Warped Passages - Lisa Randall [30]
Other branes might be parallel to ours and might house parallel worlds. But many other types of braneworld might exist too. Branes could intersect and particles could be trapped at the intersections. Branes could have different dimensionality. They could curve. They could move. They could wrap around unseen invisible dimensions. Let your imagination run wild and draw any picture you like. It is not impossible that such a geometry exists in the cosmos.
In a world in which branes are embedded in a higher-dimensional bulk, there could be some particles that explore the higher dimensions and others that stay trapped on branes. If the bulk separates one brane from another, some particles can be on the first brane, some on the other, and some in the middle. Theories tell us about many ways in which particles and forces might be distributed among different branes and the bulk. Even for branes derived from string theory, we don’t yet know why string theory should single out any particular allocation of particles and forces. Braneworlds introduce new physical scenarios that might describe both the world we think we know and other worlds we don’t know on other branes we don’t know, separated from our world in unseen dimensions.
New forces confined to distant branes might exist. New particles with which we will never directly interact might propagate on such other branes. Additional stuff accounting for dark matter and dark energy—the matter and energy that we surmise from their gravitational effects but whose identity is a mystery—might be distributed among different branes, or even in both the bulk and on other branes. And gravity might even influence particles differently as you go from one brane to the next.
If there is life on another brane, those beings, imprisoned in an entirely different environment, most likely experience entirely different forces that are detected by different senses. Our senses are attuned to the chemistry, light, and sound surrounding us. Because fundamental forces and particles are likely to be different, the creatures of other branes, should they exist, are unlikely to bear much resemblance to the life of our brane. The other branes will probably be nothing like our own. The only necessarily shared force is gravity, and even gravity’s influence can vary.
The consequences of a braneworld will depend on the number and types of branes, and where they are located. Unfortunately for the curious, particles and forces confined to distant branes are not required to influence us very strongly. They might merely determine what travels in the bulk, and emit weak signals which might never even reach us. Therefore many conceivable braneworlds will be very difficult to detect, even if they do exist. After all, gravity is the only interaction that we know for sure is shared between the stuff on our brane and the stuff on any other brane, and gravity is an extremely weak force. Without direct evidence, other branes will remain cloistered in the realm of theory and conjecture.
But some of the braneworlds I will present could lead to detectable signals. The detectable braneworlds are the ones that have implications for the physical features of our world. Even though the proliferation of possible braneworlds is in some respects frustrating, it is really quite exciting. Not only might branes help resolve long-standing problems in particle physics, but if we’re lucky, and one of the scenarios that I will describe is correct, evidence for braneworlds should appear in experiments with elementary particle physics very soon. We might really be living on a brane—and we might actually