It's a realistic anxiety for theoretical physicists, who, as a rule, burn out early. Most have done their best work by 40, and some peak even sooner than that. (Einstein came up with the theory of relativity at 26.) So the question naturally arises: Why would Greene, still in his prime, squander his energies on a popular book for the muddling innumerate, particularly when there's still so much work to be done? The Elegant Universe took two years to write, so naturally his colleagues are concerned. "I think really highly of Brian," says Ed Witten. "But I'm glad he's finished writing his book." Why? He pauses. "Oh, I think maybe, perhaps, it slowed him down," he says. "Researchwise."

Greene says it didn't, but there are other worries. String theory, for all its hype, is still a young discipline. Some fear that if Greene's book is a big success, it might mislead the public and lure too much talent away from other important areas of research. "I do see some danger," admits Frank Wilczek. "People may not realize this is not yet an established theory, and also . . ." He trails off. "You know what Gresham's law is, right? That bad currency drives out the good? One thing science doesn't need is to be turned into mysticism. So I can only hope that Brian did a good job."

On the ninth floor of the Pupin Building, at the north end of the Columbia campus, Greene is standing outside his locked office door, sifting through a Ziploc bag full of loose keys. There must be 30 or 40 of them, all of nearly identical shape and size. He broke his key chain a few months ago and has been toting around this clanking bundle ever since.

"I don't think it's here."

He checks a few pouches of his shoulder bag. He reinspects the Ziploc bag. Then he checks his pocket. Eureka.

Greene's failure to do the obvious -- buy a new key chain -- is one of the ways that he does conform to the stereotype of the befuddled, I'm-too-distracted-to-tuck-in-my-shirt kind of physicist, the kind who floats through life in a vapor of deep thoughts. Another is that he can't seem to remember names and numbers, including the room number of his second office in Columbia's math department. Ellen Archer, his girlfriend (she's an actress, naturally), says that sometimes while he's driving, she'll notice that the car has suddenly slowed to a crawl, so lost is he in the caverns of his own mind. She remembers leaving him alone once at a party with one of her overtalkative friends. An hour later she found him in the same place, her friend still gabbling and burbling away like an open wound. "I'm so sorry to have stranded you," she told Greene after rescuing him. "It's okay," he told her. "I was solving equations."

Greene throws open the door, and we step into his office. The place is a disaster. There are carpet swatches and paint samples sitting on a broken chair, and big, white blotches of spackle polka-dot the walls. Inexplicably, a psychiatrist's couch sits off to the right. The place has been under renovation for two years.

Greene readily admits that he has always been a slob. His senior year, the Harvard yearbook ran a photo of his room as the supreme exemplar of undergraduate slumminess. "When you have this sense," he tries to explain, "that at rock bottom, there is a coherence, a simplicity, an explanatory core to the universe, I don't know, perhaps it gives you a certain license . . ." He stops. "Okay. Maybe I'm just trying to rationalize not putting my clothes in the hamper."