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The Revolution That Didn't Happen STEVEN WEINBERG	6 (Back to page 1)
of motion or the theory that fire is an element ("phlogiston") are false. Kuhn himself in his earlier book on the Copernican revolution told how parts of scientific theories survive in the more successful theories that supplant them, and seemed to have no trouble with the idea. Confronting this contradiction, Kuhn in Structure gave what for him was a remarkably weak defense, that Newtonian mechanics and Maxwellian electrodynamics as we use them today are not the same theories as they were before the advent of relativity and quantum mechanics, because then they were not known to be approximate and now we know that they are. It is like saying that the steak you eat is not the one that you bought, because now you know it is stringy and before you didn't. It is important to keep straight what does and what does not change in scientific revolutions, a distinction that is not made in Structure.5 There is a "hard" part of modern physical theories ("hard" meaning not difficult, but durable, like bones in paleontology or potsherds in archeology) that usually consists of the equations themselves, together with some understandings about what the symbols mean operationally and about the sorts of phenomena to which they apply. Then there is a "soft" part; it is the vision of reality that we use to explain to ourselves why the equations work. The soft part does change; we no longer believe in Maxwell's ether, and we know that there is more to nature than Newton's particles and forces. The changes in the soft part of scientific theories also produce changes in our understanding of the conditions under which the hard part is a good approximation. But after our theories reach their mature forms, their hard parts represent permanent accomplishments. If you have bought one of those T-shirts with Maxwell's equations on the front, you may have to worry about its going out of style, but not about its becoming false. We will go on teaching Maxwellian electrodynamics as long as there are scientists. I can't see any sense in which the increase in scope and accuracy of the hard parts of our theories is not a cumulative approach to truth.6 Some of what Kuhn said about paradigm shifts does apply to the soft parts of our theories, but even here I think that Kuhn overestimated the degree to which scientists during a period of normal science are captives of their paradigms. There are many examples of scientists who remained skeptical about the soft parts of their own theories. It seems to me that Newton's famous slogan Hypotheses non fingo (I do not make hypotheses) must have meant at least in part that his commitment was not to the reality of gravitational forces acting at a distance, but only to the validity of the predictions derived from his equations. However that may be, I can testify that although our present theory of elementary particles, the Standard Model, has been tremendously successful in accounting for the measured properties of the particles, physicists today are not firmly committed to the view of nature on which it is based. The Standard Model is a field theory, which means that it takes the basic constituents of nature to be fields—conditions of space, considered apart from any matter that may be in it, like the magnetic field that pulls bits of iron toward the poles of a bar magnet—rather than particles. In the past two decades it has been realized that any theory based on quantum mechanics and relativity will look like a field theory when experiments are done at sufficiently low energies. The Standard Model is today widely regarded as an "effective field theory," a low-energy approximation to some unknown fundamental theory that may not involve fields at all. Even though the Standard Model provides the paradigm for the present normal-science period in	5 I am grateful to Professor Christopher Hitchcock for a comment after my talk at Rice that led me to include the following remark in this essay. (back) 6 Another complication: As professor Bruce Hunt pointed out to me in conversation, it can happen that two competing theories with apparently different hard parts can both make the same successful predictions. For instance, in the nineteenth century it was common for British physicists to describe electromagnetic phenomena using equations that involved electric and magnetic fields, following the lead of Faraday, while the equations of continental physicists referred directly to forces acting at a distance. Usually what happens in such cases is that the two sets of equations are discovered to be mathematically equivalent, although one or the other may turn out to have a wider generalization in a more comprehensive theory, as turned out to be the case for electric and magnetic fields after the advent of relativity. (back)

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