Warped Passages - Lisa Randall [163]
In the next section we will see that the graviton, the particle that communicates the gravitational force, is one such bulk particle. In a higher-dimensional setup, it would travel throughout the higher-dimensional space and interact with all particles everywhere, whether they are on a brane or not.
Gravity: Different Again
Gravity, unlike all other forces, is never confined to a brane. Brane-bound gauge bosons and fermions are the result of open strings, but in string theory, the graviton—the particle that communicates gravity—is a mode of a closed string. Closed strings have no ends, and therefore there are no ends to pin down on a brane.
Particles that are the vibrational modes of closed strings have unrestricted license to travel in the full higher-dimensional bulk. Gravity, the force we know to be communicated by a closed-string particle, is thus once again singled out from the other forces. The graviton, unlike gauge bosons or fermions, must travel though the entire higher-dimensional spacetime. There is no way to confine gravity to lower dimensions. In later chapters we will see that, amazingly, gravity can be localized near a brane. But one can never truly confine gravity on a brane.
This means that although braneworlds could trap most particles and forces on branes, they will never confine gravity. This is a nice property. It tells us that braneworlds will always involve higher-dimensional physics, even if the entire Standard Model is stuck on a four-dimensional brane. If there is a braneworld, everything on it will still interact with gravity, and gravity will be experienced everywhere in the full higher-dimensional space. We’ll soon see why this important distinction between gravity and other forces might help to explain why gravity is so much weaker than the other known forces.
Model Braneworlds
Very soon after physicists recognized the importance of branes to string theory, branes became the focus of intense study. In particular, physicists were eager to learn about their potential relevance to particle physics and our conception of the universe. As of now, string theory doesn’t tell us whether branes exist in the universe and, if they do, how many there are. We know only that branes are an essential theoretical piece of string theory, without which it wouldn’t fit together. But now that we know that branes are part of string theory, we have also begun to ask whether they could be present in the real world. And if they are, what are the consequences?
The potential existence of branes opens up many new possibilities for the composition of the universe, some of which might even be relevant to the physical properties of matter that we observe. The string theorist Amanda Peet, upon hearing Ruth Gregory’s expression “fully loaded” branes, interjected that branes “blasted open the field of string-based model building.” After 1995, branes became a new model building tool.
Towards the end of the 1990s, many physicists, myself included, expanded their horizons to include the possibility of branes. We asked ourselves, “What if there were a higher-dimensional universe in which the particles and forces we know about don’t travel in all dimensions, but are confined to fewer dimensions on a lower-dimensional brane?”
Brane scenarios introduced many new possibilities for the global nature of spacetime. If Standard Model particles are confined to a brane, then we are as well, since we and the cosmos that surrounds us are composed of these particles. Furthermore, not all particles have to be on the same brane. There might therefore be entirely new and unfamiliar particles that experience different forces and interactions from the ones we know. The particles and forces we observe might be only a small part of a much larger universe. Two physicists from