinking about the string landscape?
Oh, string landscape. Essentially, it followed from two of Polchinski’s works: the brane idea, and the other idea he had with Raphael Bousso that fluxes are quantized. So, there’s a lot of fluxes in the string theory. Once you have branes which are charged, charge gives fluxes, much like electric charge leads to quantized electric and magnetic fluxes. So, fluxes are quantized. So, you have many fluxes, many quantized values, which can take any value, say, 1,000 to -1,000, and you have ten of those, so you have numerous possible solutions. And that’s the beginning of the string landscape, I think. At first, people think maybe there are only one or two solutions, and then they realized there are many, and then they realized there are many, many, and then people realized maybe it is infinite. So that’s beginning of the string landscape. So, we are in one of these solutions with a particular choice of all the fluxes value, discrete values. So, we are less and less significant as we know more and more physics (laughter). That’s all I can say. You know, a long time ago, we thought we were the center of the world, and then we were the center of solar system? the center of our galaxy. Now, we are not at the center of anything. We are really just a speck of dust in the infinite vast of the string landscape.
The string landscape has made you become a bit of a philosopher, it sounds like.
Well, so the whole thing is that we want to find out where we are in that landscape, and then try to understand why we are there. So, one can combine it with inflation, because after inflation, the universe has to evolve down to somewhere. Why evolve to today’s universe and not somewhere else?
So, it is somewhere between science and philosophy; the difference is quite narrow in the sense that I would describe philosophy as something which is self-consistent by itself but may have nothing to do with our universe. Science is- as philosophy itself is self-consistent, but it describes nature and it can be checked by experiments and/or observations. The term natural philosophy is really, really an excellent description.
And, of course, religion is that it doesn’t have to be self-consistent. You just have to have faith. So the line separating religion and philosophy is not very clearly drawn. So religion to philosophy to science, I think, the boundaries keep moving, and there are overlaps in some places.
Henry, how does the cosmic landscape help us understand the wavefunction of the universe?
The wavefunction of the universe, presumably any part of the landscape you are in, you can write down a wavefunction of the universe. And presumably, the wavefunction of the universe that describes our universe is a particular wavefunction. Different people have different ways of thinking about it. Some people think that the wavefunction of the universe has been evolving from the big bang. Some other people say once we know how nature’s described, then you can write down the wavefunction of the universe, essentially, the model or rules that describe the universe. So, it’s just a nice way to put it, a fancy way to put it, or less technical way to put it. But I don’t know, yeah. It’s a fancy way to put what we hope to understand eventually.
Henry, one aspect of your career we haven’t really touched on yet is your work as a teacher to undergraduates, and a mentor to graduate students. First, I’d like to ask, during your Cornell years, what were some of the favorite classes that you taught Cornell undergraduates?
Oh, I don’t really have any favorite courses. Although, P116, the freshman advanced mechanics, I enjoyed it mostly because the students are very smart. The undergrad engineering courses, you know, it’s a service course, some excellent students, some students just wanted to get over with it, so that’s different. I didn’t teach that much undergrads, yeah, mostly service courses.
And over the years, Henry, who have been some of your most successful graduate students?
I don’t have that many, yes. Quite a few of them moved to Wall Street and industry. At this moment, let me see, Keith Dienes, Sarah Shandera, Gary Shiu, David Senechal. They are essentially faculties at research universities. I don’t know who else. Postdocs are doing better.
Henry, you’ve also done much as a science communicator, talking to lay audiences or the broader public. I’m curious, overall, given that the field that you work in is so inaccessible to non-specialists, these are such complex issues, what are some of the major themes or metaphors or ideas that you share to help communicate your research so that non-specialists from the public can gain an appreciation of how the universe works?
Yeah, I like the inflation of the universe, although I sometimes don’t even use the word “inflation,” is that the whole universe starts from nothing. The universe starts inflating when it’s a tiny, tiny, tiny point/bubble. So, if you go one step further back, my basic idea, and that of many people, is that that point can arise from a quantum fluctuation. So, we start with nothing. By ‘nothing,’ I do not mean a vacuum. Vacuum has space. So, we start with no space but with the laws of nature, and a quantum fluctuation is allowed by the quantum theory we know. Without space, there’s, for sure, no time. So, this is called nothing. And then the laws of physics allow a quantum fluctuation of a tiny, tiny bubble, you know, much, much smaller than an electron, inside which is present a vacuum energy that drives inflation. So that describes how the universe comes from nothing.
But we have a lot of matter in the universe. According to Einstein, matter equals energy. How can matter come from nothing? And then you realize that the gravitational field has negative energy. Electric (and magnetic) field has positive energy so we can store energy in the electric field. Electric field has opposite charges attract; same charges repel. But gravity has same charge (i.e., mass) attract, which is masses attract. As a result, you can see that the gravitational field has negative energy. So, we have a lot of positive energy in matter, but there’s gravitational field with negative energy, so you can imagine the whole universe can start with zero energy. So, it is possible to start from nothing, and end up with all the stars etc. I tell people that everything, including space, comes from nothing. So, I think people like that idea, and that actually is what inflation postulates.
And, Henry, to be clear, when we say that the universe comes from nothing, does that pertain to our universe, or, if there is a multi-universe, it pertains to all universes?
Yeah, it can pertain to all, yes, I would say all. In string theory, that would be all. So, essentially, there’s nothing to start from, and then you have a fluctuation, or multiple fluctuations. That fluctuation(s) creates the whole universe, multiverse, whatever. Now, the question is how to calculate that fluctuation? Why that a fluctuation from nothing to our universe and not to another universe? I did some calculation some time ago and found that it’s not unreasonable that inflation will follow after fluctuation to a universe like ours; not some other universe like a ten-dimensional one. So, it’s possible this from nothing idea is correct.
If you believe inflation, you know that at least a tiny, tiny point expands to the whole universe. So, in some sense, for most people, that tiny point is negligible, so it essentially means nothing. But I really think that nothing is really mathematically nothing, and not just a tiny, tiny, little point. So probably many cosmologists believe that nowadays. But that’s what common people feel—they may not understand but they feel they get the message.
Henry, do you think that this idea of the universe coming from nothing works well with a religious perspective that there was only God before the universe, and that God could exist and create something from nothing?
Well, okay, this is interesting. In some religions like Buddhism, their idea is the universe is always there, always will be, and there will be no change. So, Buddhists think the universe was never created. It’s always there, continue to be there, so the steady-state universe scenario fits Buddhism. But if you looked up Genesis, the Bible, from Islam or the Christian religion, the universe has a beginning. God created the universe. I am perfectly okay with that, but what do we mean by God? “God” carries different meanings to different individuals. One way to describe God is the laws of nature. So, we are given the laws of nature to create a bubble from nothing. I’m not religious. I’m more an atheist. But, still, it’s amazing. It’s wonderful that maybe the laws of nature is God or laws of nature is the name of God or it’s the face of God. I don’t know. But, certainly, nature is very amazing as well as mysterious.
Henry, to bring our discussion up to the present in the narrative, what have been some of the projects you’ve worked on in the past few years?
Oh, solving the cosmological constant (CC) problem, that’s what I’ve been working on since I came back to Hong Kong. Remember that it is precisely a large CC that drives inflation, and inflation ends when CC becomes small. So, I have been contemplating the CC problem ever since. Following a path to solve the CC problem, I tried to deduce observables to be tested. One observable I have now is that there’s no super-particles. All the super-particles conjectured will not be there. LHC will find no super-particles.
Which would tell us what?
There’s actually no super-particles to be discovered in the lab. You can search anywhere. No super-particles so there’s no WIMPs for cold dark matter. That’s one prediction, which follows from the naturalness of the small cosmological constant. So, people have studied mass hierarchy, you remember, and they want to understand why the Higgs particle is so light, the Higgs boson is so light. And they invent supersymmetry to solve the problem. But supersymmetry doesn’t solve the problem. It’s only technically natural. I don’t know if you know the term. It’s not natural. It’s only technically natural. So, you haven’t actually solved the mass hierarchy problem. Since the cosmological constant problem is a lot more severe than the mass hierarchy problem, a naturally small cosmological constant will probably have very strong implications on physics: particle phenomenology and cosmology. We have to follow those implications, expecting a tremendous amount of consequence and implications.
The reason the smallness of the vacuum energy (cosmological constant/CC) is such a tough problem is radiative instability. If there are super-particles we haven’t yet discovered, their contribution to the vacuum energy from loops will shift the CC by many orders of magnitude bigger than today’s extremely small value. If you fine-tune the tree-level CC to the observed value. You have to re-fine tune it after you include the one loop radiative correction. If you include the two-loop effect, you have to re-fine tune again, and so on. That’s totally unnatural. So, in order to make it natural, you just have to remove all the super-particles. that’s a problem I’ve been working on for ten years, so I’m getting to the end of my (laughter) career, and I’m trying to finish it enough- with enough predictions so I can truly retire.
Henry, on that note, for the last part of our discussion, I’d like to ask a few broadly retrospective questions about your career-
-and then we’ll look to the future. One that we haven’t touched on yet is the role of computational power, and machine learning, and artificial intelligence. In what ways have these things been relevant for your research, and where do you think the field will be headed as a result of all of the enormous computational power that will continue into the future?
Okay, I, myself, don’t do much computation. In fact, none. The only calculation I do nowadays can be done on the calculator on my iPhone (laughter). That’s all, okay. I don’t do anything more than that. That’s why I always need student and postdoc to work with me. They can do better. Certainly, the computational power would be very useful in making predictions, analyze data. And when that amount of data is pouring in so fast, that’s become very important, and machine learning will help one to analyze data and to make predictions. And I know that artificial intelligence will come into play. My research don’t involve them. But I know that the observational cosmologists and high energy experimentalists who provide me the data information, they have to do those things. So, I appreciate the importance of that. But, personally, all I need is paper and pencil and an eraser and I am too old to change (laughter).
(Laughter) Henry, because your research has always involved some of the most fundamental mysteries of the universe, I’m curious, if you can reflect over your career, what is the greatest satisfaction that you felt in terms of really understanding something that was not understood before, and what remained for you the biggest question marks, the biggest mysteries in our ongoing quest to truly understand how the universe works?
Hard to say. I enjoy all my research, except there’s a few years I worked on something called fractional superstring, which, looking back, is regrettable a bit because my students in those years didn’t do well, you know, so, and a waste of time. But, otherwise, I’m pretty happy with my research overall. Since coming back to Hong Kong, the one big problem I have worked on is the cosmological constant, how to make the very small observed value natural. This is the problem that have bothered me since the inflationary universe scenario in 1980. Since 2011, I have been developing an idea that started with my postdoc Yoske Sumitomo at that time. This has been a ten-year project, and I’ve more or less come to a point where I’ve done what I want to do and wait for observations. So that’s probably- I would feel happy that I am able to finish that project to a level that I can rest (or retire) on.
In the past ten years, there is another project that I had spent quite a bit of time on. Together with Sam Wong, my student then, I proposed that LHC has a good chance to observe baryon number (atomic units) violating events. I hope we do not have to wait for a higher energy collider to detect such events.
Another work I’m happy about is the so called the KLT relation, which I did with Hikaru Kawai and David Lewellen in the mid-80s. One consequence is that a graviton scattering amplitude can be expressed as the square of the non-abelian gauge theory amplitude. Nowadays, they sometimes call the relation Einstein = (Yang-Mills)2. A rather technical result can boil down to a nice simple cliché statement or something that easy to express. When we wrote the paper, nobody paid much attention to it. So [laugh] for quite a number of years, maybe one or two citations a year. Then more and more citations. And, also, you cannot cite our paper without spending a huge amount of technical effort to digest and use it. Even now, people told me that they have difficulty understanding our paper because it’s rather not trivial.
Henry, for my last question, looking forward, it’s so wonderful to hear you express optimism that the things that you’re studying in the next five or ten years will be testable, and so I’d like to-
-end- you have such optimism that all of these things can be tested, that they can be proven true in deduction, in deductible science.
I’ll ask, to end, a question that touches philosophically, and that is, are there limits to what can be tested? In other words, are you comfortable with some things being fundamentally mysterious, beyond the realm of testability, or, as a scientist, do you think that, ultimately, everything that can be known about the universe will be known?
I doubt that (laughter). As we know more, even if superstring theory proves to be correct, the more we know, the more we don’t understand. So, when you dig into more detail, you find new mysterious things which are unexpected. So, I think that science will continue to make big jumps in progress, but we’ll keep finding new, exciting questions and puzzles to be digested.
So it’s a never-ending loop?
And that’s not only in physics.
It’s a never-ending loop, essentially?
Not a loop. We are doing better, better. We know a lot more than one hundred years ago, not only physics but every subject. And at a certain point, no matter how much you understand- in particular, in my wife’s work (DNA replication) and my daughter’s work (brain-mind connection), you understand how every neurons work, or how every DNA is replicated, still the fact that you can have people come out with thinking, not only conscious, we can have feelings, we have emotions, and intelligence, it’s- I think it’s never-ending. But the thing is we’ll continue to know more, and more, and that’s- so it’s not a loop, but rather- how I say? That’s progress and that is why science is exciting.
I hope for your own- the biggest questions that you have, that they will happen in the timeframe that you envision, and you will be part of the discovery.
Hopefully, yes. I have to live long enough (laughter).
(Laughter) Henry, on that note, it’s been a great pleasure spending this time with you. Thank you so much for talking with me.
Thank you. Thank you for your time.