Gravity is arguably the most obvious and most obscure of natural phenomena. What goes up must come down. The current scientific explanation of why this happens, the general theory of relativity, invokes challenging and counterintuitive concepts like curved geodesics in a four-dimensional spacetime. With such extremes between the apparent and the arcane, gravity forces a sharp focus on scientific method. The relation between observation and theory is the heartbeat of science, and the pulse is nowhere stronger than in comparing what we observe and what we theorize about gravity. This is the motivation for a book about both the science of gravity and the scientific method.
If gravity is such a simple and obvious phenomenon, why complicate things with obscure theory? If gravity is so undeniably real and so easily observed, as in, go jump off a cliff and then tell me the law of gravity is a social construct, why does the basic description, the theoretical account of gravity, change? How could the early scientists have missed something so obvious as a force between two massive objects? But by the light of general relativity, it's not a force after all. So, maybe it's not so obvious.
What Goes Up …Gravity and Scientific Method will clarify the theories of gravity from Aristotle to Einstein. Aristotle's explanation was that a stone falls because it seeks its natural place at the center of the universe. According to Newton, a stone falls because of an instantaneous force from the massive Earth. Einstein, and most physicists now, say a stone falls because, with no forces acting, it follows the curved geodesic in spacetime. Differences and similarities between the theories will be highlighted, and there will be some surprises. For example, Aristotle's idea that the trajectory of the falling stone is guided by a point in space, the center of the universe, is not so different from Einstein's claim that the trajectory of the fall is guided by a line in spacetime, the geodesic.
Equally important to understanding these scientific results will be understanding the scientific methods. By showing how the theories were derived and tested, we will clarify what makes science scientific. The focus will be on the relation between theory and evidence, and links among theoretical ideas and principles. Again, methods will be compared, and again there will be surprises. Aristotle is often criticized for deriving his scientific ideas from principles rather than evidence, and for failing to test his theories. Einstein did very much the same thing by starting with the Principle of Relativity and the Principle of Equivalence, and he is famously dismissive of the importance of the 1919 solar-eclipse evidence, one of the so-called classic tests of the general theory of relativity.
Once a few main questions and concepts are in place, the development of the theories of gravity will be in historical order, from ancient Greece to the present, with one exception. The survey starts with Newton. The first two chapters raise the relevant questions about scientific method and introduce the helpful scientific concepts to describe gravity, but then the third chapter begins the study of gravity with a review of what you would find in any introductory physics textbook, that is, the Newtonian theory. It will be out of historical context and with no justification or proof, not a word about scientific method. From there, we go back to the historical beginning and follow the development from Aristotle to Newton, this time with both context and motivation, to Einstein and current developments. The reason for doing the basic Newtonian theory first is to start with the familiar. When most people think of gravity they think of Newton, and maybe the acceleration of gravity at 9.8 m/s2 and the basic force law F = GMm∕r2. But they generally don't ask where the ideas came from or how they differ from what came before and after. The force of gravity is taken for granted. We start by making the familiar precise, so we have a sense of our perspective and no longer take it for granted. The first look at Newtonian gravity will give us some conceptual context, a way to compare what came before and after. It's wise to do history with your current ideas out in the open. That way we'll see if our description of the past is being influenced by our understanding of the present.
There are no prerequisites for reading the book, neither scientific nor mathematical. Technical terms will be minimal and always explicitly clarified. Any important concepts, whether scientific or philosophical, will be printed in bold when they are first described in the text, and then defined in the Glossary. There is a little math, but it is never more challenging than algebra and geometry. The math will be limited and in all cases avoidable by readers who find it off-putting. It will be there for those who enjoy it and for everyone to at least see what work in gravity looks like to those doing the work. We won't derive any of the equations, or even use them to do calculations. The emphasis will be on understanding the physical implications of the components of an equation. For example, the r2 in the equation in the previous paragraph, the 1∕r2 dependence in the law of gravity, is important. It's not just that the force gets weaker as you get further away; it decreases as the square of the distance. We'll find out why the 1∕r2 is there, and what it leads to for phenomena like planetary orbits and escape velocity. By the time we get to the general theory of relativity, this sort of careful accounting of what work is done by each piece of the equation will clarify what it means to say that gravity is the curvature of spacetime.
This book started as a class in the Northern Arizona University Honors Program. I have the Honors Program to thank for the inspired proposal of a class that requires both substantive science and a study of how science works, and for allowing me the opportunity to teach the class with gravity as the focus. The students in the class were a great help in pointing out where things were confusing and in many cases clearing up the confusion. In that formative way they helped write What Goes Up…Gravity and Scientific Method. Several of my colleagues at Northern Arizona University helped along the way with conversations about relativity, astronomy, and the history of science. Thanks to David Sherry in the philosophy department, to Ed Anderson and Gary Bowman in physics, and to Andrea Holmen in both.
I should also thank a few people with whom I had the pleasure to work at Cambridge University Press. Thanks to Vince Higgs, to whom I pitched the proposal and who had the kindness to first improve my pitching and then take on the project. And to Philippa Cole, whose correspondence I always looked forward to, for both the cheerful encouragement and detailed help.
Some of the material in the book is taken from my previously published work. Parts of Chapters 4 and 5 come from “Void points, rosettes, and a brief history of planetary astronomy,” Physics in Perspective, 15 (2013), 373–390, used with the kind permission from Springer Science+Business Media. The first half of Chapter 8 is from “The discovery of Neptune,” School Science Review, 90 (September 2008), 53–58, by permission from the Association for Science Education. Chapter 13 has pieces from “Detecting extrasolar planets,” Studies in History and Philosophy of Science, 37 (2006), 224–236, and Chapter 14 comes in part from “Evidence of dark matter and the interpretive role of general relativity,” Studies in History and Philosophy of Modern Physics, 44 (2013), 143–147. Both are by permission from Elsevier.
