Wednesday, September 23, 2015

Interview with Dr. Xi Yin

                                          By Fiona Rawsontile, Sept 2015

This interview was inspired by an earlier interview of Dr. Yin I saw on the Internet, which made me think that we can’t expect someone who normally writes for entertainment to understand a physicist. To “provoke” a scientist, we need another scientist. So I volunteered (to myself) and sent an invitation to Dr. Yin, who was recently promoted to Professor in Physics at Harvard University at the age of 31. It is a great pleasure for me to share with you his experience and wisdom. 


Fiona: As a faculty member at Harvard, you must have met many outstanding students and researchers. When you recruit students into your lab, what personal traits or qualification do you particularly look for? What type of students is an absolute NO? Can you talk about your mentoring style?

Dr. Yin: Indeed, I have interacted with numerous spectacular talents at Harvard: students, postdoctoral fellows, faculties. It’s one of the best things about being at Harvard.

I should clarify a few things. First of all, I’m a theoretical physicist and I work with pens, papers, chalks, and computers. Sometimes I work with the Harvard supercomputing cluster, but I do not have a lab, nor would I need one. 

Fiona commented: You reminded me of a joke I read. “I’m a mathematician. All I need is paper and a trash basket.” “Too bad you are not a philosopher; then you wouldn’t need the basket.”

Dr Yin: Second of all, unlike many other institutes, in the physics department at Harvard each faculty does not recruit students directly from colleges. Each year an admission committee consisting of around ten professors handle the applicants to our PhD program, with a loose quota in each subfield. The admitted students will choose their thesis advisors and/or labs later on. Typically, experimental physicists require more students to run their labs. We theorists do not depend on or need students as much. Our students are more independent in their research. While I’m perfectly happy to collaborate with my students, they are also free to work on their own projects and publish on their own, if they wish to.

Initially, students that are interested in working with me will come to talk to me, often towards the end of their first year or during their second year in graduate school, and ask for research problems/projects. I would hand them a number of papers to study, and ask them to come back and report on what they have learned. If they make progress, we will discuss and they read more, and at some point we arrive at a concrete and interesting problem to develop and work on. In a way this is also a selection process. About half of the students who talk to me will not end up working with me. I rarely turn away a student. The students would turn away on their own if they do not find it productive to be working with me.

Fiona commented: Well, your model sounds like a classic Socrates-type mentorship. I run a biomedical lab, and have to spend a lot of energy in money raising and management, including motivating or firing employees.

Dr. Yin: I do not require any specific quality of a student, as long as they have the basic integrity and are capable of getting the job done. One type of students I definitely do NOT work with are those who pretend to understand what they don’t know. It’s perfectly okay to be ignorant about a subject, but it’s not okay to pretend that you know something and you really don’t.

I think in reality my selection process puts fairly high demands on the students. All the students I ended up working with have been terrific. I work quite closely with my students, we discuss our work nearly daily, and I try to keep all of my students informed on all topics I’m interested in, even if they are not actively working on it. I believe the students should not confine their knowledge into a tiny area of their own research, but rather they should learn broadly and keep their eyes open on all subjects of interest.

Fiona: The next question is related to your view of the scientific environment in China. What do you think is the critical advantage researchers have in top Chinese institutions, such as funding opportunities, student availability/quality, or the tenure system? On the other hand, what is the biggest issue that hinders the progress of their research or prevents them from being recognized internationally?

Dr. Yin: I’m not as familiar with the scientific environment in China as I should. I believe there is plenty of funding opportunities at top institutes in China for fundamental research. A key disadvantage, however, is that it is difficult for a top institute in China to attract high quality foreign researchers, especially postdoctoral fellows who contribute in an essential way to the research in my field. It’s no secret that many of the top talents in Chinese colleges apply to graduate schools in the United States, leaving a somewhat weak talent pool for graduate schools in China. I am not familiar with the tenure system in China. 

I have observed that there is a huge disparity between faculty members of different levels in China, in terms of privilege as well as salary. I believe this is deeply unhealthy and hinders the scientific progress tremendously. I strongly favor the Israeli academic system (which in my opinion is better than that of the US), where all faculties have a flat base salary, with bonus each year based on their productivity and the quality of their works. Such a system stimulates a collaborative atmosphere and curbs unhealthy competitions. Professors should never have to worry about their salaries and administrative duties. They should be able to focus entirely on research and teaching.

Fiona: A flat base salary model may be difficult to implement in the US because of the huge variation of living costs in different areas of the country, but hopefully there are other ways of promoting equity. Anyway, as a physicist, what would you say is the most important personality for your success, such as curiosity, vision, imagination, or persistence? (You don’t have to choose from the list.) Do you have a life-long goal you’d like to achieve, such as solving a long-existing problem, establishing a new area, or educating the public? 

Dr. Yin: I would say the one personality of mine that benefits my scientific work is that tendency of being obsessive. When I’m onto a problem, I can skip meals and sleep and work tireless until I’m satisfied. I would have a hard time going to sleep, say, when I know that there is a mistake in my work and I haven’t been able to identify it.

Fiona: Obsessive. You reminded me of what Oscar Wilde said, “Moderation is a fatal thing. Nothing succeeds like excess.”

Dr. Yin: I do have a few long standing problems in my mind, that I contemplate from time to time. However, my experience in research is that, more often than not, ground breaking work originates from attempts of patching up a tiny hole. I spend most of my time patching up tiny gaps in our knowledge here and there. It often happens that progress in these little problems leads to major breakthroughs.

My style of research is probably more known for problem solving than for establishing a new area of research. I wouldn’t mind doing the latter, but I don’t seem to be particularly good at it. What I really like to do is to solve a problem that many people have looked at and thought about but could not solve. If I happen to invent a new method and open up a new direction of research while doing so, it would be icing on the cake.

Fiona: So you like String Theory. I’m a biomedical engineer. Could you give me a reason why I should care about it? About gravity, Einstein said it’s space-time curvature; quantum mechanics says it’s executed by gravitons (correct me if I’m wrong). How do you reconcile the two interpretations?

Dr. Yin: Don’t you want to know what is the tiniest, most fundamental building block of our universe? What lies beyond the standard model of particle physics? Are quarks and leptons truly fundamental particles or do they have internal structures?

One may say that such questions are endless and you could always try to divide things up further and it never ends. While this could have been the case in the world of particle physics without gravity, it cannot be so in the world of quantum AND gravity. Here is why. In the quantum world, it takes energy to probe short distances, or “divide stuff into small pieces”. The energy it takes is inversely proportional to the size. Roughly speaking, the energy it takes to probe a certain tiny distance scale is of order the Planck constant times the speed of light, divided by the distance scale of question. This is why to make tiny new particles we need huge particle accelerators, like the Large Hadron Collider. That is all fine, but in a world with gravity, a large amount of energy cannot stay confined in a tiny space. This is because energy is mass (according to the famous equation E=mc^2 you see on T-shirts), and mass gravitates, and when there is enough mass in a small volume of space, it makes a black hole. To the outside observer, a black hole is as big as its horizon, and the horizon size grows with the mass (or energy) of the black hole. Now you see, gravity does not want you to probe arbitrarily short distances. You might take this as a hint that perhaps there is a fundamental size after all, beyond which there is no structure. Well, to understand all of this, you need string theory. :)

Regarding your question of reconciling the classical description of gravity as spacetime curvature and the quantum nature of gravitons, it does not touch the essence of quantum gravity and is in fact well understood within the framework of quantum field theory. The question is not different in any essential way from reconciling the description of electromagnetic fields and photons. The answer is that the notion of “field” (or curvature of spacetime, in the context of gravity) may be viewed as a classical approximation of a certain type of quantum states, known as coherent states, that are superpositions of states involving many photons (or gravitons) in such a way that they exhibit semi-classical behavior.

Fiona: I still can’t perceive gravity in the same way of an electromagnetic field and photons. For example, if, for any bizarre reason, a heavy item is suddenly “born” at a particular point of space, the rest of the universe would not instantly know its existence, because gravitons have to travel at speed of light? That is, we cannot be “attracted” by something before we can “see” it?

Dr. Yin: Firstly, one has to be careful in asking hypothetical questions in physics, because the laws of physics do not leave much room for modifications. One could easily arrive at inconsistent and contradictory conclusions based on faulty assumptions. The conservation of energy and momentum is essential for a consistent theory of gravity, just as the conservation of charge is essential in electromagnetism.

It is true that, if the sun explodes for some reason, we would only perceive the resulting gravitational disturbance eight minutes later, the same amount of time it takes for light to travel the distance from the sun to the earth. In this perspective, gravity is not all that different from electromagnetism, if you substitute charge with mass.

One thing that makes gravity different, however, is that Einstein’s equations of gravity are nonlinear, whereas Maxwell’s equations of electromagnetism are linear. This nonlinearity makes the equations of gravity a whole lot more complicated. However, the nonlinearity of Einstein’s equations is important only in the presence of strong gravitational fields, and allows for all sorts of bizarre phenomena such as black holes.

Your question though is really about classical gravity, which to the first order approximation was understood by Einstein in 1915 (even though it took decades to verify experimentally various aspects of his theory). The questions string theorists are tackling are really about quantum gravity, and effects of quantum gravity are expected to be important only in the presence of extremely high energy and/or extremely strong gravitational fields.

Fiona: In our field, theorists often collaborate with experimenters. Does your study depend on such collaborations? Have you proposed any hypothesis that you really want to verify experimentally but are unable to do so due to technological infeasibilities?

Dr. Yin: The short answer is no, and we make up for the lack of experiments with mathematical rigor. 

Physics is the most mature among all subjects of natural science. We have come to understand the theory so well, to the point that the very logical and mathematical consistency of the theory itself leaves little room for adjustments. We are not talking about models of economics where you can adjust parameters here and there to fit experiments. The laws of physics are supposed to be absolute. If there is any small violation of the laws of physics by any experiment, the entire foundation of modern physics could be shattered and we would have to rethink everything. This has happened a few times in history, most notably the black body radiation and the constant speed of light, which shattered Newtonian physics and paved the way to quantum mechanics and relativity. 

Now just because we know the principles doesn’t mean we know what theory is exactly. For instance, the theory of quantum electrodynamics is based simply on the principle of quantum mechanics and relativity, but it took decades and works of thousands of brilliant physicists to understand how to calculate and make experimental predictions with this theory. Eventually, the theory was proven to be successful, perhaps more successful than any other theory in the history of mankind. For instance it successfully predicted the anomalous magnetic moment of the electron to eleven digits. As a layman’s analogy, that is better than predicting the exact number of human population on the earth, to the accuracy of a single person.

In modern theoretical physics, we don’t simply fit models with data or come up with new hypothesis. We try to understand what the theory is based on its own mathematical consistency, and its compatibility with basic principles that we believe to hold absolutely. We are driven not by the need to explain a certain piece of experimental data, but rather questions like “what is the cross section of graviton scattering at Planck energy?” and “what is the state of a black hole at the end of Hawking evaporation?” There are a number of deep theoretical puzzles that drive us to advance our understanding of the theory itself.

That is not to say we know for sure that string theory is correct. We would like to understand how quantum gravity works, and string theory is the only theory known to mankind that works, and it works beautifully. Over the last two decades we have learned that the mathematical structure of string theory is inevitable in the study of quantum field theories, and quantum field theory is our establish framework that explains all phenomena of particle physics to date. 

I personally think we don’t understand string theory well enough yet to even attempt a direct comparison with experiments in particle physics and astrophysics. On the other hand we are learning tremendous fundamental physics by studying string theory, and it gave us deep insights into other fields as well, such as nuclear physics, condensed matter physics, and even fluid dynamics.

That being said, I am interested (as a side project) in aspects of fluid dynamics that involves turbulence, and the possibility of applying quantum field theory to understand the universality of turbulence. In this case experimental data would be helpful, but really what we need is computer simulations (which could be called experiments by the theoretical physicists’ standard).

Fiona: Truth is objective, but the pursuing of it, which we call scientific activity, is imprinted with human characters. Is there someone who had a significant influence on your professional life, without whom you might have become a different individual?

Dr. Yin: There are a few people that have made significant impact on my research career. One of them is my PhD advisor Andy Strominger. He has the inimitable skill of reaching deep conclusions with the simplest possible calculation, and he has a terrific taste in telling the good physics from the bad ones. 

Another person that shaped my approach to research is Davide Gaiotto, with whom I had collaborated extensively while he was a postdoctoral fellow at Harvard. He is now a faculty at Perimeter Institute and one of the stars of my field. One thing I learned from him is that, when you don’t know where to start in trying to solve a problem, don’t look around. Go to the blackboard, start writing down equations. You are probably wrong initially, but little by little you will correct them, until when things click.

Fiona: The academic system in the U.S. is generally reasonable and functioning, albeit not perfect. What aspects would you like to see change, including, but not limited to, job recruitment, peer reviewing, tenure, etc.?

Dr. Yin: The academic system in the US is quite tough on the young people, partially due to the publish-or-perish culture. Junior researchers are often forced to work on topics in which they can be productive in terms of publications, and are discouraged from taking risks on truly original and unexplored research directions.

In my field there are extremely few faculty jobs compared to the number of PhDs awarded each year. (Fiona sighed. “Dare I say this is the case for a lot of majors now!”) Many tremendously talented physicists spend years working as postdocs, which is quite difficult for those who have families especially kids, due to the constant need for relocations. And still, in the end most of them are forced to leave academia due to the lack of faculty position openings.

I would say the peer reviewing system in my field is acceptable but very, very far from perfect. In certain fields such as mathematics, research papers are put to great scrutiny before publication. Proofs are checked line by line. This is possible only when people don’t write many papers. Physicists tend to write a lot more papers than mathematicians. It is difficult and impractical for every physics paper to be inspected and verified line by line before its publication. The peer review system does little more than filtering out crackpots. In my field, the quality of a research paper is not judged based on the journal on which it is published, but rather through a reputation that is built based on seminars, private discussions, and follow-up works. Most of the time, the truly important and original papers do become known to the community, and that’s what matters in the end.

I think I’ve made a number of complaints here, with no immediate solutions to offer. I enjoy working in the field of string theory, particularly because in our community people are open with sharing ideas (often before publication) and most of us value the progress in our field more than the assignment of credits on a piece of publication.

Fiona: Has your Chinese background (cultural, educational) influenced your career, in either positive or negative ways? Do you have particular advice for Asian scientists who strive in the Western academia?

Dr. Yin: I think my ethnic background has had zero influence on my career. I feel completely comfortable living and working in the US, as well as during my extended visits to India, Israel, and Japan. I never feel tied to a certain place. I think Asian students tend to focus too much on course work and do not spend enough time socializing and live a balanced life. I’m an introvert myself, but I can be social when I need to be. I see many successful Asian scientists in the US, and I don't think they need any advice from me. Go vote, that’s my only advice to Asians in this country.:)

Fiona: All right. Thank you so much, Dr. Yin, for taking the time to offer us your invaluable insights. Is there anything else you would like to share with us that hasn’t been covered by the above topics?

Dr. Yin: The other day I receive an email which was practically a dating/marriage proposal. While flattered, I would like to clarify that I have been happily married for 10 years and my daughter is 7 years old and she loves AC/DC. I enjoy rock climbing so if anyone goes to Rumney, NH I’d gladly join and partner up.

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Comments highly appreciated! - Fiona