Warped Passages - Lisa Randall [180]
At the Oxford supersymmetry conference in 1998, the Stanford physicist Savas Dimopoulos gave one of the most interesting talks. He reported on work he had done in collaboration with two other physicists, Nima Arkani-Hamed and Gia Dvali. The colorful names of these three match their colorful characters and ideas. Savas gets very excited about his projects; his collaborators tell me that his enthusiasm is always contagious. He was so taken with extra dimensions that he told a colleague that all the new unexplored physics ideas made him feel like a kid in a candy store—he wanted to eat it all before anyone else got any. Gia, a physicist from former Soviet Georgia, takes great risks, both in his approach to physics and in his audacious feats of mountaineering. He was once stuck without any food on a stormy mountaintop in the Caucusus for two nights. Nima, a physicist from an Iranian family, is very energetic, stimulating, and vividly articulate. Now my Harvard colleague, he often roams the hallways enthusiastically explaining his latest research and convincing others to join in.
Ironically, Savas’s talk at the supersymmetry conference, which was not about supersymmetry at all but was instead about extra dimensions, stole some of supersymmetry’s thunder. He explained that extra dimensions, rather than supersymmetry, could be the physical theory underlying the Standard Model. And if his suggestion was correct, experimenters could expect to find evidence of extra dimensions, rather than supersymmetry, when they explore the weak scale in the near future.
This chapter presents Arkani-Hamed, Dimopoulos, and Dvali’s* idea about how very large dimensions might explain the weakness of gravity. In essence, large extra dimensions could dilute the gravitational force so much that gravity’s strength would be much weaker than estimates without extra dimensions would have you believe. Their models don’t actually solve the hierarchy problem because you still have to explain why the dimensions are so large. But ADD hoped this new and different question would be more tractable.
We’ll also consider the related question ADD asked: how big can rolled-up extra dimensions be if Standard Model particles are confined to a brane and are not free to travel in the bulk without contradicting experimental results? The answer they found was extraordinary. At the time they wrote their paper, it looked like extra dimensions could be as big as a millimeter.
Dimensions (Almost) as Large as a Millimeter
In the ADD model, as in the sequestering model I described in Chapter 17, the Standard Model particles are confined to a brane. However, the two models had very different objectives, so their remaining features are completely different. Whereas the sequestering model had one additional dimension that was bounded between two branes, the ADD models all have more than one dimension and those dimensions are curled up. Depending on the details of the implementation, space in their models contains two, three, or more additional curled-up dimensions. Moreoever, the ADD model contains a single brane on which the Standard Model particles are confined, but that brane does not bound space. It simply sits inside the extra curled-up dimensions, as is illustrated in Figure 75.33
Figure 75. Schematic drawing of the ADD braneworld. The universe’s extra dimensions are rolled up (and large). We live on a brane (the dotted line along the cylinder), so only gravity experiences the extra dimensions.
One question ADD wanted to address with their setup was how large extra dimensions could still be hidden if all particles of the Standard Model were trapped on a brane and the only force in the higher-dimensional bulk was gravity. The answer they found surprised most physicists. As opposed to the size of one-hundredth of a thousandth of a trillionth