Reality could be made of anything from blancmange to billiard balls and we'd never know.

Eugene Samuel

WHAT are the fundamental building blocks of the Universe? Once we were told they were atoms. Then it turned out that these were not fundamental at all, but made of protons, neutrons and electrons. Protons and neutrons are in turn made of quarks. Deeper still, we now learn, come tiny vibrating strings and membranes living in a space of 10 or 11 dimensions. But we all expect that one day physicists will finally discover the deepest structures of nature. Won't they?

Not necessarily. Maybe it's impossible to discover these deepest structures. What's more, maybe it doesn't matter what they are. That's the startling claim of Robert Laughlin, a Nobel laureate at Stanford University. According to Laughlin, it may be that what we call reality is a spontaneous phenomenon, emerging like a wave out of some forever unknowable cosmic medium.

In some ways, Laughlin's ultimate aim is not so different from that of other theoretical physicists. Their common goal is to find a single theory that unites quantum mechanics-the theory that describes the behaviour of matter on tiny scales-with Einstein's general theory of relativity, which describes space, time and gravity. Such a "theory of everything" would unite all the forces of nature and explain why time and space exist, as well as answering such trifles as how the Universe began and what happens at the centre of a black hole. Ambitious stuff. The well-trodden route towards this ultimate theory is to try to find the right building blocks of reality and then see whether they can account for the natural phenomena we observe. Laughlin is treading a very different path, however, because he believes you can't build a theory of everything from the bottom up. The laws that govern large-scale phenomena will not be deduced from the laws that govern tiny particles, he says. "It's in the same way that flocking behaviour can be characterised without understanding everything about birds, or superconductivity without understanding atomic theory."

Maybe the nature of reality is hidden from us.Everything is emergent but we'll never know what from.

This idea is called emergence. It's a familiar phenomenon in the theory of condensed matter, which is Laughlin's background. Solids and liquids sometimes play host to strange entities that bear little resemblance to the atoms making up the substance. For example, in some materials there are things called spin waves. Every atom acts a bit like a small magnet, with a north and a south pole aligned along its spin axis, and spin waves are oscillations in the alignment of these spins. "Somewhat like what would occur if one took a supple picket fence and rapidly twisted one end back and forth," says Laughlin. Because this is the quantum world, waves can be considered as particles, and vice versa, so spin waves behave like a kind of emergent particle.

Many other kinds of emergent creature live inside matter, including vibrational waves called phonons, electrical excitations called excitons in semiconductors, and waves of charge called plasmons. These are called variously "collective excitations" and "quasiparticles". From inside the material, these bizarre objects would seem as real as any other particle.

But if quasiparticles are indistinguishable from real particles, could it be that things we think of as real-electrons and so on-are themselves quasiparticles, emerging out of some ubiquitous but undetectable cosmic stuff?

It's a controversial idea. Sure, even in string theory and other bottom-up theories, matter particles arise from the behaviour of smaller building blocks. But for Laughlin there's a crucial difference-we can never determine what that basic "stuff" is.
In a solid, for example, quasiparticles can't be derived or predicted from the behaviour of the individual particles they are made of. In general it has proved impossible to solve the quantum equations of motion for each interacting atom to predict the existence of spin waves or phonons: there are just too many equations to handle. This means knowing about the quasiparticles may tell you nothing about what they are made of.

This isn't a problem for ordinary materials, because by studying these at higher temperatures, we already know what they are made of.But if the Universe works like this, then maybe the underlying nature of reality is hidden from us. Everything is emergent, but we'll never know what from. It would explain why physicists have so far had such trouble finding the right fundamental particles to unify the whole of physics in a theory of everything. "If what you see is model-independent then you can't learn anything about the underlying equations by observing it," says Laughlin. "You could call this the dark side of emergence."

Postcard from the edge: maybe we can never see much deeper into reality than the level of these subatomic particles.

At first sight, this is a depressing conclusion. Does it mean physicists should just give up their quest? Thankfully not. Laughlin thinks we just have to look elsewhere for the fundamental nature of reality-in the process of emergence itself.
The range of emergent particles found in most condensed-matter systems is far too limited to be a blueprint for a theory of everything. Such a theory would need to account for a plethora of particles, including their charge, spin and mass, and also spawn the whole of space and time. "Known cases of emergence are too primitive to serve as a model for real space-time," Laughlin says. "We need to find better ones."

Whatever that model will eventually be, Laughlin's betting it will exploit a phenomenon called quantum criticality. This is a kind of behaviour seen in some materials near absolute zero, when they are poised between one state and another. For example, in some magnetic solids individual spins become so highly correlated that the behaviour of one affects them all, and the collective wavefunction of the material lacks any sense of scale.

And to Laughlin, this is a highly desirable property, because scale invariance is also a fundamental property of space-time. In the standard model of particle physics, particles are thought of as collective oscillations of the vacuum of space. In this model, a small chunk of space oscillates exactly as much as a larger chunk of space. This is just like when you zoom in on a stretch of coastline while looking at a map. You see as much variation in the coast no matter what scale you're looking at, because as the scale of the map gets smaller, you lose sight of larger variations but become sensitive to smaller ones.

So modelling the Universe with quantum criticality gives you scale invariance for free. But it also means that any sense of the material being made up of building blocks is lost. In a superconducting material, for example, nothing about what the material is made of follows from the behaviour of spin waves. "If all we could observe was the quasiparticles, we wouldn't be able to tell," says Matthew Fisher, who works on the theory of quantum criticality at the University of California, Santa Barbara.

Likewise, if the very fabric of the Universe is in a quantum-critical state, then the "stuff" that underlies reality is totally irrelevant-it could be anything, says Laughlin. Even if the string theorists show that strings can give rise to the matter and natural laws we know, they won't have proved that strings are the answer-merely one of the infinite number of possible answers. It could as well be pool balls or Lego bricks or drunk sergeant majors.

Just a minute, though. If you can warm up a quantum-critical solid so that the bits become visible again, why not heat up a piece of the Universe-a little matter, say-to do the same? This is effectively what experimental particle physicists have been doing for decades with particle accelerators. The trouble is, to see the underlying medium of reality you have to reach beyond the maximum energy that a quasiparticle can carry. The quasi-particle might then start to show signs of its component parts. And according to quantum mechanics, this would be at a temperature of 10³² kelvin-ten million billion times hotter than anything we've achieved so far, and probably out of reach of any conceivable civilisation. "What we emerge from is unknowable," says Laughlin. "The underlying equations of the Universe cannot be determined from what we know."

This isn't enough to convince most theorists. "All these hopes are fine and grand," says David Gross, a string theorist at the University of California, Santa Barbara. "I'd like to see concrete examples of how you can leap to quantum gravity from this idea."
So Laughlin needs to build a testable theory from this idea, and derive the things we see from the ideas of quantum criticality. "The hope is to find the right equations that get it all to come rolling out," he says.

His first problem is a big one: explain how space and time emerge. Gross thinks it's impossible. "Space and time are coordinate systems that sit there, they're not something that can emerge." But Laughlin says Gross is voicing a mere article of faith.
Lee Smolin, a theoretical physicist at Perimeter Institute for Theoretical Physics in Waterloo, Canada, is more sympathetic. Smolin has worked on a theory called loop quantum gravity, in which space and time emerge from abstract creations called spin networks. They produce a quantum version of space-time called spin foam, which is grainy on very small scales-just what is needed for a quantum version of gravity.

Smolin has had to give his spin networks one basic property, a fundamental symmetry of the Universe called general covariance. This states that however you are moving, all the laws of physics (for example, the observed speed of light) look the same. It is the principle behind general relativity, which is still our best description of gravity, space and time. But it's at odds with all quantum theories of how subatomic particles behave. If you could derive a model of the Universe that includes generally covariant particles, then you'd finally have a theory of quantum gravity.

But Smolin says it's hard to see how general covariance could come out of quantum criticality. And Laughlin can't just assume, as Smolin did, that his building blocks have this property, because there are no building blocks in the first place. If he can't assume any property at all, is there any hope of getting general covariance to emerge?

Laughlin cites the work of Subir Sachdev, who is modelling quantum-critical systems together with Yale colleagues Matthias Vojta and Ying Zhang. In 2000, the Yale group explained some mysterious fluctuations in the electrical current in a superconductor. They worked out that it was in a quantum-critical state with a new kind of quasiparticle. Astonishingly, these quasiparticles obey their own version of special relativity. For example, there's an absolute speed that they can't surpass (although it's not the speed of light). This implies that the particles observe a spatial symmetry called Lorenz invariance-that is, frames moving with constant velocity all see the same physical laws. This is a weaker symmetry than general covariance, but if the one can emerge, says Laughlin, why not the other?

He is now working on a three-dimensional quantum-critical system in which he hopes objects similar to black holes will emerge. It builds on the work that won him a Nobel prize, in which he predicted the existence of quasiparticles with fractional electric charge.
So far, he has one very tentative prediction. He and George Chapline of Los Alamos National Laboratory speculated that black hole event horizons might be a kind of phase transition in the vacuum. By analogy with a condensed matter phase transition, they concluded that relativity might fail at the horizon, altering the spectrum of the light that comes off the black hole. So this theory could in principle be tested.

And another kind of evidence could back Laughlin's ideas up. A real material. So far, no material is known to have the right kind of quantum critical behaviour to include generally covariant quasiparticles, but if we did find such a material, Laughlin thinks it would put emergence ahead of string theory or loop quantum theory as the leading approach to quantum gravity.

With this in mind he's looking at all sorts of low-temperature materials in the hope of spotting some overlooked phenomena that might be models of reality. The nearest so far comes from Grisha Volovik at the Helsinki University of Technology, who claimed in 2000 that there are quasiparticles in liquid helium-3 that respect special relativity.

This is still a long way from modelling the whole of particle physics and general relativity. But perhaps, in some chemist's lab somewhere, or even buried in the rock under your feet, is a small crystal that will do the trick-a material that's a microcosm of the Universe.

Entangling particles is easy when you know how

THE dream of teleporting atoms and molecules and maybe even larger object has become a real possibility for the first time. The advance is thanks to physicists who have suggested a method that in theory could be used to "entangle" absolutely any kind of particle. Quantum entanglement is the bizarre property that allows two particles to behave as one, no matter how far apart they are. If you measure the state of one particle, you instantly determine the state of the other. This could one day allow us to teleport objects by transferring their properties instantly from one place to another. Until now, physicists have only been able to entangle photons, electrons and atoms, using different methods in each case. For instance, atoms are entangled by forcing them to interact inside an optical trap, while photons are made to interact with a crystal.

"These schemes are very specific," says Sougato Bose of the University of Oxford. But Bose and Dipankar Home, of the Bose Institute in Calcutta, have now demonstrated a single mechanism that could be used to entangle any particles, even atoms or large molecules. To see how it works, consider the angular momentum or "spin" of an electron. To entangle the spins of two electrons, you first need to make sure they're identical in all respects but their spin. Then you shoot the electrons simultaneously into a beam splitter. This device "splits" each electron into a quantum state called a superposition, which gives it an equal probability of travelling down either of two paths. Only when you try to detect the electron do you know which path it took. If you split two electrons simultaneously, both paths could have one electron each (which will happen half of the time) or either path could have both.

Bose and Home show mathematically that whenever one electron is detected in each path, they will be entangled. While a similar effect has been demonstrated before for photons, the photons used were already entangled in another way, even before they reached the beam splitter. "One of the advances we have made is that these two particles could be from completely independent sources," says Bose.

The technique should work for any objects-atoms, molecules and who knows what else as long as you can split the beam into a quantum superposition. Anton Zeilinger, a quantum physicist at the University of Vienna in Austria, has already shown that this quantum state is possible with buckyballs-football-shaped molecules of C60. Although entangling such large objects is beyond our technical abilities at the moment, this is the first technique that might one day make it possible.
Any scheme that expands the range of particles that can be entangled is important, says Zeilinger. Entangling massive particles would mean they could then be used for quantum cryptography, computing and even teleportation. "It would be fascinating," he says. "The possibility that you can teleport not just quantum states of photons, but also of more massive particles, that in itself is an interesting goal."
Anil Ananthaswamy
More at: Physical Review Letters (vol 88, article 05401)

Further Reading

"The theory of everything" by Robert Laughlin and David Pines, proceedings of the National Academy of Sciences (vol 97, p 28) "Quantum phase transitions and the breakdown of classical general relativity" by George Chapline and others (www.arxiv.org/abs/gr-qc/0012094)