Knocking on Heaven's Door - Lisa Randall [164]
BACK ON SOLID GROUND
String theory very likely contains some deep and promising ideas. It has already given us insights into quantum gravity and mathematics and provided interesting ingredients for model builders to pursue. But it will most likely be a long time before we can solve the theory sufficiently to answer the questions we would most like to solve. Deriving string theory’s consequences for the real world directly from scratch might just be too difficult. Even if successful models ultimately arise from string theory, the clutter of superfluous elements makes them very difficult to find.
The model-building approach in physics is fueled by the instinct that the energies at which string theory makes definite predictions are too remote from those we can observe. As with many phenomena that have different descriptions on different scales, it could be that the mechanisms that address questions in particle physics are best studied at the relevant energies.
Physicists share common goals, but we have different expectations about how best to achieve them. I prefer the model-building approach because it is more likely to receive experimental guidance in the near future. My colleagues and I might use ideas from string theory, and some of our research might have string theory implications, but applying string theory is not my primary goal. Understanding testable phenomena is. Models can be described and subjected to experimental tests, even before any connection to a more fundamental theory is made.
Model builders pragmatically admit that we can’t derive everything at once. A model’s assumptions could be part of the ultimate underlying theory, or they might simply illuminate new relationships that have still deeper theoretical underpinnings. Models are effective theories. Once a model proves correct, it can provide direction for string theorists, or anyone attempting a more top-down approach. And models already benefit from the rich set of ideas that string theory provides. But models primarily focus on lower energies, and experiments that apply at these scales.
Models that go beyond the Standard Model incorporate its ingredients as well as the results at energies that have already been explored, but they also contain new forces, new particles, and new interactions that can be seen only at shorter distances. Even so, fitting everything we know is difficult, and the resulting precise model that I or anyone else works on often loses much of its initial elegance. For this reason, model builders need to have open minds.
People are often puzzled when I tell them that I work on many different models when I know that they can’t all be correct and that the LHC should tell us more about which could be right. They are even more surprised when I explain that I don’t necessarily assign huge probabilities to any particular model I am thinking about. Nonetheless, I choose projects that illuminate a genuinely new explanatory principle or new type of experimental search. The models I consider generally have some interesting feature or mechanism that provides interesting potential explanations for mysterious phenomena. Given the many unknowns—and uncertain criteria for progress—predicting and interpreting reality poses formidable challenges. It would be miraculous to get it all right from the get-go.
One of the beautiful aspects of the extra-dimensional theories is that ideas from both the top-down and bottom-up camps converged to produce them. String theorists recognized the critical role of branes in their theoretical