Reductionism Redux

It used to be traditional for college courses on the history of philosophy to begin around 600 BC with Thales of Miletus. According to later writers, Thales taught that everything is made of water. Learning about Thales, undergraduates had the healthy experience of starting their study of philosophy with a doctrine that they knew to be false.

Though wrong, Thales and his pre-Socratic successors were not just being silly. They had somehow come upon the idea that it might be possible to explain a great many complicated things on the basis of some simple and universal principle—everything is made of water, or everything is made of atoms, or everything is made of atoms, or everything is in flux, or nothing ever changes, or whatever. Not much progress could be made with such purely qualitative ideas. Over two thousand years later Isaac Newton at last proposed mathematical laws of motion and gravitation, with which he could explain the motions of the planets, tides, and falling apples. Then in the Opticks, he predicted that light and chemistry would someday be understood “by the same kind of reasoning as for mechanical principles,” applied to “the smallest particles of nature.”

By the end of the nineteenth century physicists and chemists had succeeded in explaining much of what was known about chemistry and heat, on the basis of certain assumed properties of some ninety types of atoms—hydrogen atoms carbon atoms, iron atoms, and so on. In the 1920s physicists began to be able to explain the properties of atoms and other things like radioactivity and light, using a new universal theory known as quantum mechanics. The fundamental entities to which this theory was applied were no longer the atoms themselves but particles even more elementary than atoms—electrons, protons, and a few others—together with fields of force that surround them, like the familiar fields that surround magnets or electric charges.

By the mid-1970s it had become clear that the properties of these particles and all other known particles could be understood as mathematical consequences of a fairly simple quantum theory, known as the “standard model.” The fundamental equations of the standard model do not deal with particles and fields, but with fields of force alone; particles are just bundles of field energy. From Newton’s time to our own we have seen a steady expansion of the range of phenomena that we know how to explain, and a steady improvement in the simplicity and universality of the theories used in these explanations.

Science in this style is properly called reductionist. In a recent article in these pages1 Freeman Dyson described reductionism in physics as the effort “to reduce the world of physical phenomena to a finite set of fundamental equations.” I might quibble over whether it is equations or principles that are being sought, but it seems to me that in this description Dyson has caught the essence of reductionism pretty well. He also cited the work of Schroedinger and Dirac on quantum mechanics in 1925…

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