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     Volume 1 Issue 6 | September 10, 2006 |


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You are made of space-time?

Lee smolin is no magician. Yet he and his colleagues have pulled off one of the greatest tricks imaginable. Starting from nothing more than Einstein's general theory of relativity, they have conjured up the universe. Everything from the fabric of space to the matter that makes up wands and rabbits emerges as if out of an empty hat.

It is an impressive feat. Not only does it tell us about the origins of space and matter, it might help us understand where the laws of the universe come from. Not surprisingly, Smolin, who is a theoretical physicist at the Perimeter Institute in Waterloo, Ontario, is very excited. "I've been jumping up and down about these ideas," he says. This promising approach to understanding the cosmos is based on a collection of theories called loop quantum gravity, an attempt to merge general relativity and quantum mechanics into a single consistent theory.

The origins of loop quantum gravity can be traced back to the 1980s, when Abhay Ashtekar, now at Pennsylvania State University in University Park, rewrote Einstein's equations of general relativity in a quantum framework. Smolin and Carlo Rovelli of the University of the Mediterranean in Marseille, France, later developed Ashtekar's ideas and discovered that in the new framework, space is not smooth and continuous but instead comprises indivisible chunks just 10-35 metres in diameter. Loop quantum gravity then defines space-time as a network of abstract links that connect these volumes of space, rather like nodes linked on an airline route map.

From the start, physicists noticed that these links could wrap around one another to form braid-like structures. Curious as these braids were, however, no one understood their meaning. "We knew about braiding in 1987," says Smolin, "but we didn't know if it corresponded to anything physical."

Enter Sundance Bilson-Thompson, a theoretical particle physicist at the University of Adelaide in South Australia. He knew little about quantum gravity when, in 2004, he began studying an old problem from particle physics. Bilson-Thompson was trying to understand the true nature of what physicists think of as the elementary particles - those with no known sub-components. He was perplexed by the plethora of these particles in the standard model, and began wondering just how elementary they really were. As a first step towards answering this question, he dusted off some models developed in the 1970s that postulated the existence of more fundamental entities called preons.

Just as the nuclei of different elements are built from protons and neutrons, these preon models suggest that electrons, quarks, neutrinos and the like are built from smaller, hypothetical particles that carry electric charge and interact with each other. The models eventually ran into trouble, however, because they predicted that preons would have vastly more energy than the particles they were supposed to be part of. This fatal flaw saw the models abandoned, although not entirely forgotten.

Bilson-Thompson took a different tack. Instead of thinking of preons as particles that join together like Lego bricks, he concentrated on how they interact. After all, what we call a particle's properties are really nothing more than shorthand for the way it interacts with everything around it. Perhaps, he thought, he could work out how preons interact, and from that work out what they are.

To do this, Bilson-Thompson abandoned the idea that preons are point-like particles and theorised that they in fact possess length and width, like ribbons that could somehow interact by wrapping around each other.

He supposed that these ribbons could cross over and under each other to form a braid when three preons come together to make a particle. Individual ribbons can also twist clockwise or anticlockwise along their length. Each twist, he imagined, would endow the preon with a charge equivalent to one-third of the charge on an electron, and the sign of the charge depends on the direction of the twist.

The simplest braid possible in Bilson-Thompson's model looks like a deformed pretzel and corresponds to an electron neutrino (see Graphic). Flip it over in a mirror and you have its antimatter counterpart, the electron anti-neutrino. Add three clockwise twists and you have something that behaves just like an electron; three anticlockwise twists and you have a positron. Bilson-Thompson's model also produces photons and the W and Z bosons, the particles that carry the electromagnetic and weak forces. In fact, these braided ribbons seem to map out the entire zoo of particles in the standard model.

Bilson-Thompson publis hed his work online last year
(www.arxiv.org/abs/hep-ph/0503213). Despite its achievements, however, he still didn't know what the preons were. Or what his braids were really made from. "I toyed with the idea of them being micro-wormholes, which wrapped round each other. Or some other extreme distortions in the structure of space-time," he recalls.

It was at this point that Smolin stumbled across Bilson-Thompson's paper. "When we saw this, we got very excited because we had been looking for anything that might explain braiding," says Smolin. Were the two types of braids one and the same? Are particles nothing more than tangled plaits in space-time?

Smolin invited Bilson-Thompson to Waterloo to help him find out. He also enlisted the help of Fotini Markopoulou at the institute, who had long suspected that the braids in space might be the source of matter and energy. Yet she was also aware that this idea sits uneasily with loop quantum gravity. At every instant, quantum fluctuations rumple the network of space-time links, crinkling it into a jumble of humps and bumps. These structures are so ephemeral that they last for around 10-44 seconds before morphing into a new configuration. "If the network changes everywhere all the time, how come anything survives?" asks Markopoulou. "Even at the quantum level, I know that a photon or an electron lives for much longer that 10-44 seconds."

Markopoulou had already found an answer in a radical variant of loop quantum gravity she had been developing together with David Kribs, an expert in quantum computing at the University of Guelph in Ontario. While traditional computers store information in bits that can take the values 0 or 1, quantum computers use "qubits" that, in principle at least, can be 0 and 1 at the same time, which is what makes quantum computing such a powerful idea. Individual qubits' delicate duality is always at risk of being lost as a result of interactions with the outside world, but calculations have shown that collections of qubits are far more robust than one might expect, and that the data stored on them can survive all kinds of disturbance.

In Markopoulou and Kribs's version of loop quantum gravity, they considered the universe as a giant quantum computer, where each quantum of space is replaced by a bit of quantum information. Their calculations showed that the qubits' resilience would preserve the quantum braids in space-time, explaining how particles could be so long-lived amid the quantum turbulence.

Smolin, Markopoulou and Bilson-Thompson have now confirmed that the braiding of this quantum space-time can produce the lightest particles in the standard model - the electron, the "up" and "down" quarks, the electron neutrino and their antimatter partners (www.arxiv.org/abs/hep-th/0603022).

All from nothing at all
So far the new theory reproduces only a few of the features of the standard model, such as the charge of the particles and their "handedness", a quantity that describes how a particle's quantum-mechanical spin relates to its direction of travel in space. Even so, Smolin is thrilled with the progress. "After 20 years, it is wonderful to finally make some connection to particle physics that isn't put in by hand," he says.

The correspondence between braids and particles suggests that more properties may be waiting to be derived from the theory. The most substantial achievement, Smolin says, would be to calculate the masses of the elementary particles from first principles. It is a hugely ambitious goal: predicting the masses and other fundamental constants of nature was something string theorists set out to do more than 20 years ago - and have now all but given up on.

As with string theory, devising experiments to test for the new theory will also be difficult. This is a problem that plagues loop quantum gravity in all its guises, because no conceivable experiment can probe space down to 10-35 metres. Ironically, the best arena in which to look for experimental proof might be the largest scales in the universe, not the smallest. "The closest anyone is getting to making predictions is in the area of cosmology,"

says John Baez, a mathematician and expert on quantum gravity at the University of California, Irvine. Markopoulou is now trying to think of ways of testing the braid model using the fossil radiation left over from the big bang, the so-called cosmic microwave background that permeates the universe. Physicists believe that the patterns we see today in that radiation may have originated from quantum fluctuations during the earliest moments of the big bang, when all of the matter in the universe was crammed into a space small enough for quantum effects to be significant.

Meanwhile, Markopoulou's vision of the universe as a giant quantum computer might be more than a useful analogy: it might be true, according to some theorists. If so, there is one startling consequence: space itself might not exist. By replacing loop quantum gravity's chunks of space with qubits, what used to be a frame of reference - space itself - becomes just a web of information. If the notion of space ceases to have meaning at the smallest scale, Markopoulou says, some of the consequences of that could have been magnified by the expansion that followed the big bang. "My guess is that the non-existence of space has effects that are measurable, if you can only see it right." Because it's pretty hard to wrap your mind around what it means for there to be no space, she adds.

Hard indeed, but worth the effort. If this version of loop quantum gravity can reproduce all of the features of the standard model of particle physics and be borne out in experimental tests, we could be onto the best idea since Einstein. "It's a beautiful idea. It's a brave, strange idea," says Rovelli. "And it might just work."

Of course, most physicists are reserving judgement. Joe Polchinski, a string theorist at Stanford University in California, believes that Smolin and his colleagues still have a lot of work to do to show that their braids capture all of the details of the full standard model. "This is in a very preliminary stage. One has to play with it and see where it goes," Polchinski says.

If the new loop quantum gravity does go the distance, though, it could give us a new sense of our place in the universe. If electrons and quarks - and thus atoms and people - are a consequence of the way space-time tangles up on itself, we could be nothing more than a bundle of stubborn dreadlocks in space. Tangled up as we are, we could at least take comfort in knowing at last that we truly are at one with the universe.

Supersizing quantum gravity
For loop quantum gravity to succeed as a fundamental theory of gravity, it should at the very least predict that apples fall to Earth. In other words, Newton's law of gravity should naturally arise from it. It is a tall order for a theory that generates space and time from scratch to describe what happens in the everyday world, but Carlo Rovelli at the University of the Mediterranean in Marseille, France, and his team have succeeded in doing just that. "Essentially we have calculated Newton's law starting from a world with no space and no time," he says.

Newton's law of gravity describes the attractive force between two masses separated by a given distance. However, it is not so simple to measure this separation when space has a complex quantum architecture of the sort in loop quantum gravity, where it is not even clear what is meant by distance. This has been the biggest obstacle to showing how Newton's law can emerge from quantised space.

The naive way to measure length in quantised space is to hop from one quantum to another, counting how many steps it takes to reach the final destination. According to loop quantum gravity, however, the fabric of space seethes with quantum fluctuations, so the distance between two points is forever changing, and can even take several values at the same time.

Working with Eugenio Bianchi of the University of Pisa, Leonardo Modesto of the University of Bologna and Simone Speziale of the Perimeter Institute in Waterloo, Ontario, Rovelli circumvented the problem. The team found a mathematical way of isolating regions of space for long enough to measure the separation between two points. When they zoomed out and used this mathematics to look at space-time on much larger scales, they found that Newton's law popped out of their theory.

The calculation by Rovelli's team does not yet reproduce the full complexity of Einstein's general relativity, which also describes masses large enough to curve space appreciably. Their result does point in the right direction, however. Lee Smolin of the Perimeter Institute calls it a major step forward. "Their work shows that loop quantum gravity definitely has gravity in it," he says. "It's no longer just pie in the sky."

Source: New Scientist



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