Warped Passages - Lisa Randall [162]
Now imagine that many branes are superimposed. There would then be many new types of open string because the two string ends can be confined to any of the branes (see Figure 71). Open strings that extend between different branes, or the strings that begin or end on any single one of the branes, imply new particles, composed of the vibrational modes of these strings. Once again, these new particles include new types of gauge boson and new types of charged particle. And once again the new forces are associated with new symmetries that interchange the various superimposed branes.
Figure 71. Each string that begins and ends on the same brane or that extends between branes gives rise to gauge bosons. When the branes coincide, there are new massless gauge bosons, corresponding to each of the ways in which a string can begin and end on each of the coincident branes.
So indeed, branes come “loaded” with forces and particles; many branes mean rich possibilities. Furthermore, even more intricate situations can arise involving separated batches of branes. Branes situated in different places would carry entirely independent particles and forces. The particles and forces that are confined to one group of branes would be entirely different from the particles and forces confined to the others.
For example, if the particles of which we are composed, together with electromagnetism, are all confined to one brane, we would experience the electromagnetic force. However, particles that are confined to distant branes would not; those foreign particles would be insensitive to electromagnetism. On the other hand, particles confined to distant branes could experience novel forces to which we are completely insensitive.
An important property of such a setup, which will be relevant later on, is that particles on separated branes don’t interact with each other directly. Interactions are local: they can take place only among particles in the same place; particles on separated branes would be too far apart to interact with each other directly.
You might compare the bulk, the full higher-dimensional space, to a huge tennis stadium with separate matches going on throughout. The ball on any one court would go back and forth across the net and could move anywhere on that court. However, each match would proceed independently of the others, and each ball would stay on its own isolated court. Just as the ball in a given court should stay there and only the two tennis players on that court would have access to it, brane-confined gauge bosons or other brane-confined particles interact only with objects on their own brane.
However, particles on separate branes can communicate with one another if there are particles and forces that are free to travel throughout the bulk. Such bulk particles would be free to enter and leave a brane. They might occasionally interact with particles on a brane, but they can also travel freely in the full higher-dimensional space.
A setup with separated branes and bulk particles that communicate between them would be like a stadium with separate simultaneous matches in which the players in the separate games have the same coach. The coach, who might well want to keep an eye on several games going on at the same time, would travel from one court to another. If one player wanted to communicate something to a player on another court, he could tell the coach who could carry the message over. The players wouldn’t communicate directly during their matches, but they could nonetheless communicate via a person who