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Home , Lisa Randall, Raman Sundrum

Focus: Small Science

Exclusive Interview:

Harvard Theoretical Physicist

By Jennifer Gao and Limor Spector

arvard Professor of Lisa Ran- level, even protons and neutrons have is theory. Theoretical particle Hdall’s work on the fundamental nature quarks. But at some level you think physics is trying to understand what is of particles, the observables, you’ve reached the smallest ingredient, really going on at these deeper levels, , and other aspects of particle which is the particle. And we’re try- what underlies what we see. physics has earned her widespread recognition ing to understand physics at the most LQWKHÀHOGRI WKHRUHWLFDOSK\VLFVLQFOXGLQJ fundamental level and understand what HSR: Can you describe the basic most recently the Klopsted Award, presented by everything is made up of and what the research you’re conducting and inter- the American Association of Physics Teachers fundamental ingredients and interac- ested in? in 2006. She recently spoke to the Harvard tions are. That’s what our goal is. String Science Review about her research and her theory is the idea that particles aren’t the LR: I’m interested in a couple of dif- recent book, : Unraveling most fundamental – that instead, strings ferent directions [of research]. Most re- The Mysteries of The Universe’s Hid- are the most fundamental matter, and cently, my research focuses on extra di- den Dimensions, which was named a New particles are the oscillation modes of mensions of space and the implications York Times notable book of 2005. these fundamental oscillating strings. of in space, should The reason that people have proposed they exist in the universe. Basically, Harvard Science Review: The Har- this might be the case – we don’t know we’re trying to understand fundamental vard Science Review targets audiences if it’s the case – is that in standard particles and their interactions, what from a variety of backgrounds, includ- , we don’t fully under- gives mass to fundamental particles, ing humanities and science concen- stand gravity. We don’t know how to why gravity is as weak as it is. We’re trators. For students unfamiliar with combine together quantum mechanics trying to understand cosmological is- , could you give a and gravity at all distances. At large sues, how the universe came to be what brief description of what particle phys- distances, we can use Einstein’s theory it is. And those could all tie in with this ics, , and theoretical physics of ; we understand physics of extra dimensions of space. is all about? how that works. Quantum mechanics In particular, I’m trying to see whether just doesn’t play a big role [at these there are experimental consequences to Lisa Randall: Particle physics is based large distances]. At atomic distances, some of these theories I’ve worked on, on the idea that at a fundamental level, we use quantum mechanics, and rela- especially for particle physics accelera- everything is composed of elementary tivity doesn’t play a big role. But there tor experiments, in which particles get particles. That is, if you keep digging are distances, far beyond what we can accelerated to high energies and collide deeper [into the internal structure of see experimentally, where the theories together to make new particles. Also YLVLEOHPDWWHU@\RXÀQGWKDWWKLQJVDUH become incompatible, and that tells us I’m thinking about possible [experi- composite: an atom has neutrons, pro- theoretically that there should be some- mental consequences] for gravity wave tons, electrons. Of course, at a deeper thing else – and the proposed solution detectors, which are going to be better

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and better in the future. We can see gravitational signals from the sky that tell us things like if there was a phase transition in the early universe. We’re also thinking about what black holes would look like in the sorts of higher- dimensional theories that interest me at the moment. That’s the direction I’m JRLQJLQQRZWU\LQJWRÁHVKRXWVRPH of these theories better as well as their H[SHULPHQWDOFRQVHTXHQFHVWRÀQGRXW if they’re really right—that is, if the universe is as we propose.

HSR: Your new book is entitled Warped Passages: Unraveling the Mysteries of The Universe’s Hidden Dimensions. What are warped passages, hidden dimensions, and how do these two concepts cor- relate with each other?

LR: Passages was a word I made up to refer to extra dimensions of space. Dimensions are independent directions of space. The number of dimensions is the number of quantities you would need to pinpoint an object in space. We don’t really have a name for them once we get beyond the third dimension, so I called them passages. “Warped” has to do with the spacetime geometry that I’ve worked with, and it’s actually what we’ve found to be the solution to Einstein’s theory of general relativity in the particular context in which we studied it. In other words, we had a setup of objects in higher dimensions and we found that the solution was highly warped. Basically, it was really dramatically curved —and in ways that necessarily go straight there, you take two dimensions are analogous to our have very interesting consequences. some warped routes, so “warped” has three dimensions—that was all they The title is derived from the research many meanings. saw and experienced. So how would in the technical sense of the word they conceptualize a third dimension?” “warped.” It’s not just a Star Trek term. Hidden dimensions… we’re really talk- For them, a third dimension would be It’s actually called “warped,” the type ing about the idea of hidden dimen- an extra [unseen] dimension. So, for of geometry we found. But I was also sions of space, the idea that there is a example, if you imagine what they see sort of punning a little bit, because it’s sense of space beyond that which we when a sphere passed through their P\ÀUVWERRN²VR,FDOOHGLW´:DUSHG see. You can understand it in many two-dimensional world, you would see Passages” [laughs], which most people different ways; probably the best way that a sphere would look like a series seem to miss. It [the word “warped”] is the way Edwin A. Abbott in the of disks that increased in size and also helps to describe the way people late 19th century understood it. He then decreased in size. But the two arrive at physics results, both in doing asked the question, “Suppose you had dimensional creatures wouldn’t be able it and understanding it. You don’t two dimensional creatures. For them, to put it together and say they saw a credit: Scott D. Kominers.

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sphere—except mathematically. They a bead on a wire: a bead on a wire can wouldn’t be able to envision it—but travel, but only in the one dimension of “We’re trying to they could do it in their imagination, the wire, even though it might be on a or in words, or with math. In the same two-dimensional table in a three-dimen- understand physics at way, we don’t necessarily perceive extra sional room. In the same way, we might the most fundamental dimensions, but they could exist. We be stuck on branes so that the stuff level… what everything don’t know for sure if they do exist, but we’re made of, our universe, is stuck is made up of and they could exist. For example, if a hy- on a three-dimensional brane—even what the fundamental persphere passed through our universe, though there might be more [dimen- ingredients and it would look like a series of spheres sions]. Gravity would still [have to] that increased in size and decreased in travel [and operate] through all these interactions are… size. And again, it’s hard to visualize, but dimensions; [it would still have to be] cosmological issues, we can still imagine it and understand it spread throughout those dimensions. how the universe came mathematically. The idea is that there But we and the stuff we’re made of and to be what it is.” really might be dimensions of space the galaxy and universe in which we live beyond those that we see. The warping might be stuck on a lower dimensional [comes as a] consequence to Einstein’s object called a brane. theory of general relativity: if there is energy in these extra dimensions, it can HSR: Are there other aspects of your warp space, curve space, in very dra- work you want to highlight? matic ways which turn out to have very interesting implications. They can help LR: We [Randall and collaborator “Particle physicists us understand the weakness of gravity ] found a couple would… predict that relative to other forces in our universe. of radically new consequences of gravity is about the Another revolutionary thing we found warped geometry. One was quite dra- same strength as the was that warping could explain why matic, because since the 1920s, people extra dimensions are hidden. thought that extra dimensions, if they other forces we know existed, would have to be extremely about, yet in reality HSR: Nima Arkani-Hamed, a theo- WLQ\²FXUOHGXSWRDPLQXVFXOHVL]H it is many orders of retical physics professor at Harvard or as was later postulated, bound up magnitude weaker in University, has stated that you are most between branes. We found you could strength.” well-known for your research on the DFWXDOO\KDYHDQLQÀQLWHH[WUDGLPHQ- concept of branes. What are branes, VLRQ²VRPHWKLQJSHRSOHWKRXJKWZDV and what relevance do they have to impossible. In 1999, my collaborator current high-energy theoretical phys- Raman Sundrum and I discovered that. ics research? How and why have they And that was quite a radical observa- become, as he says, part of the current tion. People who worked on general lexicon? relativity—on gravity—hadn’t realized this was a possibility, so I think this was LR: I’ve actually worked on a lot of quite a dramatic result that might have “It’s not that the different aspects of particle physics. implications for how extra dimensions Standard Model will Most recently, I’ve worked on extra hide in our universe. The other thing be wrong, but it’s dimensions and branes. Branes are we found is that if you have these re- probably part of a lower dimension objects. The word ally warped extra dimensions, they can bigger, richer theory, comes from “membranes,” since they’re actually explain the weakness of gravity, membrane-like objects in a higher which is a huge puzzle from the view- and we’re trying to dimensional space. The idea is that point of particle physics. Particle physi- ÀQGZKDWWKDWULFKHU there could be extra dimensions [in cists would naively predict that gravity theory is.” the universe], but not everything [in is about the same strength as the other it] necessarily experiences or travels in forces we know about (electromagne- those extra dimensions. They could be tism and the weak and strong nuclear stuck on lower dimensional surfaces forces), yet in reality it is many orders called branes. A good analogy might be of magnitudes weaker in strength. The

spring 2006 • Harvard Science Review 61 Focus: Small Science

question is, “Why is that?” In warped We know that isn’t—or we believe that scenarios in which we should be using spacetime, we showed that you end up isn’t—really what’s going on, so what it, if it is correct. Physics would be a with a very natural explanation. Gravity we’re trying to do is extend the Standard bigger, richer stage in which to apply is essentially concentrated somewhere Model. It’s not that the Standard Model those sorts of laws. else, and we’re experiencing the tail end will be wrong, but it’s probably part of of gravity. We see exponentially weaker a bigger, richer theory, and we’re trying HSR: Where do you see theoretical gravity than we would if we were in WRÀQGZKDWWKDWULFKHUWKHRU\LV6R physics in 10 or 20 years? the region where gravity was highly my research will have an impact, if it’s concentrated. correct, in showing how the Standard LR: That’s a really great question. Model is embedded in a larger theory, Looking to the past, I would be hesi- HSR: In the popularization of physics be it extra dimensions of space or some tant to do that [predict its direction] research in previous years, much of the other theory. because people so often get it wrong. focus has been on the Standard Model, One of the really exciting things is that a model that physicists almost univer- HSR: So what exactly is the Standard the (LHC) is VDOO\FRQVLGHUWREHÁDZHGDQG\HWKDV Model then? going to turn on. It’s a large particle held up remarkably well to experimen- accelerator which achieves about seven tal testing. Can you explain what the LR: The Standard Model of particle times the energies we’ve achieved at the Standard Model is? If your research is physics describes the most basic mat- Tevatron, an existing particle accelera- correct, how will it, if at all, impact the ters of elements that we know and their tor at Fermi Lab. So experimenters will Standard Model? interactions. It describes particles like collide together protons at enormously electrons and quarks and also the four high energies. And basically [the data LR: First of all, I should explain what forces we know about—it really con- that comes from that is] going to set “flawed” means, because this is an centrates on non-gravitational forces: WKHGLUHFWLRQ:KHQZHÀQGWKDWRXW especially important point for non-sci- electromagnetism, and the weak and [what happens in the LHC], we’ll have entists to get straight, as a lot of science strong nuclear forces. a much better idea of where things are debates are about this. We have theories headed. we call “effective theories.” They work HSR: To what extent do you see your up to certain distances, certain ener- work as being a natural progression HSR: What led you to choose phys- JLHVDQGWKH\ZRUNMXVWÀQH,W·VWKH from the already accepted aspects of ics? same way Newton’s laws work. I don’t physics, and to what extent does your need to know general relativity to suc- work lead physics in an entirely new LR: I like math a lot, but I didn’t re- cessfully apply Newton’s laws. I don’t direction? ally see myself as a mathematician. I even need to know quantum mechan- LR: Basically, it does build on old phys- just thought the questions were too ics. These “effective theories” may not ics ideas. In fact, half my book is really abstract. I ended up doing really ab- be the most fundamental, or the most building up the old physics, which I stract physics stuff, but I like the idea complete descriptions [of physical phe- think is an important thing to do. It’s of doing something [that ties back] to nomena]. But they’re a good descrip- important for people to know not only the world, to something that we see. tion on the scales in which we use them. what is non-speculative, but also to I like the games and puzzles aspect The Standard Model falls into that know how our current research builds of it. I just enjoy problem solving. I category. It works extremely well—it’s on and is a logical extension of what is deeply like consistency, the idea that been tested to a percent precision at the known from before. It’s also the reason WKLQJVÀWWRJHWKHU,OLNHWKHLGHDWKDWD scale at which we measure things. But why what we’re talking about isn’t sci- XQLYHUVHFDQÀWWRJHWKHULQDFRKHUHQW LW·VFRQVLGHUHGÁDZHGEHFDXVHLWGRHV HQFHÀFWLRQEHFDXVHZH·UHXVLQJVFLHQ- whole. And when I see it’s not [doing not address this issue of the weakness WLÀFWKHRULHVZHNQRZWREHFRUUHFWOLNH that in our theories], I like addressing of gravity. And particularly what it general relativity. We’re using it in a dif- that and seeing why it isn’t and if we doesn’t address is why masses aren’t ferent context—namely, when there’s can solve it. much bigger than the theory seems to extra dimensions of space—and we’re —Jennifer Gao ’06 is a Chemistry concentra- predict. To make the Standard Model ÀQGLQJYHU\EL]DUUHQHZSKHQRPHQD tor in Adams House. Limor Spector ’07 is give the right answers, we have to do OLNHWKLVLQÀQLWHH[WUDGLPHQVLRQLGHD a Physics and Mathematics concentrator in something like a [mathematical] fudge, but it’s [still based upon] known physi- Lowell House. VRPHWKLQJZHFDOO´ÀQHWXQLQJµ>,W·V@ cal laws. I think in [the way in which it’s a parameter that’s put into [the theory applied] won’t necessarily predict new to achieve] sixteen digits of precision. laws of physics, but it will predict new

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