The Last Word On Nothing

The following are excerpts from a profile of Jim Peebles, who just yesterday won a Nobel Prize. Peebles is a quintessential theorist — “I spent a few days standing near telescopes getting cold,” he said, “and in the end, my attention wandered” — whose opinion of his own theories is finely balanced.

The profile is old, from 1992, and from a magazine now dead, The Sciences. At the time I wrote it, the microwaves left over from the Big Bang had just been reliably measured for the first time, even though they’d been detected 27 years before by observers who didn’t know what they’d detected until Peebles told them. The observers went on to win the Nobel Prize; Peebles wasn’t mentioned. Should he have been? I asked, and he thought not: “Anyway,” he said, “I’m too young to be famous.” Apparently he’s old enough now.

In the informal basement cafeteria of the otherwise fancy faculty club at Princeton University, the physicist and cosmologist Philip James Edwin Peebles and three younger colleagues are having lunch over the latest hot news in cosmology. The lunch is fortuitous: one by one, the cosmologists have showed up a little after one o’clock and gravitated to the same table. Ruth Daly, David Spergel and Neil Turok are theorists, like Peebles, and among them the four have three theories about what went on in the early universe. The hot news comes from COBE, the Cosmic Background Explorer, a space probe that, for nearly three years, has been mapping a remnant of the birth of the universe, the big bang. . . . COBE has given theorists some badly needed constraints on how the early universe evolved. And so they debate: Whose theory of the early universe is still tenable?

Spergel and Turok are coming close to losing, but they are being gentlemen about it. Peebles is trying to convince the others that he is not losing. Daly, whose main interest is distant radio galaxies, remains uncommitted. As they talk, the four cosmologists become so intense, so serious that the outsider, who can barely follow what they say, can nonetheless see what they see: in the air just above the sandwiches, chips, fruit and juice, entire universes appear, evolve and disappear.

But, eventually, creating and modifying universes in thin air gets too hard even for the very smart, and Peebles resorts to the napkin under his plate, requests a pen and condenses the latest universe to graphs on the napkin. In the end, having pushed chairs back and put trays away, Daly, Spergel and Turok look thoughtful, and Peebles looks just a bit smug. The game has been fair throughout: although he is by far the most famous and senior of the four, he has taken up only a quarter of the conversation, and his convictions have been judged, as they should be, on merit alone. He is fierce and tenacious about his position, but he is unfailingly courteous about those of the others. He phrases his comments gently, as questions: “Doesn’t COBE specify that?” he asks. “And those fluctuations are on what scale?”

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Peebles, although he likes a good story, favors another narrative standard. “What’s best,” he says, “is what’s true.” So when he tells the story, as he does, it is a reasonably well substantiated account of the history of the universe. Peebles has written a good part of that history himself, and he has spent his career working out the implications of his [microwave background radiation] prediction: yes, the universe is still expanding; the light–or more properly, the radiation-has cooled to a quiet background whose temperature is three degrees above absolute zero; matter has now gathered into galaxies and clusters of galaxies and superclusters of galaxies. But how did those structures form? How did the universe get from then to now?

Down a bright corridor of the Joseph Henry Laboratories in Princeton’s Jadwin Hall, Peebles’s office comes after Robert H. Dicke’s. The order is appropriate: thirty years ago Dicke was Peebles’s thesis adviser at Princeton. Dicke is now the Albert Einstein professor of science, emeritus, and Peebles is the current Einstein professor. As Peebles was finishing his postdoctoral work, Dicke was beginning to think that the background radiation, expanding with the universe, should now be vastly cooler. “[Dicke] said to me, ‘Why don’t you think about the theoretical consequences?'” says Peebles. “I may have been callow, but I knew that when Bob Dicke said something, I should pay attention. So I did.”

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Peebles is famous in his field because for a long time the field has tracked the direction of his work. That fact is not something he seems to think about. His work has never had a direction, he says; he has never made long range plans. On the contrary, he operates by “a random walk, no, an undirected walk, or rather a locally directed walk: as you take each step you decide where the next one is going to go.” The reason for his short-term plans, he says, is a short attention span: “I would go bananas if I spent my life looking for one thing.” But a list of the problems he’s worked on suggests a preoccupation with Dicke’s question, and one suspects Peebles’s intellectual walk has taken the shortest possible path toward discovering the full theoretical consequences of the big bang. In the process, he has begun or helped begin what are now some of the field’s minor industries.

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Peebles codified his taste for observables and predictables, for seeing the universe as a physics problem, in the textbook he wrote in 1971, Physical Cosmology. The book is famous for having created a new field by the same name, but Peebles denies he started it. All the questions had been asked, he says, before he wrote that book, though he will admit they had not been as heavily emphasized. “I think I can lay claim to having deflected the direction of the field,” he says, then makes a midcourse correction: “Even that is too strong, because the field was going to change direction. I just wrote the book at the right time.”

Meanwhile, he has just finished a new “darn book on cosmology,” which began as an update of Physical Cosmology and then just grew. He gets irritable when asked the title of his new book: “The Revenge of Physical Cosmology,” he says snidely, “The Decline and Fall of Physical Cosmology.” In Peebles’s office the notes for his apparently nameless book cover the top of a table and take up about fifteen cubic feet of hanging files.

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It’s a charming story. A point explodes, radiation and matter fly outward, matter falls inward into clouds, clouds spin together into galaxies, galaxies are pulled into the neighborhoods of clusters and superclusters of galaxies. The story is particularly charming because it is probably true, or much of it is-at least the part that’s observable. Peebles, with his physicist’s taste, has quantified the charm. In 1988 and 1990 he and another theorist, Joseph Silk of the University of California at Berkeley, wrote two articles for Nature, one called “A Cosmic Book,” the other “A Cosmic Book of Phenomena.” They mean book as in the phrase “making book”: placing odds on the truth, or at least believability, of various cosmological theories and the observations on which they are based. They awarded highest probability to one of the universes built on defects in space-time, which now, with the COBE results, looks much less probable. Peebles’s own baryonic universe came in at middling probability.

Discouraging? Probably. Why do it? “If I were at all good with poetry,” Peebles says, “I could imagine sitting down and writing this beautiful ‘pome,’ and it would give me great pleasure to look at it. I do science because I enjoy seeing how things will work out. It’s very personal. It’s totally selfish. I’m pleasing myself.” Then he adds, “Because I love to see the way things connect up to reality.”

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Photo taken in 1995 by Princeton University, Robert P. Matthews (1995); used with permission

Peebles’ Revenge of Physical Cosmology ended up being titled, Principles of Physical Cosmology.

The profile, called “Once Upon the Start of Time,” ran in the Sep/Oct, 1992 issue of The Sciences, a beautiful but erstwhile magazine published by the New York Academy of Sciences. The profile discusses Peebles’ science, which this post does not. It’s online but paywalled so if you think you need a copy, let me know.