Warped Passages - Lisa Randall [32]
In the next section we’ll consider the modern methods used to pursue such clues: model building—my forte—and the alternative approach to fundamental high-energy physics, namely, string theory. String theorists try to derive universal predictions from a definite theory, whereas model builders try to find ways to solve particular physical problems and then to build up theories from these starting points. Model builders and string theorists both seek more comprehensive theories with more explanatory power. They aim to answer similar questions, but they approach them in different ways. Research sometimes involves educated guesses, as with model building, and sometimes it involves deducing logical consequences of the ultimate theory you already believe to be correct, as with the string theory approach. We’ll soon see that the recent research on extra dimensions successfully combines elements of both methods.
Model Building
Although I was first drawn to math and science by the certainty they promised, today I find the unanswered questions and the unexpected connections at least as attractive. The principles contained in quantum mechanics, relativity, and the Standard Model stretch the imagination, but they barely scratch the surface of the remarkable ideas engrossing physicists today. We know that something new is required because of the deficiencies in existing ideas. Those shortfalls are harbingers of novel physical phenomena that should emerge when we do more precise experiments.
Particle physicists try to find the laws of nature that explain how elementary particles behave. These particles, and the physical laws they obey, are components of what physicists call a theory—a definite set of elements and principles with rules and equations for predicting how those elements interact. When I speak about theories in this book, I’ll be using the word in this sense; I won’t mean “rough speculations”, as in more colloquial usage.
Ideally, physicists would love to find a theory capable of explaining all observations, one that uses the sparest possible set of rules and the fewest possible fundamental ingredients. The ultimate goal for some physicists is a simple, elegant, unifying theory—one that can be used to predict the result of any particle physics experiment.
The quest for such a unifying theory is an ambitious—some might say audacious—task. Yet in some respects it mirrors the search for simplicity that began long ago. In ancient Greece, Plato imagined perfect forms, such as geometric shapes and ideal beings, that earthly objects only approximate. Aristotle also believed in ideal forms, but he thought that only observations can reveal the ideals that physical objects resemble. Religions also often postulate a more perfect or more unified state that is removed from, but somehow connected to, reality. The story of the fall from the Garden of Eden supposes an idealized prior world. Although the questions and methods of modern physics are very different from those of our ancestors, physicists, too, are seeking a simpler universe, not in philosophy or religion but in the fundamental ingredients that constitute our world.
However, there is an obvious impediment to finding an elegant theory that we can connect to our world: when we look around us, we see very little of the simplicity that such a theory should embody. The problem is that the world is complex. It takes a lot of work to connect a simple, spare formulation to the more complicated real world. A unified theory, while being simple and elegant, must somehow accommodate enough structure for it to match observations. We would like to believe that there is a perspective from which everything is elegant and predictable. Yet the universe is not as pure, simple, and ordered as the theories with which we hope to describe it.
Particle physicists negotiate the terrain connecting theory to observations with two distinct methodologies. Some theorists follow a “top-down” approach: they start with the theory they believe to be correct—for example,