David Tong is a professor of theoretical physics at DAMTP in Cambridge, a fellow of Trinity College, Cambridge, and joint recipient of the 2008 Adams Prize. He was a postdoc at the MIT Center for Theoretical Physics. He was an Adjunct Professor at the Tata Institute of Fundamental Research (TIFR). He is currently also a Simons Investigator. His main research interest is in Quantum Field Theory, while also studying aspects of Quantum Field Theory related to String Theory, Supersymmetry, Solitons, Geometry, Cosmology, and Condensed matter physics. His most-cited paper, “DBI in the sky”, provides a possible observational test of one mechanism for inflation in the very early universe. He is also well known amongst the students for his very enthusiastic lecturing and comprehensive lecture notes for courses he has taught at the University of Cambridge (most notably the ones on quantum field theory). On 26th February 2020, Prof. David was invited by the Physics and Astronomy Club to deliver a webinar lecture. The topic of the lecture was the Renormalisation Group. Watch Out! had the chance to inquire into his thoughts about theoretical physics and our attempts to make sense of the structured chaos of our universe.

WO: How did you get interested in physics when you were young? And after so many years of being involved in theoretical physics, what still excites you?

Prof. David Tong: I got a copy of Stephen Hawking’s “A Brief History of Time” for my 17th birthday, and it opened my eyes. I didn’t know anything about quantum mechanics or black holes or particle physics before reading that book. It really lit a spark in me. I also didn’t know that there was a career called “theoretical physicist” where you get to sit and think about these things for a living.

I’m very easy to excite when it comes to physics. It’s an infinitely rich playground and it’s very hard to be bored. At the moment I’m thinking about topological insulators and what they may have to say about particle physics.

WO: How can an undergraduate student start with research in an area like theoretical physics?

Prof. David Tong: It can be hard to do undergraduate research in some areas of theoretical physics, especially quantum field theory. If you do get the chance to do some research as an undergraduate, the exact area doesn’t matter so much. Just enjoy the chance to solve some problems.

WO: How do you feel about the research environment in India and how it differs from the international domain? What can be done to improve it?

Prof. David Tong: India has some of the best theoretical physics groups on the planet. The country has a wonderfully strong education system, and many talented physicists who could have taken positions in prestigious universities abroad but have decided to return home. That retention of your best scientists is really the key to generating the research environment.

One way that it differs from most countries is the large number of research institutions, where there is limited contact with undergraduates. I don’t know if that’s good or bad, but personally I find that I learn a great deal from teaching undergraduates, and I’d certainly miss that in a research institute.

WO: We’ve heard some people say that theoretical physics is in a state of slowdown. What do you think about this?

Prof. David Tong: I suspect that these people either aren’t paying attention, or have a very limited view of what theoretical physics entails. If you have a small attention span and want a revolution like quantum mechanics every few years, then research in theoretical physics isn’t for you. You should probably just write a blog or something.

But if you’re interested in the universe around us, then there’s so many exciting things going on, from breakthroughs in experiment like gravitational waves and the Higgs boson, to breakthroughs in theory like quantum computing and topological insulators, to mysteries that we don’t understand like dark energy and dark matter. It’s very hard to see how anyone can view this as a slow time.

WO: With the rise of the internet and video lectures on YouTube, a lot of educational resources are now more accessible to the general public than ever. How do you feel about that? Do you think that this has sabotaged the learning experience for students and the importance of a teacher for a student?

Prof. David Tong: The more of this stuff, the better. Physics is hard. Really hard. Anything that can help students learn has got to be good.

And there’s still a major role to be played by universities and the traditional lecturing style. Despite all the videos and materials available online, the quickest way for me to learn something new is still to sit in front of someone smart and have them explain it clearly.

WO: What are the differences between how one does theoretical physics and experimental physics in terms of the mindset required to work in it?

Prof. David Tong: Since I barely know one end of a screwdriver from another, I’m not the right person to ask about experimental physics. Does it even involve screwdrivers?

WO: How would you explain QFT to a curious 5-year-old?

Prof. David Tong: I wouldn’t start with the path integral. That would be way too hard for a 5 year old. So I guess I’d start with the Lagrangian, then move to the Hamiltonian and just go over the usual canonical commutation relations with them.

WO: What are your views on people who believe that the earth is flat and that birds are just drones spread around by the government to spy on us?

Prof. David Tong: I’ll let you guess what my views are. Here’s a hint: your first guess is probably the right one.

WO: Have you ever read a book and were captivated by a particular quote or a quirky footnote that has just stuck with you?

Prof. David Tong: Here’s a story. A few months ago I was reading a book on general relativity that included the following quote from Einstein:

“You know, when you start calculating you just shit yourself”.

For obvious reasons, that kind of stuck with me. But I didn’t understand it, so I tried to track it down. The book I read cited another biography of Einstein which, in turn, said it was from in a letter Einstein wrote to the physicist Otto Stern. That book was translated from German. I don’t speak German but thought it might be interesting to get the original. When I eventually found it, it said:

“Wissen Sie, wenn man zu rechnun anfängt, b’scheisst man unwillkürlich.”

“Scheisst” is shit yourself. But “b’scheisst” is cheat yourself. So this is my great contribution to the history of science: Einstein didn’t shit himself. But he did cheat himself.

WO: A lot of undergraduates tend to sway away from research since it’s a path which requires a lot of struggle which often amounts to nothing. People also give up on careers in research for monetary reasons. What advice do you have for undergraduates in this regard?

Prof. David Tong: There’s so many rewarding careers out there and many reasons to look at them. If you want money, you should do something other than research. If you want a career without a lot of struggle then you should definitely do something other than research.

But if you think physics is beautiful, are desperate to understand more, and get a real thrill when you understand something, even something small, then research might be the right path.