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October 8, 1998
The Revolution That Didn't Happen STEVEN WEINBERG
I first read Thomas Kuhn's famous book The Structure of Scientific Revolutions1 a quarter-century ago, soon after the publication of the second edition. I had known Kuhn only slightly when we had been together on the faculty at Berkeley in the early 1960s, but I came to like and admire him later, when he came to MIT. His book I found exciting. Evidently others felt the same. Structure has had a wider influence than any other book on the history of science. Soon after Kuhn's death in 1996, the sociologist Clifford Geertz remarked that Kuhn's book had "opened the door to the eruption of the sociology of knowledge" into the study of the sciences. Kuhn's ideas have been invoked again and again in the recent conflict over the relation of science and culture known as the science wars. Structure describes the history of science as a cyclic process. There are periods of "normal science" that are characterized by what Kuhn sometimes called a "paradigm" and sometimes called a "common disciplinary matrix." Whatever you call it, it describes a consensus view: in a period of normal science, scientists tend to agree about what phenomena are relevant and what constitutes an explanation of these phenomena, about what problems are worth solving and what is a solution of a problem. Near the end of a period of normal science a crisis occurs—experiments give results that don't fit existing theories, or internal contradictions are discovered in these theories. There is alarm and confusion. Strange ideas fill the scientific literature. Eventually there is a revolution. Scientists become converted to a new way of looking at nature, resulting eventually in a new period of normal science. The "paradigm" has shifted. To take an example given special attention in Structure, after the widespread acceptance of Newton's physical theories—the Newtonian paradigm—in the eighteenth century, there began a period of normal science in the study of motion and gravitation. Scientists used Newtonian theory to make increasingly accurate calculations of planetary orbits, leading to spectacular successes like the prediction in 1846 of the existence and orbit of the planet Neptune before astronomers discovered it. By the end of the nineteenth century there was a crisis: a failure to understand the motion of light. This problem was solved through a paradigm shift, a revolutionary revision in the understanding of space and time carried out by Einstein in the decade between 1905 and 1915. Motion affects the flow of time; matter and energy can be converted into each other; and gravitation is a curvature in space-time. Einstein's theory of relativity then became the new paradigm, and the study of motion and gravitation entered upon a new period of normal science. Though one can question the extent to which Kuhn's cyclic theory of scientific revolution fits what we know of the history of science, in itself this theory would not be very disturbing, nor would it have made Kuhn's book famous. For many people, it is Kuhn's reinvention of the word "paradigm" that has been either most useful or most objectionable. Of course, in ordinary English the word "paradigm" means some accomplishment that serves as a model for future work. This is the way that Kuhn had used this word in his earlier book2 on the scientific revolution associated with Copernicus, and one way that he continued occasionally to use it. The first critic who took issue with Kuhn's new use of the word "paradigm" in Structure was Harvard President James Bryant Conant. Kuhn had begun his career as a historian as Conant's assistant in teaching an undergraduate course at Harvard, when Conant asked Kuhn to prepare case studies on the history of mechanics. After seeing a draft of Structure, Conant complained to Kuhn that "paradigm" was "a	1 Thomas S. Kuhn, The Structure of Scientific Revolutions (University of Chicago Press, 1962; second edition, 1970), quoted below as Structure. This essay is based in part on the author's 1997 Bohner Lecture at Rice University, delivered as part of its yearlong symposium on the work of Thomas Kuhn, and on a 1998 colloquium talk given at the Department of Physics at Harvard University. (back) 2 Thomas S. Kuhn, The Copernican Revolution (Harvard University Press, 1957). (back)

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