Warped Passages - Lisa Randall [153]
Branes’ finite tension and nonzero charge tell us that they are not merely places, they are also things: their charges tell us that they interact, and finite tension tells us that they move. Like a trampoline—a surface that interacts with its environment when it is depressed and when it springs back—a brane can move and interact. For example, both trampolines and branes can be distorted. And both trampolines and branes can influence their surroundings, trampolines by pushing on people and air, and branes by pushing on charged objects and the gravitational field.
If branes exist in the cosmos, their violation of spacetime symmetries should be no more disturbing than the violation of spatial symmetries caused by the Sun or the Earth. The Sun and the Earth are also located in particular locations; when measured with respect to the Sun or the Earth, not all positions in three-dimensional space are the same. Nonetheless, physical laws preserve the spacetime symmetries of three-dimensional space, even if the state of the universe does not. Branes would be no worse than the Sun or the Earth in this respect. Branes, like all other objects at definite places in space, break some symmetries of spacetime.
A moment’s reflection reveals that this is not such a bad thing. After all, if string theory is the true description of nature, then not all dimensions are created equal. The three familiar spatial dimensions look alike, but the extra dimensions must be different; if they weren’t, they wouldn’t be “extra.” From the vantage point of the physical universe, the violation of spacetime symmetries could help explain why extra dimensions are different: branes might correctly distinguish string theory’s extra dimensions from the three spatial dimensions we experience and know.
In later chapters, I will consider branes with three spatial dimensions and describe some of their potentially radical implications for the real world. But for the rest of this chapter we’ll concentrate on why branes are so significant in string theory—so important, in fact, that they catalyzed the “second superstring revolution” of 1995. The next section gives a few reasons why branes have remained at the forefront of string theory for the past decade, and why we now think they’re here to stay.
Mature Branes and the Missing Particles
While Joe Polchinski was hard at work investigating D-branes, Andy Strominger, then his colleague at Santa Barbara, was pondering p-branes—fascinating solutions to Einstein’s equations. They expand infinitely far in some spatial directions, but in the remaining dimensions they act as black holes, trapping objects that come too close. D-branes, on the other hand, are surfaces on which open strings end.
Andy told me how he and Joe would discuss their research progress every day over lunch. Andy would talk about p-branes, and Joe would discuss D-branes. Although they were both studying branes, like all other physicists they initially thought that their two types of brane were two different things. Joe eventually realized that they were not.
Andy’s work demonstrated that the p-branes he was studying are critically important in string theory because in some spacetime geometries they give rise to new types of particle. Even if string theory’s non-intuitive and remarkable premise is true, and particles arise as oscillation modes of strings, string oscillations don’t necessarily account for all particles. Andy showed that there could still be additional particles that arise independently of strings.
Branes come in different shapes, forms, and sizes. Although we’ve focused on branes as the places where strings end, branes themselves are independent