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NS 1 sep 2001 20 sep 2001
Movers and shakers
Brain cells with a bit of get-up-and-go are what make us so smart NS sep 2001
A UNIQUE twist in the development of the human brain may explain how we evolved the capacity for complex abilities such as language and abstract thought.
At some point in our evolution the brain areas that mediate these talents began to expand. How did they do that? By sending an army of extra neurons along a route uncharted in other mammals, say Kresimir Letinic and Pasko Rakic of Yale University Medical School in Connecticut. This process allowed key brain regions that normally develop separately to bulk up in synchrony.
This study is the first to show a difference in brain development between humans and other mammals, says Yi Rao, a neurobiologist at Washington University School of Medicine in St Louis. "The general consensus has been that the vast majority of developmental processes are conserved, from flies to humans," he says.
Most uniquely human abilities arise in parts of the cortex. "The cortex is the integration place. Attributes like planning, language and high-order thinking depend on it," says Katerina Semendeferi, a physical anthropologist at the University of California at San Diego. But if the cortex expands, other related areas need to grow too. One such area is the dorsal thalamus, a key gateway for information destined for the cortex.
But these two areas of the brain develop independently. As the brain forms, neurons migrate from specialised zones where they are produced to their final destination. According to neuroscience dogma, there's no cross traffic-neurons in the thalamus start off in one zone, neurons for the cortex in another. So how the dorsal thalamus and the cortex coordinated the explosion in their growth is a puzzle.
Letinic and Rakic used a fluorescent dye to track migrating neurons in slices of brain tissue from aborted mouse, macaque and human fetuses. They found that the human cells were breaking the rules-neurons made in the ganglionic eminence (GE), a zone that normally sends brain cells to the cortex, were travelling to the thalamus too.
Rakic says this could explain how the different brain regions important for language and cognition developed together. As the GE boosted its neuron production to bulk up the cortex, some of the extra cells were diverted to the dorsal thalamus.
A second set of experiments showed that the pathway doesn't exist in mice. The researchers found that neurons taken from the human GE migrated across a culture dish towards a chunk of dorsal thalamus. But in mice, the dorsal thalamus didn't attract GE neurons. In fact, nearby structures actively repelled them (see Diagram). Rakic says this simple difference means the brain expansion in humans could have come from just a few-or even just onemutation in the genes for the signalling molecules that attract or repel migrating neurons.
But Semendeferi warns that it's too early to say this migratory pathway is the special preserve of humans. "To suggest this is a uniquely human phenomenon, you'd have to look at chimps," she says. Greg Miller
More at: Nature Neuroscience (vol 4, p 9311
Desperate measure The illicit use of an ulcer drug for abortions is leaving a terrible legacy
FAILED attempts to induce abortion using an ulcer drug called misoprostol may be causing a minor epidemic of birth defects around the world. Misuse of the drug is increasingly common where abortion is not freely available.
In Colombia, Brazil and the Philippines, the drug is readily available on the black market. An informal survey by New Scientist has also revealed that clandestine abortions with the drug are taking place in the Dominican Republic, Argentina, Spain, Nigeria, South Africa and Indonesia.
In Britain and the US, doctors prescribing the abortion drug RU486 also give misoprostol to induce contractions, although it's not licensed for this purpose (New Scientist, 28 October 2000, p 13). The combination is highly effective. But in countries where abortion is illegal, women are taking misoprostol on its own to induce abortions.
"It's really the poor person's method," says Susheela Singh of the Alan Guttmacher Institute in New York, who has written extensively about clandestine abortion practices in Latin America. The drug is very cheap-as little as 35 US cents-and is often available over the counter.
But taking misoprostol on its own only induces abortions about 40 per cent of the time, so many babies are born after failed Oabortion attempts. Several studies in Brazil, where up to 75 per cent of clandestine abortions involve misoprostol, suggest the drug causes birth defects such as fused joints, growth retardation and a condition known as Mobius syndrome, which is characterised by paralysis of the face.
One recent study found that out of 93 children with defects associated with Mobius syndrome, 34 per cent of those infants had been exposed to misoprostol, compared with just 4.3 per cent of the 279 infants in a control group. Another revealed that 49 per cent of infants born with Mobius at seven hospitals in Brazil had been exposed to misoprostol, whereas only 3 per cent of 96 infants born with neural tube defects had been exposed to the drug.
While such findings clearly suggest a link between misoprostol and certain congenital defects, the real risk might never be known because it would be unethical to do the necessary studies. "I think [these results] are real. Statistically they are highly significant," says Fernando Vargas of the University of Rio de janeiro, who took part in both studies. Because the drug is used secretly, it is hard to find out how many birth defects might be caused by it, Vargas adds.
But even critics of the drug's clandestine use recognise that, for women, attempting abortion with misoprostol is less dangerous than other methods, such as injecting saline. Misoprostol can cause bleeding, but this often allows women to enter the health system legally. And it doesn't result in the kind of complications caused by other methods.
In the US, misoprostol is controversial not just because of its use in abortions but because it is widely used to induce labour. The practice has come under scrutiny recently because several cases of uterine ruptures and deaths of babies and mothers have prompted lawsuits. Sylvia Pagin Westphat, Boston More at: American Journol of Medical Genetics ivot 95, p 302)
We're under attack!
A tiny artificial muscle could detect lethal biowarfare agents
THE molecules that make muscles contract could give an early warning of a biological weapons attack. A team of Australian researchers is using the molecules to develop a chip-based bioweapons detector small enough to fit on a wristwatch.
When a muscle contracts, filaments of two proteins, actin and myosin, slide past each other. The researchers say that if you attach myosin molecules to a biochip, you can use the movement of the adjoining actin molecules to detect whether biowarfare agents such as anthrax are present.
At the moment, bacteria and viruses are usually identified using laboratory equipment the size of a large fridge, or with tests that take days. But a quick result is vital in germ warfare, and biodetectors must be light and portable enough to be taken to the scene of a possible attack.
To produce such a device, one of the first problems that Dan Nicolau of Swinburne University of Technology in Melbourne and Cris dos Remedios of the University of Sydney had to overcome was getting thousands of actin and myosin molecules lined up in the same direction. They did this by embedding the myosin molecules in microscopic channels on a polymer chip. "Proteins attach to surfaces as they like, not as we like, so we had to engineer the surfaces again and again until we found the right composition," says Nicolau.
In the end, the team coated their chip with a polymer that doesn't attract proteins, and etched tracks into it with a laser. This exposed a new surface that the myosin sticks to.
Each of the biodetectors will contain hundreds of tracks, and each will carry thousands of actin and myosin molecules. In the right chemical environment actin molecules continuously move along the myosin molecules.
The researchers plan to detect biological weapons by attaching special antibodies to the actin molecules. These antibodies stick to proteins on the surface of biowarfare agents, such as anthrax. When the antibodies bind to the agents they will stop the actin molecules from moving along the myosin-lined channels.
Nicolau and dos Remedios are now looking at different ways to detect when the actin molecules have come to a sudden stop. One possibility, says Nicolau, is to attach tiny magnetic beads to the actin (see Figure). The researchers have already shown that the beads do not slow down the actin molecules, and when the actin is moving the beads will induce an electric current in an induction coil. The next step is to build the circuits needed to amplify these signals.
An alternative way to detect when the molecules come to a halt would be to use an alloy that exhibits -giant magnetoresistance". Moving the magnetic beads past the alloy in the presence of a magnetic field would cause a huge change in its resistance.
The researchers presented their findings at a meeting in Hawaii last month sponsored by the US Defense Advanced Research Projects Agency. "It's a tremendous idea," says Bruce Cornell, chief scientist for AMBRI, a Sydneybased biotech company that is developing its own biosensors. "They're attempting to use molecules to detect molecules. They're dealing with things on the same size scale, so it has the potential to be extremely sensitive,
Piotr Grodzinski, a microfluidics expert at Motorola in Tempe, Arizona, praised the idea. "There's a lot of creative thinking that went into this development," he says. Nicolau and dos Remedios plan to have a working prototype of the detector in two years' time. Rachel Nowak
Let's not throw away our freedoms for no good reason
ONE of the least edifying spectacles of the past couple of weeks has been the sight of political leaders using the attacks on the US to further their own ambitions or as cover for words and deeds that would be normally be unacceptable.
Even as New Yorkem were fleeing giant plumes of dust in Manhattan, Ariel Sharon, prime minister of Israel, was ordering his tanks to tighten their gnp on West Bank settlements. And vathin days, Northem Ireland's Unionists were seizing on the new mood of antipathy towards terrorism to score points against their Republican opponents.
Blatant as they were, these examples of political opportunism were at least transparent. The same cannot be said of many of the hurriedly proposed anti-terrorist measures coming out of Washington, London and Brussels. Take compulsory ID cards. Many countries already have them, of course. Now the British government has signalled its intention to be the first to Introduce plastic photo cards bearing an image of the holder's fingerprint or iris, in addition to the usual name, number and address.
There are plenty of reasons why a government might want Its citizens to carry such cards: the biological identifier would make them harder to forge and less worth stealing, and they could If used properly help governments to prevent benefit fraud and identify illegal immigrants. But that's not how the scheme is being sold. The BrMsh government is calling it an anti-terrorism measure without bothering to explain how such cards might help to prevent the kinds of attack seen In the US or assist investigators afterwards.
Perhaps that is because it is not obvious how ID cards will help. The key suspects in the New York attacks did not fly in on false passports. They were long-term residents, members of 'sleeping cells" who may well have died with their Social Security cards in their pockets. And even ff these terrorists had needed to forge high-tech ID cards, they would surely have had the means to do so.
If compulsory ID cards are such a great weapon against terrorism, why didn't the British government introduce them years ago to combat the IRA? Probably because it realised that giving police officers the power to stop virtually anyone they like on the streets to ask for identification is unlikely to catch many hardened [email protected] extremely likely to exacerbate the religious and racial tensions that drive people into the hands of extremist organisations in the first place.
But with governments keen to put up a good show of fighting terrorism, identity cards have obvious advantages. They look tough, are cheap to introduce, and are likely to make people feel more secure-even if the real threat of terrorism is undiminished.
Similar tough but empty talk has dominated the issue of how to starve terrorists of money. Following George Bush's warning to banks this week, they are now on alert not to do business with anyone calling themselves Osama bin Laden, the Salafist Group for Call and Combat, or 25 other suspected terror organisations. Does anyone imagine these people reveal their allegiance as they open accounts? One measure would be to require banks to know their customers better, and so catch any incongruous transactions (see p 6). Yet the US has led the industriallsed world in resisting such legislation and failed to [email protected] an international treaty to combat terrorist funding. Worse, the US leads the world in letting people set up fake companies to move money around. In the state of Delaware, a terrorist can register a company just by producing a driver's licence. Three such companies affiliated with bin Laden appeared on a US hit list this week. In Britain meanwhile, Gordon Brown has been calling for less banking secrecy in Switzerland, Germany and Austria. But Europe's main offshore banking centre is [email protected], where money flows with few questions asked.
Wrapped in the flag of country and outrage, much of what is being called for will serve only to pander to governments' insatiable hunger for control. In this area, no law is better than bad law.
Spot the stargate
Wormholes may be lurking at the end of a very special rainbow
IF SOPHISTICATED aliens are commuting across the Galaxy using a superfast transport network, we should be able to spot the terminuses. A multinational team of physicists has shown that "wormholes'-gateways to distant regions of space-should stamp a coloured hallmark on light from distant stars as it travels past them on its way to Earth.
One way extraterrestrials might travel through the Universe would be to use a hypothetical short cut through the fabric of space-time. Einstein's theory of gravity suggests such wormholes could exist, but they would need lots of matter with negative mass-and therefore repulsive gravityto keep them open.
Nobody knows whether negative mass can exist in such large amounts, but if it does, one way to detect it would be through an effect called "gravitational lensing". Unlike normal matter, whose gravity makes light bend towards it, negative mass would make light bend away. Several years ago, John Cramer of the University of Washington in Seattle showed that wormholes would deflect light from stars behind them to form a bright curve called a "caustic".
In practice, however, it would be tricky to spot unambiguous signs of these caustics. But now a team led by Diego Torres of Princeton University in New Jersey has found another lensing effect that's much more distinctive.
Torres and his colleagues have worked out that when light from a distant source passes a wormhole, the different colours in the light should be magnified in a peculiar way, making the coloured pattern we see on Earth very different from an image lensed by normal matter. 'It's a way of searching for a negative-mass object," says Torres. He thinks it might be possible to see this effect in existing databases of images or in future targeted searches.
"It's a very nice result and a valuable contribution to the line of investigation I and my colleagues initiated in 1995," says Cramer. He adds that scientists searching for heavy lumps of matter called MACHOs in the outskirts of our Galaxy are already looking for lensed images in several colour bands. "So at least part of what is being recommended is already in place,' he says.
"If such mass was found, it would tell us something very profound about fundamental physics," adds Torres. He stresses that the pattern his team has described is the hallmark of negative mass of any kind, not necessarily of wormholes. However, if negative mass is found to exist, the case for wormholes would be much stronger, whether they were built as a transport system by aliens in a hurry, or formed naturally in the big bang. Marcus Chown
More at: wwwarxivorg/abs/gr-qc/0109041
Beam me up
Spooky quantum link brings teleporting a step closer
CLOUDS of trillions of atoms have for the first time been linked by quantum "entanglement"-that spooky, almost telepathic link between distant particles. The feat opens new possibilities for quantum communication systems and sci-fi-style teleporting of objects from one place to another. The everyday view of atoms is of solid, independent objects a bit like billiard balls. But according to quantum theory, atoms are far less concrete entities, and can be persuaded to interact with each other so that events affecting one instantly affect another-no
'The feat opens now possibilities for sci-fi-style teleporting of objects from one place to another' no matter how far apart they are. Dubbed entanglement, this could open the way to superfast quantum communications systems and ways of teleporting objects by instantly transferring their properties from place to place.
Before now scientists only managed to entangle a few atoms close together, raising a question mark over the practicality of quantum technology. But now a team at the University of Aarhus in Denmark has entangled two clouds of trillions of caesium atoms. The method should work for very distant clouds.
The team coordinated the quantum states of two atom clouds by exploiting a loophole in Heisenberg's uncertainty principle. The principle forbids precise knowledge of the quantum state of each gas cloud. But when two clouds are in an entangled state, you can work out the overall properties of the two collections, for example, the so-called total spin state. Changes in one cloud are mirrored by changes in the other that keep the overall property of both clouds constant. To preserve the frail entanglement, the team shielded the atom clouds ftom outside disturbances. They did this using special magnetic fields to trap the atoms inside two vessels lined with paraffin wax. By shining laser light through the vessels, the team entangled the spin states of the two atom clouds then watched how long the state lasted. Full entanglement would have lasted only a million-billionth of a second, but the team kept up partial entanglement for half a millisecond-aeons by quantum standards. "The experiment shows that it is possible to create entanglement with macroscopic objects, and to do it using just laser lightwhich means one can do it even when the objects are separated by substantial distances, " says the team leader, Eugene Polzik. 'We've also shown that the state can persist for a long time, even at room temperature." "Now that the experiment has been done, it should be relatively simple to entangle more than two atomic samples, or to teleport states of atomic samples," says Ignacio Cirac, a physicist at the University of Innsbruck in Austria. Advances could lead to real-life quantum communication systems, teleportation and quantum computers. Robert Matthews More at: Nature (vot 413, p 4001
Monsters on the move
Herds of wind turbines will soon be crawling out to sea
GIANT underwater tractors are being lined up to plant wind turbine towers in offshore energy farms off the Dutch coast. Looking a little like the enormous caterpillar-tracked Crawler that trundles space shuttles out to their launch pads, the subsea tractors will be able to install turbines quickly, all year round, in any weather.
It's no mean feat, says Henk Hutting of Kema, a Dutch engineering firm based in Arnhem. Wind turbine towers are usually built on land and carried out to sea by craneequipped barges, before being lowered onto supports driven into the seabed. Bad weather makes installation next to impossible. This process is very slow and expensive, says Nick Goodall of the British Wind Energy Association, in London. "The real cost savings in offshore farms is in deployment."
Now Kema has designed a vehicle that can be driven along the seabed to make the whole process a lot less painful. It will be built by Mammoet, the Dutch engineering firm that's helping raise the Russian submarine Kursk.
The massive 20-metre-long four-tracked tractor gets its power from a hydraulic pump. The diesel generator driving the pump is housed safely above the water's surface in a pilot's cabin halfway up the turbine tower. Hydraulic lifting mechanisms in each corner of the tractor raise the four legs of the turbine tower a metre off the ground before the tractor takes its icy dip in the North Sea.
Kema hopes that the U-shaped tractor will be able to position the four legs of the tower over its support piles with an accuracy of 10 centimetres. Once the tower is lowered, the vehicle simply backs off, and trundles back to a beach. This way of working may extend the range of suitable wind energy sites, says Hutting, as turbines could be installed in water deeper than the 20 metres that is currently feasible.
Kema has further turbine developments in the pipeline. One problem with wind turbines is fatigue caused by the waves and wind vibrating the structure in rough weather. So the firm is designing a new type of lattice tower that can change its stiffness and resonant frequency by using hydraulic pistons to control the rigidity of the diagonal cross bars. Duncan Graham-Rowe
The unseen dimension
Step out into the fifth dimension, and you'll find dark matter, the key to antigravity and even what happened before the big bang, says ALison Boyte
JUST A MILLIMETRE AWAY from you, there could be another universe. You can't see it, smell it or reach out and touch it. You could only get to it by travelling in a direction that is forbidden. Because we are all prisoners, trapped in a nether dimension that physicists call the braneworld. Ifthey are to be believed, there's far more to our Universe than meets the eye, or even the most powerful telescopes. According to their theories, the familiar three dimensions that contain all the stars and galaxies we see could actually be a membrane-a brane for short-floating in a space of five, six or more dimensions, like a soap bubble in the bathroom. There could be other branes out there in the higher dimensions, other universes that we can never see. But that doesn't mean we are unaffected by the exotic regions mapped out in these extra dimensions. On the contrary, they may lie at the heart of some of the fundamental features of the Universe. Dark matter, the antigravity of empty space and even the beginnings of the Universe itself may all be bound up with the fifth dimension and beyond. Unseen dimensions are nothing new for physicists. They crop up in string theory, the leading contender for a 'theory of everything' that will bring all the forces of nature together under a single umbrella. String theory needs six extra space dimensions for its tiny elemental strings to move through. These extra dimensions are usually thought to be curled up into circles only 10-35 metres around, far smaller than the radius of a proton. No wonder they are invisible. But there is another possibility. According to a few theoretical physicists, some of the extra dimensions could be a lot larger, perhaps a tenth of a millimetre long-or even infinite, like our familiar space and time. If that is so, then shouldn't we be able to see these extra dimensions or walk sideways into them? No, say the theorists, because matter and light are stuck on our brane, leaving us blind to anything outside. We are stuck in flatiand. Only gravity is free to escape and roam in the higher dimensions-what science fiction writers call hyperspace, and physicists call 'the bulk'.
It's an attractive idea, because the existence of large gravity-only dimensions would explain why gravity appears to be so weak, many orders of magnitude feebler than electromagnetic and nuclear forces. This weakness may be an illusion. On a fundamental level, gravity might be as strong as the other forces, but appears to us to be much weaker because it leaks out into higher dimensions. The physicists scrambling to join in the brane game all have their own ideas about what the braneworld is really like. How big are the extra dimensions? How many of them are there? You can take your pick. 'At the moment it's like a playground,' says Lev Kofman of the University of Toronto. But if we're stuck in our three dimensions, what is lurking beyond them, filling the higherdimensional hyperspace? One idea is that it's the long-sought dark matter that astronomers believe must be out there somewhere, keeping galaxies and galaxy clusters in one piece. The matter we can see doesn't exert enough gravitational pull to keep these whirling bodies from flinging themselves apart. The conventional explanation forthis mystery is that space must be filled with a soup of peculiar particles that don't radiate or reflect light, but only exert gravity. Nobody has yet detected any such dark-matter particles, though, so the case is still open. Savas Dimopoulos of Stanford University thinks 'dark matter' might really be ordinary matter that is hidden f rom us. Last year, with Nemania Kaloper of Stanford, Nima [email protected] of the University of California at Berkeley and Gia Dvali of New York University, Dimopoulos described a world where this matter could be just millimetres away. This is the manyfold universe. The basic idea is that our Universe is folded over many times (see Diagram, p 28). Light f rom distant objects has to travel along the brane and so would take billions of years to reach us, but gravity could take the short cutthrough a higher dimension and trick us into thinking that there's unseen matter around us. 'Dark matter could be made out of the very same protons and electrons as we are, and we may not even know it,' says Dimopoulos.
'Higher dimensions might be home to new types of force that we could one day learn how to harness'
The 'dark matter" seems to hang around galaxies and clusters on our fold, but there's no mystery in that. Gravity between matter on the separate folds would tend to drag it together. That means a pattern of galaxies and clusters on the next fold would more or less mimic ourssuperficially, itwould look likeourlocal Universe.
Dvali, along with Gregory Gabadadze of the University of Minnesota at Minneapolis, thinks the braneworld might also explain another cosmological conundrum-why the world is made mostly of matter, and not antimatter. In the big bang, ma"er and antimatter should have been created in equal quantities. When a particle of matter meets its antimatter counterpartwhich has the same mass but opposite charge and other properties-they annihilate each other in a burst of energy. So if nature treated them equally, there would be nothing left in the Universe except radiation.
Particle physicists think that nature must be somehow lopsided: a phenomenon called CP violation has the potential to discriminate between matter and antimatter. But CP violation can't change the overall balance between matter and antimatter on its own. There has to be somewayto hidethe antimatter inthe Universe.
Dvali and Gabadadze have a braneworld explanation. They say quantum-mechanical fluctuations in the early stages of our Universe cause the brane to wriggle so much that parts of it come loose. These'baby branes'pull awayfrom our Universe and head off into the bulk, capturing some particles from the main brane and carrying them away. If CP violation somehow arranges it so that more antimatter than matter is carried off, our world is left with a net balance of matter.
Brane cosmology might also explain one of the most shocking properties of the Universe. A few years ago, astronomers discovered that the expansion of the Universe seems to be accelerating. Distant supernovae-huge explosions that mark the death of a giant star-look dimmer than they should. That can be explained if the Universe's expansion has speeded up, putting the explosions further away than we thought. What force could be responsible for this accelerated expansion?
Einstein worked out that that if space has some energy and pressure of its own, it would exert a kind of repulsive force or negative gravity. This comes into his theory of general relativity as an optional quantity called the cosmological constant-a property of space he eventually rejected, but one we may now have to take seriously. But that raises another question: why should space have its own energy?
Physicists seemed at first to have a ready explanation. Quantum theory says that space is filled with short-lived particles and anti-particles, continually appearing and disappearing. This would [email protected] space its own inherent energy, and therefore a repulsive force.
But there's a problem. When you work out the numbers, the theory predicts an energy density that iS 10^120 times too high. lt would lead to a repulsive force so powerful itwould rip apart the bonds that hold atoms and molecules together.
Branes might come to the rescue again. Arkani-Hamed, Dimopoulos, Kaloper and Raman Sundrum of Johns Hopkins University in Baltimore suggested in 1999 that the braneworld would allow most of this antigravity to leak away into the higher dimensions of hyperspace, just as ordinary gravity does.
It's a neat idea, but actually calculating whether it pans out to produce exactly the right dilution of antigravity is another matter. Braneworld physics isn't yet a precise science. Sundrum likens it to banging on a faulty TV set: sometimes you get lucky and the problem is fixed. 'Braneworlds are certainly banging on gravity, but we still have to see if they help with the cosmological constant problem,' he says.
Meanwhile, could braneworlds bang on the big bang?They provide a natural candidateforthe beginning of the Universe,' claims Neil Turok of Cambridge University. This year Turok, along with Paul Steinhardt and Justin Khoury of Princeton University and Burt Ovrut of the University of Pennsylvania,cameoutwiththeideathatthebig bang might have been more of a grand slam. In their picture, our 3-brane universewas once cold and empty. Then one day another brane came out of the fifth dimension and smacked into ours, releasing enough energy to create all the matter in out Universe (see Diagram above right). The team christened this model theekpyrotic universe", referring to an ancient Greek cosmology where the Universe is created in a burst of fire (New Scientist, 14 April, p 7).
The ekpyrotic universe does away with the mystery at the beginning of time. In conventional cosmology, if you play the film of the Universe backwards, you see all the galaxies which are now moving away from each other move back together. Do this long enough, and you will see everythi ng coalesce at a point of infinite density and temperature called a singularity. But then the film projector breaks and we are left in the dark. At a singularity, physical equations no longer make sense because everything is infinite, which is why science can't tell us what, if anything, happened before the big bang.
In the ekpyrotic universe, there is no singularity. Before the braneworld collision, our Universe was still there, but it was empty. The question of what happened before the big bang becomes a sensible one, even if the answer may be 'not much'. 'This takes you from a universe of finite age to one of potentially infinite age,' says Steinhardt.
The theory also provides an alternative to cosmic inflation, which for 20 years has been thefavoured explanation for some of the grandest properties of the Universe.
For example, the cosmic microwave background radiation, a remnant of the big bang, is almost exactly the same temperature and brightness in all directions. On the face of it, there's no reason why the Universe should have this large-scale smoothness. Right from the moment of the big bang, opposite ends of the expanding Universe would have been too far apart for light to have reached from one to other. This would have left them with no way to equalise their temperatures.
Inflation gets round this by proposing that early in its life, the Universe went through a brief but incredibly rapid period of expansion. The whole Universe we see now (and in the microwave background era)wasamplifiedfrom just one tiny patch of the original big bang. That explains why one side of the visible Universe is much the same density and temperature as the other side. lt also explains the small irregularities we see in the background radiation: they started out as minuscule quantum fluctuations, and were expanded by inflation.
Inflationary cosmology has also been successful in explaining other big-bang problems. The theory comes in many different guises and no one of these is wholly satisfactory, but there's no workable alternative-apart, perhaps, from the ekpyrotic theory.
In this view of things, when the branes collide, the conflagration occurs all along the brane with the same violence. That would account for the large-scale smoothness of the Universe. Meanwhile quantum ripples along the incoming brane's surface cause small fluctuations in temperature, and seed galaxy formation.
'A theory which competes with inflation is so important that it must be checked out,' says Andrei Linde of Stanford University, one of the fathers of inflationary cosmology. But Linde reckons it doesn't make the grade. He points out that it requires very finely balanced initial conditions. For example, the two colliding branes must be almost perfectly parallel to begin with.
Theory alone won't tell us who's right. We need some solid experimental evidenceand fortunately there may be a way to investigate the higher dimensions. Prisoners in the braneworld we may be, but we might yet be able to hear echoes from outside its walls. These echoes would come to us in the form of gravity waves.
AccordiQg to general relativity, when matter is accelerated it creates ripples in space-time. So when stars collapse into black holes, or when two black holes collide, they should emit bursts of gravitational waves. Astronomers who hope to detect these waves are building several new detectors.
BLACK HOLES litter our Universe, and loom large in sci-fi nightmares. Yet they still have physicistsprettymuchstumped.'Understanding the behaviour of black holes is really a central puzzle in modern theoretical physics,' says Andrew Strominger, a theoretical physicist at Harvard University. 'We've been wrestling with them for nearly 100 years and we still don't understand them.'
A few experiments seem long overdue. But a black hole isn't a piece of equipment you can just knock together in a lab. Not yet, at least. It is possible that in a few years we will be making black holes to order. Unlike the monsters that wander the celestial wilderness, these home-made holes won't be heavy enough to swallow the Earth. They'd be as light as a small protein molecule, and so frail that they'd live and die in a split second. But that brief life could tell us enough to revolutionise our ideas about gravity. 'It's just mind-boggling to think about making a black hole at an accelerator,' says Michael Turner of the University of Chicago.
Black holes are thought to form in space when a very massive star's core collapses under its own weight-and keeps on going until it is crushed to a point. This point is cloaked by the event horizon, a boundary from within which nothing, noteven light, canwriggle out.To make a black hole in this way you need a vast amount of matter, more than the mass of our Sun, to produce strong enough gravity to crush itself. That's hardly feasible in the lab.
To make a black hole of a more practical size, you have to give gravity a helping hand by smashing matter together at high speeds to compress it. A particle accelerator could do the job-except that, according to conventional physics, the minimum energy required is 10 million billion times as much as any existing particle accelerator can manage. All that energy would go to make a black hole weighing just 10 micrograms. Nothing lighter can make a black hole, it seems. It would have to be compressed into an impossibly small space-forbidden by quantum mechanicsin order for its gravity to get strong enough to swallow light. The problem is that gravity is terriblyyveak, manyorders of magnitudeweaker than the other forces of nature.
However, gravity's feebleness puzzles physicists (see p 26), and some of them suggest it can be explained if space has extra dimensions thatwe can'tsee.These invisible dimensions are peculiar in that only gravity can reach into them.
The idea is that gravity is so weak because it leaks into the extra dimensions. If that's true, gravity could get a lot stronger very close to a piece of matter. At a range of far less than a millimetre, it hasn't had much chance to leak sideways out of our Universe (see Diagram, p 34). So the density at the collision site of a particle accelerator might, after all, be enough to create a black hole.
The latest word is that these mini black holes might put in an appearance at the Large Hadron Collides under construction at CERN, the European laboratory for particle physics near Geneva. In 2006, the LHC will start smashing protons and antiprotons together at energies of 14 tera-electronvolts.
Eating the Earth
In two papers posted on the Web in June, Steve Giddings of the University of California at Santa Barbara and Scott Thomas of Stanford University calculate that in certain theories with two or more extra dimensions, the LHC will make about one black hole per second. They would each weigh just 5000 times the mass of the proton, and be 10 ^18 metres across.
Despite media scares that little black holes could grow by sucking in matter around them, there's no danger that these babies will eat the Earth. Cosmic rays are continually smashing into the Earth's atmosphere at even higher energies than are found in particle accelerators. If it were possible to make a stable and dangerous black hole that way, it would already have happenedand we wouldn't be here.
But how come they are so innocuous? It turns out that small black holes don't survive long. In the 1970s, Stephen Hawking showed that the intense gravitational field of a black hole can make a particle and its antiparticle pop up near the black hole's event horizon. One might escape, while the other falls in. The result is that black holes gradually lose energy and evaporate.
With big black holes, gradually is putting it mildly. The amount of radiation escaping from a black hole is inversely proportional to the square of its mass, so a hole with a mass of 30 times that of the Sun would take 1061 times the current age of the Universe to disappear. But the kind of black hole that could appear in the LHC would vanish in just 10 24 seconds-far too quickly to gobble anything, let alone a planet.
But it would leave a calling card. Savas Dimopoulos of Stanford University in California and Greg Landsberg of Brown University in Providence, Rhode Island, have worked out that 'We would know that mini black holes pop up in space every day, from collisions of cosmic rays. They would atso have thrived in the hot, youthful Universe shortly after the big bang' a black hole would decay into an unusually wide array of particles-a splash of quarks, photons, electrons and muonsamong others.'Black holes are completely democratic-they'll decay into any particle you know of,' says Landsberg. 'It would light up a detector like a Christmas tree.' Giddings and Landsberg say that by measuring the energy needed to make black holes of different sizes, we could work out how many extra dimensions there are and how tightly they are folded up. -High-energy experimentalists would become the geographers of the extra dimensions,' says Giddings.
'Itwould be a field dayforastrophysics,'says Turner.We'dknowthatmini blackholespopupin spaceeveryday,from collisions betweencosmic [email protected] ful Universe shortly after the big bang. 'We'd have to go back and think about all the different places
that tiny black holes could be made inthe Universe, and figure out their consequences,' says Turner.
Black holes at the LHC could allow scientists to test another idea. Hawking pointed out that black holes seem to devour and erase information. Regardless of what falls in, it all ends up as the same anonymous super-dense mush. The history of the matter that formed the hole has been extinguished for good. But other scientists suggest that the information is still there, and will be imprinted on the Hawking radiation that comes out of a black hole. This question is at the root of the nature of information-is it a fundamental quantity in the Universe? Is it conserved or can it be destroyed?
Strominger says that if black holes show up in the LHC we could resolve this question once and for all. The test would be simple: use accelerators to cook up lots of black holes with different ingredients but the same mass, and see if the radiation they emit as they decay looks the same. If it does, black holes really do destroy information. The perfect antidote, perhaps, to the modern blight of information overload. Fl
Gravity is too feeble to build giant planets unless it's getting a boost from hyperspace, says Marcus Chown
OUR SOLAR SYSTEM was built from the dust of dead stars, It's an often repeated fact. But if you ask how this dust actually started to form planets, you might get an embarrassed silence. Planets, it seems, grow too fast-no one knows why the dust clumps together so quickly.
Steinn Sigurdsson of Pennsylvania State University has an explanation, but it may be hard to swallow. He thinks that the dust might have been under the influence of forces from hyperspace. 'Planets may owe their existence to a space dimension that has gone unnoticed until now,' says Sigurdsson.
So what's the problem with the conventional view of how planets formed? The idea is that you start out with an interstellar cloud of gas, sprinkled with dust grains made mostly of iron, silicon and ice. The cloud begins to shrink under its own gravity, perhaps jolted by the shock wave from a nearby supernova, and soon settles into a thick rotating disc called a protoplanetary nebula.
The tiny dust grains each just a few micrometres across-drift around slowly within this disc. There is a little turbulence within the cloud, so occasionally grains collide and stick. When the grains get to about a centimetre across, they begin to move independently, sweeping up dust at a much faster rate. Only when they reach 1 0 metres across does gravity come into its own, pulling in material more rapidly. These rocks merge into planetesimals, many kilometres across, which collide to form planets.
The problem is that this process can be desperately slow. The bottleneck, according to Sigurdsson, is with the dust: to grow into pebbles those tiny dust particles have to hit each other head on and stick. In the inner parts of discs, where planets such as the Earth formed, dust is dense and grains collide frequently. However, in the outer disc, where planets like Uranus and Neptune formed, the material is rarefied and cold. Collisions are rare, and even when two grains do collide, they probably rebound most of the time. According to simulations, the typical timescale for forming a planet in the outer disc is 300 million years.
But proto-planetary discs don't last that long. Astronomers see dense proto-planetary discs around stars that are 2 to 3 million years old, but only thin and depleted discs around stars that are 20 to 30 million years old. 'An obvious inference is that the process of planetformationthereforetakesatmostafew tens of millions of years,' says Sigurdsson. If it takes any longer, the material will all be gone-blown away by the star's radiation and the stellar wind.
So planets like Uranus and Neptune should not exist at all. Jupiter and Saturn would also have had difficulty forming in the time available. That's especially important to us, as Jupiter is responsible for sweeping up most of the Solar System's rogue comets and protecting the Earth from catastrophic impacts. 'Something must intervene to speed things up,' says Sigurdsson.
One suggestion is that the grains become charged and so get pulled together by electrostatic forces. Ultraviolet light from a nearby hot star could charge up the grains by knocking out electrons from their surface. Some of these electrons would stick to neutral grains, making them negatively charged and therefore attracted to the positive grains. However, there is some doubt about whether ultraviolet could penetrate far into the thick, choking dust of a proto-planetary disc. And, even if it did, most of the ejected electrons wouldfloat about among the grains or attach themselves to floating hydrogen atoms. 'The problem is that a grain would be more likely to attract one of these than another charged grain,' says Sigurdsson.
Another way dust aggregation might be speeded up is if the proto-planetary disc is ultra-thin, like a CD. This would boost the density and so the chance of grain collisions. 'The problem here, however, is that thin discs are unstable,' says Sigurdsson. 'Their own gravity causes them to buckle, or vertical turbulence puffs them out.'
A final possibility is that grains are extremely sticky and readily cling together when they collide. But the surface chemistry of these substances in space isn't well understood.
Sigurdsson wasn't convinced by any of these explanations, and began to look at alternatives. In particular, he considered the controversial idea that, on a submillimetre scale, gravity might become much stronger than Newton's law predicts.
In 1998, Nima Arkani-Hamed and Savas Dimopoulos at Stanford University and Gia Dvali of the Abdus Salam International Centre for Theoretical Physics in Trieste, Italy, suggested that there could be an extra dimension of space in which gravity alone acts and which until now has gone unnoticed. If this is so, then gravitywhich is weak over large distances-gets stronger at the tiny distances encompassed by the extra dimension (see Diagram, p 34).
'Actually, there have to be two gravity-only dimensions,' says Sigurdsson. 'With only one, gravity is significantly modified on the scale of the Solar System-something which we do not observe.'At scales smaller than these two extra dimensions, gravity would extend into five space dimensions rather than the usual three.
'The risk is that this delicate fractal structure would be squashed or broken apart if the speed of impacts were too great'
'The two extra dimensions would cause gravity to drop off with an inverse fourth law rather than an inverse square law,' says Sigurdsson. 'If, for instance, there was an extra dimension 1 00 micrometres in extent, gravity would be 100 times stronger than predicted by Newton on a scale of 10 micrometres.' This short-range gravity boost is just what astronomers need to speed up dust aggregation. Grainsflyingclosetoeachotherwouldfeel this force yank them together.
It doesn't yank too hard, though. Like many others who simulate this process, Sigurdsson assumed that interstellar grains grow into fluffy, snowflake-like structures, whose large surface areas provide large targets and help to speed up the process of grain aggregation. The risk is that this delicate fractal structure would be squashed or broken apart if the speed of impacts were too great. So the turbulence within the disc can't be too strong, and the acceleration caused by Sigurdsson's modified gravity can't be too extreme. Assuming only a small degree of turbulence, he calculated that the impact speeds would be less than a millimetre per second-not fast enough to disrupt the aggregates. And a by-product of assuming the low turbulence is that gravity has a much better chance of attracting grains to each other.
The net result is to remove the bottleneck in planet formation. 'If there was a gravity-only dimension of about 80 micrometres in extent, I calculate that planets like Uranus and Neptune would form in a few tens of millions of years," says Sigurdsson. So the giants of our Solar System would have had a chance to grow before all their raw material was lost. And with them to take the flak, Earth would have been protected from bombardment by life-destroying rogue comets. Sigurdsson confessesthat hisworkwas done as a bit of fun. 'I publicised it simply to see if anyone could easily shoot it down,' says Sigurdsson. "But no one has-yet.' 'The idea does not strike me as crazy," says Stevenson. 'But it is unlikely to be correct." Stevenson doesn't ascribe to the need for low levels of turbulence. 'The problem as I see it lies in the assumption that encounter velocities of dust particles would be so low that modified gravity matters at this length scale. lt is far more likely that grain stickiness is what matters." If Sigurdsson's mechanism for speeding up planet formation were merely a theorists' fantasy, it wouldn't need to be taken too seriously. But it isn't. His proposal also explains a deeply puzzling result obtained by an experiment carried out on a space shuttle in 1999. A team led byjurgen Blum of Jena University in Germany released micrometre-sized silicate spheres in microgravity to see how the grains would aggregate. What the physicists found was that the grains formed just the kind of fractal grains everyone had hoped for. However, the experiment also threw up a puzzle. To everyone's surprise, all the grains grew at the same rate. Small grains stuck to small grains and then, when they were all used up, big grains stuck to big grains, and so on (Physical Review Letters, vol 85, p 2426). There was never a mixture of small and large grains. The explanation turned out to be that the growing aggregates were much fluffier than expected-more like wriggly fractal strings, or 'seedlings" as the researchers call them. These have a very large cross section, so they are extremely efficient at sweeping up any small fry. The question then is, why should they take on these stringy, open shapes? The only way for them to be so tenuous is if all collisions between aggregates are glancing blows, so that any new material sticks on somewhere near the extremities of the growing fluff. But you would expect particles to collide at random angles, leading to aggregates that are more densely packed.
'The great attraction of Sigurdsson's idea is that it is easily testable. In fact, it's likely to be proved right or wrong within a year'
According to Sigurdsson, modified gravity makes glancing collisions much more likely. Grains which by rights should have flown past each other without sticking would be snared by the powerful non-Newtonian force. Because it gets stronger as you get closer in, grains would spiral in towards each other. 'There are no stable orbits in an inverse fourth law force field so, once caught, a grain spirals in to make its glancing collision," says Sigurdsson. 'The result is a kind of gravitational focusing.' The great attraction of Sigurdsson's idea is that it is easily testable. In fact, it is likely to be proved right or wrong within a Vear. Experiments at three American universities are currently probing Newton's inverse square law of gravity on submillimetre scales. 'The latest data I've seen, from the group at the University of Washington in Seattle, is that Newton's law holds down to 218 micrometres,' says Sigurdsson. 'I'll be holding my breath when they get down to 80.'
Further reading: 'Experimental hints of gravity in large extra dimensions?' by Steinn Sigurdsson www.arxiv.org/abs/astro-ph/0107169