Hold fire! This epic vessel has only just set sail...

For those of you out there slinging mud at string theory, just pause a minute to reflect, insists Leonard Susskind. No one is saying that it's the final word.

Strange controversies reverberate through the blogosphere but few are as bizarre as the caustic attacks on string theory. "Did you say string theory - the arcane mathematical theory invented to describe the quantum theory of gravity? Public attacks? You gotta be kidding."

Yup, that's what I said. A vitriolic blog and two recent books have stirred up a firestorm of criticism about a subject that in other times would have interested no more than a few dozen nerdy physicists. But in the blogosphere everyone is an expert, everyone has an opinion and all opinions are equal. The criticism has become a drumbeat. Journalists with no more than high-school physics have condemned the subject in publications as unlikely as The Wall Street Journal , the Financial Times and The New York Times Book Review .

What in the multiverse is going on? Could it really be that a secret cabal of scientific priests have plotted to overthrow the rules of good scientific method and have absconded with the nation's scientific funding? Have America's greatest universities - Harvard, Princeton, Stanford, Berkeley, Massachusetts Institute of Technology, California Institute of Technology - all become infected with the same cancer of meaningless metaphysical speculation? Has serious science been driven out by string theorists bent on world domination?

Or are the critics a bunch of disgruntled conspiracy theorists, angry at being ignored? And might there be a bit of opportunism at work, an opportunity for gaining 15 minutes of scientific fame - without the real work?

Physics, especially at its frontiers, is fundamentally an exploration of the deep unknown: it can involve a huge payoff, but not without high risk.

Until the end of the 15th century, European geographic exploration (with the exception of the Norse) was restricted to the known world: the safe portion of the Earth's surface that lies within a hundred miles of the closest shore. By the second half of the 15th century, Europe had become aware of a much wider world. The great Atlantic was a fearful sea of ignorance, but its time had come. Curious adventurers - was it courage or stupidity? - could not rest without knowing what was on the other side.

Most people considered them foolhardy, if not crazy. The ever-present Chicken Little Society - frightened fuddy-duddies and derisive kibitzers - would proclaim the folly of sailing more than 50 miles from shore.

According to their way of thinking, the newly advanced art of shipbuilding provided an opportunity for geographers to complete the investigation of the shorelines, rivers and nearby islands of the known world. Crossing the endless Atlantic? What a waste of time, money, sailors and ships.

Physics today is much like the geography of the late 15th century. Until now we have been exploring the known world. One hundred years ago, the smallest known objects were atoms, a billion times smaller than a human hand. Over the 20th century, a series of incremental steps - both theoretical and experimental - have pushed the frontier to objects a billion times smaller still. It was a century of discovery, but at each stage theoretical physicists never had to wander far from the safety of laboratory experiments.

But, like our 15th-century counterparts, we have become aware of a much bigger ocean. On the near shore, all the known phenomena of physics: on the distant shore, a unified theory of quantum mechanics, gravity and the origin of the universe. All things point to the same conclusion: the smallest constituents of matter, the elementary quantum bits of information that control everything from elementary particles to black holes and that may even underlie the big bang, are probably 1015 times smaller than anything yet explored. An accelerator powerful enough to study these tiny, so-called Planckian objects would have to be as large as the entire galaxy.

Powering it would take trillions of barrels of oil a second. But with either courage or folly, theoretical physicists have taken the first tentative voyages.

But courage is not enough. For those who would venture beyond sight of shore, a sturdy vessel is a necessity. The right way to look at the past 35 years of string theory is as a time to create the necessary (mathematical) tools and learn how to construct a seaworthy vessel. That is not to say that no exploration has taken place - black holes and minor forays into cosmology being cases in point - but it is clear that a satisfactory string theory of the universe is well beyond our current knowledge. And most distant of all, in my view, is a complete theory of elementary particles.

Nonetheless, despite the limited degree of exploration, string theory has more to say about these areas than any other mathematical framework. But it is clear that the real revolution is yet to come.

Why, then, the angry blogs, books and newspaper articles? The seeds of discontent were sewn by string theorists themselves. Back in 1985, a time that should have been seen as the earliest days of an extremely long voyage, some of the most influential string theorists, based mainly at Princeton, began to hallucinate that their goal - a final Theory of Everything - was a few short leagues away, based on two mathematical discoveries.

Enthusiasm in science is a good thing and honest mistakes are generously forgiven. But in the mid-Eighties there was an unusual degree of hubris in Princeton, a smug, arrogant dismissal of any ideas that didn't fit the string theory agenda - or so it seemed to those who were not part of the in-group. The implication was that if one was not at Princeton doing string theory, one might as well give up physics. Personally, not being at Princeton was no hardship for me. I always found being there a mild form of punishment. But, for many young people, not being at the right place, at the right time, doing the right thing, seemed like a career disaster. Even being at Princeton was not protection enough, especially if you had chosen to work on something other than string theory.

Sure enough, within a few years the physics job market had suffered a downturn, and many perfectly competent young physicists, lacking the proteczia of string theory, found themselves employed as computer programmers instead of theoretical physicists. Many of them left physics embittered by the experience. Peter Woit was one of those Princeton mavericks who had the guts to work on other questions, in particular modern nuclear physics. Today, Woit is a computer administrator and untenured mathematics instructor at Columbia University. To his credit, he has kept up with developments in physics, even having published a lone technical paper on the internet and a collection of non-technical polemical articles laying out the ideological case against string theory. He is the author of a book and a blog, both titled Not Even Wrong , devoted to a strenuous criticism of string theory and also string theorists. Incidentally, the clever phrase "not even wrong" was coined by physicist Wolfgang Pauli who used it to express disdain for ideas that he disapproved of. Pauli was also famous for dismissing ideas that turned out to be correct and important.

Woit's criticism is chiefly of the Chicken Little variety. He sits at the edge of the wharf grumbling about foolish seamen following a delusional captain over the edge of the Earth. Woit assumes that the fact that we have not yet arrived at the distant shore is proof that the whole enterprise is doomed to failure.

Since the premature land sighting of 1985, string theory, like all explorations, has undergone important course corrections. These course corrections make it clear that the exploration will be a very long one. But Woit seems to regard the course corrections, especially those involving the large landscape of possibilities, as proof that string theorists are on a wild goose chase.

Woit is enough of a physicist and mathematician to appreciate the sturdy mathematical framework that the theory is based on. He is also enough of a historian to know that string theory originated in 1969 from the attempts by Y. Nambu, myself and Holger Nielson to apply it to the known world of nuclear physics. What he and his incurious followers believe is that the whole enterprise should have stopped right there. He believes that string theorists should devote the lion's share of their energy and mathematical power to refining "real-world physics", that is, the safe well-explored world of nuclear physics. Woit seems to not realise (or not care) that this would rob the subject of all the romance of exploration and leave it to the dullest plodders. The brightest, bravest and boldest young explorers want to go where no one else has gone before. But Woit is correct to remind us how important diversity and humility are in the face of the vast sea of ignorance.

Lee Smolin's The Trouble With Physics is another anti-string theory jeremiad. Smolin is a mid-level theoretical physicist, but his popular book-writing activities and the related promotional hustling have given him a platform high above that merited by his physics accomplishments. Smolin is no Chicken Little. He is, if anything, a Don Quixote figure. If physical theories are like ships, then Smolin is a most enthusiastic ship designer.

His "ships" include a theory in which universes reproduce by Darwinian selection, a theory called loop quantum gravity, doubly special relativity, triply special relativity and many more. These are all highly ambitious efforts, any of which, if correct, would change the course of physics.

How do Smolin's vessels fare after launching? As one of my unsympathetic colleagues put it: "One after another, the press releases go out, the champagne bottles explode, the ships slide down the ramp and sink into the mud at the bottom of the sea. They seem to have very well-ventilated hulls." But Smolin never seems to notice that he is standing astride the bridge of a sunken ship. He cannot understand why the best physicists in the world don't pay as much attention to loop gravity as they do to string theory.

Although much of Woit's and Smolin's grumbling is badly misplaced, there are legitimate reasons for criticism of string theory. Perhaps the most renowned physicist of our age, at least among other physicists, is the Dutchman Gerard 'tHooft. It is a product of a perverse public relations-driven society that among the scientifically interested public, the names Woit and Smolin are better known than 'tHooft. Although he has contributed a good fraction of the string theorist's tools, 't Hooft remains a sceptical critic - but a very thoughtful one.

'T Hooft is not sceptical because he thinks that string theory has failed the test of falsifiability. He is not troubled that 20 years have gone by without a definitive confirmation of the theory. More than anyone else, he recognises the enormous task before us. His estimates for the time scale for complete success is in the hundreds of years. And far from being content with studying the known world, 't Hooft has been way out in front, his neck stretched far beyond the crowd.

And he does not think there is anything technically wrong with string theory - he understands the mathematics extremely well. Moreover, he has been very impressed by the ability of string theory to address the most puzzling questions about black holes.

What bothers 't Hooft is the hubristic assumption that string theory, at least in its present form, is the final theory of nature. 'T Hooft's view is that the physics of future centuries will look as much like string theory as modern jet planes look like 15th-century galleons.

I share his scepticism. String theory has too many loose ends. For now it is our most reliable vessel - at least it floats - but between now and the final reckoning, there will be astonishing surprises, reversals of fortune, insights from completely unexpected directions. This leads 't Hooft to another important point: diversity of viewpoints is to be cherished, not suppressed. This is something that Woit and Smolin have properly reminded us of, and string theorists should not be allowed to forget it.

Leonard Susskind is a theoretical physics professor at Stanford University and is widely regarded as the father of string theory. His most recent book, The Cosmic Landscape , is published by Little, Brown.