Knocking on Heaven's Door - Lisa Randall [160]
CHAPTER EIGHTEEN
BOTTOM-UP VERSUS TOP-DOWN
Nothing substitutes for solid experimental results. But we physicists haven’t just been sitting on our thumbs for the last quarter century waiting for the LHC to turn on and produce meaningful data. We’ve thought long and hard about what it is that experiments should look for and what the implications of the data are likely to be. We have also studied results from experiment s that have been operational during this time frame, and these have taught us details about known particles and interactions and helped orient our thinking.
This interim period has also been a tremendous opportunity to think more deeply about ideas that at least for the time being are more removed from data. Some of the more interesting and speculative models and theoretical insights of the last twenty-five years resulted from these more mathematical pursuits. I doubt that I, for one, would have thought about extra dimensions or more mathematical aspects of supersymmetry had data been more plentiful. Even if measurements that would ultimately support these ideas had been made, the implications would have taken a while to unravel without the luxury of previous mathematical pursuits.
Experiments and mathematics both lead to scientific advances. But the road to progress is rarely clear, and physicists have been divided as to the best strategy. Model builders use the “bottom-up” approach introduced in Chapter 15 to start with what is known from experiments and then address residual puzzling unexplained features—often employing more theoretical mathematical developments. The last chapter presented some specific examples of models and how they influence the searches experimenters at the LHC will perform.
Others, most notably string theorists, apply a “top-down” way of thinking, in which they start with the theory that they believe is true—namely string theory—and try to use its underlying concepts to formulate a consistent quantum theory of gravity. Top-down theories are defined at high energies and small distances. The label refers to the theoretical notion that everything can be derived from fundamental premises defined at high-energy scales. Although the name can be confusing since high energies correspond to short distances, recall that the ingredients at small distances are the fundamental building blocks of matter. In this way of thinking, everything can be derived from basic principles and fundamental ingredients, which are defined at small distances and high energies—hence the label “top-down.”
This chapter is about the top-down and bottom-up approaches and the ways in which they contrast with each other. We’ll explore the differences, but also reflect on how they occasionally converge to yield remarkable insights.
STRING THEORY
Unlike model builders, more mathematically inclined physicists try to work from pure theory. The hope is to start with a single elegant theory and derive the consequences, and only then apply the ideas to data. Most any attempt at a unified theory embodies such a top-down approach. String theory is perhaps the most prominent such example. It is a conjecture for the ultimate underlying framework from which all other known physics phenomena would in principle follow.
String theorists take a major leap in the physics scales they try to conquer—jumping from the weak scale to the Planck scale at which gravity becomes strong. Experiments probably won’t directly test these ideas anytime soon (although the extra-dimensional models of the last chapter might be an exception). But even though string theory itself is difficult to test, elements of string theory do provide ideas and concepts that potentially observable models have incorporated.
The question physicists ask when deciding on model building versus string theory is whether to follow the Platonic approach,