Trinh Thuan (BS '70), Observational Astrophysicist and Founding Member of the International Society for Science and Religion

Professor Trinh Thuan is an eminent authority in both astrophysics and exploration at the interface of science and spirituality. If the "how" questions that animate the scientific study of the universe - its origins, structure, and evolution over time - are generally disassociated from the "why" questions relating to the reason of the universe and our place in it, Thuan's intellectual output over the past five decades provides an important and compelling point of convergence.

In his scientific realm, Thuan has conducted foundational work on the structure and evolution of galaxies, and specifically on the role of Blue Dwarf Galaxies in the creation of larger galaxies, including our own Milky Way. As a founding member of the International Society for Science and Religion, and as a prolific author whose books have explored the way we perceive and make sense of reality, Thuan is a leading public intellectual seeking to bridge the perception that science and spirituality offer no connecting points. In this realm his specific expertise is Buddhism, and rather than trying to apply a tortured logic that reconciles science and religion, he takes both for what they are, offering "radically different" pathways in the pursuit of truth.

There is no doubt that Thuan's unique perspective is a product of his unique life trajectory. Born in Hanoi in 1948, Thuan's family left for the south of Vietnam to escape living under the regime of the communist Viet Minh. After distinguishing himself in high school, Thuan enrolled as an undergraduate at Caltech during the height of the Vietnam War. At Caltech, Thuan reveled amidst the scientific greatness of faculty members including Richard Feynman, and he fell in love with astronomy after conducting research at Palomar Observatory. As a graduate student at Princeton under the direction of Lyman Spitzer, Thuan studied the intercloud medium of the Milky Way, using data from a space telescope conceived by Spitzer. After a delightful return to Caltech as a postdoc, Thuan joined the faculty at the University of Virginia, where he has contributed over the decades to the growth and stature of the Department of Astronomy.

Over the course of his career, Thuan has also taken numerous research visits to France, including at the Institut d'Astrophysique de Paris, where he remains a research associate. Today Thuan splits his time between Virginia and Paris, a duality that ideally suits his wide-ranging interests. At the conclusion of the discussion below, Thuan strikes an optimistic note regarding the prospects of breakthrough discovery for the two most confounding topics of astrophysics and cosmology, namely, the nature of dark matter and dark energy. Whether it is through transformational technologies, the arrival of a singular genius (or both), Thuan is confident that our eventual understanding will arrive, and of course, sooner better than later. Is this assessment a product of Thuan's faith, or is it scientific extrapolation? Perhaps one lesson from his life is that such distinctions are not very meaningful.

DAVID ZIERLER: This is David Zierler, Director of the Caltech Heritage Project. It's Wednesday, May 8, 2024. It is my great pleasure to be here with Professor Trinh Thuan. Trinh, it's so nice to be with you. Thank you for joining me.

TRINH THUAN: Well, I'm very glad to talk to you. I'll be very, very curious to see how this comes out.

ZIERLER: Wonderful. To start, please tell me your institutional affiliation and your title.

THUAN: I'm now a Professor emeritus at the University of Virginia (UVA), in Charlottesville, Virginia. I retired recently, on February 1, 2021, from the University where I have been since September 1, 1976. It has been a long span, more than 40 years on the campus in Charlottesville.

ZIERLER: What have you been doing since you went emeritus?

THUAN: I divide my time into two main types of activity. First, I continue to do rmy astrophysical research on the formation and evolution of galaxies, based mainly on observations obtained with the Hubble Space Telescope and the ground-based 8.5 meter Large Binocular Telescope in Arizona. Second, I continue to write popular astronomy books, not in English, but in French. That's why I'm in Paris at this moment. French is my literary language because I was French-educated. I attended the French Lycée in Saigon during the sixties. In fact, English was for me a secondary language that I picked up during my years at Caltech. At that time, I didn't have any formal training or learning in English, so I never felt very at ease writing literary texts in English. Of course, I do use English as a language to write my scientific papers. English is a precise and concise language, so it is perfectly suited for expressing scientific ideas.

ZIERLER: The books that you write in French, what are they? What do you focus on?

THUAN: I like to write about cosmology, the history of the universe, because, as said before, galaxy formation and evolution is my field of research. My first book, the one that led me to break through with the French public, was published in 1988 with the title La Mélodie secrète (The Secret Melody) in Paris, France. It contains scientific and philosophical reflections on the Big Bang. It has been translated into English and published in 1995 by Oxford University Press as The Secret Melody. That book and subsequent ones (I have written some twenty books) have made me known as an author in the French-speaking world (Belgium, Switzerland, Quebec, Morocco, etc). Another book that is well-known world-wide and that has been translated in some twenty languages is the Quantum and the Lotus (Crown 2001), which is about the possible parallels between cutting-edge science and Budddhism. My writing has had a considerable impact. It has drawn positive feedback from my readers. I am often interviewed about astronomical matters by French newspapers (such as Le Figaro, Le Monde, Libération, etc.) and magazines (such as Le Point, L'Express, Le Figaro magazine Paris-Match, etc.). I also make radio and television appearances, and give many public talks.

ZIERLER: Do you have any affiliation in Paris? Are you connected with a university?

THUAN: Yes. I am a Research Associate at the Institut d'Astrophysique de Paris (or IAP, Institute of Astrophysics of Paris). I have past and ongoing collaborations with French colleagues at the IAP since 1978. This is one of the premiers institutions in France for astrophysics . I usually spend my summers doing research there, when I do not have teaching duties at the University of Virginia. Every six years, I have a year of sabbatical leave from UVa which I may also spend at the IAP. Thus my collaboration with the IAP scientists has been quite a long and productive one.

A Society for Science and Religion

ZIERLER: Tell me about the International Society for Science and Religion and some of your interests in helping to found it.

THUAN: Science teaches us about the nature of the physical world around us and about our own minds. From the sixteenth century on, at the epoch of the scientific revolution, people have generally considered science to be synonymous with knowledge. The exponential increase in imformation driven by the rise of science is relentless and is not about to slow down. Meanwhile, religious practice has sharply declined in democratic, secular states, while often becoming more radical in religious states. The great spiritual traditions, whether they are dogmatic (like Judaism or Christianity) or based on pure contemplative experience (such as Buddhism), provide powerful ethical and moral rules that can help people to structure and inspire their lives. On the other hand, science is silent when it comes to providing wisdom about how we should live. Thus, as a complement to science, I believe we must cultivate a "science of the mind", which we can call "spirituality". For me, this spirituality is not a luxury but a necessity (I prefer the word "spirituality" to "religion" since I am a Buddhist, and Buddhism does not have the concept of God). So, when my Cambridge University colleagues invited me in 2002 to join them in founding the International Society for Science and Religion (ISSR) for "the promotion of inter-disciplinary learning and research in the fields of science and religion conducted in an international and multi-faith context", I was glad to accept.

ZIERLER: Do you think that it's easier to reconcile Buddhism and science than it is for Judaism, or Christianity, or Islam?

THUAN: Well, I wouldn't claim to be an expert in all those religions, but I can comment about Buddhism. Buddhism tries to understand reality by using logical arguments and reasoning, like science does. Thus, if both science and Buddhism describe the same reality, and if both of them are true, they should intersect somewhere. If they never meet, then either one of them is false or both are false.

Science and Buddhism have radically different methods for investigating reality. In science, intellect and reason have the leading roles. By dividing, categorizing, measuring, analyzing and comparing, scientists express the laws of nature in the highly abstract language of mathematics. Intuition is not excluded from science, but it can only play a part only if can be formulated in a coherent mathematical structure. By contrast, it is intuition – or inner experience – that has the leading role in the contemplative approach of Buddhism. It adopts a holistic approach, refusing to break up reality like science does in its reductionist approach, but intead aims to understand it in its entirety. Buddhism has no use for measuring apparatus such as particle accelerators, microscopes or telescopes that help to gather the sophisticated observations that form the basis of experimental and observational science. Its statements are more qualitative than quantitative.With these considerations, I was afraid that Buddhism would have very little to say about the nature of the world of phenomena.

But as my conversations with the French buddhist monk Matthieu Ricard (who is a biologist himself) developed and as the writing of our joint book "The Quantum and the Lotus" progressed, I soon realized that my fears were groundless. I discovered that not only has Buddhism thought about the nature of the world, but it has done so in a deep and original way. Its purpose is not, like science, to find out about the world of phenomena for its own sake, but rather its aim is therapeutic: it is by understanding the true nature of the physical world that we can clear away the mists of ignorance which lead to suffering, and open the way to enlightenment.

ZIERLER: I wonder if you've ever thought about how Buddhism has made you a better scientist or how science has made you a better Buddhist.

THUAN: In fields like neurobiology, I do think that Buddhism, because of its focus on meditative techniques of the mind, can greatly illuminate science in certain ways. In my own field of astrophysics, buddhism has considerably sharpened my view of physical reality. For me, it has shed new philosophical light on the natural world. I find that there is a definite convergence and resonance between the Buddhist and scientific visions of reality. Some of Buddhism's views on the world of phenomena are strikingly similar to the underlying notions of modern physics – in particular, its two main grand theories: quantum mechanics, which is the physics of the infinitely small; and relativity, the physics of the infinitely large. Even though Buddhism and science have radically different ways of investigating the nature of reality, this does not lead to an insuperable opposition, but rather to a harmonious complementarity.

Take, for example, one of Buddhism's central tenets, "the interdependence of phenomena". "Interdependence" says that nothing can exist inherently, or be its own cause. An object can be defined only in terms of its relation to other objects. Interdependence is essential to the manifestation of phenomena. A given phenomenon can come about only if it is linked to others.This implies that reality cannot be localized and fragmented, but should be considered as holistic and global. Physics experiments of the EPR-type (initials of Einstein and his collaborators Podolsky and Rosen who proposed the experiment) have now imposed this global view on us. They have shown that reality is "inseparable", that two ‘entangled' particles that have interacted with each other continue to act as parts of a single reality, to behave in an instantaneously correlated way. For example, if a particle changes to be spin up, the other one instantly changes to be spin down, without any exchange of information occurring, even if the entangled particles are separated by thousands of miles. In 2022, the Nobel prize in physics has been awarded to the physicists Alain Aspect, John Clauser and Anton Zeilinger for their experiments on entangled particles showing that reality is non-local and non-separable.

Another central tenet of Buddhism is the concept of impermanence. According to Buddhism, the world is constantly changing. It is like a vast stream of events and dynamic currents that are all interconnected and interacting. Nothing is still and immutable. Everything is moving and changing. This concept of perpetual, omnipresent change in Buddhism chimes well with modern physics and cosmology. Aristotle's immutable heavens and Newton's static universe are no more. The Universe was born in a Big Bang 13,8 billion year ago, and since then it has not ceased to expand and to become larger, cooler and less dense. One day, it will die perhaps in an icy freeze. It has acquired a history: a beginning, a present and a future. From the tiniest atom to the entire universe, including all the structures it contains -- galaxies, stars, planets and mankind --, everything is moving, changing and impermanent. They, too, have a history. Thus, they are born, reach maturity, then die. Stars have life cycles that span millions or even billions of years.

The same goes for the atomic and subatomic world. There too, everything is impermanent. Particles can change their nature: a quark can change its family or "flavor". A proton can become a neutron and emit a positron and a neutrino. Matter and antimatter annihilate each other to become pure energy. Because of the quantum uncertainty of energy, space around us is filled with an unimaginable number of ‘virtual' particles with fleeting, ghostlike existences.With their infinitely short life cycles of 10-43 second, they are a perfect illustration of impermanence.

So reality can be perceived in various ways, and different approaches – the contemplative one and the rational one – can lead to the same truths.

Has Buddhism made me a better scientist? It has not influenced directly the kind of astronomical research I do, except for clarifying its philosophical basis, as discussed before. But by guiding me in my moral and ethical choices, it has helped me to become a more compassionate and altruistic person. And by being a better and a happier person, I can do better science. We are faced at the present time with numerous ethical problems such as global warming, the destruction of our planet's biodiversity, nuclear proliferation, cloning, genetic manipulation, etc. Science does not provide moral guidance. Only spirituality is able to do it. Science can operate without spirituality, and spirituality can exist without science. But I have the firm belief that man, to be complete, needs both.

The Duality of Enlightenment

ZIERLER: One of the great lessons in science is, the more that you discover, the more you know you don't know. I wonder if there's an analog to that in Buddhism.

THUAN: Buddhism is essentially a path toward "enlightenment". This is a state of supreme knowledge, combined with infinite compassion. But not many people can reach that state. In the last 2500 years, only Buddha has attained enlightenment. As remarked before, I also think that a scientist will never know everything about the universe. To me, science is an asymptotic process. A scientist will get closer and closer to the true view of reality, but he will never know everything. We know we have notes of music sent to us from nature, but we'll never be able to unravel the whole melody. There will always be some mystery left. And Buddhism is the same way. It leaves open the field of investigation.

ZIERLER: Let's now cover your academic research. What have been the big questions you've pursued in cosmology and astrophysics?

THUAN: The main theme of my astronomical reaearch is the formation and evolution of galaxies. I've been concentrating my research efforts on the so-called "star-forming Blue Compact Dwarf" or BCD galaxies. Because I think that BCDs are the bricks of galaxy formation. In the hierarchical picture of galaxy formation, massive big galaxies like the Milky Way form from the assembly by the attractive force of gravity of small and less massive dwarf galaxies. Thus the Milky Way which contains 100 billion stars can result from the gravitational assembly of 1000 dwarf galaxies containing 100 million stars each. We still do not know the exact birth date of the first star-forming dwarf galaxy (we call them a ‘primordial' galaxy). In fact, we do not even know whether stars formed first and then assembled under the influence of gravity to form galaxies, or galaxies formed first and then broke up to form stars. But now that the James Webb Space Telescope (JWST) has been launched (on Christmas day 2021), it's starting to discover faint primordial dwarf galaxies that have formed only 300-400 million years after the Big Bang, at times earlier than were thought before.

Before the launch of JWST, those early times were not accessible even to the largest existing space (the Hubble Space Telescope) and ground-based telescopes. These telescopes could only go back in time to a period of only some 2-3 billion years after the Big Bang. In astronomy, seeing back in time is seeing far since light takes time to reach us, and seeing far is seeing faint. So I was faced with the question: how can I study primordial galaxies since they are too faint for telescopes in the pre-JWST era?

I had the following idea : what if, instead of carrying out studies of primeval galaxies in the far-away universe, I can find and study them in the local one? These galaxies in the nearby universe would act as proxies of the primordial galaxies in the distant universe. They would be much easier to study because nearby galaxies would be considerably brighter and larger (in angular size) than distant ones. But how does one find these nearby primeval galaxies ? By searching for very metal-deficient objects. Primordial galaxies are those that are undergoing their first burst of star formation. So they have not formed many massive stars previously. Once a massive star is born, with a mass between 5 to 90 solar masses, it makes by nuclear fusion heavier and heavier elements : carbon, nitrogen, oxygen, and so on up to iron. After a few million years, it consumes all its nuclear fuel and dies in a fantastic explosion called "supernova". The supernova makes all heavier elements than iron, such as uranium or plutonium. Thus to find primordial galaxies in the local universe, the strategy would be to search for star-forming dwarf galaxies that have made not much metals and are metal-deficient, i.e. they have metallicities varying from one half to 1/50 of solar metallicity. The most metal-deficient ones would be young "baby galaxies" in the sense that they do not have a substantial old stellar population. Most of their stellar populations are young and most of their massive stars did not form until the last billion years or so. Recall that the age of the universe is 13.8 billion years.

In 1981, I have dubbed these metal-deficient galaxies to be "Blue Compact Dwarf galaxies" or BCDs: "blue" because BCDs contain many young massive stars, and these are blue; "compact" because their star formation occurs in very dense regions. And lastly, BCDs are dwarf galaxies that are 1,000 -- 10,000 times less massive than a normal galaxy.

Since the beginning of the eighties, my research has concentrated on BCDs. I have searched the literature to assemble as many of these objects as I could. Some BCDs can be found in the "Catalog of Compact Galaxies" that Fritz Zwicky, a professor at Caltech, has assembled. One of the most famous BCDs listed in that catalog is named I Zwicky 18. I and my colleague Yuri Izotov have written many papers on that object. It is famous because it has a very low metallicity, only 2 percent (or 1/50) that of the Sun. I Zwicky 18 stood for a long time and still stands today, more than a half-century after its discovery, as one of the most-metal deficient star-forming dwarf galaxy in the local universe. Most galaxies have a similar metallicity to our sun in the Milky Way. Dwarf galaxies tend to have lower metallicity, with a mean metallicity around 1/10 that of the Sun. For comparison, the Large Magellanic Cloud, the nearest dwarf galaxy orbiting around our Milky Way, has 1/2 of the Sun's metallicity. The second dwarf galaxy nearest to us is the Small Magellanic Cloud, with a metallicity of about 1/7 of the Sun's metallicity.

My work concentrates on extremely metal-deficient (XMD) BCDs. These objects have metallicities are 1/40 to 1/50 that of the Sun, comparable to that of I Zwicky 18, i.e. 20-25 times more metal-deficient than the Large Magellanic Cloud. XMDs are the best local proxies to primordial galaxies. As said before, we don't know the exact date of first star or galaxy formation yet, but the first primordial galaxies observed by JWST formed about 400 million years after the Big Bang, quite early in the history of the universe.

Studying local proxies of primordial galaxies will allow to shed light on the reionization sources in the early universe. The universe was mot transparent until the year 380,000. Before, it was filled with a soup of elementary particles: protons, electrons, neutrons, neutrinos and antiparticles, all moving about at great speeds. And because the electrons were free, they impeded the propagation of the photons (the particles of light) making the universe opaque.There were also hydrogen and helium nuclei. A hydrogen nucleus is made of a proton while a helium nucleus is made of 2 protons and 2 neutrons. When the year 380 000 arrived, when the universe had cooled down to about 3000 degrees Kelvin (slightly cooler than the surface of the Sun), the electrons combined with the hydrogen and helium nuclei to form neutral hydrogen and helium atoms. This allows photons to travel freely through space, giving birth to a sea of radiation called the Cosmic Microwave Background. There were no stars yet. The Universe was plunged in deep darkness. That was the Dark Ages of the Universe.

Gravity acts to assemble the clouds of hydrogen and helium to large enough masses so that they collapse to form the first stars. That happened sometime around the first 400 million years, a period called the Cosmic Dawn. The newly-born O and B stars and galaxies emit plenty of ultraviolet ionizing photons that ionize the neutral hydrogen and helium gas clouds: that is the re-ionizaion period of the universe.

To summarize, from the beginning of the Universe to the year 380,000, the Universe was ionized. It then became neutral at year 380,000, and a few hundred millions years later, it became re-ionized again. By studying the local primordial galaxies, I can use them as proxies to understand what happened in the first 400 million years after the Big Bang.

I have been studying BCDs during most of my professional life. I joined the Astronomy department of the University of Virginia (UVA) in September 1976, and right away, I knew I wanted to concentrate on a research subject that hadn't been worked on much before so I could make a dent in it. By focusing on the BCDs, I had another advantage : on the campus of UVa in Charlottesville are located the headquarters of the National Radio Astronomy Observatory (NRAO) which runs the national radio telescopes for the US community. The BCDs, being metal-deficient, have not had much star formation in the past, and they must contain large fractions of neutral hydrogen gas. I could use the NRAO 300-ft radio telescope at Green Bank, West Virginia, to measure the neutral hydrogen content of the BCDs. The hydrogen atom has a spin flip which gives rise to the 21-centimeter line. That line allows to measure the neutral hydrogen gas content of galaxies. I spent several years gathering the radio data in the course of many observing trips to Green Bank, and I published the results of the neutral hydrogen radio survey of BCDs in 1981.

Since then, I've continued to work on BCDs using the largest telescopes on ground and in space. I have observed them in all types of light, spanning the entire electromagnetic spectrum, from X-ray to radio, including UV, optical and infrared. I've been studying the metallicity, the physical properties of the ionized gas, the morphological characteristics, the surface brightness distribution and the stellar populations of BCDs.

In December 2004, thanks to images obtained with the Hubble space telescope, I and Yuri Izotov were able to demonstrate that the BCD I Zwicky 18 is truly a young galaxy with the age of its oldest star less than 1 billion years.

The BCD research has also led me to measure the primordial He abundance, which in turn constraints the amount of dark matter that exists in the Universe. The luminous matter (100 billion galaxies each containing 100 billion stars) accounts for only 0.5% of the content in mass and energy of the Universe. The remaining 99.5% is made of dark matter (29.5%) and dark energy (70%). One important question is: what is the dark matter made of? Since the He nucleus is made of 2 protons and 2 neutrons (called generically "baryons"), one can measure the amount of baryonic dark matter, made of protons and neutrons, by measuring the primordial He abundance. Since BCDs are very metal-deficient and have not had much previous massive star formation and nucleosynthesis, its He abundance is nearly primordial. Thus one of my major topic of research is the determination of the He abundance in BCDs. I and Yuri found that the baryonic dark matter amounts to about 4.5% of the content in mass and energy of the universe. This is compared with the exotic non-baryonic dark matter which constitutes 25% of the content of the universe, the nature of which we know nothing.

Those are some of the issues I've worked on, and I must say, working on these problems have given me a lot of intellectual pleasure. I don't regret having chosen BCDs as topic of research. They are indeed wonderful proxies for studying the high redshift primordial universe.

Theory and Observation

ZIERLER: Are there theoretical aspects to your work?

THUAN: I'm more an observer than a theorist. At the beginning of my career, I did do some theoretical work on the dynamical evolution of stellar clusters with my thesis advisor Lyman Spitzer, and on galaxy formation with Jerry Ostriker, a professor at Princeton, and Richard Gott, a fellow graduate student at Princeton. However, my work is mainly observational. But that does not mean that I ignore current theoretical ideas. Observations are always planned in the context of some paradigm, and to interpret and explain my data in terms of current theoretical models, I have to be familiar with the most recent theories. An observer does not work in a theoretical vacuum: he is not content just to observe, he uses his data to test and constrain theories, to support or to eliminate models. For example, observations of BCDs constrain theories of galaxy formation in the early universe.

ZIERLER: You mentioned radio astronomy. What are some of the other observational techniques that are most important for you?

THUAN: Spectroscopy, the decomposition of galaxy and stellar light into its different energy components, is quite important. It helps to determine the relative chemical abundances abundances of elements, such as He, C, N, O in galaxies and stars. It thus sheds light on stellar nucleosynthesis and the chemical evolution of galaxies. For the BCDs, it's by doing optical spectroscopy of their light that I can measure their metallicity (the abundance of elements heavier than hydrogen and helium are referred to as "metals" in astronomy) and determine whether they are metal-deficient or not. Spectroscopy allows also to study the properties of star-forming regions in BCDs. Emission lines from H II regions (ionized hydrogen regions) provide information about their physical conditions (i.e. their electron temperatures and densities).

To carry out spectroscopy of BCDs, I need to use the largest optical telescopes on Earth, because the objects I am studying are tiny compact blue dwarf galaxies that are very faint points of light. Fortunately, UVA is part of a consortium that allowed me to get access to the Large Binocular Telescope (LBT) on Mount Graham, near Tucson, Arizona. Composed of two mirrors of 8.4 meter diameter, it is indeed one of the largest telescopes in the world. In addition, we have been doing spectroscopy of BCDs with the largest space telescopes as well. The Hubble Space Telescope allows us to do UV spectroscopy while the James Webb Space telescope permits us to do near- and mid-infrared spectroscopy. These supplementary UV and NIR and mid-IR permit us to paint a much more detailed and richer picture of star formation processes in BCDs.

The Vietnam War from Both Sides of the Pacific

ZIERLER: Let's go back now, I want to learn about your childhood. What was it like growing up in Hanoi?

THUAN: I spent the first six years of my life in Hanoi, the capital of North Vietnam. I was born there in 1948 and left with my parents for South Vietnam in 1954, after the military defeat of the French at Dien Bien Phu. During all those years, the communist troops led by Ho Chi Minh, the Viet-Minh, were waging war against the French colonialists with the aim of giving back to Vietnam its national independence. The Geneva Accords of 1954 between the French and the Viet-Minh divided Vietnam into two parts along the 17th parallel -- communist North-Vietnam and non-communist South-Vietnam -- with the intention of holding nationwide elections in 1956 to reunify the country. With the communists taking over North Vietnam, the life of my father was at risk. Being a very high-ranked civil servant in the old French administration, he could be subject to punitive retaliation by the communists. Even assuming that my father would not be outright executed, he could be put in jail or imprisoned in a camp. My father made quickly the decision to leave Hanoi and move to South Vietnam. My parents scrambled and packed up in a hurry some basic items in a few suitcases. Then our family (my parents, me and my four sisters) boarded a plane to fly down South. We left everything behind: our house, our belongings, my childhood things, i.e. all of our ancient way of life. We left Hanoi with a heavy heart, not knowing when we will see it again. My family was not the only one to be part of this great exodus southward. In total, nearly a million of North-Vietnamese made the trip.

Our destination was Dalat, a city located in the Central Highlands region of Vietnam. It is known for its cooler climate, French colonial architecture, beautiful landscapes, and spectacular waterfalls. I have nice nemories of Dalat, a very pleasant place to live and where I began my schooling. .My father worked in Dalat for two years as a judge (he had a law degree from the University of Hanoi), and then our family moved south again when he got promoted to the High Court of Appeals in Saigon. There, I and my sisters attended French schools. Because the French system of education was among the best and the most rigorous that was offered in South Vietnam, the well-to-do Saigonese "bourgeois" families sent their offsprings to French lycées. I attended the Lycée Jean Jacques Rousseau, named after the famous Swiss-French writer. I had good professors and I enjoyed a lot of fun and excitement learning about various topics such as maths, physics, philosophy, literature and history, all taught in the French language. We had also classes in Vietnamese history and literature, viewed through the French prism. I found out that I liked and had a gift for science but also for literary subjects. In 1966, when I was 18 years old, I took the ¨Baccalaureate" exam, which marks the end of Lycée in the French system. An exam which I passed with flying colors.

At about the same time, in 1965, the US began a large-scale military intervention in Vietnam, called ¨the American war¨, in an attempt to stop the invasion of South Vietnam by the North Vietnamese army. US troop levels increase rapidly, reaching a peak of more than half of a million soldiers in 1968.

ZIERLER: Did you see the American soldiers around Saigon?

THUAN: Oh, yes. There were GIs everywhere. Barbed wire and sand bags were visible around military and administrative buldings (such as the US embassy), The US presence greatly modified the economy of Saigon, the capital of South Vietnam. Shops, bars and night clubs sprouted in great number everywhere to cater to the needs of US personel. Rentals of houses and apartments to Americans drove rental prices through the roof.

There was a real atmosphere of war. Although security was tight everywhere, Saigon (now Ho Chi Minh City) experienced numerous terrorist bombings, with many casualties, primarily carried out by the Viet Cong (VC) as part of their strategy to destabilize the South Vietnamese government and its American ally. I remember in particular the Caravelle Hotel Bombing on Christmas Eve 1964, and the U.S. Embassy Bombing in 1965. These terrorist attacks were direct assaults on symbols of the American presence in Vietnam.

ZIERLER: How did you feel about the War? Did you support the American presence in Vietnam?

THUAN: Yes, I did support the US intervention at the time. As said before, our family was firmly anti-communist. We have left North Vietnam for South Vietnam, leaving everything behind, because we did not want to live under a communist regime. We felt that the US presence in Vietnam was necessary to stop the communist expansion in South East Asia.

ZIERLER: What did you do after the French Baccalaureate?

THUAN: After having completed my work in the French Lycée, I wanted to pursue university studies in Paris, France. At that time, the Vietnamese government still gave draft deferment to academically-gifted students so they can pursue advanced studies before joining the army. My French teachers in Saigon all advised me to go to France to study and learn how to do research at two French elite institutions in Paris: the École normale supérieure or the École polytechnique. Those are the cream of the cream of the French system of education. I was all set to depart for France, but all my plans were derailed by the then president of France, Charles de Gaulle. In 1966, he gave a major speech in Phnom Penh, the capital of Cambodia, about the US presence in Vietnam. He was very anti-Vietnam War. According to him "The war is an internal conflict between North and South-Vietnam, and the US have no business being there. All American troops should be withdrawn."

The South Vietnamese government who wanted US troops to stay in Vietnam was very angry at that declaration by de Gaulle. It decided to immediately cut off all diplomatic relationship with France. A consequence of this action was that South-Vietnamese students were not allowed anymore to go to study in France.. I went instead to Switzerland, enrolling in the École Polytechnique at Lausanne, which is a very good engineering school. It had the advantage that the teaching was carried out in French.

From France to California

ZIERLER: How did you end up doing your undergraduate work in the US ?

THUAN: After one year at this École Polytechnique de Lausanne, I realized that it was not the right place for me. I didn't really want to learn engineering, Rather, I wanted to become a scientist. In my view, scientists discover new connections and novel phenomena. They unravel new laws in nature while engineers are content to apply the laws that scientists discover. I had more the soul of a scientist than that of an engineer. Although I did not master English very well, I made the decision to go to study in the US. I had made some preliminary investigations in Saigon after I learned that I could not go to France to study. Before my departure for Switzerland, I'd gone to the US Cultural Center in Saigon. There, I could find catalogs of well-known US universities. Given my interest in science, I was advised to investigate in particular MIT, Caltech, and Princeton. By looking through their catalogs, I could only marvel. The three universities all had beautiful sprawling campuses with an active cultural and social life, laboratories equipped with the latest instrumentation and top-notch science professors, some of them being Nobel prize winners. I sent in my application to all three institutions, asking them to be admitted as a transfer second-year student because I wanted to get credit for the year spent in Switzerland. Because of my low proficiency in English, I did well on the maths and physics entrance tests (math symbols and equations are the same in any language), but only average on the English tests. But that was enough to get me admitted to all three universities. Admission came with the award of a scholarship because I had explained that tuition and fees alone would have been more than several times my father's annual salary. I was faced with an embarrassment of riches as to which university to attend because I was sure that all three would give me an outstanding education.

ZIERLER: Why did you choose Caltech ?

THUAN: The choice was not easy. I was leaning for Princeton because of a sentimental reason: it was the town where Einstein, my scientific idol, spent the last 22 years of his life. He was the man who inspired me to go into science. I had read his book The World as I See It as a teenager. I was admirative of not only his scientific genius, but also of his great humanity. But in the end my choice of college was dictated by not so much scientific factors (all three universities had outstanding physics departments), but by somewhat more frivolous reasons. [Laugh]

During my year in Switzerland, I had discovered for the first time snowy winters, which Vietnam, being a tropical country, did not have. I did not enjoy the cold weather. So I said to myself "California is the best choice since it does not have winter." In addition, there were also all the myths that surrounded California : its sandy beaches without end, its pretty girls in bikinis, its surfers chanted by The Beach Boys, its glamourous movie stars in Hollywood, the capital of cinema. Furthermore, during my adolescence days in Saigon, I was an ardent listener of the radio broadcasts of the US Armed Forces and an avid watcher of US TV shows. That gave me some kind of a glimpse into American society. I was quite intrigued by the American way of life, and in particular, by California culture. These factors tipped the balance in favor of the West coast and I decided to go to Caltech to do my undergraduate work.

I majored in physics. Richard Feynman and Murray Gell-Mann were the uncontested "stars" of the Caltech physics department, with Feynman winning the 1965 Nobel prize in Physics for his work in quantum electrodynamics, and Gell-Mann, the inventor of the "quark", being awarded the 1969 Nobel prize for his work on the classification of elementary particles and their interactions. Those two made Caltech the center of the particle physics world. Feynman was one of my favorite professors, although it took me some time to get used to his New York Queens accent. His view of physics was quite personal and original. He always looked at nature with new eyes, reinterpreting everything – classical mechanics, electromagnetism, gravitation, quantum mechanics -- in his own way. He had an unusual physical intuition. When he tackled a problem, one had the feeling that he already knew the answer, and that mathematical reasoning only serves to validate his intuition. With the inspiration provided by professors like Feynman, it would have been natural that I became a particle physicist. I graduated in 1970, obtaining a bachelor in physics with honors. But when the time came to apply for graduate school, I chose not particle physics, but astrophysics.

A Golden Age for Astrophysics

ZIERLER: What motivated you to choose astrophysics?

THUAN: Indeed, another field of research, astronomy, has increasingly drawn my attention. I discovered that Caltech was not only the world Mecca in particle physics, it was also at the forefront of astronomy. Until the 1960s, it has been associated with the largest telescopes built on Earth. In 1922, the steel magnate Andrew Carnegie financed the construction of a 2.5 m diameter telescope on Mount Wilson, near Los Angeles. Edwin Hubble, the astronomer after whom the Space Telescope is named, used the Mount Wilson 2.5 m telescope to demonstrate that the Universe was much larger than the Milky Way and that it contained many galaxies. He also showed that the Universe is expanding, an observation that gave birth to the Big Bang theory. In 1948, with a grant from the Rockefeller Foundation, Caltech built a 5-meter telescope located at Mount Palomar, at about 100 kilometers south of Los Angeles. This telescope remained the largest in the world until the end of the 1960s and could look some 7 billion years back in time.

The sixties was the golden age of astrophysics. There were so many discoveries, some by Caltech professors using the Palomar telescope. For example, Maarten Schmidt discovered quasars in 1963. These are fabulous galaxies at the edge of the universe that are the most intrinsically luminous in the cosmos. They harbor in their heart a very massive black hole with the mass of several billions suns that eats up stars in the galaxy and convert their mass into luminosity.

In 1965 was the discovery of the cosmic microwave background (CMB) by Arno Penzias and Robert Wilson at Bell Telephone Laboratories. Penzias would later on become one of my professors at Princeton. This epochal discovery gave a firm observational proof of the Big Bang theory. Then, there was the discovery in 1967 of pulsars, these neutron stars of 10 kilometer radius that are rotating so fast on themselves that they can make 1000 turns per second. And then, there was the Mars exploration program run by the Jet Propulsion Laboratory (JPL), a NASA center managed by Caltech. I will always remember the first pictures of Mars being beamed down in 1969 to my classroom, by the Mariner 9 spacecraft, during a class on nucleosynthesis by Professor William Fowler. My classmates and I discovered with wonder the reddish, dusty and waterless Martian surface, without any of the little green men and Martian cities that I'd been reading about in science-fiction books. And 1969 was also the year which saw Neal Armstrong land on the moon. The 1960s were indeed very special for space and astronomy. These explorations and discoveries could not help but draw me toward studies of the cosmos.

However it was a trip to Palomar that definitely tipped the balance. During my 3rd year at Caltech, I had a summer job with Professor Gordon Garmire in the Physics department. He used a X-ray telescope in space to image X-ray sources in the sky. This very energetic radiation is generally linked to very violent celestial events, for example, the explosive death of massive stars (called "supernova"), or hot gas (tens of millions of degrees) from the atmosphere of a star falling into a black hole or a neutron star. He had the project to use the Palomar telescope to look for optical counterparts of X-ray sources. He asked me to accompany him in the observing run. My first contact with the legendary telescope was an unforgettable experience. My heart filled with happiness when I saw, at the turn of the road, the big dome rising, like a futuristic cathedral, toward the sky. At nightfall, when it was time to point the technological marvel to the heavens and gather the precious cosmic light coming from thousands of light-years away, I felt for the first time the immensity of the universe and the ineffable feeling of cosmic connexion. I told myself that this vast cosmos contained so many secrets and puzzles, that even without having the extraordinary abilities of a Feynman, I still would be able to contribute to the unraveling of some of the problems of the universe. I decided to become an astrophysicist, a decision that I have never regretted.

ZIERLER: In the 1960s, what were your feelings about all of the protests against the Vietnam War when you were a student?

THUAN: I was sympathetic to the motivations of the protesters. University campuses were in turmoil. The students felt (just like de Gaulle in his Phnom Penh speech) that the Vietnam war was an internal conflict between the North and the South, and that the US has nothing to do with it and should withdraw all its troops. They did not believe, like the US government did, that letting South Vietnam fall into the hands of communist North Vietnam would result in all of South East Asia becoming communist (this is known as the "domino" theory: one domino falls, triggering the fall of all the others), which would threaten the American way of life. Young Americans were being drafted and sent to a far-away country to fight for a cause they did not believe in. The protests were virulent as the American people believed that they were being lied to by the administration regarding the evolution of the war. The government kept saying that "the light is being seen at the end of the tunnel" and that the war would soon be won. Yet in 1968, America was shocked to see on live television the Viet-cong flag float during a brief instant over the US embassy of Saigon, during the Tet (the Vietnamese New Year) attacks.

I find myself in a very awkward situation. Here I am, being deferred from military service and studying in the US, far from the war, while my classmates and contemporaries are being drafted and sent to my country to fight a war they did not approve of. But I never felt any personal animosity and reproach from my classmates. I think that they all realized that we were just pawns caught in the turmoil of History, and that matters of life and death, and of war and peace were out of our hands.To avoid to be sent to Vietnam, a few of my classmates went to Canada. Others fasted to lose weight, with the aim of failing the physical exam by starving themselves.

In addition to the anti-war protests, I also lived through the societal and moral turmoil of the 60s. A society based on only bourgeois materialistic values was questioned, and new moral and sexual mores emerged. With the hippie movement, the baby-boomers rejected the values of their parents and of the consumer society they have built. They lived in communities and adopted other cultures (particularly those from the Orient), experimenting with drugs and new sensorial perceptions (that gave rise to psychedelic art). There was also great political upheaval with the assasinations of Martin Luther King and Robert Kennedy, both in 1968.

ZIERLER: Were you able to stay in touch with your family? Were you worried about their safety?

THUAN: Yes, I constantly kept in touch with my family. Internet was not available in those times. Telephone was very expensive. So communication was best done by letters which took about a month to go from Saigon to Pasadena. Yes, the safety of my family was always a big concern and constantly in the back of my mind. In addition to occasional terrorist bomb attacks and shelling of Saigon by mortars (one fell into our house that caused lasting psychological scars to one of my sisters), there were always the lingering worry of the withdrawal of US troops from South Vietnam and the possible ensuing military invasion of the country by North Vietnamese communist troops.

My father had been named Chief Justice of the Supreme Court of South Vietnam, i.e. he was a highly-ranked official in the South Vietnamese government. I had the disquieting foreboding that he would be emprisoned or killed if South Vietnam were to be taken over by the communists. I told him, for having seen the war protests in the US, that the withdrawal of US troops and the subsequent invasion of the South by the North was a real possibility. But my father assured me that the US embassy had gotten in touch with him and promised to help him leave the country in case of a military invasion: helicopters would transport him, my mother and my sister from the US embassy to American aircraft carriers stationed in the China sea.

Unfortunately, my worst fears came true. After the Paris treaty of 1973, the US troops left South Vietnam, putting an end to the American war. Left on its own, South Vietnam saw its military situation rapidly deteriorate. At the beginning of March 1975, the communists launched a general offensive against the South. With nearly no resistance, the cities were falling into the hands of the enemy one after the other. Finally, on 30 April 1975, Saigon fell to the enemy. Filled with anguish, I followed all these heart-wrenching events on television. I did have any news from my family. One week before the fall of Saigon, I tried to contact my parents by phone, but to no avail. Telephonic lines were completely saturated due to the heavy call volume: all Vietnamese expatriates were trying to call home ! On April 30, Saigon was completely chaotic. Everyone was in a state of panic, trying to flee the city before the arrival of the communist soldiers. With increasing alarm, I was watching powerless all these events unfold on television. A huge crowd was surrounding the US embassy in Saigon, trying to climb into the helicopters which were transporting US citizens and employees from the American embassy to the plane carriers in the China sea. Filled with disquieting thoughts, I wondered whether my family was in that crowd.

During a long agonizing month, I stayed without any news from my family. Finally, a letter came from my mother. It brought bad news. On April 30, my father did go to the US embassy in the hope of being evacuated, but the chaos and confusion was so great and the crowd so dense that he could not reach the gates of the embassy. So he was left behind. My father was imprisoned by the communists in what they called a "reeducation camp", but which is really a ‘gulag' a la Vietnamese: extremely hard daily manual work in the rice paddies, "autocriticism" to "confess crimes" commited against the communist state, brain washing and learning of Marxism and Leninism. The prisoners were fed with only a bowl of rice per day and lived in deplorable hygienic conditions, with no medical care. Unlike Pol Pot and his "Khmers rouges" who shot outright the intellectuals, the Vietnamese made them die slowly by malnutrition and sickness. I knew that, to see my father alive again, I needed to act fast.

ZIERLER: What did you do?

THUAN: My mother and one of my sisters were still in Saigon. The communists took over our house, but let them stay in a small room. I was getting very desperate letters from my mother telling me that she haad been able to go to the camp (in North Vietnam) to visit my father. His health was failing and he would not survive many more months in the camp. It was a very, very stressful period for me. I was, by that time (1978), Assistant Professor at UVA. My father had already spent 32 months in the camp. Time was pressing. How do I get my father out of the camp, and then out of Vietnam? The President of Caltech Harold Brown, who had high connections in Washington, D.C, kindly helped me to get in touch with the US State Department to ask if it could intervene on behalf of my father and get him freed from the camp. Unfortunately, the response was negative. After the fall of South Vietnam in 1975, the US had cut off all diplomatic relationship with the reunified Vietnam, and there was no channel to go through to negotiate the release of prisonners.

It was deeply discouraging. But, as I was losing all hope, a series of events happened that still leave me flabbergasted every time I think about them. Jean Audouze, the director of the Institut d'Astrophysique in Paris, had invited me to spend a sabbatical leave during the 1978 Fall semester at his institution, doing research. The invitation could not be more opportune. Contrary to the US, France has maintained diplomatic ties with Vietnam, so a possible way to help my father would be to travel to France and go the Vietnamese embassy in Paris to fill an application to ask for my father to be freed and reunite with his family. I did that as soon as I arrived in Paris. But several months passed without any reply from the Vietnamese embassy. Probably, my application was just discarded. Profoundly demoralized, I was lamenting one day about my lack of success in helping my father to a French colleague and friend. She advised me: "You should go to the Observatoire de Meudon, not far from Paris, and talk to the astrophysicist Henri van Regemorter who works there. He may be able to help you. I heard that he is a close friend of the actual Prime Minister of Vietnam, Pham van Dong." I hastily took the advice of my friend. My meeting with van Regemorter went well. I explained to him my father's predicament, and at the end of our conversation, without hesitation, he wrote before me a letter to the Prime Minister whom he addressed as "Dear friend". In the letter, he invoked humanitarian reasons and asked the prime minister to release my father from reeducation camp so he can reunite with his family. Van Regemorter explained to me that the Prime minister knew him well because he has spent many years working on cultural and university exchange programs between France and Vietnam. Profoundly affected by the German occupation of France during World War II, that was his way of helping a country torn by two successive wars. Then he handed the letter to me with the firm admonition: "Please make sure that this letter reaches the Prime minister". Just at that time, one of my aunts was departing for Hanoi and I gave her the letter.

Meanwhile, my sabbatical leave in Paris was ending. and I had to go back to Charlottesville to teach. I did so with a heavy heart and the uneasy feeling that I have not made much progresss and that time was running out. I didn't believe much in the success of the letter. Even if it gets to the Prime minister, why would he agree to van Regemorter's request?

Little did I know. One day in March 1979, I almost fell off my chair when I received a telegram from my father with the atonishing message that the Prime Minister has ordered his release from the camp and allowed him to leave Vietnam to join me in France for family reunification. My father asked me to fly instantly to Charles de Gaulle (CDG) airport in Paris to meet his flight from Ho-Chi-Minh city. I was overwhelmed with joy: van Regemorter's letter did miraculously work and the long nightmare was over. When I met my father in Paris, he was only the shadow of himself. He weighed only 32 kilograms, having lost some 40 kilograms. Under his skin were only bones, His hair has whitened and he could barely walk, so I had to check him immediately into a French hospital where he had to stay for three months before recovering his health. We had to decide whether my parents would stay in France or live with me in the US. My father told me: "I am French-educated, so I feel at ease in French culture. While I am fluent in French, I do not speak English well. Furthermore, I don't drive, so that would make me very dependent in the US. That's not the case in France which has a very good public transport system. Maybe it'd be better that we stay in France. You can come to visit us here during your summer vacation." Thus, both of my parents lived in Paris until the end of their lives. My father passed away in 1994. I'm glad that during the last 15 years of his life, he could live a quiet and peaceful life in a country the culture of which he held in high esteem. I never saw him harbor any hate toward those that made him suffer in the camp, nor any bitterness for all which he has lost. I admire his wisdom.

As I look back, the chain of events that led to my father's liberation still looks like a miracle. As I don't believe in random chance, I called it "fate" or "destiny". Somehow, fate led me to meet the right person at the right time. Otherwise, my father would not have been saved.

Lyman Spitzer and the Milky Way

ZIERLER: Speaking of the right person at the right time, back to your education in astronomy, did you go to Princeton specifically to work with Lyman Spitzer?

THUAN: Yes, I did.

ZIERLER: What was the connection? Was there a Caltech professor who suggested you?

THUAN: Well, there was a Princeton professor, Jerry Ostriker, who was spending a sabbatical leave at Caltech in 1970. He encouraged me to go to Princeton for graduate school. The top two astronomy departments in the US at that time were Caltech and Princeton. Caltech with its many large telescopes was the observational center while Princeton, not having access to large observational facilities, has made its reputation by concentrating mainly on theoretical work. I had done some investigations on the work of Spitzer and knew that his main research interests were on the interstellar medium, i.e. the gas and dust in the space between the stars, on the dynamical evolution of globular clusters and on plasma physics. Spitzer was also famous for being the father of the Hubble Space Telescope. As far back as in 1946, a whole 11 years before the launch of Sputnik, he wrote a groundbreaking paper which outlined the benefits of placing telescopes in space, free from the absorbing and distorting effects of Earth's atmosphere. This visionary idea eventually led to the development of the Hubble Space Telescope (HST). I thought I could not go wrong by doing my doctoral work under the supervision of such an extraordinary man. Furthermore, doing graduate work at Princeton would bring me to the town of Einstein, my scientific hero. I spent the period from 1970 to 1974 doing a PhD at Princeton, with Spitzer as my thesis advisor during the last two years.

ZIERLER: What was Spitzer like as a person?

THUAN: Oh, he is a wonderful and kind person, a true gentleman in the old-fashioned way. Although he was busy with many administrative duties (he was then the chair of the department), committee meetings (with many trips to Washington,D.C. to persuade Congress to fund the HST) and his own scientific work, he always found the time to discuss my thesis work with me. He gave me one hour per week, during which I would have to succinctly summarize the progress made since I last saw him. He expected me to come up with my own ideas to solve any problem that may arise. Then, in one or two sentences, he would tell me whether that idea was good or bad. It was wonderful to see his intuition and his mastery of science at work.

Our relationship was more than professional. It was also personal. He and his wife Doreen were well aware that, being far away from home, foreign students sometimes are subject to solitude and loneliness. During major holidays such as Thanksgiving, the Spitzers would always make a point of inviting me to their home. Both were also very Francophile and enjoyed to speak French. That was a supplementary bond that united us.

ZIERLER: Tell me about your dissertation research. What did you work on?

THUAN: I worked on modeling the ionization structure of the intercloud medium (ICM) of the Milky Way, based on data obtained by Copernicus, an ultraviolet space telescope conceived by Spitzer and launched in 1972. I explored in my thesis the astrophysical situation in which the sources of ionizing ultraviolet photons were young and massive so called "runaway stars". These stars have high space velocities (50 to 100 kilometers per second) and thus can travel large distances and ionize a significant volume of the Milky Way during their lifetimes. I found that, under certain conditions, the ionization structure of the ICM observed by Copernicus could be well explained by runaway stars as the main ionizing sources.

However, my main research interests were more observational cosmology than studies of the ISM. But, because I wanted to have the experience of working with Spitzer, I would have accepted any topic that he proposed me.

ZIERLER: With your interest in cosmology, did you interact with Bob Dicke?

THUAN: Not really. I did attend some of his lectures on the Brans-Dicke theory of gravity. He gave them as a guest lecturer in Jim Peebles's course on cosmology in which I was enrolled. I was glad when Peebles, a student of Dicke, obtained the Nobel Prize in physics in 2019.

Professional Origins and Cosmology Focus

ZIERLER: When did you start to do research in cosmology?

THUAN: I didn't really do any direct work on cosmology until I went back to Caltech as a post-doc, working with Professors Jim Gunn and Bev Oke during the period September 1974- August 1976. Jim Gunn was an observational cosmologist. He wanted to measure the deceleration of the universe. At that time, the astronomical community still thought that, although the universe was expanding, that expansion must be decelerating because the attractive gravity of the galaxies pull them toward each other.

How do we measure the deceleration of the universe? If we wish to measure the deceleration of our car when we brake, we measure the speed of the car at two different instants, and then we divide the speed difference by the time difference. In the case of the universe, we need to rely on a class of celestial objects that can be observed at different cosmic times, for which we can determine both velocities and distances. The velocities are relatively easy to obtain. The light from all distant celestial objects is redshifted due to the expansion of the universe. By measuring the redshift, we can determine how fast the universe is expanding at different distances (and hence at different times) in the past. The redshift is determined using spectroscopy, which splits the light from a celestial object into its component wavelengths and shows how much the light has been stretched. As for the distance to the celestial object, it is calculated by using its apparent brightness and its known intrinsic luminosity. One needs to use so-called "standard candles" i.e. objects with a known constant intrinsic luminosity which does not vary in space nor time.

To map out the expansion of the universe, Gunn wanted to use, as standard candles, giant elliptical galaxies in clusters of galaxies (groupings of thousands of galaxies). Because of their large intrinsic luminosity, these giant ellipticals can be seen at large distances, and allow to go back in time to at least 7 billion years, i.e. half of the age of the universe. I spent a large fraction of my 2-year post-doctoral stay at Caltech (1974-1976) to use the Palomar telescope to gather the necessary data on these giant elliptical galaxies.

Our project to measure the rate of change of the expansion of the universe could only be successful if the giant elliptical galaxies in clusters were true standard candles, i.e. that their luminosities did not evolve either in space or time. If the candles were themselves evolving, we would not be able to distinguish their intrinsic evolution from the evolution of the universe. Unfortunately, it is now known that giant elliptical galaxies do evolve. Galaxies are made of stars that are born, live and die, and hence their luminosity diminishes with time. Furthermore, giant elliptical galaxies can also become more luminous and more massive with time because of a processus of "galactic cannibalism" : they have a strong tendency to attract by gravity their smaller companions and "eat" them up. Thus giant elliptical galaxies do evolve, they are not true standard candles.

Thus we could not measure the rate change of the expansion of the universe by using giant elliptical galaxies. If we had done it correctly, with true standard candles, we would have perhaps discovered that the expansion of the universe, instead of decelerating was on the contrary accelerating, its space being pushed apart by a mysterious anti-gravity force called "dark energy" . That discovery was made independently by two groups in 1998, using "supernovae of type Ia" (thermonuclear explosions of white dwarfs) as standard candles. The finding was rewarded by the 2011 Nobel Prize in physics.

ZIERLER: This was your postdoc research at Caltech, looking at what was thought to be the deceleration of the universe. You worked also on other galaxy topics?

THUAN: That's right. At the same time as the cosmology reseach, I was working on understanding various properties of galaxies such as: how giant elliptical galaxies acquire their large stellar envelopes, how spiral and elliptical galaxies form, how galaxies acquire their angular momentum, etc.

ZIERLER: Tell me about joining the faculty at University of Virginia.

THUAN: After two years of postdoc at Caltech, the time has come to apply for a faculty position at an university that would allow me to pursue my research. I consulted the job register and applied to several universities. Of the several offers I had, I thought that the University of Virginia (UVA) best met my objectives. Academically, it was certainly not on a par with the top two institutions, Caltech and Princeton, with which I had been associated. But the UVA astronomy department was on the way up. It had a young good faculty I could interact with. Furthermore, the presence of NRAO on campus, with its excellent staff, made Charlottesville a major center for astronomical research in the US. I thought UVA was a good place for me to develop scientifically on my own. There would be no pundit in the department that would hover over me and tell me what science to do.

There were also non-academic reasons for my choice of UVA. I was a great admirer of its founder Thomas Jefferson. Author of the Declaration of Independence of the USA, 2-times US President, Jefferson was convinced of the virtues of education. He built the University in 1819, near his home at Monticello. He drew himself the plans of what he called his "academical village". UVa was the first public university in the US. However, despite his many great achievements, there is a zone of shadows in the life of Jefferson that is deeply disturbing and had to do with slavery. The author of the famous opening sentence of the Declaration of Independence "All men are created equal" always kept slaves in his domain of Monticello. Worse, after the death of his wife, he had several children with a woman slave named Sally Hemmings, which he never freed. He never had the courage to stand up against slavery either in his public or his private life. However, despite my misgivings about Jefferson on the subject of slavery, he remains my American intellectual hero and I was glad to come to his University.

ZIERLER: The perfect place for you to be a professor then.

THUAN: Yes. I should also mention that I have a great admiration for Jefferson's architectural skills and the way he designed the heart of the campus, known as "the Lawn" . Jefferson included Palladian architecture in his Academical Village to express the Enlightenment ideals of truth, reason, and learning. I like the Palladian structures that Jefferson built on the Lawn : the columns, the Rotunda and the Pavilions. Monticello and the University of Virginia in Charlottesville have been designated in 1987 as UNESCO World Heritage Sites in the US for their cultural value. For me, it's quite a pleasure to walk through the Lawn each day and enjoy such beauty.

There is another event that links me to Jefferson. I became an US citizen on Independence day July 4, 1981. The moving naturalization ceremony occurred at Monticello, Jefferson's home. I could not think of a better and more symbolic and appropriate place for becoming American.

ZIERLER: In the beginning of your faculty career, what were some of the big questions on the blue dwarf galaxies, and how did you want to answer them?

THUAN: One of the important questions was the space density of Blue Compact Dwarf (BCD) galaxies: how many of these star-forming compact galaxies exist in a given volume of space, and how do I locate them in the local universe? I had to devise criteria to find them in the many published lists of galaxies which I examined. BCDs are currently undergoing bursts of star formation that give birth to massive stars in a compact region. Those massive young stars produce energetic photons that ionize the neutral hydrogen gas, giving rise to an emission-line spectrum. So I use the following selection criteria to select BCDs from the "objective prism survey " lists (these lists catalogued strong emission-line objects) of Markarian, Haro and Zwicky: 1) low-luminosity; 2) compactness and 3) emission-line spectrum.

I thus assembled a list of more than a hundred BCDs which I could use to study in detail their properties. Imaging and spectroscopy allowed me to investigate their morphology, stellar populations, metal abundance, star, gas and dust components. I found out that very metal-deficient (and hence very young) BCDs are very rare. If you look at 10,000 star-forming dwarf galaxies, maybe there's one that has a metallicity as low as 1/50 that of the Sun. Today, more than 40 years later, I and my colleagues (mainly Yuri Izotov) are still using large galaxy surveys (such as the Sloan Digital Sky Survey) to search for these very metal-deficient nearly primordial galaxies in the local universe.

Radio Astronomy and Angular Momentum

ZIERLER: What about your work in angular momentum? Tell me about that.

THUAN: Yes, that was a theoretical project I carried out in collaboration with Richard Gott at Princeton. It has been argued that tidal interactions of protogalaxies in the early universe generate sufficient torques to give galaxies an angular momentum in order of magnitude agreement with the observed values.We applied the tidal interaction picture to elliptical and spiral galaxies. It gave the correct amount of angular momentum for our Milky Way. Monte Carlo calculations were used to predict the distribution of total angular momenta for galaxies of a given mass (or luminosity). The observational data were in good agreement with the predicted individual distribution shapes and also with systematic properties with mass.

In a second article, Gott and I investigated the angular momentum of galaxies in the Local Group composed of the Milky Way, Andromeda (or M31) and less massive smaller dwarf galaxies. The protogalaxies were originally frozen in the Hubble flow in the recombination era and they gained angular momentum by tidally interacting with their neighbors. We showed that there were a number of geometrical constraints on the orientations of binary galaxies like the Galaxy and M31, if the spins of the two galaxies were produced by mutual tidal interactions. The most likely scenario is probably one in which a significant fraction of the Galaxy's angular momentum was produced by M31 (which explains the fact that they lie nearly in each other's planes), with the remaining small fraction produced by more distant galaxies.

ZIERLER: Did you use the NRAO radio facilities?

THUAN: Yes, I did use the NRAO radio telescopes. I observed quite extensively with the 300-foot Telescope at Green Bank, West Virginia before its collapse in 1988. I had a long-term project to use the 21 centimetre line of atomic neutral hydrogen to measure the neutral hydrogen content of the list of more than one hundred blue compact dwarf galaxies that I had compiled, as described before. Green Bank is about a 2-hour drive from Charlottesville and I have the souvenir of many nice trips through the spectacular scenery of the mountains of West Virginia.

I have also used the Jansky Very Large Array (JVLA) in Soccoro, New Mexico, another NRAO facility. It is composed of 27 individual radio telescopes linked together to form an interferometer. It can see much finer details than a single-dish telescope like the one in Green Bank, and can give a map of the spatial distribution of the neutral hydrogen. i.e. provide a radio picture. I have observed several BCDs with the JVLA. By comparing their optical and radio images, I find that the extent of the neutral hydrogen gas is always much larger (by a factor of 3 or more) than the optical size. However, the visible compact star-forming region of a BCD is always nearly coincident with but not necessarily on top of the highest density location of the HI distribution. Radio investigations of BCDs have been indeed very fruitful.

ZIERLER: I wonder if you can explain generally your interest in spectroscopy, why it's so useful.

THUAN: Spectroscopy is indeed extremely useful in astronomy. It allows scientists to dissect light into its constituent colors (or wavelengths) and examine the resulting spectrum to gain detailed information about the nature of the light source and the medium through which it has passed.

For BCDs, spectroscopy helps to determine their chemical composition. By examining the spectral emission lines (distinct lines at specific wavelengths) of each BCD, we can identify the elements present in it, because each element has a unique spectral fingerprint. With the line intensity ratio measured for each emission-line in the spectrum, we can measure the abundance of each element (relative to that of hydrogen). Abundance determination is crucial for deciding whether the BCD is metal-deficient (and hence nearly primordial) or not.

Spectroscopy allows also to determine the physical conditions of the ionized gas in the BCD such as its temperature and its density. It is also useful for measuring the Doppler shift of spectral lines, which indicates whether the light-emitting object is moving towards (blueshift) or away (redshift) from the observer. This information is crucial for understanding the motion of the gas in BCDs. Finally, the redshift of the BCD due to the expansion of the universe gives its distance. Without spectroscopy, I would not be able to do my BCD work.

ZIERLER: When the Hubble Space Telescope was operational, what did that make possible for you?

THUAN: Because it's orbiting above the Earth's atmosphere, HST can operate not only in the optical, but also in the UV. Concerning the BCD studies, the UV spectroscopy which I am carrying out with HST has been instrumental in complementing optical spectroscopy for studying the composition, structure, and dynamics of the interstellar medium of BCDs. For example, it allows to detect elements such as hydrogen, helium, carbon, nitrogen, and oxygen in their highly ionized states, that are not easily observed in optical or infrared wavelengths. but which are prominent in the UV range.

UV spectroscopy also helps study the energetic processes in massive stars and supernovae, including the impact of their winds, shocks, and radiation on surrounding environments. It is crucial for understanding the intense bursts of star formation occurring in BCDs, providing data on star formation rates and the properties of young stellar populations.

Finally, UV spectroscopy is essential for studying the early universe, particularly the reionization era when the first stars and galaxies reionized the neutral hydrogen in the primordial universe. As metal-deficient BCDs are good proxies for the first galaxies, their UV spectroscopic studies will allow to estimate the Lyman continuum escape fraction of primordial galaxies, and determine whether the observed number of low-mass star-forming compact dwarf galaxies at high redshifts is large enough to reionize the early universe.

ZIERLER: What about the Spitzer Space Telescope? Was that important for your research?

THUAN: Yes. I was very glad when the Spitzer Space Telescope was launched in 2003. Not only for scientific but also for sentimental reasons. It would have been appropriate to name HST after Spitzer since he was the first scientist to advocate placing telescopes in space to avoid the distortion and absorption caused by Earth's atmosphere. His work and advocacy laid the groundwork for the development and eventual launch of HST. But NASA wanted to honor Edwin Hubble's discoveries which were foundational for the field of observational cosmology. Hubble's work in the 1920s showed that "nebulae" are not located within the Milky Way but are extragalactic systems. Most importantly, he provided evidence that the universe was expanding, His work led eventually to the Big Bang theory.

However, Lyman Spitzer's contributions were not forgotten. Another major space telescope was named after him: the Spitzer Space Telescope. This telescope, launched in 2003, focused on infrared astronomy, complementing the work of the Hubble Space Telescope which functions mainly in the optical and ultraviolet wavelength ranges.

Foray Into Science Communication

ZIERLER: When did you start to get interested in science communication and being more public about writing and thinking about Buddhism in science? When did that happen in your career?

THUAN: My first popular book was published in 1988, so I was 40 years old then. At that time, I had already established myself as a reasonably successful researcher in my field of blue compact dwarf galaxies. As a researcher and a professor, I spent much time communicating and sharing knowledge with my colleagues and students. However, because I find the beauty and the harmony of the universe so wonderful and inspiring, I have always nourished in the back of my mind the desire to share my astronomical knowledge with a larger public. I have always felt some frustration that the scientific articles on which I had intensely worked for months or years would be read and understood in detail by only a handful of experts, because knowledge is becoming so specialized. It seems sometimes that scientists know everything about almost nothing.

Again, there were a happy combination of circumstances that led me to become a science popularizer. Although it was not a matter of life and death like the events that led me to send the letter to the Prime Minister of Vietnam, asking him to free my father from camp, it seems like the succession of events that led to the publishing of my first popular book was no less magical. In 1984, I got an invitation to write a popular cosmology article on the Big Bang theory and the Birth of the Universe for the French magazine La Recherche, the French equivalent of Scientific American. The La Recherche article attracted attention from the public, as witnessed by the many letters that I received from readers. There was a letter from an editor at the Editions Fayard, a well-known Parisian publishing company. The Fayard editor wrote: "I really appreciated your Big Bang popular article. I would like to ask you to write in French an entire book on the subject, telling the whole history about how human representations of the universe evolved from the earliest cosmological ideas to the Big Bang theory." Just at that time, I had a 1-year sabbatical leave from UVA coming up, so I had an extended period of time I could use to write my book. I accepted the proposition of the Editions Fayard and went to Paris in 1986 (I had invitations from the Institut d'Astrophysique and from the Astrophysics department of the Center for Nuclear Studies at Saclay) to write the book. I chose Paris because the book was going to be written in French, and that would give me the opportunity to interact with my editor. Most importantly, I could see my parents. I finished The Secret Melody (La Melodie Secrete in French) at the end of my sabbatical year and it was published in 1988. Meanwhile, I had gone back to teaching and research at UVA. It was with some trepidation and anxiety that I awaited the verdict of the French critics and the reaction of the public to my book. At the time, I was of course a complete unknown in France. Fortunately, the director of Fayard noticed and appreciated my work, and decided to start a vigorous press campaign to launch the book. I had good press reviews in the major French newspapers like Le Figaro and Le Monde and magazines like l'Express and Paris-Match. Most importantly, I was invited to the famous TV talk-show Apostrophes hosted by the well-known journalist Bernard Pivot. This show that gave authors the opportunity to talk about their books is broadcasted in France on primetime every Friday night and is watched by millions of people. If the author manages to generate interest in his book during the show, it is sure to become an instant best-seller. One can consider Apostrophes to be a more sophisticated version of Oprah's Book Club in the US.

I remember the chain of events that led to the success of The Secret Melody like it was yesterday. When I got the invitation of Pivot, I was in the misdt of an observing run with a colleague at Kitt Peak National Observatory in Tucson, Arizona. I had to fly instantly to Paris so to be on time for the TV show on Friday night. Despite jet lag and fatigue, I managed somehow to give a good presentation of my book. I kmew that I had succeeded when the next day, while I was at Charles de Gaulle airport on the way back to Charlottesville. I was stopped by several passengers who saw me on TV the evening before and congratulated me on my performance. La Melodie Secrete shot immediately to the best-seller list. It has been translated into six languages, including English and Vietnamese. More than three decades later, it still sells hundreds of copies in its paperback version. Apostrophes gave me my status of popular science writer in France. Once you have put your foot in the door and have acquired a following, the writing and publishing of subsequent books is much easier. I have now written some 20 books, some more important than others. Four have been translated in English. Besides The Secret Melody and Chaos and Harmony, my best-known book in the US is The Quantum and the Lotus, written in collaboration with the Buddhist monk Matthieu Ricard on the relationship between science and Buddhism.

ZIERLER: How were you regarded by your astronomy colleagues when you were writing these books? Were they supportive? Did they consider this a productive use of your time?

THUAN: Well, I never slowed down in my astronomical research, so I was still producing scientific research papers at the same steady rate. I don't think there was any problem with my colleagues, at least from my perspective. My colleagues considered my book writing as a hobby of mine which I accomplished in my spare time, and which did not hurt my research efforts.

ZIERLER: Were these things that you always wanted to write about, but you needed to be more established in the field before you felt comfortable doing so?

THUAN: I have always wanted to write about the magnificence of the sky amd the origin and destiny of the cosmos. I have an advantage that scientists in other fields do not : it is much easier to write about stars and planets than about protons and electrons. Stars and galaxies are much more poetic and beautiful than elementary particles. There are other reasons that motivate me to write for the general public. In a world that is more and more interdependent, it is vital that citizens in democratic countries possess a basic understanding of all aspects of science and technology, and the good or bad consequences they may have. This knowledge will help those citizens to select wisely their leaders. Beyond the description of phenomena – Big Bang, dark matter, black hole, quasars or pulsars – questions are raised that touch philosophy and spirituality. Modern cosmology has modified profoundly our notions of space and time, our ideas on the origin of matter, life and consciousness. The questions that the cosmologist asks are surprisingly similar to those raised by the theologian; does the universe has a beginning and end? Was the universe self-created? Is the presence of mankind in the universe by pure chance or is it inscribed in the properties of every atom, star and galaxy?

As to your question on when to start popular writing, that's certainly true that I felt I needed to be well established as a scientist first, before taking on the task of writing for the general public. That's why I waited until I was 40 years old, when I already had tenure at UVA and was about to be promoted to Full Professor. Yes, I do think it's not good for a young person who's just coming out of grad school, finishing his thesis, to embark on this kind of popular science writing. If he had not developed before the reputation of being an outstanding researcher, he would never be able to find a job at a good research institution. When you are starting out, not spending most of your time to do research may hurt irremediably your professional career.

ZIERLER: Did you find yourself spending more and more time in Paris?

THUAN: No, the time spent doing research and teaching at UVA during the 9 academic months did not change. I was in Paris only during the 2 summer months. That gave me the opportunity to talk to my Parisian editor. While I was in Charlottesville, I did not wait until the summer to write. I always reserve some time in the evening for writing after the day's work.

The Meanings of Chaos and Harmony

ZIERLER: We talked about The Secret Melody. Tell me about the 2000 book, Chaos and Harmony. What is chaos, and what is harmony?

THUAN: I wrote this book because I wanted the general public to know that there has been a profound paradigm shift in the way we see the world at the end of the 20th century. After having dominated western thought for some 300 years, the Newtonian vision of a fragmented, mechanistic and deterministic world has given way to the new vision of a holistic, indeterministic and brimming with creativity universe.

For Newton, the Universe was merely an immense machine composed of inert material particles, subjected to blind forces. With a small number of physical laws, the history of the entire universe can be explained and predicted if we can manage to characterize it perfectly at a given instant. Laplace summarized this triumphant determinism in the following way: "If an intelligence was able to embrace in a single formula the movements of the largest bodies in the universe and those of the lightest atoms, for it, nothing would be uncertain, the future and the past would be equally present to its eyes." Time is, in a way, abolished. Newton thought that once the laws of physics are determined, all God has to do is to wind up the cosmic clock, then you let it go. He does not need to do anything more. The universe just evolve according to the laws set at the beginning. This sterile, rigid and dehumanizing determinism lasted until the end of the 19th century. In the 20th century it was swept away by the liberating vision of chaos theory and quantum physics.The role of chance, or what we could call contingency, was recognized in such diverse fields as cosmology, astrophysics and biology. Our world has also been molded by a succession of historical events. The most famous example of such a contingent event is the asteroid that hit the Earth 65 millions years ago, causing the disappearance of the dinosaurs, and thus giving our mammal ancestors (and us) the chance to proliferate. The French mathematician Henri Poincaré, one of the pioneers of chaos theory, replied as follows to Laplace's deterministic credo: "It can happen that small differences in the initial conditions create very large ones in the resulting phenomena. A tiny error in the initial state then leads to an enormous error in the final state. Prediction becomes impossible." Chaos isn't a lack of order, as is often believed. It has more to do with long-term unpredictability. Chaos is often illustrated by what physicists call the "butterfly effect": the flapping of a butterfly's wings in Rio de Janeiro, Brazil can trigger a rainstorm in Paris. It's our inability to know perfectly the initial conditions that makes it impossible to predict the future.

In the atomic and subatomic world, Heisenberg's uncertainty principle also says that the dream of knowing all the initial conditions with perfect precision is mere delusion. The uncertainty principle does in fact state that, given that any measurement implies an exchange of energy, it cannot be made in zero time. The shorter the time for the measurement, the more energy is needed. An instantaneous measurement would therefore require infinite energy, which is impossible.So the Laplacian dream of knowing all the initial conditions with perfect accuracy is mere delusion. We can't accurately pin down a particle's position and speed at the same time, and so can't trace its trajectory. Liberated from its deterministic straightjacket, nature can give free rein to its creativity. The laws of physics provide the universe with themes for variation and improvisation.

ZIERLER: Do you see chaos and harmony as artifacts of human perception? In other words, do chaos and harmony exist in and of themselves?

THUAN: No, I do not think that chaos and harmony are pure intellectual concepts invented by the human brain. I really do believe that nature behaves in that way. To me, "Nature plays jazz instead of a Bach cantata". In a Bach cantata, not a music note could be changed without making the whole musical structure collapse. The structure is rigid like the deterministic world of Newton. However, Chaos theory and Quantum mechanics have liberated Nature from its deterministic straight-jacket. It is now free to create and innovate, just like a jazz player who invents new sounds and patterns, embroidering around a basic main theme, but changing depending on the reaction of the audience and his own inspiration.

ZIERLER: Is chaos something that exists outside the boundaries of science? Is it simply not predictive?

THUAN: No, chaos theory does not lie outside the boundaries of science. It is a bona fide scientific theory which is non-predictive. A tiny change in the initial conditions can lead to exponentially divergent results. In scientific terms, chaos is not a lack of order, as in the general use of the word. It has more to do with long term unpredictability. For example, it is impossible to forecast the weather more than a week ahead, because weather events are extremely sensitive to initial environmental conditions. In order to predict long-term weather, we would need to know those initial conditions with an infinite precision, which is not possible. Chaos is at work all the time in our daily lives. We have all experienced occasions when apparently innocent events led to dramatic consequences. An alarm clock fails to go off, so a man misses his interview and the job he wanted. The car of a woman breaks down, so she misses her plane ans escapes death when it crashes into the ocean a few hours later. Chaos theory (along with quantum mechanics) presents an ineluctable limit to our knowledge, but it sits squarely in the domain of science.

The Path to Future Understanding

ZIERLER: Thinking about chaos and harmony, there are some things in physics and astronomy that seem to work perfectly, like the Standard Model of particle physics. No matter how hard we try, it always works. And there are some things that are seemingly forever mysterious to us, like dark energy, or dark matter, or how gravity truly works. Do you see things in those terms, harmony and chaos, things that can be harmonized and things that seem chaotic?

THUAN: I do not think that dark matter and dark energy will remain forever mysterious to us. For me, the reason why we don't yet know their nature is because we haven't worked on them hard enough. I think that eventually, given all the intensive observational and theoretical work that is currently being done, we'll unravel their nature.

ZIERLER: You're confident one day, there will be breakthroughs in things that are mysterious now, such as dark energy and dark matter?

THUAN: I believe so. I don't think they're insurmountable problems. Maybe we need a change in paradigm. May be we need another Einstein, somebody who can see nature with completely new eyes. The theories of quantum mechanics and relativity are now about one century old. General relativity was born in 1915 and quantum mechanics between 1905 and 1930 . May be the time has come to have a new theory. I do not despair.

ZIERLER: What was the intellectual transition to your book The Quantum and the Lotus?

THUAN: Buddhism is the spiritual tradition that nourished my adolescence. I'm also a scientist. As an astrophysicist studying the formation and evolution of galaxies, my work brings me constantly to confront notions of matter, space and time. I often asked myself, "What did Buddha think of these notions, and how do they compare to those of a scientist ?". If both systems of thought use logical tools to describe reality, then the two should meet somewhere. There would be something wrong if they did not intersect. However, because I was busy with research and teaching, I never had the time to explore these issues until my encounter in 1997 with the French Buddhist monk Matthieu Ricard at a meeting on "Science and Society" in Andorra. He was the ideal person with whom to discuss these sujects: he was a scientist himself (he got a doctorate in molecular biology at the Institut Pasteur under the supervision of the Nobel Prize winning biologist François Jacob), so I did not have to explain to him the scientific method. Furthermore, he knew well the Buddhist texts and philosophy having become a Buddhist monk and moved to Nepal where he has lived for more than half a century. Our dialogue has been mutually enriching and has resulted in the publication of the Quantum and the Lotus in 2001.

ZIERLER: Besides writing books, where and how else did you communicate with the public? Did you get involved in debates, did you do public lectures?

THUAN: Yes, I am often invited to deliver public lectures. I also participate in debates, but to a lesser extent. I also interact with the public in signing books at book fairs. I often give press interviews and I am occasionally invited to TV and radio shows.

ZIERLER: And these were always enjoyable to you, you liked interacting with the public?

THUAN: Yes, I enjoy immensely interacting with the public. This give me the occasion to have a direct contact with my readers. I can thus meet persons in wider circles, outside of the academic environment. I am greatly moved when some readers confide that a particular book of mine has helped them during difficult periods of their lives. I am touched when they thank me for celebrating by my writing the beauty and the harmony of the universe. Thanks to my books, I have met people from different walks of life -- academics, authors and artists that work in variuos fields such as history, literature, music, art, poetry, etc. -- people that I would have never met otherwise. I enjoy transmitting knowledge and I am moved when that's successful.

ZIERLER: Moving into the more recent era, what have been some of the advantages in the new tools in astronomy such as computation, more powerful telescopes? What are things that can be accomplished today that simply weren't possible or were much less efficient when you started your career?

THUAN: Because I am an observer, the building and launching of more powerful ground and space-based telescopes has had the most influence on my astronomical research. Since a large part of my work on BCDs is based on spectroscopy of faint objects, large-aperture telescopes are extremely helpful. I started my career in 1976 with 4-5 meter class optical telescopes and am now using 10-meter class telescopes such as the Large Binocular Telescope (the LBT has a size of 2 x 8.4 meter) in Arizona or the Very Large Telescope in Chile (the VLT has a size of 4 x 8.2 meter) . The advent of large space telescopes, such as Hubble (2.5 meter) and JWST (6.5 meter), have totally changed my scientific life. Because they orbit above the Earth's atmospheres, the celestial light they capture is not subject to blurring and absorbing effects. They can thus operate over the whole electromagnetic spectrum. I have thus used Hubble for UV spectroscopy, Spitzer for infrared spectroscopy and Chandra for X-ray imaging of BCDs. These observations at different light energy are complementary and give a more complete picture of the physical processes at work.

For me, astronomical advances are driven not so much by theoretical, but by observational work. For example, Einstein's General Relativity (GR) predicted a dynamic universe, but without the 1929 observation by Edwin Hubble of the expansion of the universe , nobody would have believed the results of GR by themselves. Even Einstein didn't believe in his own equations. He thought that the universe was static. And he rejected the work of Georges Lemaître who showed in his Ph.D thesis that the universe must have been expanding from a very small, hot and dense state which Lemaitre called a "primeval atom". To me, the driver of progress are observations. But maybe I'm biased. I think that way because I'm an observer. But theory without observation, I think, is just metaphysics. On the other hand, observations cannot stand alone by themselves. They need theory to be interpreted and modeled. Theory and observation must go hand in hand.

ZIERLER: What do you think it will take for us to gain a greater understanding of what happened between the Big Bang and 380,000 years after?

THUAN: At around year 380,000, the universe cools sufficiently for electrons to combine with nuclei to form neutral atoms. This period is known as ‘recombination era". After recombination, the universe becomes transparent. Photons decouple from matter and travel freely through space, giving birth to the cosmic microwave background radiation (CMB). To study the events between the Big Bang and the year 380,000, we need to continue to measure the tiny temperature fluctuations of the CMB. This wil help us understand the density fluctuations in the early universe, which eventually led to the formation of galaxies and large-scale structure. Detecting the B-mode polarization pattern in the CMB can provide evidence for primordial gravitational waves and help constrain models of inflation.

We should continue to study primordial nucleosynthesis, i.e. measure the abundance of light elements such as hydrogen, helium, deuterium and lithium to help test models of Big Bang nucleosynthesis. Detecting the 21 cm radiation from neutral hydrogen can provide a direct probe of the "Dark Ages" and the epoch of reionization, shedding light on the formation of the first stars and galaxies.

These observations require a combination of ground-based telescopes, space missions, and specialized instruments to cover the different wavelengths and phenomena associated with the early universe. Collaborations among international scientific communities and advanced technologies are essential to gather and analyze the data needed to understand the universe's evolution from the Big Bang to the formation of the first structures.

ZIERLER: Tell me about some of the ways you've been recognized for your work at the interface of science and communication. For example, UNESCO's Kalinga Prize and the Prix Mondial of the Cine del Duca.

THUAN: My work in the popularization of science is very important to me and I am deeply grateful that this work has been acknowledged by the international community. I was born Vietnamese, got educated in the French system, and learned all my science in the US, so I really feel I'm a citizen of the world. Thus I am deeply moved by any recognition I received from people and international organizations around the globe that are outside my own country, outside the place where I live and work. This is tremendous and fills me with joy and pride. It's a great encouragement for my efforts. I will cite a few examples of recognition. In 2009, I got awarded the UNESCO Kalinga Prize for "communicating science to society".With it came the award of a Kalinga teaching chair from the government of India that gave me the wonderful opportunity to visit Indian universities and research institutes. In 2012, I got awarded the Cino del Duca World prize for my work that "embodies a message of modern humanism". A previous recipient of this prize is writer Jorge Luis Borges which I deeply admire. I have already talked about my love for French culture and language. Thus I was very proud and grateful that the Académie Française awarded me the Grand Prix de la Francophonie in 2022. It is given to an author that contributes to the flourishing of the French language throughout the world. I was deeply moved when in 2013, President François Hollande awarded me France's highest decoration, the Légion d'honneur, for "the promotion of scientific culture and the transatlantic collaboration in the field of astrophysics".

ZIERLER: We'll move the conversation closer to the present. Was the pandemic a factor in the timing of your retirement?

THUAN: Yes, I really suffered through the Covid years. It was a tough period for me. I was stuck in Charlottesville. I could not see my colleagues and my students, except through Zoom sessions. I hated teaching remotely, preferring by far in-person contact. Fortunately, I could write, and that was my salvation. I worked on a book on the subject of exoplanets and extraterrestrial life.

ZIERLER: Do you maintain connections? Do you still have a home in Charlottesville?

THUAN: Yes, I do.

ZIERLER: But now, you spend most of your time in Paris?

THUAN: No, I go back and forth.

ZIERLER: It's nice to have that duality.

THUAN: Yes. And I want to keep that duality. That's what makes me distinctive from other people, I think.

ZIERLER: Today, do you keep up with the literature? Are you still doing research?

THUAN: Yes. I still keep up with the current scientific literature. It is easy to get access to the latest astronomical preprints because they are published on the Internet in the Astrophysics Data System, operated by the Smithsonian Astrophysical Observatory and funded by NASA. I still enjoy attending scientific seminars, either by Zoom or in person.Thinking about astronomy still provides me with much joy and excitement. I still enjoy intellectually the discoveries and advances that are being made, especially in the field of cosmology and the more restricted one of BCDs. In collaboration with colleagues, I still participate in obtaining new observational data, mainly by using the Hubble Space Telescope and the LBT (Large Binocular Telescope). I continue to write papers and still get a kick out of thinking of new problems. For me, retiring just means no more teaching. But research and writing go on, although at a slower pace. Doing astrophysics is not a job but a passion, and it will remain so until my last breath.

ZIERLER: What's next for you in your ongoing explorations of spirituality in science?

THUAN: The relationship between science and Buddhism is a vast field of investigation. I've barely scratched the surface. So I'll just keep reading more and thinking more about it. For the moment, I don't have any precise agenda, but I am certain there are a lot of things to learn and discover still.

The Gift of Great Mentors

ZIERLER: For the last part of our talk, I'd like to ask a few retrospective questions about your career, and then we can end looking to the future. Let's start with Caltech. What has stayed with you from Caltech, both as an undergraduate and as a postdoc? What has influenced your approach to science, the way you see the universe?

THUAN: First of all, I had fabulous and inspiring teachers at Caltech who were at the same time top-notch researchers. I am thinking of people like Richard Feynman. At Caltech the professors considered undergraduates like adults. They never looked down on you, and were always ready to help you further your scientific learning. And then, what most impressed me during my Caltech years is how the whole campus is geared toward research, toward helping scientists to discover new phenomena. The discovery of quasars by Maarten Schmidt at Mount Palomar had a great impact on me. I got to know him better while I was a post-doc.

As an undergraduate, I was astonished at how easy it was to get a research job on campus. The scholarship that Caltech gave me by no means covered all my expenses, so I had to find a job for supplementary income. At Caltech, the research is so varied that it was easy to go find a professor whose research interests aligned with yours and ask him for a job. I got my first summer job during my sophomore year, working in the laboratory of William Fowler, a Nobel-prize winning founder of the field of stellar nucleosynthesis. I had to use data from a particle accelerator to derive cross-sections of nuclear reactions in stars. That was nuclear physics. I moved to astrophysics in my junior year, I already mentioned thar Professor of Physics Gordon Garmire gave me a job to assist him in identifying the optical counterparts of celestial X-ray sources by using the Palomar 5-meter telescope. He took me to observe at Palomar, making me fall irremediably in love with astronomy.

ZIERLER: I wonder if you can reflect on the greatness of Lyman Spitzer, both for his scientific achievements and the kindness that he showed his students, including you.

THUAN: I miss Spitzer's presence. He is an outstanding person, not only from the point of view of his considerable scientific legacy, but also because of his great humanity. He made fundamental contributions to the field of the interstellar medium. He also contributed to plasma physics, laying the groundwork for the development of controlled nuclear fusion. He was a visionary advocate for the use of space-based telescopes and was instrumental in proposing the idea of what would eventually become the Hubble Space Telescope.

He was a wonderful thesis advisor, teaching me how to do good science with his marvelous physical intuition. I learned more from him during our weekly half-hour long meeting than in longer dialogues with others. I already talked about how he and his wife treated me and my fellow students with compassion and kindness.

ZIERLER: For all of the discovery that you've made, what has been most satisfying about your contributions to what we now understand about the universe?

THUAN: I think the intuition I had at the end of the 70s in thinking that metal-deficient star-forming Blue Compact Dwarf galaxies would be excellent proxies for primordial galaxies did turn out to be correct and most satisfying.

ZIERLER: What have been your major satisfactions in communicating these topics, these discoveries to the public?

THUAN: One major satisfaction is that, with my books, I have been able to communicate to the general public the wonder of the universe and its beauty and harmony. I have had letters from readers telling me how some of my books have brought solace to them in time of great distress. The realization of our interconnectedness with the vast universe was of great help to them when they went through bad patches.

ZIERLER: Do you think that you have made the topic of spirituality in science more accessible? Are there more scientists who are willing to explore these issues as a result of your work?

THUAN: Yes, I think that my books have made the topic of spirituality in science, especially Buddhist spirituality, more accessible to the general public. I have shown that despite very different means of investigation of Reality, reason in science and contemplation in Buddhism, there is a definite convergence between the two view points about the nature of the world. Some of my scientific colleagues find this convergence intriguing, but the majority of them still keep science and spirituality apart in their lives.

ZIERLER: Have you ever reestablished connections with Vietnam? Have you followed Vietnam's contributions in physics and astronomy?

THUAN: Yes, I have made several trips to Vietnam since its reunification in 1975. For a long while, I had cut off all connections with my native country because of what the Communist Vietnamese government did to my father. But in February 1993, totally unexpectedly, I got an invitation from French president François Mitterand to accompany him in his state visit to Vietnam, as part of the French delegation. I never knew exactly the precise reason for the invitation, but I presume that the French President, who is a very cultured man, had read and appreciated my book La Mélodie Secrete. I spoke to my father who, being a true Buddhist, did not harbor any hate or sentiment of revenge towards the Communist regime. He encouraged me to go back and see for myself, and so, it was with a great emotion that I set foot again in Hanoi, my native city, nearly forty years after leaving it. Since then I have gone back to Vietnam several times for scientific meetings, lectures at universities and book signing. All my books have been translated in Vietnamese and I have a large following in my native country.

During my trips, I have had occasions to speak to university leaders. I must say that I'm not very happy about the present situation in Vietnam in physics and astronomy in particular, and in higher education in general. Although there has been progress, things have not advanced as fast as I would wish, since the reunification of Vietnam some 50 years ago. I was hoping that the government would develop universities and higher education. I am convinced that Vietnamese students have the same brain power as their counterparts in Taiwan or South Korea, and given the right conditions, they can transform Vietnam into a "little tiger" like those countries. But changes have been slow because the government has not invested enough in higher education.

Another interesting example to consider is that of China. Although China has a communist political system, it went capitalistic for science. It poured money into higher education. That allowed it to send Chinese students to the best US universities to learn American science, build well-equipped laboratories equal to the most sophisticated ones in the West, boosting up salaries of professors to a level comparable to that of the best western universities, so much so that China has succeeded in attracting back some of the best Chinese scholars working in US universities. And the progress had been just amazing. The Chinese publication rate is exploding. China has now a space program. It wants the next man to return to the surface of the moon to be Chinese.

ZIERLER: Are you hopeful for the future, that things would change in Vietnam?

THUAN: All I can do is hope that the political system will evolve and that a future leader will be far-sighted enough to invest in higher education and research. Things will have to change one day because they cannot stay the same forever. Buddhism says that all things are impermanent. I mentioned the Chinese system of higher education as a possible example to follow. I hope that future Vietnamese leaders will find the courage to make reforms, and that Vietnam will one day find the right way to improve its educational system.

ZIERLER: A generational question. For young scientists today who are starting their research careers in astrophysics and astronomy, what do you see as the most important and exciting areas for discovery in the next decades?

THUAN: I may be biased , but I think cosmology will be one of the most important areas for discovery. We need to elucidate the nature of dark matter and dark energy which constitutes 99.5% of the content of the universe. Then we need to study the era of the formation of the first stars and galaxies, and the era of re-ionization of the universe. All that is still not well known. Then comes the field of planet formation and evolution. Great strides have been made in that area as well. And then there is the one-million dollar question: will life and consciousness emerge on those exoplanets? Will E.T contact us one day?

ZIERLER: On that basis, if humanity ever discovers life beyond ourselves, what do you think the spiritual impact of that will be on our collective consciousness?

THUAN: There will be a huge impact on the collective consciousness of humanity, as for the first time, we will know for sure that we are not alone in the universe, that the universe is bio-friendly and that it is probably teeming with life. The realization that we are not alone might prompt a reassessment of human importance. This could lead to a humbler understanding of our place in the cosmos and foster a greater sense of unity among humanity. The discovery could also spark a renewed sense of wonder and curiosity about the universe. Humanity might explore deeper existential questions about the purpose of life, the nature of consciousness, and the possibility of a shared spiritual destiny with extraterrestrial beings.

E.T. will probably be much more advanced than us in the areas of science and technology. He is likely to have already answers to such questions as: what is the nature of dark matter and dark energy? What is the origin of life and consciousness? He can certainly teach us a lot about advanced scientific questions. Hopefully, E.T. would be very wise as well. But I am not so sure. The problem is that progress in science does not necessarily imply progress in wisdom.

ZIERLER: Sadly, not. We'd be a lot wiser if it did. [Laugh] Finally, one last question, looking to the future, what do you hope to discover? What's attainable in your lifetime that you hope to understand then what you don't understand now?

THUAN: I think that understanding the nature of dark matter and dark energy would be wonderful. I think that those are the keys to the problem of the evolution of the universe.

ZIERLER: And if we understand the evolution of the universe, what will that mean then?

THUAN: I think that we'll never understand all the secrets of the universe. We can approach the truth asymptotieally, but never quite reach it. The melody will always remain secret. But even if you can understand everything scientifically, you will still need to learn how to be compassionate and wise. Spirituality is the necessary companion of knowledge.

ZIERLER: I want to thank you so much for spending this time with me.

THUAN: Thank you for interviewing me.

[END]