New York Times, March 13, 2001, Tuesday, F4
Scientists have come up with theories in the shower, on barren mountains, while driving to work and even in their sleep. But what all theories have in common is that their predictions are eventually tested in experiments, where nature determines which inspirations are right and which are wrong.
Then there is string theory, the ambitious, profoundly mathematical attempt to knit together all of physics -- from gravity to quantum mechanics to subatomic forces -- into a single sublime formalism. Though string theorists first suspected they might be onto a "theory of everything" in the mid-1980's, and the field is the hottest area in theoretical physics, string theorists have yet to devise a make-or-break laboratory test for their ideas.
In part, that is because the theorized strings cannot be observed directly; they are thought to be vibrating entities smaller than a trillionth of a trillionth the size of an atom. Different string vibrations somehow correspond to different particles in nature, but scientists have yet to develop more than fragments of what they presume will ultimately be a complete theory.
Nevertheless, string theorists are already collecting the spoils that ordinarily go to the experimental victors, including federal grants, prestigious awards and tenured faculty positions. Less than a decade ago, there were hardly any jobs for string theorists, said Dr. David Gross, director of the Institute for Theoretical Physics at the University of California in Santa Barbara.
"Nowadays," Dr. Gross said, "if you're a hotshot young string theorist you've got it made."
Dr. Gross has no problems with that success; he was one of string theory's early developers. But some physicists are dismayed by the dominance of a theory that has yet to prove itself experimentally.
"I think the whole theory is a long shot," said Sir Roger Penrose, a physicist at Oxford University. He said he had nothing against an interesting long shot but that string theory had "taken over at the expense of all other areas."
Dr. John Baez, a scientist in the mathematics department at the University of California in Riverside, who studies a different approach to unification based more directly on relativity theory, said, "String theorists keep saying that they're succeeding."
"The rest of us can wonder whether they are walking along the road to triumph," Dr. Baez said, "or whether in 20 years they'll realize that they were walking into this enormous, beautiful, mathematically elegant cul-de-sac."
A number of physicists discussed the question at a conference in Santa Barbara this month to honor Dr. Gross on his 60th birthday.
Their thoughts revealed how in lean experimental times physicists rely on their aesthetic senses, purely mathematical clues, suggestive connections with established theories, and a Houdini-like taste for escaping roadblocks. While physics has always called these tricks into play, researchers are relying on them as never before in the case of string theory, simply because it attempts to reach so far into the unknown. Still, said Dr. Jeffrey Harvey of the University of Chicago, scientists in the field are confident their ideas are based in reality; they have, as he put it, "the feeling that string theory is something we discover rather than invent."
String theory has constantly changed since it first emerged several decades ago, and even its ardent adherents concede that they still do not understand more than what Dr. Gross called "the tail of the tiger," or a few suggestive parts of what is believed to be a complete theory. Until recently the physical crux of the theory was thought to be vibrating, 10-dimensional loops of string roughly a billion trillion times smaller than a proton. Different modes of vibration of the strings (made of what, no one is sure) represented different particles in nature.
Now physicists believe the ultimate objects are 11-dimensional membranes. Either way, the extra dimensions beyond the usual four would be curled up so as to be nearly imperceptible. And because the vibrations would include the graviton, the particle thought to transmit gravity, as well as particles involved in the strong and weak nuclear forces and electromagnetism, string theory offered the prospect of unifying physics.
But with that aesthetic attraction came deep problems. First, in the 1980's, it seemed that the strings had a basic inability to cope with known differences between particles and their mirror images, and other such broad facts of nature. But closer study showed that, contrary to all expectation, various terms in the theory canceled each other, fixing the problem.
A decade later, the field again narrowly escaped when what seemed to be several different string theories all turned out to be different facets of the same underlying theory -- a fortunate development, because physicists would not know what to do with more than one "ultimate" theory of the universe. Those episodes tell physicists they are onto something important, said Dr. Edward Witten, a physicist at the Institute for Advanced Study in Princeton, N.J.
"If it would turn out that string theory, which has led to so many miraculous-looking discoveries over so many decades, has nothing to do with nature, to me this would be remarkable cosmic conspiracy," Dr. Witten said.
At the same time, string theory's strong influence on several branches of pure mathematics have provided a good test of the "intellectual horsepower" of the idea, said Dr. Stephen Shenker, a Stanford physicist. He said the wider impact suggested both that physicists had not missed some crucial mathematical error and that the theory was rich enough to be an encompassing theory of nature.
All those developments, Dr. Gross said, "do convince people that there's something here."
But despite these checks, said Dr. Stephen W. Hawking, the University of Cambridge physicist, who also attended the conference, physicists should also keep in mind that they have been wrong before and that string theory still contains many mysteries.
"For most of the last hundred years, we have thought that the theory of everything was just 'round the corner," Dr. Hawking said. "We keep making new discoveries, but I don't think we can yet say the end is in sight."
Copyright 2001 New York Times. All rights reserved. International copyright secured. File Date: 3.13.01