Warped Passages - Lisa Randall [36]
Physics always strives to predict the largest number of physical quantities from the smallest number of assumptions, but that doesn’t mean that we always manage to identify the most fundamental theories right away. Advances have often been made before everything was understood at the most fundamental level. For example, physicists understood the notions of temperature and pressure and employed them in thermodynamics and engine design long before anyone had explained these ideas at a more fundamental microscopic level as the result of the random motion of large numbers of atoms and molecules.
Because models relate to physical “phenomena,” (meaning experimental observations) model builders with stronger ties to experiment are sometimes called phenomenologists. “Phenomenology” is a poor choice of word, however. It does not do justice to data analysis, which in today’s complex scientific world is deeply embedded in theory. Model building is far more tied to interpretation and mathematical analysis than phenomenology, in the philosophical sense of the word, would suggest.
The best models do, however, have an invaluable feature. They yield definite predictions for physical phenomena, giving experimenters a way to verify or contradict a model’s claims. High-energy experiments are not merely searching for new particles—they are testing models and looking for clues to better ones. Any proposed particle physics model will involve new physical principles and new physical laws that apply at measurable energies. It will therefore predict new particles and testable relationships among them. Finding these particles and measuring their properties can confirm or rule out proposed ideas. The goal of high-energy experiments is to shed light on underlying physical laws and the conceptual framework that gives them their explanatory power.
Only some models will prove correct, but models are the best way to investigate possibilities and build up a reservoir of compelling ingredients. And if string theory is right, we might eventually learn how some models follow as consequences of it, much as thermodynamics was rooted in atomic theory. However, for about a decade the two communities were sharply divided. As Albion Lawrence, a young string theorist from Brandeis University, commented recently when he and I were discussing this schism, “One of the tragedies is that string theory and model building were distinct intellectual subjects. Model builders and string theorists weren’t talking to each other for years. I always thought of string theory as the granddaddy of all models.”
Both string theorists and model builders are searching for a tractable, elegant route that connects theory to the observed world. Any theory will be truly compelling and likely to be correct only if this path, and not just the view from the top, manifests this elegance. Model builders, who start from the bottom, run the risk of many false starts, but string theorists, who start at the top, run the risk of finding themselves at the edge of a precipitous, isolated cliff, too remote for them to find their way back to base camp.
You might say that we are all searching for the language of the universe. But whereas string theorists focus on the inner logic of the grammar, model builders focus on the nouns and phrases that they think are most useful. If particle physicists were in Florence learning Italian, the model builders would know how to ask for lodging and acquire the vocabulary that would be essential to finding their way around, but they might talk funny and never fully comprehend the Inferno. String theorists, on the other hand, might aspire to grasp the subtleties of Italian literature—but run the risk of starving to death before learning how to ask for dinner!
Fortunately, things have now changed. These days, theory and low-energy phenomena bolster each other