Warped Passages - Lisa Randall [190]
Theorists had a more mixed reaction. On the one hand, large extra dimensions seemed outlandish; no one had considered them before, since no one knew of any reason why extra dimensions should be so large. On the other hand, no one could identify a way to rule them out. In fact, before the first paper about large extra dimensions was written, Gia Dvali, one its the authors, spoke about them at Stanford. The authors, who were aware of the radical nature of their proposal, awaited the talk with trepidation, and were relieved when there were no serious objections. But they were also dismayed—how could people accept this pretty radical idea with such equanimity? Nima told me that when they first posted their paper on the Internet, they had a similar experience. Although they had expected a flood of responses, they received only two. Apparently the Italian physicist Riccardo Rattazzi and I were the only ones to comment on some potential problems. And even these two messages were not really independent: Riccardo and I had just discussed the paper at CERN, where we were both visiting.
Subsequently, as physicists absorbed the implications of the ADD model, they investigated the real-world consequences in more detail, considering tests of gravity, accelerator searches, astrophysical consequences, and cosmological implications. Reactions varied according to research interest or style.
Physicists whose research explored details of the Standard Model were happy to accept the possibility of a new idea, one which was in any case interesting. Surprisingly, there was more hostility from some model builders, who were unwilling to forfeit ideas about supersymmetry that had become entrenched over the years. Admittedly, altering the Standard Model so dramatically poses formidable challenges. Any new model would have to reproduce those features of the Standard Model that have already been experimentally verified, and theories that alter the Standard Model too dramatically will have a tough time meeting this challenge. Furthermore, the shining light of supersymmetry—the unification of couplings, the fact that at high energy all forces would have equal strength—would have to be abandoned. However, younger theorists not so wedded to supersymmetry were more excited. The topic of extra dimensions was a new, not-yet-cornered idea, and posed new challenges and open questions.
The reaction from string theorists was mixed as well. When Savas Dimopoulos began his project, he foresaw that work on extra dimensions would bring string theory and particle physics closer together. And string theorists did pay attention, though most of them viewed large extra dimensions as an interesting idea that would never be relevant to string theory. For string theorists the major problem was theoretical: it is very difficult to understand how dimensions could be as large as assumed in the ADD proposal.
Personally, I don’t believe that extra dimensions, even if they exist, will turn out to be this large.* Both for theoretical reasons (it’s hard to get dimensions that are this large) and for observational ones (it’s very tough to get cosmology to work out), the idea seems like a long shot. Even Nima, one of the protagonists, is skeptical at this point. But this was a very important theoretical idea. This new, previously unexplored suggestion highlighted the extent of our ignorance about gravity and the shape of the universe. The ADD paper stimulated a good deal of new thought, and whether or not the idea proves correct, it has had an important impact on physicists’ thinking. The large-dimension scenarios have led to many new proposals for extra dimensions and many ideas for experimental tests. After the LHC turns on, theoretical prejudices will be irrelevant in any case, since the implications of hard data will be irrefutable. Who knows? They might turn out to be right.
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