Knocking on Heaven's Door - Lisa Randall [131]
MODEL BUILDING
In the end, just as in life, science doesn’t have just a single criterion for beauty. We merely have some intuitions—along with experimental constraints—that we use to guide our search for knowledge. Beauty—both in art and science—might have some objective aspects, but almost any application involves taste and subjectivity.
For scientists, however, there is one big difference. Ultimately experiments will decide which, if any, of our ideas are correct. Scientific advances might exploit aesthetic criteria, but true scientific progress also requires understanding, predicting, and analyzing data. No matter how beautiful a theory appears, it can still be wrong, in which case it must be thrown away. Even the most intellectually satisfying theory has to be abandoned if it doesn’t work in the real world.
Nonetheless, before we reach the higher energies or distant parameters needed to determine the correct physical descriptions, physicists have no choice but to employ aesthetic and theoretical considerations to guess what lies beyond the Standard Model. In this interim, with only limited data, we rely on existing puzzles coupled with taste and organizational criteria to point the way forward.
Ideally, we’d like to be able to work through the consequences of a variety of possibilities. Model building is the name of the approach we use to do this. My colleagues and I explore various particle physics models, which are guesses for physical theories that might underlie the Standard Model. Our goals are simple principles that organize the complicated phenomena that appear on more readily visible scales so that we can resolve current puzzles in our understanding.
Physics model builders take the effective theory viewpoint and the desire to understand smaller and smaller distance scales very much to heart. We follow a “bottom-up” approach that starts with what we know—both the phenomena we can explain and those we find puzzling—and attempt to deduce the underlying model that explains the connections among observed elementary particle properties and their interactions.
The term “model” might evoke a physical structure such as a small-scale version of a building used to display and explore its architecture. Or you might think of numerical simulations on a computer that calculate the consequences of known physical principles—such as climate modeling or models for the spread of contagious diseases.
Modeling in particle physics is very different from either of these definitions. Particle models do, however, share some of the flair of models in magazines or fashion shows. Models, both on runways and in physics, illustrate imaginative new ideas. And people initially flock toward the beautiful ones—or at least those that are more striking or surprising. But in the end, they are drawn toward the ones that show true promise.
Needless to say, the similarities end there.
Particle physics models are guesses for what might underlie the theories whose predictions have been already tested and that we understand. Aesthetic criteria are important in deciding which ideas are worth pursuing. But so are consistency and testability of the ideas. Models characterize different underlying physical ingredients and principles that apply at distances and sizes that are smaller than those which have yet been experimentally tested. With models, we can determine the essence and consequences of different theoretical assumptions.
Models are a means of extrapolating from what is known to create proposals for more comprehensive theories with greater explanatory power. They are sample proposals that may or may not prove correct once experiments allow us to delve into smaller distances or higher energies and test their underlying hypotheses and predictions.