Warped Passages - Lisa Randall [35]
So, the approach to advancing beyond the Standard Model that I will emphasize is model building. The term “model” might evoke a small-scale battleship or castle you built in your childhood. Or you might think of numerical simulations on a computer that are meant to reproduce known dynamics—how a population grows, for example, or how water moves in the ocean. Modeling in particle physics is not the same as either of these definitions. However, it’s not entirely different from the use of the word in magazines or fashion shows: models, both on runways* and in physics, demonstrate imaginative creations and come in a variety of shapes and forms. And the beautiful ones get all the attention.
Needless to say, the similarities end there. Particle physics models are guesses at alternative physical theories that might underlie the Standard Model. If you think of a unified theory as the summit of a mountain, model builders are trailblazers who are trying to find the path that connects the solid ground below, consisting of well-established physical theories, to the peak—the path that will ultimately tie new ideas together. Although model builders acknowledge the fascination of string theory and the possibility that it could turn out to be true, they are not as certain as string theorists that they know what theory they will find if they ever get to the top.
As we will see in Chapter 7, the Standard Model is a definite physical theory with a fixed set of particles and forces that reside in a four-dimensional world. Models that go beyond the Standard Model incorporate its ingredients and mimic its consequences 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. Physicists propose these models to address current puzzles. Models might suggest different behaviors for known or conjectured particles, behaviors determined by a new set of equations that follow from a model’s assumptions. Or they might suggest a new spatial setting, such as the ones we’ll explore with extra dimensions or branes.
Even when we fully understand a theory and its implications, that theory can be implemented in different ways, which might have different physical consequences for the real world in which we live. For example, even when we know how particles and forces interact in principle, we still need to know which particular particles and forces exist in the real world. Models allow us to sample the possibilities.
Different assumptions and physical concepts distinguish theories, as do the distance or energy scales at which a theory’s principles apply. Models are a way of getting at the heart of such distinguishing features. They let you explore a theory’s potential implications. If you think of a theory as general instructions for making a cake, a model would be a precise recipe. The theory would say to add sugar, but the model would specify whether to add half a cup or two cups. The theory would say that raisins are optional, but the model would tell you to be sensible and leave them out.
Model builders look at the unresolved aspects of the Standard Model and try to use known theoretical ingredients to address its inadequacies. The model building approach is fueled by the instinct that the energies for which string theory makes definite predictions are too far away from those we can observe. Model builders try to see the big picture so they can find the pieces that could be relevant to our world.
We model builders pragmatically admit that we can’t derive everything at once. Instead of trying to derive string theory’s consequences, we try to figure out which ingredients of the underlying physical theory will explain known observations and reveal relationships among