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Jul 31st 2021

AS HE LIKED to tell it, there were three epiphanies in Steven Weinbergs life. The first came in a wooden box. It was a chemistry set, passed on by a cousin who was tired of it. As he played with the chemicals in it, and found that each reacted differently because of atoms, a vast thought struck him: if he learned about atoms, he would know how the whole world worked.

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The second epiphany came when, as a teenager, he paid a routine visit to his local library in New York. On the table was a book called Heat, open to a page of equations. Among them was the elegant, unknown swirl of an integral sign. It showed that with a mathematical formula, and a magic symbol, science could express something as rudimentary as the glow of a candle flame. His third awakening, when he was in his 20s and already a professor of physics, was the discovery that a mathematical theory could be applied to the whole dazzling array of stars and planets, dark space beyond them and, he concluded, everything.

All regularities in nature followed from a few simple laws. Not all were known yet; but they would be. In the end he was sure they would combine into a set of equations simple enough to put on a T-shirt, like Einsteins E=mc2. It was just a matter of continually querying and searching. In the strange circumstance of finding himself conscious and intelligent on a rare patch of ordinary matter that was able to sustain life, doggedly asking questions was the least he could do.

His signal achievement was to discover, in the 1960s, a new level of simplicity in the universe. There were then four known universal forcesgravity and electromagnetism, both of which operate at large scales, and the strong and weak nuclear forces, both of which are appreciable only at small scales. Electromagnetism was explained by a quantum field theory; similar theories for the nuclear forces were eagerly being sought.

In quantum field theories, forces are mediated by particles called bosons; the boson involved in electromagnetism is the photon, the basic particle of light. He and others showed that a theory of the weak force required three bosons: the W+ and the W-, which carried electric charges, and the Z0, which did not. The W particles were at play in the observable universe; they were responsible for some sorts of radioactive decay. The Z was notional until, in 1973, researchers at CERN, Europes great particle-physics lab, observed neutral currents between the particles they were knocking together. These had never been seen before, and could be explained only by the Z. In 1979 the Nobel prize duly followed.

In his understated way, he called his contribution very satisfactory. It was not just that the weak force and the electromagnetic force could be explained by similar tools. At high energies they were basically the same thing.

That triumph of unification increased his curiosity about the only point where such high energies were known to have existed: the Big Bang. In his book The First Three Minutes, in 1977, he described the immediate aftermath, to the point where the hyper-hot cosmic soup had cooled enough for atomic nuclei to form. He saw early on how deeply particle physics and cosmology were intertwined, and became fascinated by the idea of a universe dominated by unobservable dark energy and dark matter in which ordinary matter (the stars and the planets and us) was merely a small contamination. He longed for CERN s Large Hadron Collider to find evidence of dark matter. It caused him lasting frustration that Congress in 1993 had cancelled the Superconducting Super Collider, which was to have been even bigger.

Whatever was found, he was sure it would fit into the simple scheme of natures laws. Quantum mechanics, however, troubled him. He worried that its determinism implied that the world was endlessly splitting, generating myriad parallel histories and universes in which the constants in nature would have different values. Goodbye to a unified theory of everything, if that were so.

Such a unified law would have given him satisfaction but, he knew, no comfort. Natures laws were impersonal, cold and devoid of purpose. Certainly there was no God-directed plan. As he wrote at the end of The First Three Minutes, the more the universe seemed comprehensible, the more it seemed pointless. No saying of his became more famous, but the next paragraph softened it: humans gave the universe their own point and purpose by the way they lived, by loving each other and by creating art.

He set the example by marrying Louise, his college sweetheart, devouring opera and theatre, revelling in the quirky liberalism of Austin, where he taught at the University of Texas for almost four decades, and looking for theories in physics that would carry the same sense of inevitability he found so beautiful in chamber music, or in poetry. He still thought of human existence as accidental and tragic, fundamentally. But from his own little island of warmth and love, art and science, he managed a wry smile.

What angered him most was the persistence of religion. It had not only obstructed and undermined science in the age of Galileo and Copernicus; it had also survived Darwin, whose theory of evolution had shocked it more sharply than anything physics did. And it was still there, an alternative theory of the world that corroded free inquiry. For even if the laws of nature could be reduced to one, scientists would still ask: Why? Why this theory, not another? Why in this universe, and not another?

There was, he reflected, no end to the chain of whys. So he did not stop asking or wondering. He liked to review and grade his predecessors, from the ancient Greeks onwards, chastising them for failing to use the data they had, but also sympathising with their lack of machines advanced enough to prove their ideas. The human tragedy was never to understand why things were as they were. Yet, for all that, he could echo Ptolemy: I know that I am mortal and the creature of a day, but when I search out the massed wheeling circles of the stars, my feet no longer touch the EarthI take my fill of ambrosia, the food of the gods.

This article appeared in the Obituary section of the print edition under the headline "Natures laws"

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In search of nature's laws Steven Weinberg died on July 23rd - The Economist