Warped Passages - Lisa Randall [219]
What’s New
Localized gravity is a local phenomenon. It doesn’t depend on distant regions of spacetime.
Gravity can behave as if the world has different dimensions in different regions, since a localized graviton does not necessarily extend over all of space.
We could be living in an isolated pocket of space that appears to be four-dimensional.
24
Extra Dimensions: Are You In or Out?
But I still haven’t found what I’m looking for.
U2
Athena’s dreams about OneDLand, branes, and five dimensions were passed down for generations. When Ike XLII heard them, he wanted to check whether there was any truth to her stories. So he took out his Alicxvr and went down to a very small scale—not so small that strings would appear, but sufficiently small to check whether there was a fifth dimension. The Alicxvr answered Ike’s question by sending him off to a five-dimensional world.
But Ike was not completely satisfied. He remembered the bizarre things that had happened earlier on when he had fooled around with the hyperdrive option. So he once again cranked up the hyperdrive lever—and once again, everything changed drastically. Ike couldn’t identify a single familiar object. He could tell only one thing: the fifth dimension had disappeared.
Ike was mystified, so he searched the spacernet to see what it could tell him about “dimensions.” He waded through numerous sites that he recognized from his more embarrassing spam, but soon realized that he’d have to refine his search. When he still couldn’t find anything definitive, he conceded that he wouldn’t know the fundamental origin of dimensions any time soon. So he decided to turn his attention to time travel instead.
Physics has entered a remarkable era. Ideas that were once the realm of science fiction are now entering our theoretical—and maybe even experimental—grasp. Brand-new theoretical discoveries about extra dimensions have irreversibly changed how particle physicists, astrophysicists, and cosmologists now think about the world. The sheer number and pace of discoveries tells us that we’ve most likely only scratched the surface of the wondrous possibilities that lie in store. Ideas have taken on a life of their own.
Nonetheless, many questions have yet to be fully answered, and our journey is far from over. Particle physicists still want to know why we see the particular forces we see, and are there any more. What is the origin of the masses and properties of familiar particles? We also want to know whether string theory is right. And if it is, how does it connect to our world?
Recent observations of the cosmos point to even more mysteries we want to address. What composes most of the energy and matter in the universe? Was there a brief phase of explosive expansion early on in the universe’s evolution, and if so, what caused it? And everyone wants to know what the universe looked like when it started.
We now know that gravity can behave very differently on different length scales. At very short distances, only a quantum theory of gravity such as string theory will describe gravity. On larger scales, general relativity applies admirably well, but recent observations across the universe at very big distances pose cosmological puzzles, such as what accelerates its expansion. And at longer distances still, we reach the cosmological horizon beyond which we know nothing.
One of the intriguing aspects of extra-dimensional theories is that they naturally have different consequences on different scales. Gravity in such theories exhibits behavior at distances smaller than curled-up dimensions, or where the curvature is too small to have an effect, that is different