Warped Passages - Lisa Randall [151]
In this chapter, I’ll describe the evolution of branes from an amusing, neglected curiosity into a central player in the string theory story. We will see several ways in which branes helped to resolve some bewildering aspects of string theory since the mid-1990s. Branes helped physicists to understand the origin of mysterious particles in string theory that couldn’t possibly arise from strings. And when physicists included branes, they discovered dual theories—pairs of theories that seem very different from each other but have the same physical consequences. The opening story refers to one remarkable example of duality that this chapter will explore: an equivalence between ten-dimensional superstring theory and eleven-dimensional supergravity, which is a theory that contains branes but no strings.
This chapter will also introduce M-theory, an eleven-dimensional theory that embraces both superstring theory and eleven-dimensional supergravity, and whose existence was inferred using the insights from branes. No one really knows what the “M” stands for—the term’s originator, Edward Witten, deliberately left it ambiguous—but suggestions have included “membrane,” “magic,” and “mystery.” At this point, I’ll just say that M-theory is still a “Missing theory” which is postulated but not fully understood. However, even though M-theory still leaves many questions unanswered, the advances made with branes revealed theoretical connections that called for M-theory’s more complex, more enveloping structure. That is why string theorists study it today.
This chapter updates the string theory picture that began in the 1980s, presenting some aspects of the more modern viewpoint that physicists developed in the 1990s. Much of this material will not be central to branes’ applications to particle physics, and later brane-world conjectures won’t explicitly rely on any of the phenomena described below. You should therefore feel free to skip ahead if you choose. But if you like, take this opportunity to get acquainted with some of the remarkable developments in string theory that were in large part responsible for placing branes squarely on string theory’s theoretical map.
Nascent Branes
In Chapter 3 we saw that branes extend over some, but not necessarily all, of space’s dimensions. For example, a brane might extend only over three dimensions of space, even if the bulk space contains many more. Extra dimensions might terminate on branes; in other words, branes can bound extra-dimensional space. We also know that a brane can house particles that move only along its dimensions. Even if there were many additional spatial dimensions, particles confined to a brane would move only along the more limited region occupied by that brane; they wouldn’t be free to explore the full extra-dimensional bulk.
We will now see that branes are more than just a location; they are objects in their own right. Branes are like membranes, and, like membranes, they are real things. Branes can be slack, in which case they can wiggle and move, or they can be taut, in which case they will probably sit still. And branes can carry charges and interact via forces. Furthermore, branes influence how strings and other objects behave. All these properties tell us that branes are essential to string theory; any consistent string theory formulation must include branes.
In 1989, Jin Dai, Rob Leigh, and Joe Polchinski,