The theory that may have been stringing scientists along for years

String theory, which hopes to end Einstein's quest for a Theory of Everything, has lacked definitive evidence for decades but events over the coming months underground in Switzerland may make or break the theory, and its proponents.

Michael Green, pioneer of string theory, Lucasian Professor of Mathematics and Professor in Department of Applied Mathematics and Theoretical Physics in his office at the University of Cambridge. Martin Godwin / Getty Images
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Over the next few days, the southern Indian city of Bangalore will be playing host to an exclusive group of people.

No amount of money can buy you membership and it doesn’t matter how well connected you are. But rumour has it that it helps if you have a brain the size of a planet.

The city is the venue for this year’s gathering of theorists working on the hardest problem in science – the quest for the Theory of Everything.

Not even Albert Einstein got close to creating such a thing – a single, unified account of all the forces in the universe and the particles they influence.

But those attending the conference are convinced they’ve got the best shot of succeeding – and that’s because they believe in strings.

That’s the oddly ho-hum name given to bizarre multidimensional entities so small you’d need to magnify subatomic particles to the size of galaxies to notice them.

Despite their size, the contortions of these strings can account for all of the forces and particles in the cosmos – just as a Theory of Everything demands.

That, at least, has been the claim of string theorists for the last 30 years. There’s just one problem: there’s not a shred of evidence to support it. And that is now leading to awkward questions about just what all these very smart people have been doing with their time and funding.

This month, The New York Times carried an editorial by two distinguished astrophysicists asking whether string theorists are doing science at all.

It is a row that has been brewing for some time among academics, and is now spilling over into the public arena.

At its heart is the definition of science – and how to tell when it turns into pseudo-scientific navel gazing.

Most scientists insist they know real science when they see it: unlike pseudo-science it makes hard and fast predictions that can be tested by experiment.

But what if the science focuses on phenomena so extreme that experimental tests may never be possible? That’s been the problem with string theory since thinkers first suspected it might hold the keys to the cosmos.

Any candidate for the Theory of Everything must succeed where Einstein failed and unify the theories for all the universe’s forces.

The most familiar of these is electromagnetism, and since the 1970s this has been successfully unified with more esoteric forces – the “strong and weak interactions” – at work inside atoms. But the biggest hurdle has been unifying these to the most familiar force of all, gravity.

The problem lies in the fact that the best theory we have for gravity – General Relativity, devised by Einstein a century ago – is utterly unlike the theories for the other forces, which are based on quantum theory, the rules of the subatomic world.

But during the 1970s, the American theorist John Schwarz at the California Institute of Technology spotted something intriguing. Einstein’s theory of gravity seemed to emerge automatically from a quantum theory based on string-like entities.

Exactly what these are remains hard to explain, not least because they exist in many more than the four dimensions of space and time we see around us. But what has always been clear is that studying them directly is not going to be easy.

Calculations suggest that reaching the energies needed to witness string-like behaviour needs a particle accelerator bigger than our entire galaxy.

In the absence of such a machine, theorists have pressed on in the hope of finding evidence that strings really do tie Everything together.

By 1984, Mr Schwarz and his British collaborator, Michael Green, had shown that strings seem to dodge a host of technical problems that killed off previous attempts at a Theory of Everything.

That persuaded other theorists to take notice. Ever since, string theory has entranced many of the most brilliant minds in theoretical physics.

Over the years, they have discovered it has a host of elegant features and intriguing implications – some of which may be relevant for other areas of physics.

But still there is no sign of what many scientists regard as the hallmark of real science – a testable prediction.

This has prompted the authors of the Times editorial to draw parallels between string theory and the elegant celestial machinery invented by the Greeks 2,000 years ago to explain the motion of the planets.

Its elegance ultimately proved empty when confronted by cold, hard data.

The American Institute of Physics has now gone further, with an editorial in Physics Today asking if the insouciance of string theorists over the issue of testability may harm public faith in science.

To veteran string theorists at this week’s conference, among them Mr Schwarz and Mr Green, all this will be familiar stuff.

But they also know that this is a critical time for their decades-long quest.

This month, the Large Hadron Collider fired up again, smashing particles together with unprecedented violence. While the energies reached are still far below those needed to observe strings directly, they are enough to start testing one of their characteristics: “super-symmetry”.

This esoteric mathematical property underpins some of the most alluring features of string theory. It also makes a testable prediction, that every known particle should have a super-symmetric partner.

The collider should be able to detect these particles within the next year or so – if they exist. But if they do, that does not mean string theory is correct; merely that it gets to live another day.

The real fun and games will start if the particles are not found. That is because, even after all these years, string theory is not tied down very tightly.

As a result, theorists can tweak their equations to explain away the failure.

Such flexibility is a classic symptom of pseudo-science, and it is what particularly irks physicists about string theory – along with the arrogance of some of its practitioners, who insist it is the only way to complete Einstein’s quest.

But there is another source of resentment, and one with which we can all sympathise: that an entire generation of brilliant minds may have been lost to an esoteric mind-game.

That is why we should all hope to hear of some spectacularly surprising discovery from the LHC over the coming months.

Robert Matthews is visiting reader in science at Aston University, Birmingham