Yesterday, Discover.com 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.
Discover.com: I'm Amos Kenigsberg, the editor of Discover.com and the moderator of the chat.
Discover.com: We're chatting, as I'm sure you know, with Harvard theoretical physicist Lisa Randall. (Hi, Prof Randall.)
Lisa Randall: Good afternoon.
Discover.com: 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.
Discover.com: 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.
Discover.com: 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.
Discover.com: "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.
Discover.com: We have some physics sophisticated readers here...
Lisa Randall: Excellent.
