This raises the question of how the next revolution in physics might break out: Will it be an episode of Kuhnian irrationalism, as hordes of rank-and-file physicists jump aboard the superstring bandwagon for shabby reasons of professional politics? Or an aesthetic revolution?
In his forthcoming Beauty and Revolution in Science, the philosopher James W. McAllister argues that all revolutionary episodes in science have really been of the aesthetic variety. Consistency with evidence, predictive success, explanatory power--these criteria for scientific truth have survived revolutions unscathed. It is an idea of beauty, even of simplicity itself (a terrifically complicated notion), that is toppled and replaced. "Every property that has at some date been seen as aesthetically attractive in theories has at other times been judged as displeasing or objectively neutral," McAllister writes.
This is no whim of intellectual fashion. Scientists fancy some aesthetic feature of a theory to the extent that it is linked with empirical success at the time. For instance, René Descartes, Pierre Gassendi, and Robert Boyle, all rivals to Newton, found "corpuscularism" attractive. This was the doctrine that all phenomena should be explained as the motion and collisions of solid corpuscles that possess no occult properties. Newton, however, could find no corpuscularist story that accounted for celestial motion. So, hearkening back to Renaissance magic, he appealed to gravitational "action at a distance." This was at first a serious blemish in a theory that was eventually viewed as the soul of beauty.
Looking back through the history of science, it is plain that beauty is a harbinger of probability only in retrospect. Most aesthetic convictions held by scientists--like the Aristotelian notion that uniform circular motion is the loveliest of all--ended up impeding the advance of knowledge. Innovations like Kepler's ellipses or quantum mechanics usually come surrounded by an aura of ugliness, which predictive success replaces with the splendor of truth. The great exception is Einstein's relativity theory, a singularly heroic discovery that set the tone for the rest of the century. Today's physicists have returned to the classical Greek project of understanding the cosmos by thought alone. The beautiful Final Theory will be the true one, not because it accords with the empirical evidence, but because it reflects the mind of God--a God who geometrizes or arithmetizes, depending on your taste.