Lisa Randall Chat Transcript

Harvard physicist Lisa Randall explains multiple dimensions, thinks about creativity, and ponies up for bets on upcoming experiments.


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Yesterday, had an open discussion with Harvard physicist Lisa Randall, our first live, online chat. We had a great turn-out, and lots of interesting questions were posed of Randall. Here's the proof—the transcript. Look forward to more chats with scientists in the future, and thanks to all those who dropped in yesterday. I'm Amos Kenigsberg, the editor of and the moderator of the chat. We're chatting, as I'm sure you know, with Harvard theoretical physicist Lisa Randall. (Hi, Prof Randall.)

Lisa Randall: Good afternoon. She's most well-known for her theories about the existence of extra (beyond 4) dimensions, and that's the subject of her great book, Warped Passages. For people who are not familiar with your work, what are "extra dimensions"?

Lisa Randall: Extra dimensions are directions in space beyond those 3 we experience and observe. It's a concept that seems pretty strange to us humans—why do you think they exist?

Lisa Randall: They can exist even thought we don't directly see them. We think they might exist for several reasons. One is string theory (a proposed theory of quantum gravity). One is because they help explain mysterious features of the universe we do observe. And finally is, "Why not?" We know there are many things we can't see. Why should our physiology prove only three dimensions exist?

Xman: Does your research tell us why we live in a 3D universe? Why not two dimensions, or seven?

Lisa Randall: The research I find most reliable assumes we live in a space that appears to be three-dimensional. But with the physicists Andreas Karch we asked the question whether three dimensions might end up being favored simply by letting a higher dimensional universe evolve in which there are restricted regions of space called "branes." And we found that three (and seven!) dimensions were indeed favored. "And seven!"? Could you explain that just a bit?

Lisa Randall: Well, we think there might be surfaces with fewer numbers of dimensions in higher-dimensional space. We assumed they all existed, but got diluted through interactions and the expansion of the universe. Three and seven dimensions were special. It turns out, in string theory matter might live in seven dimenisons even if we end up thinking we're in three, so it's suggesive.

delmartian: Do you feel that as we try to get closer and closer to an understanding of the singularity, the initial point of the big bang, that the theoretical physics has gotten so complicated that it has become impossible for the non-physicist to comprehend? I'm a pretty smart guy, but trying to understand exactly what happened in that very first fraction of a second, and why it's so important, is getting me pretty frustrated!

Lisa Randall: Well, I agree in some sense. The big problem is no one knows the answer to that question! Although I touch on string theory and quantum gravity in my book the primary emphasis is on physical phenomena at lower energies that might be explained with extra dimensions. There is a lot involved (gravity, quantum mechanics, particle physics, string theory) but I do think it's all accessible. Clearly not all the details, but the basic, relevant concepts.

zeezee: What's your view on the recent discovery of evidence for dark matter?

Lisa Randall: I think it's pretty doubtful at the moment. We'll believe it more when confirmed by other experiments.

Eric: Is time a dimension? Clearly it is unidirectional, which makes it different than the others. Can there be other dimensions that are unidirectional also?

Lisa Randall: It's a good question about time. There are certainly some senses in which time is a dimension and it's well known that Einstein (and Minkowski) introduced spacetime geometry. But time is different. Technically, it's term appears with opposite sign in the metric. And as you point out, we make different assumptions about it and formulate questions differently when dealing with time. So far no one knows how to include more than one dimension of time consistently.

And as for unidirectional space, that would assume some symmetry of space is broken. It's possible for example in a world with an extra dimension and a single "brane" (one that we considered). You can go in both directions but there is a boundary in one direction which cuts off the space. We have some physics sophisticated readers here...

Lisa Randall: Excellent.


Michael: I'm reading Warped Passages right now and find that while your explanations are beautifully clear, it's almost impossible for me to actually visualize some of the things you describe. Can you personally, actually "see" this stuff in your mind, or is it more a matter of mathematics?

Lisa Randall: It's really math for me (and concepts which aren't always in equations). I can see projections just like anyone but for whatever reason we are not physiologically designed to "see" more than three dimensions.

SJRabadi: Which particles do you hope to observe after running the LHC? [Note from the moderator: LHC is the Large Hadron Collider, a new accelerator being being in Europe.] And how long after the completion of the LHC would you say that a definitive information about the particles will be available?

Lisa Randall: The particles are called Kaluza-Klein particles. They are particles that travel in extra dimensions and have mass associated with the momentum in the extra dimensions. I talk a lot about them in my book because they are the most likely experimental evidence of extra dimensions. If we're lucky (that is if the energy and interactions are right) we could see them within a couple of years of running (which really begins in 2008). We might also see higher dimensional black holes or even warped strings that are much lighter than you typically expect! Great. Excited for the LHC results. We've gotten a few questions on string theory, so I'll grab one of those.

Lisa Randall: Sure.

bigdirtymosh: There's been quite a bit of backlash against string theory lately--some physicists are saying that there is no way for humans, based on our limited ability to perceive the universe, to find any evidence for it—and it can't, therefore, ever be anything more than theology. How does string theory respond to this?

Lisa Randall: There are a couple of responses. Not to sound too self-promoting but that's really the essence of what my book is about—how to connect the abstract ideas of string theory possibly to what we can see in exeriments. Even if we don't directly test string theory we can test ideas that come out of string theory like extra dimensions and branes if they are related to physical phenomena we observe.

Lisa Randall: Plus there are the other issues which are simply that it's a hard theory. People have made progress but certainly don't know all the answers. There is room for more than one approach. That doesn't mean simply more than one approach to quantum gravity. It means also asking other questions such as why forces and masses are related the way they are. That last response brings up an interesting question asked a few minutes ago:

skdh: Do you think that there is more than one mathematically consistent fundamental "theory of everything," and that only one of them actually describes reality? Or that there is, under it all, only one, and that all approaches—if mathematically consitent—will eventually turn out to be part of the same truth?

Lisa Randall: I'm not even sure what we mean by that exactly. Do we mean will science continue to work as we probe higher energies and shorter distances? I think it will but the only way to know is to try.

Lisa Randall: I doubt there is more than one actually distinct theory that describes the world. After all. if two theories give the same predictions they really are the same! There seems to be some some money riding on the thoughts in this question, so I'll jump to that one.

qd_survivor: Dr. Randall, recently I placed a $1,000 bet on my blog, that LHC experiments will not see 5-sigma discrepancies from the Standard Model, with two colleagues (an experimentalist working at the D0 experiment, and a string theorist, your colleague Jacques Distler). What side of the bet would you pick? Would you bet on large extra dimensions being found at LHC in two years of running?

Lisa Randall: That wasn't one side of the bet as I understand it. I'd bet we'll see something new. We have good reasons to believe the Standard Model is not all there is [again, described so excellently in my book :) ]. Extra dimensions, and the kind of warped dimension I describe, is definitely one of the possibilities. I'd say there is a finite but hardly 100% chance this is what we'll find. But a finite probability is pretty high in my book. If you want to make the bet about seeing something I'm definitely in. Huh. Maybe has a future as a science-prediction bookie...

Lisa Randall: Is that legal? We can ferret our earnings away to extra dimensions. We digress...

HaileKofi: Prof. Randall, your work on the Randall-Sundrum braneworld scenario seems to me to be an excellent example truly creative work. Can you comment on the process by which you arrive at such ideas? In particular, do you think such creativity is inborn, or that there are particular things a well prepared student/researcher might do to aspire to such innovation?

Lisa Randall: Hard to say—not my field of study. But I do think strong acquaintance and commitment plays a big role in intuition. If you think about something a lot, and discrepancies bug you, and you really know what's happening, you will inevitably have something new to say. I think I am creative—probably comes in part from being easily bored—but I do think having thought a lot about the problems I work on informs my intuition. In fact, in some ways Raman and I were lucky. We were attacking a different problem but recognized something interesting when we happened across it.


Jefferson: Prof. Randall, congratulations to the German edition of your book. Do you think that the small cosmological constant can be explained by the exponential warping in a Randall-Sundrum braneworld?

Lisa Randall: Thank you. I'm excited about it (and just back from Berlin).

Lisa Randall: I don't think the CC can be explained by the exponential warping. Problem is everything would be low energy—not just CC. If the cutoff on energy really was low there wouldn't be a CC problem in the first place. But we know there are plenty of particles much heavier than such a cutoff would imply.

Dumbfisy: I've watched the talk you gave at PI. How likely do you think it is that we could observe a 4D [four-dimensional] object moving through our 3D space? Have we seen it already and possibly just dont realize it? Or was that analogy more for understanding that there is more than just 3D?

Lisa Randall: Sorry, just an analogy. I don't see any real reason we should be so lucky to happen to be looking when a 4D object comes through. But more importantly, it's likely it interacts only via gravity. In which case it would be very difficult to identify and detect. That brings up a question I very much wanted to ask: How much chance is there for interplay between our 3D world and other worlds with more dimensions? Will we be able to see anything that comes from extra dimensions? Will we ever be able to probe into them (with photons, for instance)?

This may just be silly, but I suspect a lot of people (e.g., me) wonder this when you mention extra dimensions: Can a person ever go to an extra-dimensional realm? Can she come back? And do you have any inside track with multidimensional explorers? You seem like the person to ask.

Lisa Randall: Well, I try to stick to what we can do with the physics we know. I realize what I do is speculative, but the idea of traveling to extra dimensions is far more so. What I do think we can detect potentially is the influence of the existence of extra dimensions on our world—affecting the weakness of gravity and leading to KK particles for example. Okay, too much fiction, not enough science. Gotcha.

Lisa Randall: If photons are in the extra dimensions, we know they can't be very big since otherwise electromagnetism would be very dilute. So in some sense in that case the dimensions are right here and we're spread through them and we just don't know it. If photons are stuck on our brane, very likely we have just gravity interacting with extra dimensions.

Lisa Randall: Eventually maybe we can detect gravitational waves. Would be exciting. So these dimensions, I take it, are extremely unlike the ones we know and love.

Lisa Randall: They are in the sense that they are either finite in size or warped. Also not everything we know and love and hate necessarily travels in the extra dimensions.

qd_survivor: Dr. Randall, does your work bring you to browse through the blogosphere of physicists and mathematicians? That seems to be a very fertile ground for lively discussions lately. Of course, in part a waste of time, but in part also a birthplace of new ideas and of continuous development. What do you think about that?

Lisa Randall: You won't like this answer but I don't really find I get new ideas there—at least ones that are likely to be right. The fact is I have too little time to work on my own stuff these days so I have to go with high-probability paths. These topics are difficult and it's just more likely that people who do physics for a living will come up with good ideas. Sorry if I've alienated half the readers here. I think the question was about professional physicists' blogs. [I don't think the readers would be insulted—you're the physicist!]

Lisa Randall: Well as far as that goes, I know the physicists, I go to talks, and I read the papers. So the blogs are more for the public domain. The non-physicists are the only people on the blog sites that I wouldn't be familiar with already. I see. Okay, it's already 3pm, so I'll look through the chat-bag and grab one more quicky for you to tackle, if that's okay, Prof Randall, and then you're off the hot seat.

Mike: I'm a 54-year-old electronics engineer. I've read science magazines for 40 years. Every time we find the "smallest" particle, someone divides it into smaller pieces. Is it possible there is no "matter" and it's all just manifestations of energy? Does this "energy" propagate from one dimension to another?

Lisa Randall: We know E=mc^2. That's pretty specific. It says matter can be converted to energy. So in some sense it's a manifestation of energy. But it has other properties, like charge, for example. Energy can be exchanged from one dimension to another but not necessarily with all forms of matter. Quarks, for example, might be stuck on a brane. Okay, 3pm has just sprung right upon us. Thanks very much, Prof. Randall. We appreciate it.

Lisa Randall: Thanks for this opportunity to chat. The questions have been great.

Lisa Randall: Signing off... For all the other folks, thanks very much for the great questions, and we hope you'll come back for more chats in the future. A shout-out to the Norwich KS physics class, too—thanks for coming. Bye bye. Oh, yes, we will be posting the transcript of the chat on soon. Thanks again!

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