Physicists tend to shy away from philosophy (and religion, although there are some exceptions). Any dealings with philosophy – other than those concerning quantum theory – tend to be superficial allusions to Karl Popper’s notion of falsification or to the “paradigm shifts” championed by Thomas Kuhn in his book The Structure of Scientific Revolutions . Put simply, Popper said that theories can never be proved right, only wrong, and that any new theory worthy of the name should make new predictions that can be tested by experiment. Kuhn described science as a cyclic process in which periods of normal science are interspersed with revolutions in which the paradigm of the time is replaced by a new one. An example is the shift from the classical to the quantum paradigm.

Physicists are, by and large, happy with both these notions in the simplistic way expressed here. Falsification certainly happens and paradigms do shift from time to time. However, Steven Weinberg, the Nobel prize winning theoretical physicist, has looked more closely at Kuhn’s work and, in a stylish essay entitled The Revolution That Didn’t Happen, he concludes that Kuhn was wrong about many points.

Weinberg’s assault on Kuhn has been inspired, at least in part, by the recent controversy between various scientists, mostly physicists, and certain sociologists of science on the meaning of “truth” and “reality” in science. It is not so much the basic notion of a paradigm shift that Weinberg objects to, but two of the boundary conditions that Kuhn has attached to it and the consequences that follow from these conditions. Weinberg explains at length how the first condition – that different paradigms are “incommensurable” – is wrong. What concerns him more, however, is Kuhn’s claim that “we may have to relinquish the notion… that changes of paradigm carry scientists and those who learn from them closer and closer to the truth”. As Weinberg writes: “It is just these conclusions that have made Kuhn a hero to the philosophers, historians, sociologists, and cultural critics who question the objective character of scientific knowledge, and who prefer to describe scientific theories as social constructions, not so different from democracy or baseball.”

Weinberg views theories as having a “hard” (as in durable, not difficult) part, such as Maxwell’s equations, and a “soft” part within which the theory is interpreted, for example the ether in the days of Maxwell. Science essentially involves building on the hard part of the theory while revising or changing the soft part as necessary. “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, ” writes Weinberg. “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 it going out of style, but not about its becoming false. 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.”

Weinberg concludes: “The birth of Newtonian physics was a mega-paradigm shift, but nothing that has happened in our understanding since then – not the transition from Newtonian to Einsteinian mechanics, or from classical to quantum physics – fits Kuhn’s description of a paradigm shift.” How philosophers and others respond to Weinberg remains to be seen – his last appearance in this debate provoked a large reaction – but his attempt to overturn one of the most influential theories in the philosophy of science could result in the paradigm shift to end all paradigm shifts.