Warped Passages - Lisa Randall [160]
The results about branes from this chapter will not be relevant later on. These results do, however, explain some of the excitement about branes in the string theory community.
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Bustling Passages: Braneworlds
Welcome to where time stands still.
No one leaves and no one will.
Metallica
Icarus III was becoming increasingly disillusioned with Heaven. He had expected it to be a liberal, forgiving environment. But instead, gambling was prohibited, metal silverware was forbidden, and smoking was no longer allowed. The most restrictive constraint of all was that Heaven was stuck on a Heavenbrane; its residents were forbidden to travel into the fifth dimension.
Everyone on the Heavenbrane knew about the fifth dimension and the existence of other branes. In fact, the righteous Heavenbraners often whispered about the unsavory characters sequestered on a Jailbrane not too far away. However, the Jailbraners couldn’t hear any of the slander that Heavenbraners spread about them, so all remained peaceful in the bulk and on the branes.
From the perspective of the “duality revolution,” you might think that branes were a great boon to people trying to connect string theory to the visible universe. If all of the different formulations of string theory are in fact one and the same, physicists would no longer be faced with the daunting task of finding the rules by which nature chooses among them. There is no need to play favorites if all of the string theories appearing in different guises are actually the same.
But nice as it would be to think that we are closer to finding the connection between string theory and the Standard Model, the task is not so simple. Although branes were critical to the dualities that reduced the number of distinct manifestations of string theory, they actually increased the number of ways in which the Standard Model might emerge. That is because branes can house particles and forces that string theorists didn’t take into account when they originally developed string theory. Because of the many possibilities as to what types of brane exist and where they are situated in the higher-dimensional space of string theory, there are conceivably many new ways to realize the Standard Model in string theory that no one had thought about before. The forces of the Standard Model do not necessarily arise from a single fundamental string: they could instead be new forces that emerge from strings extended among different branes. Although dualities tell us that the original five versions of superstring theory are equivalent, the number of braneworlds that are conceivable in string theory is stupendous.
Finding a unique Standard Model candidate looked like it would be as hard as ever. String theorists’ euphoria about duality was tempered by this realization. However, those of us who were looking for new insights into observable physics were in heaven. With the new possibilities for forces and particles confined to branes, it was time for us to rethink the starting point for particle physics.
The feature of branes that is essential to their potentially observable applications is that they can trap particles and forces. The purpose of this chapter is to give you a flavor of how this works. We’ll begin by explaining why the branes of string theory confine particles and forces. We’ll consider the braneworld idea and the first known braneworld, one which was derived from duality and string theory. In the chapters that follow, we’ll proceed to those aspects of braneworlds and their potential physical applications that I find most exciting.
Particles and Strings and Branes
As Ruth Gregory, a general relativist from Durham University, puts it, branes in string theory come “fully loaded” with particles and forces. That is, certain branes always have particles and forces that are trapped on them. Like housebound cats that never venture beyond the walls of their domicile, those particles that are confined to branes never venture off them. They can’t. Their existence is predicated on the presence