MIT's Lisa Randall: Two Branes are Better Than One

MIT's Lisa Randall: Two Branes are Better Than One

s theoretical physicists grow ever more enamored of the mathematical beauty of string theory, they have left the world of experimental physics increasingly far behind. Until recently, that is. In the past two years, physicists have raised the possibility that at least one of the extra dimensions postulated by string theory could be large enough to have experimental implications. This counter-intuitive proposition might solve some of the thornier theoretical problems of string theory, while providing experimental physicists with some testable predictions. June 2002: read an interview with Lisa Randall discussing the Special Topic of Brane in ESI Special Topics.

Lisa Randall Gray

"It took a while for people to be convinced of the significance, or even to believe infinite-dimensional theory," says Lisa Randall of MIT. "It was actually quite a radical idea that you don’t have to compactify space."

The particular theory that the New York Times recently described as "currently causing all the intellectual commotion" was proposed by Massachusetts Institute of Technology professor Lisa Randall and Raman Sundrum, then a postdoc at Boston University and now a professor at Johns Hopkins. Their relevant paper in Physical Review Letters, "Large mass hierarchy from a small extra dimension," (see paper #1 in the table on page 3) has now been cited in more than 300 ISI-indexed papers (with many more electronic citations recorded online by the Stanford Public Information Retrieval System, or SPIRES). In fact, the paper's only competition for the top spot in Science Watch's Physics Top Ten recently has been from Randall and Sundrum themselves: their other 1999 Physical Review Letters paper, "An alternative to compactification" (page 3, paper #2), has also topped 300 citations. The duo, in other words, now accounts for two of the hottest papers in physics—as can be seen from the top of the "What's Hot in Physics" table on page 6.

Randall, 38, earned her bachelor’s degree in physics in 1983 and her doctorate in 1987, both from Harvard University. After spending three years as a postdoc at the University of California, Berkeley, and one year in a prestigious junior fellowship at Harvard, she joined the physics department at MIT in 1991 and became a full professor in 1998. That same year she also became professor of physics at Princeton. She has now returned to MIT.

Professor Randall spoke with Science Watch correspondent Gary Taubes.

How did you get into the large-dimension work?

Randall: I’ve only been doing it a couple of years. I had a long stretch where I worked on supersymmetry, because I’m concerned with physics at the electro-weak scale—the energy scale of the massive W and Z gauge bosons—and supersymmetry might be relevant to what’s happening there. But there were some frustrating features of supersymmetry breaking at the electro-weak scale. Working with Raman Sundrum, I recognized that having an extra dimension could actually be quite relevant in addressing some of the complicated aspects of supersymmetry breaking. People had been independently thinking about the topic of extra dimensions, motivated in part by string theory, which predicts the existence of extra dimensions. That led into studies of extra-dimensional geometry, and it took on a life of its own.

So what do we call this theory?

Randall: Well, right now it’s actually called Randall-Sundrum theory. Although it’s also called warped geometry in extra dimensions.

There are several different theories floating around that evoke large extra dimensions. Could you explain what they are and what makes Randall-Sundrum theory special?

Randall: I’ll start by telling you about the theories that aren’t ours. In the simplest type of theory there are extra dimensions, but the extra dimensions are finite in size. Why finite? Well, at long-distance scales we see strong evidence for the existence of only four dimensions: three of space and one of time. One way of explaining the fact that we only see four dimensions is that other dimensions exist but they’re so tiny that we just don’t feel any effect from them. That was the idea in string theory—that these extra dimensions get compactified on some consistent manifold, and the manifold is very tiny. A lot of people still probably believe that.

How does your theory differ?

Randall: One of our ideas is that you don’t actually have to compactify the extra dimensions. In other words, instead of saying gravity is forbidden to go beyond a certain length in the extra dimension, suppose instead there was a strong force—I’ll explain why there should be in a moment—such that gravity was sort of attracted to a specific location in space-time. In principle, it could wander far out in the extra dimension, but this external force centers gravity in one place and it looks effectively four dimensional.
What’s interesting is that this precise theory involves something called a brane, which is a lower-dimensional subspace of the higher-dimensional space. Brane is short for "membrane." So, for example, in our theories we imagine that the brane is only four dimensional, even though it lives in this five-dimensional space. There are three space dimensions and one time dimension that we see and one extra dimension that we don’t. We’re not very sensitive to it. Because the brane itself carries energy, it basically applies an attraction acting on gravity so that gravity stays very localized near the brane. Even though there is an infinite extra dimension, gravity extends so little into that dimension that it is almost as if space were compactified and you only see four dimensions. This is really very different from the compactified theory, however, because it says you can consistently have an infinite extra dimension, but still see gravity as only four dimensional.

So what exactly does this extra dimension give you?

Randall: According to what I've said so far, it's not giving us anything. It's simply telling us that we don't necessarily have to compactify and we can still have a consistent theory as far as gravity goes. However, there are problems associated with conventional compactification. When you have these compactified scenarios, you don't know what size or shape those manifolds should be. You have too many parameters. It means you have lots of possible theories. If you could avoid all that it would be great. We think we're on the road to accomplishing that.

But the idea is eventually to come up with a testable theory? We don’t seem to be there yet.

Randall: Okay. Now I’ll tell you what happens if you have a second brane in the theory. One of the very interesting things about this theory, called warped geometry, is that they have something called a warp factor.
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June 2002: read an interview with Lisa Randall discussing the Special Topic of Brane in ESI Special Topics.