麻豆传媒

AS THE bell rings for the final round, both fighters are in their corners. The heavyweight champion is panting, against the ropes. The underdog is sprawled on the opposite side, bruised, tired but determined.

Everyone is expecting an exhilarating finale. The challenger has put on a good show, landing a few square punches. Admittedly, it was accused of cheating by ignoring the rules of physics. But that was nothing compared with the favourite鈥檚 big trick: dancing in seven invisible dimensions.

This is no ordinary fight: it鈥檚 for the honour of successfully explaining the fundamental make-up of the universe. The heavyweight incumbent is string theory, a complex construct that has long had pretensions to be 鈥渢heory of everything鈥�. The upstart contender, loop quantum gravity, has more modest aspirations 鈥� but could still change the face of physics as we know it.

Physicists have long believed they must pin their hopes on just one contender. But as the two rivals walk back into the ring, there鈥檚 a turn-up for the books: they reach out for a handshake. If the latest indications are right, it seems the two theories are not so different after all. They might in fact be the same theory in disguise 鈥� with implications that reach far and wide. 鈥淭he next revolution in physics is going to come from people who can cross the boundary,鈥� says Laurent Freidel, a theorist at the Perimeter Institute in Waterloo, Canada.

At its root, this fight is all about that most perplexing of forces, gravity. In his general theory of relativity, Einstein told us that gravity is the product of an omnipresent fabric, space-time, that warps smoothly around massive objects. It works soundly for things like planets, stars and galaxies, but does far less well for all things small.

Here, it is forced to confront that other totem of modern physics, quantum theory, which tells us that everything comes in discrete chunks. This would suggest that, on the finest scales, space-time isn鈥檛 smooth but must be in some way twisted and frayed.

So quantum theory and relativity are fundamentally incompatible. The problem is most acute around singularities, tiny regions of space-time with humongous gravitational fields, such as appear at black holes or the big bang. Here, physics just breaks down.

That鈥檚 where our the heavyweight champion enters the ring. String theory started in the 1970s as a way to describe a single force of nature, the strong force, which binds particles called quarks together inside atomic nuclei. But its aspirations eventually expanded to describe nature鈥檚 other three forces too, including gravity. It aims to do so by painting matter and energy as made of vibrating strings. This goes against the grain of accepted physics, which sees all matter as made from fundamental point particles 鈥� electrons, quarks, photons and so on 鈥� devoid of any shape or substructure.

Just as a musical string sounds different on a violin or a cello, so in string theory, a string鈥檚 vibration pattern determines what kind of particle it manifests as: it can 鈥渟ound鈥� like a quark, electron or photon. And among the oscillation modes of these strings is something that looks not like an established particle, but like a graviton 鈥� a quantum particle that transmits gravity and gives structure to space-time.

String theory may be successful, but like many ageing competitors, it has its share of hang-ups. Strings need to vibrate in 11 dimensions, so backers believe that to tally with the three we observe, the extra dimensions must be folded up around the strings so tightly that we can鈥檛 see them 鈥� and indeed, no conceivable experiment can probe them. Even if we could, the folding can be done in so many ways that the universe isn鈥檛 big enough to write the number down, so we wouldn鈥檛 know what to look for.

All this has led to string theory receiving a deluge of brickbats, with some suggesting its lack of predictive power disqualifies it from scientific contention.

But string theory does in fact makes one prediction: supersymmetry. A mirror symmetry in the string vibrations means that our familiar particles should all have heavier, supersymmetric twins. But we have so far failed to spot them at the world鈥檚 most powerful particle generator, the Large Hadron Collider near Geneva, Switzerland. The absence of supersymmetric particles may not have killed string theory, but it has injected a dose of humility, says Carlo Rovelli at the Centre of Theoretical Physics in Marseille, France. 鈥淭his has not been said yet, because people are afraid of saying it.鈥�

Rovelli is a pioneer and supporter of the underdog theory, loop quantum gravity. It got into the competition in the 1980s, a decade later than string theory, but likewise promised to tackle the mismatch between general relativity and quantum theory.

It does this in a far more modest fashion, however. Rather than creating a whole stringy apparatus from which space-time ultimately derives, it says simply that space-time itself must fit in with the quantum picture: it must come in finite chunks. Do the maths and these chunks turn out to be tiny loops, which evolve by themselves into a bubble-like geometry known as a spinfoam. The spinfoam is space-time written in the language of quantum theory.

While some theorists cheer string theory for its brash ambition, others are drawn to loopy space-time for its modesty. It tries to do nothing more than is necessary to reconcile quantum theory and general relativity 鈥� a slow and steady approach that some see as more likely to win out in the end. And although loops have remained just as theoretical as strings, theorists like Rovelli have generated some testable predictions (see 鈥�Closing the loop鈥�). That said, there is a hitch. Spinfoams are too rigid to readily adhere to Einstein鈥檚 picture of a universe where space and time squeeze and stretch depending on who鈥檚 looking.

Loopy space-time and vibrating strings offer very different visions of the universe鈥檚 make-up. The only thing most adherents of either camp can agree on is a long history of not talking to each other. 鈥淔ew people now have worked in detail on both approaches,鈥� says Lee Smolin, another pioneer of loop quantum gravity and also at the Perimeter Institute. 鈥淢ost people using one approach are ignorant of and have misapprehensions about the other,鈥� he says.

Freidel believes the split actually goes right back to the ancient Greeks, and the birth of science. On one side, he says, are reductionists who, like string theorists, think we must understand smaller and smaller components of nature. On the other are those who think progress comes from understanding the bigger picture, and the nature of space, as loop theorists do. 鈥淧hysics has been successful for 2000 years without settling this divide.鈥�

Emerging hints

But with neither strings or loops landing a killer blow, things are changing. Theorists, young ones in particular, are starting to move between the camps for the first time, looking for connections between the two. 鈥淚鈥檓 surprised at how it鈥檚 changed in recent years,鈥� says Rovelli.

The first hints of something in common emerged in 2011, when Norbert Bodendorfer and colleagues at the University of Warsaw in Poland and its supersymmetric particles on the space-time described by loop quantum gravity, showing that the two very different frameworks could at least live with each other.

Another hint came to light three years later with an essay by loop theorists Rodolfo Gambini of the University of the Republic in Montevideo, Uruguay, and Jorge Pullin of Louisiana State University in Baton Rouge. They argued that to make relativity and loop quantum gravity fully compatible, you were the range of possible particles using a trick borrowed from string theory.

Little came of those isolated investigations. But just recently, a flurry of studies has suggested a more solid link 鈥� one whose foundations were unknowingly uncovered in the early 1990s. At this time, the Dutch Nobel prize winner Gerard 鈥榯 Hooft and Leonard Susskind of Stanford University in California proposed that the three-dimensional world in which we live, and gravity鈥檚 effects on it, could be a mere projection of stuff happening on its flat, two-dimensional boundary.

In this radical picture, the bulk of the universe is no more 鈥渞eal鈥� than the three-dimensional hologram on the back of a credit card. Reality is the flat boundary, and everything else is an illusion.

Since then, the holographic principle has matured into a major research area in string theory. From a perspective on the boundary, frightfully difficult physics begins to make better sense. But now, it seems, the boundary could also provide a place for string theory to entwine with loop quantum gravity. 鈥淏oth sides are making a step towards each other,鈥� says Rovelli. 鈥淭here is common ground there, on the boundary.鈥�

In November last year, loop theorists Valentin Bonzom of the University of Paris-North in France and Bianca Dittrich of the Perimeter Institute found that a calculation of holographic gravity that others had performed in the context of string theory could be achieved using loop quantum gravity, with . A month later, Bodendorfer made another breakthrough. Normally, when holographic string theory is used for calculations of gravity it gets stuck at those pesky singularities, like black holes. But Bodendorfer showed that his loopy maths could . 鈥淢ine was a proposal for how loop quantum gravity could fit inside string theory,鈥� he says.

鈥淭he bulk of the universe is no more real than the hologram on a credit card鈥�

These results could be flukes, but even more fundamental studies are now weighing in. Also in November last year, working with two other collaborators, Freidel went back to basics: he attempted to describe a small region of space surrounded by a boundary, using only general relativity. It turned out that the mathematical quantities defining his boundary resembled quantities in both string theory and loop quantum gravity, despite neither theory being a starting point.

Meanwhile, loop theorist Muxin Han at Florida Atlantic University in Boca Raton teamed up with string theorist Ling-Yan Hung at Fudan University in Shanghai, China, to try to calculate the probability of one spinfoam evolving into another in loop quantum gravity. Like the soap bubbles that billow while running a bath, spinfoams evolve of their own accord, but calculating just how they evolve can be tricky. To make their spinfoam calculation easier, Han and Hung mapped it on to a boundary. Once more, out popped mathematical features . 鈥淎ll this is teaching both string theorists and loop quantum gravity theorists something they had not considered before,鈥� says Hung. Rather than being competitors, on the boundary at least, string theory and loop quantum gravity could be one and the same.

That brings up questions about what and where this boundary is exactly. Holographic theory began by imagining it at the universe鈥檚 edge, but today鈥檚 string and loop theorists are less strict, thinking of a boundary as being anywhere in space. The part-loop, part-string physics that Hung and others are working on might appear on the finest scales if you took a random slice through space-time.

Casting the same shadow

That might seem arbitrary. But when you think about it, says Rovelli, we impose boundaries all the time, as portals through which to observe the world. To measure temperature, we place a thermometer on the edge of an object; to record light, we catch photons striking the flat surface of photographic film. The important point is that this flat picture can be the result of more than one happening, in the same way that a butterfly-shaped shadow can be made by an actual butterfly, or by linking your thumbs and waving your fingers. String theory and loop quantum gravity may look different in the whole, then, but they may, in a sense, cast the same shadow.

If true, that could make experimental results ambiguous. Observing a supersymmetric particle, say, could be evidence of a string-loop theory or of string theory proper; on a flat detector, there would be no way to distinguish the two.

That鈥檚 assuming strings and loops can even be fully joined in the first place. 鈥淭here is no string-loop theory yet,鈥� says Rovelli. 鈥淢y own opinion is that we still have two different theories. One could still be right and one could be wrong, or they could both be wrong.鈥�

That may well be true. But among younger physicists there is genuine excitement for where a reconciliation between loops and strings could lead. Even if it does not end at an answer in itself, it could point the way to an even deeper reality, one that has its own unique predictions. In that case, finding a place where string and loop theorists can at least translate between their mother tongues is no bad thing. Diplomacy, after all, first requires the antagonists to stop fighting.

This article appeared in print under the headline 鈥淲hen loops become strings鈥�