Warped Passages - Lisa Randall [29]
But although forces and matter can be stuck on a brane, brane-worlds are interesting precisely because we know that not everything is confined to a single brane. Gravity, for example, is never confined to a brane. According to general relativity, gravity is woven into the framework of space and time. That means that gravity must be exerted throughout space and in every dimension. If it could be confined to a single brane, we would have to abandon general relativity.
Fortunately this is not the case. Even if branes exist, gravity will be felt everywhere, on and off branes. This is important because it means that braneworlds have to interact with the bulk, even if only via gravity. Because gravity extends into the bulk, and everything interacts via gravity, braneworlds will always be connected to the extra dimensions. Braneworlds do not exist in isolation: they are part of a larger whole with which they interact. In addition to gravity, there could conceivably exist other particles and forces in the bulk. If there are, such particles could also interact with particles confined to a brane and connect brane-bound particles to the higher-dimensional bulk.
The string theory branes that we will briefly consider later on have specific properties aside from the ones I have mentioned: they can carry particular charges, and they will respond in particular ways when something pushes on them. However, I will rarely bring in such detailed properties later on when I talk about branes. It will be enough to know the properties we have considered in this chapter: branes are lower-dimensional surfaces that can house forces and particles, and they can be the boundaries of higher-dimensional space.
Braneworlds: Blueprints for a Jungle Gym of Branes
Because branes could trap most particles and forces, the universe we live in could conceivably be housed on a three-dimensional brane, floating in an extra-dimensional sea. Gravity would extend into the extra dimensions, but stars, planets, people, and everything else that we sense could be confined to a three-dimensional brane. We would then be living on a brane. A brane might be our habitat. The concept of braneworlds is based on this assumption (see Figure 28).
If there can be one brane suspended in a higher-dimensional spacetime, there is no denying the possibility of many more. Brane-world scenarios often involve more than a single brane. We don’t yet know the number or types of branes that could be present in the cosmos. Multiverse is a name that is sometimes attached to theories with more than one brane (see Figure 29). People often use the word to describe a cosmos with non-interacting or only weakly interacting pieces.
Figure 28. We could be living on a brane. That is, the matter we are made of, photons, and other Standard Model particles can all be on the brane. But gravity is always everywhere—on the brane and in the bulk, as is illustrated by the squiggly lines.
Figure 29. The universe can contain multiple branes that interact only via gravity or don’t interact at all. Such set-ups are sometimes called multiverses.
I find the term “multiverse” a bit strange, since a universe is defined as the whole that is the unity of its parts. It is possible, however, to have different branes that are too far apart ever to communicate with one another, or that can communicate with one another only weakly, through mediating particles that travel between them. Particles on distant branes, then, would experience entirely different forces, and brane-bound particles would never have direct contact with particles bound on another brane. So when there is more than one brane with no force in common aside from gravity, I will sometimes refer to the universe housing them both as a multiverse.
Thinking about branes makes you aware of just how little we know about