Warped Passages - Lisa Randall [166]
Finally, branes and the bulk can carry energy. This energy can be stored by the branes and by the higher-dimensional bulk; it doesn’t depend on the particles that are present. Like all forms of energy, it curves the bulk spacetime. We will soon see that such curvature of spacetime caused by energy spreading throughout the space can be very important to braneworlds.
The HW braneworld certainly has many tantalizing features. But it also suffers from the problems all realizations of string theory seem to have in reproducing known physics. Hořava-Witten theory is very difficult to test experimentally because its dimensions are so small. The many unseen particles have to be heavy enough to have avoided detection, and six of its dimensions have to be curled up, even though neither the size nor the shape of the curled-up dimensions has been determined.
Proceeding along these lines, one might accidentally hit on the version of string theory that correctly describes nature; this possibility is not definitively ruled out. For this to happen, we would have to be very lucky indeed. But problems in particle physics also beckon, and it is worth investigating how these problems might be resolved in a world with extra dimensions of space and branes that extend along only a restricted subset of these dimensions. That is what the rest of this book is about.
What to Remember
Braneworlds are possible within the framework of string theory. Particles and forces in string theory can be trapped on branes.
Gravity is different from other forces. It is never confined to a brane and always spreads through all dimensions.
If string theory describes the universe, it could contain many branes. Braneworlds are very natural in this context.
17
Sparsely Populated Passages: Multiverses and Sequestering
Just turn around now
(’cause) you’re not welcome anymore.
Gloria Gaynor
Despite its explicit prohibition on the Heavenbrane, Icarus III ultimately returned to gambling. After ignoring repeated reprimands, he was sentenced to confinement on the Jailbrane, a distant brane separated from the Heavenbrane along a fifth dimension. Even after he was sequestered on the Jailbrane, Ike doggedly tried to contact his former buddies. But the distance between their two branes made communication difficult. He was reduced to flagging down passing bulk mail carriers, many of whom ignored his entreaties altogether. The few who did stop always conveyed his messages to the Heavenbrane, but at a frustratingly leisurely pace.
Meanwhile, back on the Heavenbrane, disaster loomed. The guardian angels, who had so bravely rescued the hierarchy, had no respect for the other residents’ family values and were on the verge of creating intergenerational instability. Heaven’s fallen angels considered all pairings acceptable and encouraged everyone to mix with a trophy partner from another generation.
When Ike learned of the threat, he was aghast and he resolved to redeem the situation. Ike realized that by using the slow and deliberate manner with which he was constrained to communicate with the Heavenbrane, he could judiciously feed the massive egos of the unruly angels living there. Thanks to Ike’s helpful intervention, the angels stopped threatening the social order. Although Icarus III still had to serve his sentence, the relieved residents on the Heavenbrane honored him forevermore in urban myth.
This chapter is about sequestering, one of several reasons that extra dimensions could prove to be important for particle physics. Sequestered particles are physically separated on different branes. By confining different particles to different environments, sequestering might explain the distinctive properties that distinguish one particle from another. Sequestering might also be the reason that the anarchic principle, which says that everything should interact, doesn’t always hold true. If particles are separated in extra dimensions, they are less likely to interact with one another.
In principle, particles could have been sequestered in three spatial dimensions.