String fellows - [interview with string theorist Edward Witten]

100 years after Einstein changed physics for ever, Alok Jha visits a leafy corner of Princeton to meet his intellectual heirs - still hunting for a theory of everything

Edward Witten is so softly spoken that his voice sometimes threatens to drift away completely. His desk is a jumble of papers and his blackboard a mess of equations. But his hushed words come straight to the point and are infused with understanding and passion.

Witten's quiet manner belies his status. In his role as de facto scientist-in-chief of string theory, Witten, the Charles Simonyi professor of mathematical physics at the Institute of Advanced Study (IAS) in Princeton, is undoubtedly the heir to Albert Einstein's title of greatest living physicist. If Einstein were alive today, he would probably be a string theorist, engaged in a remarkable, but still very controversial, theory that claims to explain absolutely everything around us.

"Critics of string theory say that it might be too big a step. Most physicists in other fields are simply agnostic and properly so," says Witten. "It isn't an established theory. My personal opinion is that there are circumstantial reasons to suspect that it's on the right track. "

As personal opinions go, Witten's make other scientists sit up and listen. His ideas in the development of string theory are legendary.

"There is no question of the extraordinary quality of Witten's intellectual achievements," says renowned physicist Roger Penrose in his latest book, The Road to Reality. "Where Witten goes, it does not take long for the rest to follow."

Witten himself does not approve of the cult of celebrity, keeping a relatively low public profile. Outside the arcane world of theoretical physics, few have heard of him. But he has been key to the development of a theory that may one day be how textbooks describe the universe. What sealed his claim to the theoretical physics crown was a lecture at the University of Southern California in 1995, where he introduced the world to M-theory.

It came at a time when string theory had been in the doldrums. First proposed as a way to describe the strong nuclear force in the 1960s, but later abandoned, string theory had been developed by different physicists into five competing ideas, each laying claim to be the fundamental theory of nature. With the ensuing arguments and criticism from outside that the theory was simply not part of science, the string community had been left broken up and demoralised over the decades.

Witten's lecture came as a much-needed shot in the arm. He argued that the five competing theories were nothing more than different aspects of a single, even more fundamental, idea. His M-theory at once united the physicists and led to an interest in string theory like never before. Incidentally, the M was left undefined by Witten and string theorists have spent many an idle hour speculating what it could stand for: they have come up with a range of possibilities including matrix, muffin, mystery, murky, membrane, monstrous, mother or magic - some scientists believe it is an upside down W for Witten.

Nathan Seiberg, a colleague of Witten's at the IAS, uses the analogy of blind men examining an elephant to explain the course of string theory until 1995. "One describes touching a leg, one describes touching a trunk, another describes the ears," he says. "They come up with different descriptions but they don't see the big picture. There is only one elephant and they describe different parts of it."

Predictably, Witten is modest about his achievement. "It's an exaggeration to say that I came up with M-theory," he says. "We came up with bits and pieces but there's a long history behind it."

Wherever the credit lies, there is no doubt that M-theory took the already bizarre world of string theory into even stranger territory.

Before M-theory, strings existed in a world with 10 dimensions. These included a dimension of time, the three familiar space dimensions, as well as six extra dimensions, curled up so small that they are invisible. M-theory demanded an extra dimension for space, taking the total to 11.

The extra dimensions were necessary to satisfy the equations drawn up by Witten in his formulation of M-theory. But there were surprises: the theory suggested, for example, that this 11-dimensional world contained not only strings but objects that looked more like surfaces or membranes. These "branes" could exist in three or more dimensions and, with enough energy, could grow to huge sizes - even as large as the whole universe.

Even more peculiarly, Witten suggested our universe could be sitting on a brane in some higher dimensional space. Brian Greene, a string theorist at Columbia University, describes this idea as akin to sitting on a slice of bread contained in a loaf. "That's our one candidate for super-unification of the laws of nature," says Witten. "We don't understand it, we can't promise it's right, we're still groping in the dark. It's very exciting to understand that there's this one fascinating theory that has all these incredible properties. When nice things like that happen, it makes one convinced that one must be on the right track."

Witten can indulge even his most esoteric ideas, a freedom afforded, in part, by the IAS, an almost unique research institute based in what looks like a serene stately home in front of an 800-acre forest in Princeton. Set up in 1930 as a place for researchers to pursue their work without the distraction of university teaching or bureaucracy, it is probably most famous for being the place where Einstein spent his final years.

After spending the first part of the 20th century coming up with general relativity and sowing the seeds for quantum physics, Einstein was convinced that there was a single fundamental theory that must describe nature. Many of his colleagues around the world felt he was wasting his time but intelligent inquiry, even if it does not produce any useful results, is part of the institute's founding principles.

Einstein died in 1955 without fulfilling his dream, but the question he was asking was the inspiration for what has now become string theory.

"At the time of Einstein, we now understand that it was really premature to embark on such a long-term project, partly because of the things that happened in the decades afterwards," says Seiberg. "A lot of developments that he could not have seen shaped our understanding of particle physics - mostly the forces which act within the nucleus and what the elementary particles are. These are ideas which were developed after Einstein's work."

Einstein himself had no idea that the fundamental forces of nature had yet to be fully described, never mind united into one theory. He had spent his time trying to unite electromagnetism with gravity - the strong and weak nuclear forces were only formalised by theorists and discovered by atom smashers after he died.

Most of his work, therefore, went in the wrong direction, but the physicist's spirit undoubtedly lives on. "Being in the place where Einstein was is clearly an inspiring idea," says Seiberg.

The IAS is no museum, though. "If this had not been a good question to work on, then there would not have been a group working on this subject here," says Seiberg. "I think this place will remain focused on the most exciting thing that is going on, rather than preserving a legacy."

String theory has been moving particularly fast in the decade since M-theory came on the scene. Just over a year ago, Witten again came up with a big idea: using a 40-year-old idea called twistor theory, originally developed by Penrose at Oxford University, Witten showed that strings may not need all those extra dimensions after all. It sparked a whole slew of papers from his fellow theorists and interest is still growing. Last week, string theorists from around the world met in Oxford to discuss progress. Witten is not convinced yet. "I think twistor string theory is something that only partly works," he says.

The future for the string theorists looks bright but there are still some basic questions for physicists to answer. The first is simply - what is the theory describing? "This is unlike general relativity where Einstein laid out the principles and then he derived the consequences," says Seiberg. "We are in the very bizarre, unprecedented situation where we know how to derive some of the consequences but we don't know what the fundamental principles are."

The second, and for some critics the biggest, gap is the lack of experimental data for string theory. For this, Witten and his colleagues are looking to the new particle accelerator under construction at Cern in Switzerland. The Large Hadron Collider (LHC) will operate at such high energies that some of the things predicted by string theory might make an appearance. One of these is supersymmetry, the idea that fundamental particles, (such as electrons and quarks) all have supersymmetric partners at high energies (in this example, called selectrons and squarks). If the LHC found these, champagne corks would no doubt be popping at string theory research groups around the world.

But nature is ultimately a tricky beast. Even if string theory is shown to be correct, could there possibly be something even more fundamental?

"Most string theorists are very arrogant," says Seiberg with a smile. "If there is something [beyond string theory], we will call it string theory."

String theory really could be the most fundamental thing, though. Traditionally, physicists found more fundamental forces at work as they went to shorter and shorter distances. With string theory, the concept of distance breaks down. Spacetime is an assumed concept, the arena of quantum mechanics and general relativity, but string theory does not assume anything. Our notion of space and time will have to be a result of the eventual equations of string theory. At that level, there is no parameter that can get smaller, so no possibility of a more fundamental theory.

The future of string theory may not even involve strings as they were first defined. But M-theory's branes, surfaces and parallel universes ensure that the world of fundamental physics is a truly strange place; with or without the strings.

String theory is the name given to the arcane and fiendishly complicated mathematical world that was first dreamed up in the late 1960s as a way to describe the strong nuclear interaction, the force that stops protons flying apart in the nucleus of atoms.

Protons are made of even more fundamental particles called quarks and these are held together by particles, called gluons, that transmit the strong nuclear force. The mystery was why quarks and gluons were never seen by themselves, even when atoms were smashed apart in particle accelerators.

String theory was coined as a way to answer this (in simple terms, perhaps gluons and quarks were at the ends of a string of energy, and you can't have a one-ended string), but eventually gave way to what became known as quantum chromodynamics - a precise quantum mechanical description of the interaction between quarks and gluons. String theory was later salvaged as a way to explain not just the strong force but all the fundamental forces. The idea is that everything in the universe, from the Earth to the paper you are holding now, and all the forces that are acting upon them, is made of tiny vibrating strands of energy called strings. The theory is an attempt to fix something that has troubled scientists for a century - the fact that their two greatest theories of nature don't agree with each other.

At the beginning of the 20th century, our understanding of the world began to turn upside down. The new ideas behind quantum mechanics said the world was unpredictable, that the behaviour of everything lay firmly in the hands of chance. Einstein's general relativity described how the space around us was warped by gravity, turning our understanding of the force into an exercise in geometry. But these two concepts share a problem: they don't agree with each other. To answer some of the biggest questions in physics, such as what actually happened in the big bang, this disagreement is a big problem - do you use the equations of general relativity because there is an enormous amount of mass? Or do you use quantum mechanics because it's all in such a small space?

Finding a way to bridge the chasm - the so-called theory of everything - has consumed theoretical physicists, including Einstein, for decades. Not without some controversy, string theory is the lead contender to fulfil Einstein's dream.

While string theory is the only thing that can explain both quantum mechanics and gravity, it is as yet unproven by experiment. And it goes so far beyond our physical experience of the world that some critics say that it should be considered more as a work of philosophy than the ultimate scientific description of nature.

Even if string theory turns out to be mistaken or only a stage on the way towards a more fundamental physical theory, Ed Witten will still be recognized and remembered as one of the smartest men to ever walk this planet.

When reading about string theory it is easy to become strung out and lose the thread of the discussion. :-)

He's as smart as they come at science and math, but politically he's a conventional liberal and he's a relatively poor chessplayer. (I live nearby and know this stuff....)

...politically he's a conventional liberal and he's a relatively poor chessplayer.

Not surprising. He's unwilling to spend his limited time thinking about politics or how to beat opponents in a board game. Fortunately for him (and for us, too), he lives in a society that allows him to do what he does better than almost everybody else—think about physics and mathematics.

There have been over 10,000 papers on string theory. Everyone is either writing on string theory or desperately trying to learn string theory. In less than 150 years there will not be a sane Westerner left.

When reading about string theory it is easy to become strung out and lose the thread of the discussion. :-)

Here's some advice with no strings attached: never give yourself enough rope to hang yourself.

(There's a million of 'em, eh?)

I would say that if he turns out to be right, or, as you say, it is stage on the way to something else, then Witten will go down in history has an equal of folks like Einstein and Newton. On the other hand, if he is totally wrong he will go down as one of the biggest smart-alexes (sp?) in history.

What would be really funny would be to find out that the energy levels at the LHC are not even close, that noting new happens, but there is no definitive disproof of anything. We shall know soon enough.

And all this time, I thought that because I was bad at chess, I didn't have a chance at theoretical physics. Maybe it's time to go back to grad school!

I would say that if he turns out to be right, or, as you say, it is stage on the way to something else, then Witten will go down in history has an equal of folks like Einstein and Newton. On the other hand, if he is totally wrong he will go down as one of the biggest smart-alexes (sp?) in history.

I worded my initial post pretty carefully. Note that I said nothing about his greatness as a physicist; I spoke only about his intelligence, which, by all accounts, is remarkable. It may well happen that he'll be remembered as much for his contributions to mathematics (which are substantial) as for his contributions to physics.

Good to hear from you again, BTW.

I met him once when he gave a lecture at the IBM's Watsons Research center. He is extremely funny and witty in person, and only a little bit eccentric for that kind of brains. Yes, you are right, I think that Von Neumann and his role in American science after the war might be a good analog to this one.

My point was that there is something about the way he presents this stuff that suggest that he really does not believe that all of this is really the case. He seem elusive about it. I am not criticizing him either.

He seems to be coming from a purely mathematical point of view. THe whole thing seems to richly amise him.

"Here's some advice with no strings attached: never give yourself enough rope to hang yourself."

"(There's a million of 'em, eh?)"

Oh my, never spin a yarn like that to rope me in. I always fall for them hook, line, and sinker?

How long can we keept this up? :-)

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