Knocking on Heaven's Door - Lisa Randall [50]
However, string theory poses enormous conceptual and mathematical challenges. No one yet knows how to formulate string theory to answer all the questions we would want a theory of quantum gravity to address. Furthermore, the string scale of 10-33 cm is likely to be beyond the reach of any experiment we can think about.
So a reasonable question is whether investigating string theory is a reasonable expenditure of time and resources. I am often asked this question. Why would anyone study a theory so unlikely to yield experimental consequences? Some physicists find mathematical and theoretical consistency reason enough. Those people think they can repeat the type of success Einstein had when he developed his general theory of relativity, based in large part on purely theoretical and mathematical investigations.
But another motivation for studying string theory—one that I think is very important—is that it can and has provided new ways of thinking about ideas that apply on measurable scales. Two of those ideas are supersymmetry and theories of extra dimensions, ideas that we will address in Chapter 17. These theories do have experimental consequences if they address problems in particle physics. In fact, if certain extra-dimensional theories prove correct and explain phenomena at LHC energies, even evidence of string theory could possibly appear at much lower energies. A discovery of supersymmetry or extra dimensions won’t be proof of string theory. But it will be a validation of the utility of working on abstract ideas, even those without direct experimental consequences. It will of course also be a testimony to the utility of experiments in probing even initially abstract-seeming ideas.
CHAPTER SIX
“SEEING” IS BELIEVING
Scientists could decipher what matter is made of only when tools were developed that let them look inside. The word “look” refers not to direct observations but to the indirect techniques that people use to probe the tiny sizes inaccessible to the naked eye.
It’s rarely easy. Yet despite the challenges and the counterintuitive results that experiments sometimes display, reality is real. Physical laws, even at tiny scales, can give rise to measurable consequences that eventually become accessible to cleverer investigations. Our current knowledge about matter and how it interacts is the culmination of many years of insight and innovation and theoretical development that permit us to consistently interpret a variety of experimental results. Through indirect observations, pioneered by Galileo centuries ago, physicists have deduced what is present at matter’s core.
We’ll now explore the current state of particle physics and the theoretical insights and experimental discoveries that have led us to where we are today. Inevitably, the description will have a rather list-like aspect to it as I enumerate the ingredients that compose the matter we know and how they were discovered. The list is a lot more interesting when we remember the very different behaviors of these diverse ingredients on different scales. The chair you are sitting on is ultimately reducible to these elements, but it’s quite a train of discoveries to get from here to there.
As Richard Feynman mischievously explained when talking about one of his theories, “If you don’t like it, go somewhere else—perhaps to another universe where the rules are simpler… I’m going to tell you what it looks like to human beings who have struggled as hard as they can to understand. If you don’t like it, that’s too bad.” 31 You may think that some of what we believe to be true is so crazy or cumbersome that you won’t want to accept it. But that won’t change the fact that it’s the way nature works.
SMALL WAVELENGTHS
Small distances seem strange because they are unfamiliar. We need tiny probes to observe what is happening on the smallest scales. The page (or screen) you are currently reading looks very different from what resides at matter’s core. That’s because the very act of seeing has to do with observing visible light. That light is emitted from electrons