Searching for a theory of quantum gravity

The array of research fields ‘is ideally suited to quantum gravity,’ says Bianca Dittrich, the latest hire at the Perimeter Institute for Theoretical Physics in Waterloo, Ontario. ‘Apart from having excellent conditions for research,’ she says, PI ‘is very scientifically broad. There is quantum foundations, quantum gravity, superstring theory, particle physics, and condensed matter physics. This is important for quantum gravity because, after all, we don’t have a finished theory yet.’

The idea for PI was hatched in 1999 by Mike Lazaridis, founder and co-CEO of Research in Motion, maker of the BlackBerry, with the goals of doing world-class research, training young scientists, and bringing science to the public. Now about 100 researchers strong, this month PI is celebrating its expansion with a series of public lectures and events . A new building, named for Stephen Hawking, will more than double the institute’s capacity, and the aim is to grow into the largest center of theoretical physics anywhere.

Dittrich is no stranger to PI. She spent three years there as a postdoc after earning her PhD in 2005 at the University of Potsdam in Germany. She then returned to Potsdam to lead a research group at the Albert Einstein Institute at the Max Planck Institute for Gravitational Physics. She holds a five-year position intended to kick-start her career. Some of her graduate students and postdocs will join her at PI, when she moves there next year for a tenure track post. PHYSICS TODAY‘s Toni Feder caught up with her by phone in August, when she was briefly—between conferences—in Potsdam.

PT: How did you happen to go into physics?

DITTRICH: I was always quite good at science. I first chose chemistry, but then I saw that everything was explained by physics. I also read a couple of popular books about different things connected to physics. One of them on complexity theory, James Gleick’s Chaos: Making a New Science. There was also one on particle physics.

PT: Did you know from the outset you would do theoretical physics?

DITTRICH: I don’t think I had a concept that there was a division between experimental and theoretical physics. But I knew quite fast that I probably would not be doing experiments. One reason is that I have two left hands. Practical issues are not my strong point.

PT: Describe your area of research.

DITTRICH: We know that gravity describes our universe at large scales in a very good manner. At small scales, we have quantum theory which governs all matter interactions. However, both theories are inconsistent in their own way. The question is to have a unified theory, usually called quantum gravity, from which both classical gravity and particle physics emerge in some limits.

Quantum gravity has a long history. After he came up with general relativity, even Einstein tried to unify gravity with quantum theory, but he didn’t have a complete picture of quantum field theory. That came only later.

I am in the tradition that starts on the gravity side. If you want to widen the quantum theory of gravity, you need to understand the quantization of space-time. The main question I am concerned with is how to get back to large scales. Usually in physics it’s difficult to have theories which are valid over many length scales. The Planck scale is very far away from particle physics scales or even longer scales, where space-time appears smooth and classical. An analogy would be to derive the properties of water from atomic physics.

PT: Is there something about your work that keeps you up nights?

DITTRICH: It would always be better to sleep than to think at night—and then to wake up with a good idea in the morning. The large-scale limit is my main thing at the moment. Then it boils down to many more technical questions. And a related question is, What are the fundamental symmetries of the quantum theory of gravity?

PT: Can you comment on the kinds of experiments that test quantum gravity theories?

DITTRICH: I think the most advanced experiments involve looking at gamma-ray bursts. These things could test the granularity of space-time. If light at different frequencies, which all started at the same time and same place—the same GRB event—were to arrive here at slightly different times, it would hint at which theories to exclude or which would be consistent. If the speed of light is found to be frequency dependent, it would be quite extraordinary. We have been waiting for experimental signatures for a very long time.

PT: What attracted you to the Perimeter Institute?

DITTRICH: Quantum theory, gravity, condensed matter and statistical physics are all very strongly represented at PI. All kinds of questions that come up in quantum gravity are covered there.

PT: How do you feel about moving to Canada from Germany? What do you look forward to? What will you miss?

DITTRICH: Doing quantum gravity, it’s usually difficult to find a position. So it was clear it was not likely to be in Germany. I will miss Berlin. But the PI is a very energetic place, with lots of exchanges going on between researchers. And to begin with, I will be cross-appointed at both institutes.

PT: There are not many women in theoretical physics. Any thoughts?

DITTRICH: I am not sure if theoretical physics is much different from experimental physics. Discrimination is subtle. So far, I have had a quite successful career. Sometimes, you know that other people question the reason for that. Success in science sometimes requires, on an unscientific level, kinds of behavior that women are not trained for. For example, you have to be very self-confident. Sometimes in discussions you have to present yourself, defend yourself, and attack others.

PT: Do you do that?

DITTRICH: Yes, I give talks, so I do.

There are management courses for women, things like that. But it would be good to have men made more aware of these differences in men’s and women’s behavior. And I don’t think we should all get more aggressive.

PT: What are your interests outside of physics?

DITTRICH: At some point, I did lots of reading. Generally I was interested in history. But running a research group doesn’t give me lots of time. I play tennis.