To Explain the World: The Discovery of Modern Science Read online

  To Explain the World: The Discovery of Modern Science

  Steven Weinberg

  HarperCollins Publishers (2015)

  * * *

  Rating: ★★★☆☆

  In this rich, irreverent, compelling history, Nobel Prize-winning physicist Steven Weinberg takes us from ancient Miletus to medieval Baghdad and Oxford, from Plato's Academy and the Museum of Alexandria to the cathedral school of Chartres and the Royal Society of London. He shows that the scientists of ancient and medieval times not only did not understand what we now know about the world, they did not understand what there is to be understood, or how to learn it. Yet over the centuries, through the struggle to solve such mysteries as the curious apparent backward movement of the planets or the rise and fall of the tides, science eventually emerged as a modern discipline. Along the way, Weinberg examines historic clashes and collaborations between science and the competing spheres of religion, technology, poetry, mathematics, and philosophy.

  An illuminating exploration of how we have come to consider and analyze the world around us, To Explain the World is a sweeping, ambitious account of how difficult it was to discover the goals and methods of modern science, and the impact of this discovery on human understanding and development.

  Dedication

  To Louise, Elizabeth, and Gabrielle

  Epigraph

  These three hours that we have spent,

  Walking here, two shadows went

  Along with us, which we ourselves produced;

  But, now the sun is just above our head,

  We do those shadows tread;

  And to brave clearness all things are reduced.

  John Donne, “A Lecture upon the Shadow”

  Contents

  Dedication

  Epigraph

  Preface

  PART I: GREEK PHYSICS

  1. Matter and Poetry

  2. Music and Mathematics

  3. Motion and Philosophy

  4. Hellenistic Physics and Technology

  5. Ancient Science and Religion

  PART II: GREEK ASTRONOMY

  6. The Uses of Astronomy

  7. Measuring the Sun, Moon, and Earth

  8. The Problem of the Planets

  PART III: THE MIDDLE AGES

  9. The Arabs

  10. Medieval Europe

  PART IV: THE SCIENTIFIC REVOLUTION

  11. The Solar System Solved

  12. Experiments Begun

  13. Method Reconsidered

  14. The Newtonian Synthesis

  15. Epilogue: The Grand Reduction

  Acknowledgments

  Technical Notes

  Endnotes

  Bibliography

  Index

  About the Author

  Also by Steven Weinberg

  Credits

  Copyright

  About the Publisher

  Preface

  I am a physicist, not a historian, but over the years I have become increasingly fascinated by the history of science. It is an extraordinary story, one of the most interesting in human history. It is also a story in which scientists like myself have a personal stake. Today’s research can be aided and illuminated by a knowledge of its past, and for some scientists knowledge of the history of science helps to motivate present work. We hope that our research may turn out to be a part, however small, of the grand historical tradition of natural science.

  Where my own past writing has touched on history, it has been mostly the modern history of physics and astronomy, roughly from the late nineteenth century to the present. Although in this era we have learned many new things, the goals and standards of physical science have not materially changed. If physicists of 1900 were somehow taught today’s Standard Model of cosmology or of elementary particle physics, they would have found much to amaze them, but the idea of seeking mathematically formulated and experimentally validated impersonal principles that explain a wide variety of phenomena would have seemed quite familiar.

  A while ago I decided that I needed to dig deeper, to learn more about an earlier era in the history of science, when the goals and standards of science had not yet taken their present shape. As is natural for an academic, when I want to learn about something, I volunteer to teach a course on the subject. Over the past decade at the University of Texas, I have from time to time taught undergraduate courses on the history of physics and astronomy to students who had no special background in science, mathematics, or history. This book grew out of the lecture notes for those courses.

  But as the book has developed, perhaps I have been able to offer something that goes a little beyond a simple narrative: it is the perspective of a modern working scientist on the science of the past. I have taken this opportunity to explain my views about the nature of physical science, and about its continued tangled relations with religion, technology, philosophy, mathematics, and aesthetics.

  Before history there was science, of a sort. At any moment nature presents us with a variety of puzzling phenomena: fire, thunderstorms, plagues, planetary motion, light, tides, and so on. Observation of the world led to useful generalizations: fires are hot; thunder presages rain; tides are highest when the Moon is full or new, and so on. These became part of the common sense of mankind. But here and there, some people wanted more than just a collection of facts. They wanted to explain the world.

  It was not easy. It is not only that our predecessors did not know what we know about the world—more important, they did not have anything like our ideas of what there was to know about the world, and how to learn it. Again and again in preparing the lectures for my course I have been impressed with how different the work of science in past centuries was from the science of my own times. As the much quoted lines of a novel of L. P. Hartley put it, “The past is a foreign country; they do things differently there.” I hope that in this book I have been able to give the reader not only an idea of what happened in the history of the exact sciences, but also a sense of how hard it has all been.

  So this book is not solely about how we came to learn various things about the world. That is naturally a concern of any history of science. My focus in this book is a little different—it is how we came to learn how to learn about the world.

  I am not unaware that the word “explain” in the title of this book raises problems for philosophers of science. They have pointed out the difficulty in drawing a precise distinction between explanation and description. (I will have a little to say about this in Chapter 8.) But this is a work on the history rather than the philosophy of science. By explanation I mean something admittedly imprecise, the same as is meant in ordinary life when we try to explain why a horse has won a race or why an airplane has crashed.

  The word “discovery” in the subtitle is also problematic. I had thought of using The Invention of Modern Science as a subtitle. After all, science could hardly exist without human beings to practice it. I chose “Discovery” instead of “Invention” to suggest that science is the way it is not so much because of various adventitious historic acts of invention, but because of the way nature is. With all its imperfections, modern science is a technique that is sufficiently well tuned to nature so that it works—it is a practice that allows us to learn reliable things about the world. In this sense, it is a technique that was waiting for people to discover it.

  Thus one can talk about the discovery of science in the way that a historian can talk about the discovery of agriculture. With all its variety and imperfections, agriculture is the way it is because its practices are sufficiently well tuned to the realities of biology so that it works�
�it allows us to grow food.

  I also wanted with this subtitle to distance myself from the few remaining social constructivists: those sociologists, philosophers, and historians who try to explain not only the process but even the results of science as products of a particular cultural milieu.

  Among the branches of science, this book will emphasize physics and astronomy. It was in physics, especially as applied to astronomy, that science first took a modern form. Of course there are limits to the extent to which sciences like biology, whose principles depend so much on historical accidents, can or should be modeled on physics. Nevertheless, there is a sense in which the development of scientific biology as well as chemistry in the nineteenth and twentieth centuries followed the model of the revolution in physics of the seventeenth century.

  Science is now international, perhaps the most international aspect of our civilization, but the discovery of modern science happened in what may loosely be called the West. Modern science learned its methods from research done in Europe during the scientific revolution, which in turn evolved from work done in Europe and in Arab countries during the Middle Ages, and ultimately from the precocious science of the Greeks. The West borrowed much scientific knowledge from elsewhere—geometry from Egypt, astronomical data from Babylon, the techniques of arithmetic from Babylon and India, the magnetic compass from China, and so on—but as far as I know, it did not import the methods of modern science. So this book will emphasize the West (including medieval Islam) in just the way that was deplored by Oswald Spengler and Arnold Toynbee: I will have little to say about science outside the West, and nothing at all to say about the interesting but entirely isolated progress made in pre-Columbian America.

  In telling this story, I will be coming close to the dangerous ground that is most carefully avoided by contemporary historians, of judging the past by the standards of the present. This is an irreverent history; I am not unwilling to criticize the methods and theories of the past from a modern viewpoint. I have even taken some pleasure in uncovering a few errors made by scientific heroes that I have not seen mentioned by historians.

  A historian who devotes years to study the works of some great man of the past may come to exaggerate what his hero has accomplished. I have seen this in particular in works on Plato, Aristotle, Avicenna, Grosseteste, and Descartes. But it is not my purpose here to accuse some past natural philosophers of stupidity. Rather, by showing how far these very intelligent individuals were from our present conception of science, I want to show how difficult was the discovery of modern science, how far from obvious are its practices and standards. This also serves as a warning, that science may not yet be in its final form. At several points in this book I suggest that, as great as is the progress that has been made in the methods of science, we may today be repeating some of the errors of the past.

  Some historians of science make a shibboleth of not referring to present scientific knowledge in studying the science of the past. I will instead make a point of using present knowledge to clarify past science. For instance, though it might be an interesting intellectual exercise to try to understand how the Hellenistic astronomers Apollonius and Hipparchus developed the theory that the planets go around the Earth on looping epicyclic orbits by using only the data that had been available to them, this is impossible, for much of the data they used is lost. But we do know that in ancient times the Earth and planets went around the Sun on nearly circular orbits, just as they do today, and by using this knowledge we will be able to understand how the data available to ancient astronomers could have suggested to them their theory of epicycles. In any case, how can anyone today, reading about ancient astronomy, forget our present knowledge of what actually goes around what in the solar system?

  For readers who want to understand in greater detail how the work of past scientists fits in with what actually exists in nature, there are “technical notes” at the back of the book. It is not necessary to read these notes to follow the book’s main text, but some readers may learn a few odd bits of physics and astronomy from them, as I did in preparing them.

  Science is not now what it was at its start. Its results are impersonal. Inspiration and aesthetic judgment are important in the development of scientific theories, but the verification of these theories relies finally on impartial experimental tests of their predictions. Though mathematics is used in the formulation of physical theories and in working out their consequences, science is not a branch of mathematics, and scientific theories cannot be deduced by purely mathematical reasoning. Science and technology benefit each other, but at its most fundamental level science is not undertaken for any practical reason. Though science has nothing to say one way or the other about the existence of God or an afterlife, its goal is to find explanations of natural phenomena that are purely naturalistic. Science is cumulative; each new theory incorporates successful earlier theories as approximations, and even explains why these approximations work, when they do work.

  None of this was obvious to the scientists of the ancient world or the Middle Ages, and all of it was learned only with great difficulty in the scientific revolution of the sixteenth and seventeenth centuries. Nothing like modern science was a goal from the beginning. How then did we get to the scientific revolution, and beyond it to where we are now? That is what we must try to learn as we explore the discovery of modern science.

  PART I

  GREEK PHYSICS

  During or before the flowering of Greek science, significant contributions to technology, mathematics, and astronomy were being made by the Babylonians, Chinese, Egyptians, Indians, and other peoples. Nevertheless, it was from Greece that Europe drew its model and its inspiration, and it was in Europe that modern science began, so the Greeks played a special role in the discovery of science.

  One can argue endlessly about why it was the Greeks who accomplished so much. It may be significant that Greek science began when Greeks lived in small independent city-states, many of them democracies. But as we shall see, the Greeks made their most impressive scientific achievements after these small states had been absorbed into great powers: the Hellenistic kingdoms, and then the Roman Empire. The Greeks in Hellenistic and Roman times made contributions to science and mathematics that were not significantly surpassed until the scientific revolution of the sixteenth and seventeenth centuries in Europe.

  This part of my account of Greek science deals with physics, leaving Greek astronomy to be discussed in Part II. I have divided Part I into five chapters, dealing in more or less chronological order with five modes of thought with which science has had to come to terms: poetry, mathematics, philosophy, technology, and religion. The theme of the relationship of science to these five intellectual neighbors will recur throughout this book.

  1

  Matter and Poetry

  First, to set the scene. By the sixth century BC the western coast of what is now Turkey had for some time been settled by Greeks, chiefly speaking the Ionian dialect. The richest and most powerful of the Ionian cities was Miletus, founded at a natural harbor near where the river Meander flows into the Aegean Sea. In Miletus, over a century before the time of Socrates, Greeks began to speculate about the fundamental substance of which the world is made.

  I first learned about the Milesians as an undergraduate at Cornell, taking courses on the history and philosophy of science. In lectures I heard the Milesians called “physicists.” At the same time, I was also attending classes on physics, including the modern atomic theory of matter. There seemed to me to be very little in common between Milesian and modern physics. It was not so much that the Milesians were wrong about the nature of matter, but rather that I could not understand how they could have reached their conclusions. The historical record concerning Greek thought before the time of Plato is fragmentary, but I was pretty sure that during the Archaic and Classical eras (roughly from 600 to 450 BC and from 450 to 300 BC, respectively) neither the Milesians nor any of the other Greek students of nature were re
asoning in anything like the way scientists reason today.

  The first Milesian of whom anything is known was Thales, who lived about two centuries before the time of Plato. He was supposed to have predicted a solar eclipse, one that we know did occur in 585 BC and was visible from Miletus. Even with the benefit of Babylonian eclipse records it’s unlikely that Thales could have made this prediction, because any solar eclipse is visible from only a limited geographic region, but the fact that Thales was credited with this prediction shows that he probably flourished in the early 500s BC. We don’t know if Thales put any of his ideas into writing. In any case, nothing written by Thales has survived, even as a quotation by later authors. He is a legendary figure, one of those (like his contemporary Solon, who was supposed to have founded the Athenian constitution) who were conventionally listed in Plato’s time as the “seven sages” of Greece. For instance, Thales was reputed to have proved or brought from Egypt a famous theorem of geometry (see Technical Note 1). What matters to us here is that Thales was said to hold the view that all matter is composed of a single fundamental substance. According to Aristotle’s Metaphysics, “Of the first philosophers, most thought the principles which were of the nature of matter were the only principles of all things. . . . Thales, the founder of this school of philosophy, says the principle is water.”1 Much later, Diogenes Laertius (fl. AD 230), a biographer of the Greek philosophers, wrote, “His doctrine was that water is the universal primary substance, and that the world is animate and full of divinities.”2

  By “universal primary substance” did Thales mean that all matter is composed of water? If so, we have no way of telling how he came to this conclusion, but if someone is convinced that all matter is composed of a single common substance, then water is not a bad candidate. Water not only occurs as a liquid but can be easily converted into a solid by freezing or into a vapor by boiling. Water evidently also is essential to life. But we don’t know if Thales thought that rocks, for example, are really formed from ordinary water, or only that there is something profound that rock and all other solids have in common with frozen water.