October 21, 2005

Preposterous Universe

One of these days, I'll actually be able to wrap my head around those extra dimensions in space that string theorists always talk about. One of these days.
The simplest way to hide extra dimensions from view is to imagine that they are "compactified"—curled up into a tiny ball (or other geometrical configuration) with an extent much smaller than what can be probed by current experimental apparatus. In the 1990s, however, a new possibility arose, as scientists came to appreciate the role of "branes" in higher-dimensional physics. A brane, generalizing the concept of a membrane, is simply an extended object: A string is a one-dimensional brane, a membrane is a two-dimensional brane, and so on, up to however many dimensions may exist. A remarkable feature of such objects is that particles may be confined to them, unable to escape into the surrounding space. We can therefore imagine that our visible world is a three-dimensional brane, embedded in a larger universe into which we simply can't reach.

Gravity, as the curvature of spacetime itself, is the one force that is hard to confine to a brane; the extra dimensions must therefore have some feature that prevents gravity from appearing higher-dimensional. (For example, in four spatial dimensions, the gravitational force would fall off as the distance cubed, rather than the distance squared.) One possibility, proposed by Nima Arkani-Hamed, Savas Dimopoulos and Georgi ("Gia") Dvali, is that the extra dimensions curl up into a ball that is small without being too small—perhaps as large as a millimeter across in each direction. Randall, in collaboration with Raman Sundrum, showed that an extra dimension could be infinitely big, if the higher-dimensional space was appropriately "warped" (hence the title of her book).
That's from a review of Lisa Randall's new book, Warped Passages.
-- Brad Plumer 12:05 PM || ||