## Beautiful Losers: Plato's Geometry of Elements

Dec

29

This essay is part of the series Beautiful Losers.

Plato believed that he could describe the Universe using five simple shapes. These shapes, called the Platonic solids, did not originate with Plato. In fact, they go back thousands of years before Plato; you can find stone models (perhaps dice?) of each of the Platonic solids in the Ashmolean Museum at Oxford dating to around 2000 BC, as pictured below. But Plato made these solids central to a vision of the physical world that links ideal to real, and microcosm to macrocosm in an original, and truly remarkable, style.

AN1927.2727-31 Neolithic Carved Sandstone Balls, Copyright Ashmolean Museum, University of Oxford.

Let me explain, first, what the Platonic solids are. To begin, consider something simpler: regular polygons. Regular polygons, by definition, are two-dimensional shapes bounded by sides of equal length, each making the same angles with its neighbors. Equilateral triangles, squares, regular pentagons, and so on are all regular polygons. Platonic solids are the three-dimensional analog of regular polygons, and prove to be far more interesting. Platonic solids are bounded by regular polygons, all of the same size and shape. One can prove mathematically that there are exactly five Platonic solids. Here they are:

The Platonic solids and their proposed identification with fundamental world-elements

The tetrahedron has four triangular faces, the cube six square faces, the octahedron eight triangular faces, the dodecahedron twelve pentagonal faces, and the icosahedron twenty triangular faces. Plato proposed that four of these solids built the Four Elements: sharp-pointed tetrahedra give the sting of Fire, smooth-sliding octahedra give easily-parted Air, droplety icosahedra give Water, and lumpish, packable cubes give Earth. The dodecahedron, at last, is the shape of the Universe as a whole. Later Aristotle emended Plato's system, suggesting that dodecahedra provide a fifth essence—the space-filling Ether.

Plato's ideas lent dignity and grandeur to the study of geometry, and greatly stimulated its development. The thirteen and final book of Euclid's Elements, the grand synthesis of Greek geometry that is the founding text of axiomatic mathematics, culminates with the construction of the five Platonic solids, and a proof that they exhaust the possibilities. Scholars speculate that Euclid planned the Elements with that climax in mind from the start.

From a modern scientific perspective, of course, Plato's mapping from mathematical ideals to physical reality looks hopelessly wrong. The four (or five) ancient “elements” are not simple substances, nor are they usable building blocks for constructing the material world. Today's rich and successful analysis of matter involves entirely different concepts. And yet...

In its general approach, and its ambition, Plato's utterly mistaken theory anticipated the spirit of modern theoretical physics. His program of describing the material world by analyzing (“reducing”) it to a few atomic substances, each with simple properties, existing in great numbers of identical copies, coincides with modern understanding.

Deeper still penetrates his insight that symmetry defines structure. Plato sensed enormous potential in the fact that asking for perfect symmetry leads one to discover a small number of possible structures. Based on that foundation, and a few clues from experience, the outlandish synthesis that his philosophy suggested should be possible, to realize the World as Ideas, might be achievable. And clues were there to be found: Near-coincidence between the number of perfect solids (five) and the number of suspected elements (four); suggestions of how observed qualities might reflect underlying shapes (e.g., the sting of fire from the sharp points of tetrahedra). One must also admire the boldness of genius in seeing an apparent defect in the theory—five solids for four elements—as an opportunity for crowning creation, either with the Universe as a whole (Plato) or with space itself (Aristotle).

Modern physicists, when seeking equations to describe the unfamiliar laws of the microcosm, must make guesses based on fragmentary information. Optimistically—and lacking constructive alternatives—they have turned, as Plato did, to symmetry as their guide. Symmetry of equations is perhaps a less familiar idea than symmetry of shapes, but there is nothing obscure or mystical about it. We say an equation, like a shape, displays symmetry when it allows changes that make no change. So for instance the equation

X = Y

has a nice symmetry to it, because exchanging X for Y changes it into this:

Y = X

and this transformed equation expresses exactly the same content as the original. On the other hand X = Y + 2, say, turns into Y = X + 2, which expresses something else entirely. As this baby example demonstrates, symmetric equations can be rare and special, even when the symmetry involves quite simple transformations.

The equations of interest for physics are considerably richer, of course, and the "changes that make no change" we hope they allow are much more extensive and elaborate. But the central idea inspiration remains, as it was for Plato, the hope that symmetry defines a few interesting structures, and that Nature chooses one (or all!) of those most beautiful possibilities.

Plato's Beautiful Loser was, in hindsight, a product of premature, and immature, ambition. He tried to leap directly from beautiful mathematics, some imaginative numerology, and primitive, cherry-picked observations to a Theory of Everything. In this his ambition was premature. Also, Plato failed to draw out specific consequences from his ideas, or to test them critically. He sketched an inspiring world-model, but was content to "declare victory" without engaging any serious battles. The mature and challenging form of scientific ambition, which aspires to understand specific features of the world in detail and with precision, emerged only centuries later.

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