I have to discuss different possibilities. One is that the universe we live in is the Planck brane, which localizes the graviton. Another is that we live on a second brane, which is the case where we discussed the hierarchy problem. We generally assume that we live on a brane, but it may not be the brane on which gravity is concentrated. Suppose that gravity is highly concentrated near what I'll call the Planck brane. So gravity is concentrated on one brane, the Planck brane, and we live on a second brane, not precisely on top of the first brane but a little apart. Gravity on our second brane would appear to be weak. And that's precisely what we wanted to explain: why gravity appears to be so weak. That's the hierarchy problem—why gravity is so weak. And this follows from the key insight that we don't actually have to talk about how to get a huge mass scale; we can talk about why gravity is so weak.

  So why has this paper generated so much interest and so many citations?

Well, it went in the face of what everyone who has studied gravity has believed. We always thought if we have extra dimensions, we had to do what's called compactifying them, which is to say they curl up so that they can't be seen from our point of view. If you look at distance scales larger than the size of this curled-up dimension, the physics would reduce to being four-dimensional again. In the standard "conventional" scenario, if you have extra dimensions, they are curled up very small. The reasoning is as follows: if you probe distance scales smaller than the extra dimension, you would see higher dimensional physics, for example, one over r-cubed as the gravitational force scale. However, if you look at distance scales bigger than these extra dimensions, it would look like four-dimensional physics. You can understand it in a very intuitive way: if you look at a garden hose, for instance, far away it looks like a one-dimensional line. Only up close do you realize it's actually a cylinder. So if you look from far enough away—that is, only look on large distance scales—you wouldn’t know these other dimensions were there. So we've known this compactification could work. You could have the extra dimensions of string theory, but you curl them up and you wouldn't see them. It wasn't thought to be possible to have a theory with extra dimensions that wouldn't compactify and still have the physics reduce to four-dimensional physics here. So our theory, that you could have an infinite extra dimension that wasn't compactified, was a radical departure from conventional wisdom.

  What was the first reaction from the community?

I guess it's fair to say that we weren't believed by everyone at first. Some people assumed there must be something wrong.

  Why did you choose to publish in Physical Review Letters?

Well, in addition to it being an important journal in the field, we knew the article would be read by a broader audience. Although we are particle physicists, we thought the work would be of interest to general relativists and cosmologists who might not otherwise read some of the other journals we might publish in.

  Is it widely accepted now as a viable theory?

Certainly, it's now widely accepted in the sense that it clearly does reproduce the four-dimensional physics and is clearly a viable possibility. Is it really a theory of the world? Who knows?

  Does it offer the possibility of predictions that could be tested by experiment?

That’s difficult. The cosmological implications have not been sufficiently well explored at this point. Certainly, as far as particle physics goes, unless there's a second brane and we go back to the hierarchy problem, the effects would be hard to notice. That’s what's so amazing: we have a theory with an infinite fifth dimension, but it's indistinguishable from four-dimensional gravity. In some sense, you can say that all evidence we have for four dimensions might equally well be evidence for these other theories.

  Is there anything you would do differently if you could go back and do this research again?

I can't actually say yes to that. Because one of the things that was so interesting about all these theories is that they, in effect, told us what was going on. So it was a matter of figuring out the implications of what they were telling us. We were trying to solve one problem and found ourselves led into this situation where we were working out the consequences of what we were observing. With hindsight, you could say if we knew this was the problem we were going to end up solving, we could have done things differently, but we didn't know.

  Are you surprised by the impact the work has had?

Well, I actually thought they were important papers, but I often think my papers are important and it’s not true that the rest of the community always agrees with me. So I guess the answer is yes and no. And one of the things that is very nice is it has had an impact not just with people doing physics similar to what we do, but in the more general community, as well, including string theorists, particle physicists, general relativists, and cosmologists.

  What do you think the future holds for your research?

That’s a good question. I'm not sure I have a good answer. I think the cosmology will be interesting, trying to understand the relationship between some of the things were thinking about and cosmology. As for particle physics, once the Large Hadron Collider turns on at CERN, we will actually start having data that will change the whole topography in which we’re doing physics. And one of the great things about physics is that we often get surprises when we turn on a new accelerator.

  Is there one message you'd like to convey about your work to the general public?

I think it is important to recognize that particle physics is actually an evolving field. The pace of experiments has slowed down, but the pace of ideas has not.

Lisa Randall, Ph.D.
Harvard University
Department of Physics
Cambridge, MA, USA