Why Does the World Exist: An Existentia - By Jim Holt Page 0,86

to convince some of the world’s leading philosophers that he might not be entirely daft in doing so. Yet somehow I wasn’t surprised to learn that he lived in Canada.

Interlude

It from Bit?

Mathematical Platonism turned out to be a nonstarter as an ultimate explanation of being. But its shortcomings invite deeper reflection on the nature of reality.

Of what does reality, at the most fundamental level, consist? It was Aristotle who supplied the classic answer to this question:

Reality = Stuff + Structure

This Aristotelian doctrine is known as “hylomorphism,” from the Greek hyle (stuff) and morphe (form, structure). It says that nothing really exists unless it is a composite of structure and stuff. Stuff without structure is chaos—tantamount, in the ancient Greek imagination, to nothingness. And structure without stuff is the mere ghost of being, as ontologically wispy as the smile of the Cheshire Cat.

Or is it?

Over the last few centuries, science has relentlessly undermined this Aristotelian understanding of reality. The better our scientific explanations get, the more that “stuff” tends to drop out of the picture. The dematerialization of nature began with Isaac Newton, whose theory of gravity invoked the seemingly occult notion of “action at a distance.” In Newton’s system, the Sun reached out and exerted its gravitational pull on Earth, even though there was nothing but empty space between them. Whatever the mechanism of influence between the two bodies might be, it seemed to involve no intervening “stuff.” (Newton himself was coy on how this could be, declaring Hypotheses non fingo—“I frame no hypotheses.”)

If Newton dematerialized nature on the largest of scales, from the solar system on up, modern physics has done the same on the smallest of scales, from the atom on down. In 1844, Michael Faraday, observing that matter could be recognized only by the forces acting on it, asked, “What reason is there to suppose that it exists at all?” Physical reality, Faraday proposed, actually consists not of matter but of fields—that is, of purely mathematical structures defined by points and numbers. In the early twentieth century, atoms, long held up as paragons of solidity, were discovered to be mostly empty space. And quantum theory revealed that their subatomic constituents—electrons, protons, and neutrons—behaved more like bundles of abstract properties than like little billiard balls. At each deeper level of explanation, what was thought to be stuff has given way to pure structure. The latest development in this centuries-long trend toward the dematerialization of nature is string theory, which builds matter out of pure geometry.

The very notion of impenetrability, so basic to our everyday understanding of the material world, turns out to be something of a mathematical illusion. Why don’t we fall through the floor? Why did the rock rebound when Dr. Johnson kicked it? Because two solids can’t interpenetrate each other, that’s why. But the reason they can’t has nothing to do with any sort of intrinsic stufflike solidity. Rather, it’s a matter of numbers. To squash two atoms together, you’d have to put the electrons in those atoms into numerically identical quantum states. And that is forbidden by something in quantum theory called the “Pauli exclusion principle,” which allows two electrons to sit directly on top of each other only if they have opposite spins.

As for the sturdiness of individual atoms, that too is essentially mathematical. What keeps the electrons in an atom from collapsing into the nucleus? Well, if the electrons were sitting right on top of the nucleus, we’d know exactly where each electron was (right in the center of the atom) and how fast it was moving (not at all). And that would violate Heisenberg’s uncertainty principle, which does not permit the simultaneous determination of a particle’s position and momentum.

So the solidity of the ordinary material objects that surround us—tables and chairs and rocks and so forth—is a joint consequence of the Pauli exclusion principle and Heisenberg’s uncertainty principle. In other words, it comes down to a pair of abstract mathematical relations. As the poet Richard Wilbur wrote, “Kick at the rock, Sam Johnson, break your bones: / But cloudy, cloudy is the stuff of stones.”

At its most fundamental, science describes the elements of reality in terms of their relations to one another, ignoring any stufflike quiddity those elements might possess. It tells us, for example, that an electron has a certain mass and charge, but these are mere propensities for the electron to be acted upon in certain ways by other particles and forces. It tells us that mass is equivalent