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All for one: Group Selection is alive NS 8 Jul 2000
THERE'S no such thing as society. If you accept that evolution is an about selfish genes, then the group has no role to play. Survival of the fittest means survival of the fittest DNA. Which makes you and I mere vehicles in which our genes are hitching a lift on the road to posterity. Or maybe not? For forty years this rather bleak, reductionist view of life has reigned supreme. Biologists ridiculed the idea that groups of organisms might gain a survival advantage over other groups because they shared some beneficial trait. Now that is changing. We are starting to understand that evolution happens on a variety of levels. Natural selection may favour certain genes, but it can also favour particular societies. Provided a group of individuals can cooperate without any cheats trying to sneak an unfair advantage, then it may evolve as a single entity. In the most recent breakthrough, the high priest of group selection, David Sloan Wilson from Binghamton University in New York state has found that even groups made up of many different species can possess traits that are passed from one generation to another. To some it looks like the biological equivalent of quantum weirdness, but this is not just an academic oddity. Wilson's finding could allow biologists to create designer ecosystems that increase plant productivity, change the pH of water or break down pollutants. In fact, the implications of group selection apply to any situation where individuals congregate-from hens in the farmyard to parasites inside a human body. Darwin first suggested group selection to explain why animals sometimes behave in ways that reduce their reproductive success. In an extreme case, a worker bee is unlikely to produce any offspring at all. How can this be, if natural selection favours those individuals who send more of their progeny into the next generation than their rivals? The objection applies to altruistic behaviour of any kind, and Darwin addressed it in his discussion of the evolution of humans, who show quite unnatural charitable tendencies. The anomaly can be explained, he wrote, if you imagine natural selection operating among groups of organisms, as well as among individuals. A group of people who are kind and helpful to each other may not do so well individually, but as a team they may do better than other groups of people, and so the tendency to work as a team spreads through the population. For almost a century, the group selection idea went unchallenged. But in me very influential biologists raised serious objections. Their was that within groups of generous individuals anyone selfishly would have a huge advantage: so cheats and usurpers would always take over the groups from the inside. Put another way, group selection is a very weak force, compared with selection at the individual level. Over the next couple of decades, other theories emerged to explain the evolution of altruistic behaviour. They involved helping relatives, or helping only those who reciprocate the favour, and they were backed up by mathematical models. Mainstream biologists rejected group selection. It was swept under the carpet and forgotten.
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Natural progression
But the idea is experiencing a revival. In part this is due to the efforts of mavericks like Wilson and his colleague Elliott Sober from the University of Wisconsin at Madison, who never gave up on group selection. But, perhaps surprisingly, the idea's new respectability can also be traced to a shift in thinking on the part of biologists who in the past have opposed group selection. The newly emerging view of evolution, proposed by John Maynard Smith from the University of Sussex and Ebrs SzathmAry from the University of Budapest, Hungary, describes the entire development of life as a series of major transitions in which successively more complex levels of organisation have become dominant. Each transition was a point when individual entities began working together 'm a group and natural selection kicked in at a higher level. When cells joined forces to make multicellular organisms, for example, cells that cooperated fared better than cells that exploited the resources of the group, because all the cells in an organism have a single, sealed fate. In this new multilevel selection" view of life, group selection is a natural progression. Maynard Sniith is convinced that group selection has been important at certain points in the history of life, particularly between human groups in more recent times. "There is no doubt that we were way too hasty in trashing group selection," agrees Joel Peck, also at the University of Sussex. "The theoretical models of the 60s and 70s were very oversimplified and should be taken with a pinch of salt." Peck points out that along with the theoretical shift, a growing number of experiments demonstrate that selection between groups does occur and that it can be more powerful than selection between individuals. In the early 1980s, for example, David Craig of the University of Illinois, Chicago, conducted artificial evolution experiments with communities of flour beetles living in glass vials. Each vial was allowed to produce a batch of young beetles, some of which were selected and redistributed among new vials to form the next generation. Craig tried selecting for and against the tendency to leave the vial, which benefits the rest of the vial community because there is extra space for those left behind. When he selected only individual beetles that stayed in their vials, the proportion of beetles leaving the vials did not change, even after 14 generations. But when he selected any group of beetles from populations where there was a high proportion of emigrants, the number of beetles climbing out of each vial doubled in 14 generations. "In these experiments, individual selection is weak, but group selection is very effective," says Peck.
Which is exactly the opposite of what we have all been taught." Craig's findings give clues about when and why group selection occurs. "The early models which rejected group selection assumed that altruistic traits were controlled by a single gene, with two possible forms, that was directly inherited from one generation to the next," says Peck. "Environmental variation did not exist in those models." But group selection seems to be strong when a trait is controlled not just by genes, but by interactions with the environment and with other organisms. In genetic jargon, this is a trait with "low heritability". The importance of interactions between individuals is shown by a group selection experiment that could shake up the poultry industry. If you try to improve the egg production of chickens kept in cages by selecting individual birds that are the most fertile, you will find that productivity actually goes down. This is because you are unwittingly selecting hens that are also aggressive, who will snatch food from other birds in their cage. Put these birds together and they will clash horribly, lowering overall fertility. In 1996, WiHiam Muir and his team from Purdue University, Indiana, selected for high egg production by picking out whole cages of birds that did weu, rather than selecting individual birds. They managed to increase annual egg production by 160 per cent, and the chickens lived so well together that they no longer had to suffer the indignity of having their beaks cut off-a standard practice in battery farming. Group selection worked in this case because the trait "egg production" was not just a property of the individual bird, but depended on the interaction of many other birds in the group. Wilson and his research student William Swenson wondered whether they could take this a step further. If selecting at group level works on chickens, why shouldn't it work on whole ecosystems, which have many characteristics that are functions of the interactions between organisms?
To test this possibility, Wilson and Swenson used soil-an ecosystem made up of hundreds of thousands of individuals, from many different species of fungi, bacteria and protozoa. They placed samples from a nearby forest in several transparent containers and grew plants in these artificial microcosms. After 35 days they selected those that had grown the most plant biomass, and created a second generation of microcosms using soil from them. After 16 generations, the communities under selection were producing three times as much plant biomass as other communities. The researchers conclude t some ature of the soil ecosystem that enhances plant growth passes from one soil community to another and is open to group selection. "People said it would never work," Wilson says. But it did. "The concept of selecting ecosystems without knowing about the actual organisms involved is incomprehensible to some microbiologists, who think that the ordy good science is to work with carefully isolated organisms ... We would love to know what the actual mechanism is. It's likely to be something to do with interactions between different organisms, but at the moment it's a black box."
Wilson and Swenson have also found other ecosystems in which group selection appears to work, for example the community of organisms living in pond water. They found they could select for ecosystems that decreased the acidity of the water. And in another experiment, they dramatically improved the ability of soil ecosystems to digest a common chemical pollutant called 3-chloroaniline. This method could have extensive commercial applications, especially because the breakdown of contaminant chemicals often involves more than one type of organism. "What we are doing here is developing designer ecosystems to do certain tasks," says Wilson.
Inside evolution
If it is simple to do in the lab, then who's to say that many features of ecological communities have not evolved by group selection in nature? There are countless examples of natural populations that are structured in discrete groups-from communities living on microscopic particles in the sea to patchy populations of plants. T'he best example is parasites, which live in groups of hundreds of thousands confined inside the bodies of their hosts. 'Much of the current interest in group selection is in parasitology," says Peck. Disease organisms often evolve towards non-virulence, with each individual parasite restraining its own reproduction so that the host can survive. This means the rest of the parasite group can survive and disperse effectively. 'Evolution of non-virulence cannot be discussed without invoking group selection," he adds.
Despite all the evidence, group selection remains unacceptable to some biologists. Richard Dawkins, author of The Selfish Gene, has little time for Wilson's latest work. "They are interesting experiments, but have no connection with group selection," he says. Dawkins accuses Wilson of trying to resurrect an old biological heresy. "Enormous credit would accrue to anybody who could pull off the seemingly impossible and rehabilitate group selection," he says. "But actually, such rehabilitation can't be achieved, because the great heresy really is wrong." Dawkins argues that group selection is just a kind of kin selection, because members of a group are always going to be related to one another, so helping others means furthering the genes they have in common. 'There is only a revival of group selection among people who have arbitrarily redefined kin selection as group selection. It is particularly galling, since the term kin selection was originally invented to distinguish it from group selection," he says. "Let's get on with pushing evolutionary theory ahead, without this tiresome, timeconsun-dng, backward-looking distraction." Wilson responds that you could just as well say that kin selection is a type of group selection. "It is all a question of perspective," he comments, "and we need different perspectives because they hold different insights." While theoreticians bicker among themselves, the implications of group selection may be extending far beyond the purely biological. Computer scientists designing artificially intelligent software are showing an interest in these techniques.
Imposing group selection on software agents or robots, by selecting groups of components that work well for whatever reason, might just provide the quantum jump that is needed in software development," says Peck. If he's right, group selection could be one of the most commercially successful ideas to have emerged from biology for a long time.
Lynn Dicks is an ecologist and science writer based at the University of Cambridge
Further reading: Unto Others by Elliott Sober and David Sloan Wilson (Harvard University Press, 1998) The Origins of Life by John Maynard Smith and E6rs Szathm.Ary (Oxford University Press, 1999) "Artificial ecosystem selection" by William Swenson and David Sloan Wilson, Proceedings of the National Academy of Sciences (forthcoming)
Born with the munchies Cannabis-like compounds may allow newborn babies to thrive
NS 8 Jul 2000
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CHEMICALS related to the active ingredient of cannabis might be vital for our survival. Naturally occurring cannabinoids in newbom mice trigger feeding, and without them the animals may die within days, says a biologist in Israel. She believes the chemicals could play a similar role in people. Cannabinoids produced in the body are known to be natural painkillers. They also coordinate the dopamine system, helping to control movement. But smoking cannabis increases appetite, a property that is sometimes exploited to help cancer and AIDS patients. Cannabinoids have also been detected in human and cow's milk, and levels here are at their highest the day after giving birth. Ester Fride of the Hebrew University of Jerusalem thought these observations might be a clue that naturally occurring cannabinoids are important in the early development of newborns. To test this, she injected newbom mice with a chemical that blocks cannabinoids by competing for receptors in the brain. None of the treated pups fed from their mothers. Some died within a week, and those that survived developed more slowly, Fride told the meeting. When Fride and her colleagues treated the pups with the active component of cannabis, delta-9-tetrahydrocannabinol, in a dose sufficient to swamp the effect of the blocker, the pups fed and grew normally, confirming that the blocker chemical was not itself toxic. "It seems that the pups are completely unable to ingest food without endogenous cannabinoids," she says.
So you think you're in love? NS 8 Jul 2000
SOME say love is blind. Others say it defies explanation. But two cognitive neurologists in Britain say that love is just a specific type of brain activity. Andreas Bartels and Semir Zeki at University College London used a functional magnetic resonance imager to scan the brains of 1 7 volunteers who described themselves as "truly and madly" in love. During the scans, each was shown pictures of their loved one, or a friend of the same sex as their partner. Seeing a lover prompted activity in four brain regions that were not active when looking at pictures of a friend, and caused a significant reduction in the activity of one other area. "We were really struck by how clear-cut the activity was," says Bartels. Two active areas lay deep in the cortex. the medial insular which may be responsible for "gut" feelings, and a part of the anterior cingulate, Which is known to respond to euphoriainducing drugs. Two lie in a deeper region known as the striatum, which is active when we find experiences rewarding. The inactive region was in the right prefrontal cortex-the region that is overactive in depressed patients.
WORKERS exposed to radiation in uranium processing plants may run an increased risk of developing lung cancer two decades later, according to a study commissioned by British Nuclear Fuels. Although there is some evidence that uranium miners can contract lung cancer from the dust they inhale, there is scant evidence that workers in facilities where the radioactive metal is processed are at risk. But now David McGeoghegan from Westiakes Scientific Consulting in Cumbria has found a link between radiation and lung cancers at the Springfields uranium fuel fabrication plant near Preston in Lancashire. He analysed the health records of 19 500 people employed at the plant between 1946 and 1995 and discovered that workers exposed to higher levels of radiation suffered more lung cancers. In all, 225 people were diagnosed with the disease. Of those, only two had received cumulative radiation doses during their time at Springfields of more than 200 millisieverts. The recommended annual safety limit for workers is 20 millisleverts. The association between dose and disease only becomes statistically significant 20 years after the dose was received, McGeoghegan says. He thinks it's possible that radiation caused some of the cancers, but points out that the doses recorded are based solely on measurements of radiation outside the body. He now plans to analyse radioactivity levels within the workers' lungs, which could be more directly related to the cancers. The connection between lung cancer and radiation dose is potentially important, says Michael Clark from Britain's National Radiological Protection Board. "There is no proof that the association is causal, but it would be sensible to investigate further." Rob Edwards
Source: Journal of Radiological Pmtection (vol 20, p 1 1 1)
Double trouble
Are twins more likely to have infertile aunts?
NS 8 Jul 2000
IT SOUNDS impossible, but infertility can be inherited. Reproductive biologists have discovered a gene that makes female sheep sterile when they carry two defective copies. When they carry only one defective copy, however, they are superfertile and give birth to twins or triplets, keeping the gene in circulation. No one knows whether a similar genetic disorder could affect humans. "The big question is whether sisters of infertile women are having twins," says team leader Susan Galloway of the University of Otago in Dunedin, New Zealand. Left to their own devices, sheep usually produce a single lamb. But selective breeding has created breeds that produce twins and even triplets. Inverdale sheep, a strain of Romney sheep, are unusual in that roughly 50 per cent of ewes produce more than one lamb at a time, while the other 50 per cent are infertile. "It's fantastic. They breed sheep for prolificacy, and they find that they are infertile," says team member Olli Ritvos of the University of Helsinki. Because the gene is carried on the X chromosome, farmers can easily get around that problem. They cross Inverdale rams with breeds that don't carry the defective gene so that each daughter produces lots of lambs. The lambs from the next generation are slaughtered for meat to prevent infertility being passed on. The biologists have discovered that the sheep gene responsible for the strange fertility pattem is the same as the human gene BMP15. This gene is active in human ovaries, but its exact role was a mystery. In the sheep, BMP15 is essential for reproduction. It makes a protein in the ripening egg, or oocyte, which seems to instruct the granulosa cells around the egg to multiply These cells supply the egg with nutrients and their numbers need to be increased to keep the egg well nourished as it grows. In sheep with two defective copies of BMP15, the granulosa cells do not multiply and the oocytes die of starvation. "It shows that there's a factor made by the oocyte in sheep and humans that influences its local surroundings," says Jock Findlay of the Prince Henry's Institute of Medical Research in Melboume. "The suspicion had been building, because a s@ar factor is found in rodents. This shows it's a general principle." But Findlay admits to being mystified by how one defective copy of BMP15 can make a sheep superfertile. "I frankly find it difficult to understand," he says. Galloway's team has a hunch, though. Sheep with one defective copy of the gene produce enough granulosa cells to nourish the developing oocytes, but fewer than in a normal sheep. These granulosa cells are also known to be more sensitive than the cells of normal sheep to pituitary hormones LH and FSH, which trigger ovulation. That increased sensitivity could be responsible for the increase in the number of mature eggs that are released from the ovary. Rachel Nowak, Melbourne
Source: Natum Genetics (vol 25, p 279)
Quantum mirage: There are some spooky goings-on in atomic world. Get them under control and we are on our way to making a computer with atoms. Robert lrion investigates
NS 8 Jul 2000
SOMETHING bizarre has popped up in the laboratory of physicist Donald Eigler, and no one is quite stire what to make,,, of it. It's the world's smallest ghost, a phanjom on an atomic scale. Eigler and colleagues at IBM's Almaden Research Center in San Jose, California, call it a 'quantum mirage" because it projects an atom's electronic $ignatur,i to another place while the atom itself stays put. The projection spans only a tiny distance:,ibout 10 nanometres, or 10 000 times narrower than q huoian hair. Still, it offers the tantalising promise of transferring information within tiny circuits of the future in which 'wires are obsolete and the components are single atoms.
The eerie image may also let physicist probe an atom without disturbing it directly. That's an intriguing prospect in a miniworld where even a few photons of,light can alter the states of particles. Eigler and his team think it may even be possible to forge a chemical bond with the mirage by moving a compatible atom next to it. That would create a weird hybrid molecule that IBM physicist Hari Manoharan calls "half real and half ghost". The mirage is yet one more curious feature of the quantum world, in which electrons and ot. behave neither like particles nor like wav ture of the two. In this case, the researchers co trons on a copper surface within a "quantum corral", a picket" fence built of several dozen closely spaced cobalt atoms. This corral kept the surface electrons from flitting about the surface of the metal with their normal freedom. What's more, Eigler's corral is in the shape of a near-perfect ellipse. That special geometry compelled some of the electrons to cluster around two spots: the left and right foci of the ellipse. Using the precise motions of a scanning tunnelling microscope (STM), Eigler's team placed an atom of cobalt on the surface. When the physicists probed the cobalt atom with the STM, they saw a swarm of electrons around it. Then, when they scanned the other focus, they an unmistakable
signature. The swarm surrounding the real atom was mirrored at the empty focus, even though th s no atom there. The reflection wasn't complete, as t detected the mirage atom at only one-third the intensity of the real one. Still, it was a startling apparition. So what is it? Of one thing Eigler is sure: it's not just an illusion. "There are real electrons there," he says. "It's a physical object." But when it comes to the nitty-gritty physics of describing the genesis of the mirage and its implications, Eigler and his team admit they're puzzled. "What exactly are we doing? What properties are really there?" asks Manoharan. It all seems rather surreal, and that other-worldliness is heightened by the manner in which Manoharan, physicist Christopher Lutz and Eigler use exaggerated iertical rehef and garish colours to depict their quantum obrral and the mirage it contains. There it was, on the covei of the 3 February issue of Nature: a ring of aggressive yellow atomic spikes enclosing a wavy orange-and-green sea of electrons and two bright violet islands, the atom and its mirage. One almost could envision the cursive writing of Rene Magritte under the i@@: "Ceci nest pas un at@@" The quantum mirage is the natural denouement of progress made during the past decade along several fronts: STM technology, quantum corrals, and "Kondo resonance" around a magnetic atom.
The basic concept of the STM hasn't changed since IBM physicists Gerd Binnig and Heinrich Rohrer developed the appa ratus in Zurich in 1981. Researchers use a fine wire-in Eigler's case, made of pure iridium-that tapers to a single atom at the point. When this tip approaches a conducting surface, such as a metal, the physicists apply a small voltage. This induces electrons to "tunnel" across the gap between the tip and the metal. By keeping the tunnelling current constant as the tip is scanned across the surface, the researchers create a topographic map of its atomic peaks and valleys. An STM can work at room temperature, but Eigler's machine has to operate at just four or five kelvin to keep the atoms in place. Eigler's group was the first to use an STM to drag atoms into desired spots. The researchers raised the voltage in the tip, and brought it close enough to the surface to form brief chemical bonds with single atoms. Eigler and IBM colleague Erhard Schweizer made an intemational splash in 1990 by spelling "I-B-M" with 35 atoms of xenon atop a layer of nickel. Three years later, Eigler and Lutz teamed up with IBM physicist Michael Crommie-now at the University of California at Berkeley-to build the first quantum corrals. Their circular barrier of iron atoms on a copper surface caused another sensation, for it revealed in dramatic fashion the wavelike behaviour of the electrons trapped within. In this case, the electron densifies peaked sharply in the centre of the ring. Surrounding that peak were concentric rings of lower electron densities, looking for all the world like ripples from a pebble plopped into a pond. The third advance that paved the way towards quantum mirages came in 1998 when physicists first observed the Kondo resonance. This effect was proposed in 1964 to explain the strange way that metals interact with atom-sized magnefic impurities, but until the invention of the STM it was impossible to verify. Two years ago separate teams led by Schneider and Crommie found unambiguous evidence that the Kondo resonance actually occurs. T'he surface of a conducting metal, such as copper, is covered with a sheen of electrons that swarm freely across it-which is why they conduct electricity so well. Place a single atom of cobalt or another magnetic element on this surface, and its electrons disrupt the smooth flow of the conduction electrons. "You can think of the tightly bound cobalt electrons as a little hard ball," says Crommie. "The copper electrons swimming around are repelled from the ball." In addition, a tiny cloud forms, and within this the copper electrons spin in the opposite direction from those around the cobalt atom. This alignment effectively screens the disruptive magnetic field of the intruder atom. At the atomic scale, nothing looks quite like the Kondo resonance. That's why Eigler's team chose the cobalt-on-copper system. Project a Kondo resonance to a remote location and you have the unmistakable signature of a quantum mirage. Another critical factor that Eigler exploited is the special geometry of an ellipse. At school, you probably teamed to draw an ellipse by sticking two tacks on a board-the foci of the ellipse-then forming a loop by tying a length of string around them. A pencil that pulls the loop tight traces out an ellipse. In the atomic world, this means that if a signal starts at one focus and bounces off the ellipse wall toward the other focus, it travels the same distance no matter which direcfion it goes. The consequences for electrons trapped within an elliptical corral are fascinating. "One path length connects the atoms at each focus," Manoharan says. "It changes the two-dimensional problem to a onedimensional problem. The entire ellipse acts like a single wire connecting the atoms at the two foci." An electronic disturbance at one focus, such as the Kondo resonance, echoes off the walls of the corral and appears, ghostlike, at the other.
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Whispering galleries
For those who can't wrap their brains around quantum mechanics, acoustic analogies abound. "Whispering galleries" are often elliptical chambers, in which sounds from a speaker at one focus echo into the ears of someone standing at the other. Many musical instruments also feature resonant chambers in which waves vibrate with characterisfic modes. "To some extent, what happens within our corral is nothing more than what happens when you have waves inside a resonant structure," Eigler says. "Where it is special is that the elliptical structure helps to make the effect we are observing very obvious in one special spot." But acoustic descriptions don't quite illustrate what exists at the mirage site and what is illusory. For that aspect, Lutz's favourite analogy is inspired by the image of a solar eclipse in a pinhole camera. "It's not the Sun itself, but it has some characteristics of the Sun," he says. "It's made of photons that really came from the Sun, and it shows the changing shape of the Sun. You could even start a fire with it if you intensified the image with a lens." One can learn some of the Sun's properties, but by no means all, by studying the projection. The same may be true of the quantum mirage, Lutz believes. For now, the only properties of the real atom that show up conclusively in the mirage are the energy states of its electrons. A spectrum taken with the STM reveals how many electrons exist at different energy levels around the atoms. That analysis shows that both the real atom and its evanescent twin display the distinctive Kondo resonance. The team is optimistic that other properties will also come into view. For instance, the spins of the electrons surrounding the cobalt atom also form a unique pattern, but the STM isn't yet sensitive to those signatures. Manoharan thinks it may also be possible to place a simple molecule, such as carbon monoxide, at one focus, and detect the characteristic vibrations of its atoms at the other. It's one of the golden rules of physics that you can't probe the properties of an atom without simultaneously affecting that atom in some way. Does the discovery of mirages change all that? Might it be possible to sense some properties of the real atom by scanning the mirage, leaving the atom itself unmolested? Sadly, the answer seems to be no. Whenever physicists investigate the mirage site, about 10 billion electrons flow through the STM tip into the corral each second, flooding the ellipse and influencing the real atom. "Anything you do at one focus is felt by the other," observes Paul McEuen of the University of Califomia, Berkeley. "You're not getting a free lunch." Still, remote sensing disturbs the real atom much less than placing the STM tip directly above it. For IBM, Eigler's work has important implications in a different direction. The company has its sights set on one day building electronic circuit out of assemblies of atomic structures rather than wires, and for this the quantum corrals show obvious promise. As Eigler points out, the corral transmits information. That could be very useful when it comes to achieving the electronics industry's goal of building components measuring a mere 100 angstroms (10 nanometres) across. This, Eigler says, is still "many generations" away. "How do I build something that adds two numbers together, has input and output channels, and fits in a 100angstrom package?" Eigler asks. Eliminating wires would be a great start, as IBM has not been slow to point out. Its news release on the Nature paper touted the discovery of a "nanotech communication method". That's rather premature, as the researchers can't yet code information in a way that would allow it to be sent from one focus of the ellipse to the other. But there appear to be plenty of possibilities. "We could use the location of the atom, the energies of the electrons, the location of the electrons, the spins of the electrons, or a complex mixture of those," says Lutz.
Upping the level
Other STM physicists were impressed when they learned of this latest work from Eigler's lab. "My first reaction was that this is simply fantastic," says Wolf-Dieter Schneider. "It's a marvellous and novel aspect of how magnetic atoms behave." Paul McEuen says that Eigler's team has "upped the level of the game" for precise manipulation of atoms. 'This shows that you can tailor the interactions between different atoms with an exquisite degree of control," he says. In the past few months, Manoharan and his colleagues have been doing what he calls "all kinds of weird experiments." Their tinkering was motivated by the musings of physicist Charles Rettner, a colleague at Almaden, that perhaps another atom could form a molecule with the mirage. After aff, the n-drage is the projection of an electronic structure. "We know that those structures underlie chemical bonding," Manoharan explains. "If we brought a real atom next to the mirage, would it make a chemical bond?" Though this initially struck Manoharan as unlikely, subsequent research has muted his scepticism. In one experiment the team observed what appears to be a magnetic interaction between two real atoms within the corral. Manoharan says that this interplay is "a step below chemical interactions", suggesting that a real chemical bond might be possible. The team has not yet attempted to form a full chemical bond between a real atom and the mirage. "It might work, depending on how much of the electronic structure we're projecting," says Manoharan. "But we are not projecting the nucleus of the atom or the orbitals of the electrons." Ultimately, turning quantum corrals into electronic components will require mass production on some sort of atomic assembly line. Eigler's group doesn't plan to try that, at least not soon. But across the continent at the National Institute of Standards and Technology in Gaithersburg, Maryland, another STM group is moving in that direction. Joseph Stroscio and his colleagues have completed a machine based on a cryogenic microscope that will assemble corrals and other structures containing many thousands of atoms. The system will start by randomly depositing atoms on a surface. Then, armed with blueprints of the desired structures, a computer will direct the STM tip to build them in the most efficient way. "It will do it ovemight while we're sleeping," Stroscio says. Once it's working, this set-up will let physicists produce countless atomic structures with subtly different configurations, allowing them to investigate how their behaviour varies. Stroscio's team also plans to apply strong magnetic fields to alter the dynamics of electrons in their corrals, which he expects will be 10 times the size of those in Eigler's lab. The hope is that physicists will be able to use these "quantum laboratories" to work out how electrons behave in different environ-rnents. It's not easy to surprise physicists who routinely play with single atoms as if they were marbles. Yet the new frontier of manipulating the electronic properties of atoms has energised the STM community. "I'm amazed each day that I can build these structures," Manoharan says. "The prospect of what we will discover excites me to no end. We can go in any direction we want."
Robert lrion is a freelance science joumalist based in Santa Cruz, California.
Further reading: "Quantum mirages formed by coherent projection of electronic structure" by H. C. Manoharan, C. R Lutz and D. M. Eigler, Nature, vol 403, p 512