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Mean and green New Scientist 17 Jun 2000

Does a deadly spider hold the key to eco-pesticides?

VIRUSES given a gene for a toxin from one of the world's deadliest spiders could replace chemical pesticides, say researchers in the US. They plan to carry out field trials, although there are fears about the wisdom of releasing such viruses. Glenn King of the University of Connecticut Health Center in Farmington and his colleagues recently identified a unique family of toxins in the venom of a funnelweb spider. These neurotoxins are lethal when injected into insect tissues, yet have no effect if eaten by insects or other animals (Nature Structural Biology, vol 7, p 505). King's team is now engineering the gene for one of these toxins into baculoviruses, common viruses that infect certain moths and butterflies, and have long been used as "biopesticides". When the modified baculovirus infects an insect, the insect's cells should start to produce the toxin, killing it faster than wild viruses. Because the host dies quickly, before much virus can replicate, the modified virus shouldn't persist in the environment, say the researchers. "I welcome a potentially environmentally friendly pest control but it's abundantly clear we need to be more firm about risk issues," comments George McGavin, an entomologist at Oxford University. "If we are not 100 per cent sure, it shouldn't be in the field." There have already been several field trials worldwide of baculoviruses given a gene for a scorpion toxin (New Scientist, 21 January 1995, p 6). However, most of the scorpion toxin made in infected insects fails to fold into the correct shape, says King. By contrast, tests in bacteria suggest that ahnost 100 per cent of the spider toxin should fold properly, making the virus deadlier. King thinks engineering toxin genes into viruses is preferable to adding them to plants, such as Bt maize. Not only does it mean that people do not have to eat plants that produce insecticidal toxins, but only target insects will be affected, he says. "These viruses can be exquisitely specific, right down to infecting individual species," King claims. "This means that only the pest insects will be killed whilst beneficial insects such as bees remain unaffected " However, critics fear that the virus will spread into the environment and affect other kinds of butterflies and moths. "A conta'mment environment could not possibly hold a virus," says McGavin, who opposed trials of a scorpion toxin virus in Oxfordshire in the 1990s. "If you could get a specific baculovirus it would be great, but baculoviruses do pass on [to other speciesl." There are also fears that the toxin gene might be transferred to other viruses. "There is no instance of a toxin gene jumping from virus A to virus B," says Bruce Hammock of the University of California, Davis, who is also working on modified baculoviruses. 'But if it jumped, the new virus would become less effective." Jenny Cory of the Centre for Ecology and Hydrology in Oxford agrees transfer of the toxin gene is unlikely, but thinks further tests would be helpful. "It's a vicious circle," she says, "you have to do a risk assessment before you do the experiment but we don't know all the risks without doing field experiments in the first place." Mark Robins and Michael Le Page

Fatal attraction New Scientist 17 Jun 2000

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SEXUAL selection is driving some fish towards extinction, say researchers from New Mexico. They have found that female Pecos pupfish prefer to mate with males of another species. "It's like seeing speciation in reverse," says biologist Jonathan Rosenfield at the University of New Mex 15 ico in Albu querque. in freshwater fish, roughly 40 per cent of endangered species are at risk of hybridising with related exotic species. But few studies have investigated the biological mechanisms that bring about the genetic swamping and assimilation of one species by another. Rosenfield and his colleague Astrid Kodric-Brown have found that sexual selection-a process of non-random mating known to play a role in the formation of new species-may be to blame. The researchers found that female Pecos pupfish (Cyprinodon pecosensis) much prefer males of another species, the introduced sheepshead minnow (C. variegatus). Hybrid offspring of these crosses are more successful than either parent species at defending breeding territories, and so enjoy greater reproductive success. "The hybrids are almost super-pupfish," observes Kodric-Brown. The consequences of this behaviour have been dire. In just five years in the 1980s the hybrids replaced the Pecos pupfish across most of its native range, a 500-kilometre stretch of the Pecos River in southern New Mexico. The pure pupfish today survive only in isolated springs and sinkholes in the river flood plain. "The hybrid zone spread extremely quickly," says Craig Stockwell, a conservation biologist and pupfish researcher at North Dakota State University, Fargo. "This work now lets us understand how and why that happened."

Snug as a bug New Scientist 17 Jun 2000

Bacteria enveloped the ancient Earth in a warming blanket of gas

TRACES of methane-eating bacteria that lived in a pond over 2.7 billion years ago may help to explain the puzzle of how the early Earth kept warm. The discovery suggests that methane, a powerful greenhouse gas, was abundant at that time and could have kept the Earth from freezing. About 2.8 billion years ago the Sun was 20 per cent fainter than it is now. With today's atmosphere, that would have left the Earth's average surface temperature below freezing, says climate modeller Jim Kasting of Pennsylvania State University in University Park. But we know that liquid water was present then, so something else must have been warming the planet. This enigma has been dubbed the "faint young Sun paradox". Today's most important greenhouse gases are carbon dioxide and water vapour. But ancient soils show that the air contained too little CO, to have kept the planet from freezing, says Rob Rye of the California Institute of Technology. So attention has turned to methane. Last month, Kasting showed that 100 parts per million of methane in the atmosphere could have done the trick. Now Rye and Heinrich Holland of Harvard University have found organic carbon trapped in ancient soil from Mount Roe in Western Australia, which indicates that methane was present 2.76 billion years ago. The carbon contains unusually low levels of carbon-13, a stable isotope that is depleted in living cells. Rye says that the low level is the hallmark of two successive depletions: the first by methane-producing bacteria, the second by methanotrophic bacteria, which gobble up the gas in the presence of oxygen. Methane producers "were probably some of the very earliest organisms", says Kasting. The carbon cycle began shifting away from a methane-rich atmosphere after photosynthesis evolved, says John Hayes of the Woods Hole Oceanographic Institution. However, methane and oxygen could not both have come from the air, because they would have reacted. Rye believes that photosynthetic bacteria in the water supplied the oxygen, while the carbon-13 isotope levels suggest that the methanotrophs were feeding off an atmosphere with at least 20 parts per million of methane. Other studies of ancient soils support the idea that the atmosphere contained very little oxygen at the time. If methane was being produced at present levels and released into an oxygen-free atmosphere, Kasting says, methane levels of 100 to 1000 parts per million are plausible. That would have kept the early Earth warm. As photosynthetic bacteria continued producing oxygen, methane concentrations gradually fell, eventually triggering the first widespread glaciation 2.4 to 2-3 bithon years ago, Rye believes. The Earth then thawed as increasing oxygen levels caused more o)adation raising C02 levels and [email protected] the greenhouse effect. Jeff Hecht

Journal of Geophysical Research (vol 105, p 11 981)

Sources:Geo/ogy (vol 28, p 483)

Junk DNA helps females avoid double trouble New Scientist 17 Jun 2000

IF IT weren't for "junk" DNA, women would be in trouble. Some of this apparently useless DNA may be essential for silencing one of their X chromosomes, say researchers in Ohio. In mammals, one X chromosome is enough for both sexes, so females would get a lethal double dose if one X weren't shut down (New Scientist, I April, p 4). This is done by wrapping up one X very tightly, with the help of RNA made by a gene called )list. Two years ago, Mary Lyon of the MRC's Mammalian Genetics Unit in Oxfordshire suggested that junk DNA might be just the handhold XIST needs to package up the DNA. Nearly half the human genome is made up of "jumping genes", or retrotransposons, bits of "selfish" DNA thought to do little except make more copies of themselves. But now, with data from the Human Genome Project, Evan Eichier and his colleagues at Case Western Reserve University in Cleveland have discovered that the X has twice as much of a class of selfish DNA called Ll as any other chromosome. "if parasitic DNA has accumulated by a truly random process, you wouldn't expect the X to have more or less," says Eichier. Because other types of selfish DNA are evenly distributed, he says, Ll elements seem to have an important function on the X. Eichier's team also found that the regions of the X that are most effectively silenced had the highest density of Ll elements. "It's not proof yet," says Stanley Gartier, a geneticist at the University of Washington who has studied X inactivation for decades, "but they have taken Lyon's idea quite far." Jonathan Knight

Source: Proceedings of the National Academy of Sciences (vol 97, p 6634)

They do it with Mirrors New Scientist 17 Jun 2000


WHEN he woke up in the recovery room, Bobby Wyatt knew straight away that something was wrong. It was his right arm. He couldn't see it and he couldn't feel it. Wyatt panicked-he'd gone in for heart surgery, not amputation. It didn't take the staff long to figure what had happened-Wyatt had had a stroke during the operation. He couldn't see his arm, because it had slumped off the side of the bed. But he couldn't feel where it was either, because the stroke had damaged part of his brain. As far as Wyatt was aware, he'd lost his arm. Usually we just know what our legs, arins and body are up to without having to look. We take for granted the powerful impression of a stable, embodied "self". But it's an impression that's possible only because the brain constructs a "body image" for us. One important component of that image is a mental map of our body surface, generated by the cortex, the brain's outer shell, using the touch signals it picks up from the skin. Other components, handled by other parts of the cortex, include the position of our muscles and joints, the intention to move, and also what we see our body doing. But, as Wyatt's experience shows, our body image can become distorted, and when it does the disability it causes can be every bit as devastating as injuring part of our body. If a stroke or accident damages the region of the brain housing the body map, patients may lose the use of a perfectly healthy limb, even though the parts of the brain that directly control movement remain intact. At the other end of the scale, amputees can have "phantom" limbs, the strong sensation that a limb exists even though they know perfectly well that it doesn't. You can even alter your body image temporarily with a simple trick (see "Try this at home, folks", p 29). The usual treatments for conditions hke these range from stimulating nerves with electrodes to physical therapies such as forced use of the affected limb. Although these treatments can be very effective, they don't work for everyone. Now a handful of neurologists led by Vilayanur Ramachandran, a neurologist and Director of the Center for Brain and Cognition at the University of California in San Diego, are developing a new type of treatment that works by beating the brain at its own tricks. They have found ways to retrain and refashion the distorted body image so that useless limbs can move again and phantom hmbs give up the ghost. Ramachandran got his inspiration in a roundabout way from a controversial study of monkeys that began more than 20 years ago (New Scientist, 6 July 1991, p 15). A group of monkeys had the sensory nerves from one of their arms severed so they wouldn't feel any sensations from that arm. Eleven years later, when researchers looked at the monkeys'brain activity, they found that despite the lack of sensory input from the arm, the arm region of the body map in the cortex hadn't gone silent. Instead, signals from the face-next door on the map-had taken over from the arm (Science, vol 252 p 1857). When Ramachandran heard about that study he instantly thought of a group of his patients who had lost limbs, and experienced bizarre phantom sensations. Might their body maps have been rearranged too? With the help of a man who had lost part of 1-ds left arm in a car accident-but swore he could still feel it-Ramachandran soon discovered that his hunch was correct. When he touched the man's face, he said it was as if Ramachandran were touching parts of his missing hand, as well as his face. The whole surface of the man's hand was mapped out beautifully on his cheek. A second map of the missing arm was inscribed on the stump of the arm. Those strange sensations are probably the result of just the sort of changes that happened in the monkeys' brains, says Ramachandran. When the brain region that had once received messages about physical sensations from the arm suddenly found itself bereft of input, it compensated by responding to signals from the parts of the body that are mapped next door to the arm-the face and stump-while still considering them to be from the arm. What's more, the whole range of feelings had shifted en masse, including differences between cold and hot, light and heavy touch, vibrations and steady pressure. Ramachandran believes that this is the essence of many a phantom bmb The brain generates the feeling of the limb from the signals coming from another part of the body. So every time Ramachandran's patient smiled or scratched his face, he stimulated the arm region of the body map. Transferring sensations of touch and temperature are one thing. But some patients have vivid experiences of movement, spasms, or-even more bizarrely-of paralysis, in their missing limbs. Ramachandran suspects that this "learned paralysis" and the other strange sensations come about in the following way. Once the limb is severed, the brain continues to send signals telling the missing limb to move. For a while, this creates the illusion of movement because the brain is still monitoring the intention. But the patients clearly don't see anything moving, so the different signals feeding into the body image contradict one another. Eventually the brain learns to interpret the lack of response as paralysis. TO treat the paralysis, Ramachandran reasoned that you needed to remove the contradiction by allowing the patient to see the movement they intended to make. He pondered for a while the practicalities and cost of virtual reality systems, but eventually hit on a much simpler idea-a mirror. He placed a large mirror sideways on in front of each patient, so that they saw a reflection of their good arm where the phantom was-just as if the limb had grown back. Next, he asked them to try making mirror-symmetric movements like conducting an orchestra. Six of the ten patients instantly felt their "paralysed" phantom limbs moving. Most found the sensation pleasant in itself, and it even enabled a few to shift their paralysed phantom limbs out of painfully awkward positions. "A couple of patients we have seen have a clenching spasm, several times a day," says Ramachandran. With no way to control the painful clenching of a fist that didn't actually exist, the patients just had to wait for it to pass. "But you put the mirror there andit unclenches instantly," he says. One amputee did something even more dramatic. He exercised his phantom arm every day for several weeks in front of the mirror and managed to correct his body image. The phantom gradually shrank and disappeared. "The first example of amputation of a phantom limb," says Ramachandran. The effect of mirrors on phantoms has yet to undergo the large-scale, placebocontrolled testing that Ramachandran says is necessary, but quite a large number of neurologists have tried it informally and liked what they've seen. At a meeting on phantom limb pain in Oxford this March, a quick show of hands revealed that around 30 therapists and neurologists had tried it with some success. "The mirror is no universal remedy for all patients with phantom limb pain," says Peter Brugger, a neurologist at the University Hospital in Zurich. "But we should all use it more, if only to find out which patients it helps and which it does not." Leamed paralysis may not be limited to phantom limbs, according to Ramachandran. The monkey studies, and work dating back decades earlier to experiments by Oxford physiologist Charles Sherrington, show that cutting the sensory signals from the arm also paralyses it. Though the motor nerves are intact, somehow the animals need the whole loop of sensory and motor signals to move. Ramachandran wondered whether sometimes the paralysis that often follows a stroke was also learned, possibly when swelling in the brain temporarily restricts nerves, cutting off sensory signals. And if a conflict between the different signals feeding into the body image was to blame, not permanent physical damage to the nerves, could the mirror help these patients too? Ramachandran and Eric Altschuler, also at UCSD, tested the mirror training with nine stroke patients who were left weak on one side-a condition known as hemiparesis. They found that mirror training seemed to improve the strength and fluidity of movement in at least half of the patients (The Lancet, vol 353, p 2035). Perhaps seeing the arm move-albeit as an optical [email protected] for missing sensory input and resolves ttie conflict. Krishnankutty Sathian, a neurologist and Medical Director of the Program in Restorative Neurology at Emory University in Atlanta, Georgia, is also enthusiastic about the potential of mirror therapy for stroke patients. "We tried a number of patients with the mirror, with not a lot of luck," he says-then Bobby Wyatt came along. "This particular patient is unusual," says Sathian. "His problem was motor, he couldn't control the limb. But the main reason for his problem was sensory loss."

Unlike Ramachandran's patients, Wyatt's sensory loss wasn't temporary, but a direct result of the brain damage from the stroke. "He couldn't feel anything from the right hand, and he couldn't tell where his right hand was in space. When he was trying to use his hand he had to look at it. He needed visual input, so we thought the mirror might help."

On reflection

Almost immediately, Sathian noticed that Wyatt's right hand moved a little more flu idly, although Wyatt didn't notice anything himself for about a week. The mirror made it look as though he was moving his arm again, but he didn't feel any movement. "Then Dr Sathian told me to look [over the mirror]. I had moved my right hand," says Wyatt. "It blew my mind. I tell you, at 57 years old, tears came into my eyes." Sathian and his colleagues believe that the mirror helped Wyatt learn to use non-standard sensory cues, such as the sensations from the upper arm, as well as vision, to substitute for the normal, fine grained sensory map. But few treatments are without side effects, even mirror therapy. Once he started moving his right arm, Wyatt found that it would sometimes tag along with the left, without him intending it. Now, after more than a year and a half of training every day with the mirror-he even set one up at home-he's learned to control his arm more independently. "I even took my driver's test a few months ago," says Wyatt. So useful is mirror therapy looking, that Ramachandran has even tried it out with a small number of patients with a rare and notoriously difficult to treat condition known as hemispatial neglect, which hap; pens after a particular type of right brain ! stroke. Not only are these patients often I paralysed on their left side, they are also completely unaware of anything to their left-even the paralysis. By putting a mirror on the patients' right, so they could see the normally neglected left side reflected to their right, Ramachandran hoped he could reverse that condition. -"The question is, if you put a mirror on the right, will it somehow enable them to suddenly pay attention?" he asks. Results have been mixed so far. In some patients, things seem to get even more confused. "They start reaching into the mirror," he says. But a couple of patients actually reach over to the left with their unparalysed arm when the mirror is there, seemingly aware again of their neglected side. The acid test will be if repeated practice with the mirror eventually helps them pay attention to their left when the mirror is gone. "At the moment it's still a maybe," says Ramachandran. Ramachandran, Sathian, and others say they wouldn't be too surprised if one day a simple mirror becomes routine therapy for a host of conditions in which the brain's ability to sense and represent the body causes a physical disability. "I don't like to go around touting n-dracle cures," says Ramachandran. "You don't want to give false hopes." On the other hand, he says, such a straightforward treatment is unlikely to do any harm. "At best, you have a procedure that genuinely works in some patients," says Ramachandran. "At the very least, you have the world's least expensive, most effective placebo."

Further reading: Phantoms in the Brain by Vilayanur Ramachandran and Sandra Blakeslee (Fourth Estate, 1998) "Doing it with mirrors: a case study of a novel approach to neurorehabilitation' by Krishnankutty Sathian and others, Neurorehabilitation and Neural Repair, vol 14, p 85 (2000) "Consciousness and body image: lessons from phantom limbs, Capgras syndrome and pain asymbolia" by Vilayanur Ramachandran, Philosophical Transactions of the Royal Societ'l of London B, vol 353, p 1851 (1998)

SHADOW WORLDS New Scientist 17 Jun 2000



THE WORLD on the other side of the mirror is a favourite of fantasy writers: a place where everything is subtly twisted, where things impossible in our world can happen. Scores of fictional adventurers, from Alice onwards, have stepped through into that reflected realm. But it could be that the world on the other side of the mirror is more than a fantasy Physicists suspect that we might be surrounded by a parallel universe of mirror matter, where mirror particles assemble themselves into mirror galaxies, mirror stars and mirror [email protected] mirror life. And two scientists will soon try to get a first glimpse into this mirror world. just like the mirror worlds of fantasy, it is a dominion subtly different from our own. If you look at yourself in a mirror, the reflected you seems to behave in just the same way as the real one, only backto-front. But look more closely and you'll find that nature's symmetry is flawed. When physicists examine some known processes involving fundamental particles, they find that the mirror-image process is not always possible. For instance, when an atom decays to create a neutrino, the neutrino always spins in the same direction, left-handedly. If it were coming towards you, you'd see it spinning clockwise-in other words it twists like a left-handed corkscrew. But we never see right-handed neutrinos. Wouldn't it be more pleasing if both types of neutrino existed, and nature were mirror symmetric? In the 1950s, Tsung Dao Lee and Chen Ning Yang, the Nobel prizewinning physicists who had first pointed out that asymmetry, speculated that perfect left-right symmetry could be restored to the Universe if a "mirror" or "shadow" world existed, complete with mirror-reflected particles. Mirror particles would differ only in the way they interacted with each other-every interaction would work as if reflected in a mirror. So processes that produce right-handed particles in our world, for example, would produce lefthanded particles in the mirror world. Mirror particles would have the same masses as their ordinary counterparts, and in the mirror world, mirror quarks and other particles would interact via mirror forces very like ours. But mirror particles would interact with ordinary particles only through gravity. Otherwise they would be essentially invisible to us-just as our Universe would be all but invisible to them. A hazy glimpse of the mirror world is provided by neutrinos. Physicists see these ghostly particles coming from nuclear reactions inside the Sun and from the upper atmosphere of the Earth, where they are created by cosmic rays. But there are fewer than expected. These shortfalls could be explained if the three known types of neutrino-the muon, tau and electron neutrinos-are spontaneously changing into each other. Unless, that is, you include the results of experiments at Los Alamos National Laboratory in New Mexico. If those results hold up, we might need a new form of neutrino, one so shy it makes the other three particles seem positively sociable (New Scientist, 13 March 1999, p 32). Could the fourth neutrino be from the mirror universe? "If neutrinos can oscillate into mirror neutrinos, this can nicely explain the solar and atmospheric neutrino anomalies," says Robert Foot, a physicist at the University of Melbourne in Australia. Neutrino research hasn't proved the existence of mirror matter, but more convincing evidence may come from an ephemeral substance called orthopositronium. Positronium is a type of matter that looks a little like an atom, but instead of electrons orbiting a nucleus made of protons and neutrons, there are just an electron and a positron orbiting each other. If the spins of the two particles point in the same direction, the system is called orthopositronium. In 1990, physicists at the University of Michigan at Ann Arbor made a batch of this short-lived substance by firing a lowenergy beam of positrons into matter, so that some of them slowed down and captured electrons. They then measured its lifetime. Theorists calculate that orthopositronium should decay into three photons after, on average, 142 nanoseconds-but the Michigan physicists discovered that its lifetime is actually 0.1 per cent shorter. At first, nobody worried too much. It was assumed that when theorists made more detailed calculations, the predicted lifetime would agree with the one observed. But when those calculations were finally done in March, by a team led by Gregory Adkins of Franklin and Marshall College in Lancaster, Pennsylvania, the discrepancy remained-leaving physicists scratching their heads (Physical Reviezv Letters, vol 84, p 5086). Enter Sergei Gninenko of CERN, the European laboratory for particle physics near Geneva, Switzerland. In a paper published with Foot in the 11 May issue of Physics Letters B (vol 480, p 171), he claims that the lifetime can be explained by a mirror universe. The idea that orthopositronium might be peculiarly sensitive to the mirror world was pointed out in 1986 by the Nobel prizewinning physicist Sheldon Glashow of Harvard University. Bob Holdom of the University of Toronto had proposed that ordinary matter and mirror matter might interact in ways besides gravity: there might be heavy particles that interact with both ordinary and mirror matter, carrying a tiny force between them. According to Glashow, this coupling between ordinary and mirror orthopositronium would mix the two states. The effect would be that orthopositronium could oscillate between mirror and ordinary orthopositronium-jumping back and forth through the mirror. Gninenko and Foot say these oscillations could explain the short lifetime. If some orthopositronium oscillates into the mirror world, and decays there, it can never oscillate back. "Extra losses mean a faster decay," says Gninenko. "We find that if the oscillations take about 3000 nanoseconds, it is possible to explain the observed reduction in decay lifetime." It's a remarkable claim, but Gninenko and Foot believe it is testable. They are preparing a proposal for a vacuum experiment to be carried out at CERN (see Diagram). The idea is to enclose orthopositronium inside the evacuated cavity of a calorimeter, which measures total energy changes. "If oscillations are occurring, then particles will disappear from our world," says Gninenko. "The umnistakable signature which we will look for is missing energy. We could check for the influence of a possible mirror world within a few years." The vacuum is crucial because frequent collisions with matter can stop the oscillations. But achieving a vacuum in such an experiment is difficult, because to create positronium in the first place you need a source of electrons-in other words, some matter. According to Gninenko, this was enough to spoil a similar experiment at the University of Tokyo in 1995, where grains of silicon dioxide supplied the necessary electrons but masked the effect. Gninenko will try to create enough positronium from collisions with electrons in the wall of his vacuum vessel. Gninenko's experiment isn't the only way to seek the mirror world. Another sign would be the existence of particles with tiny electric charges. All the particles we know of have electric charges that are multiples of the charge on an electron, or thirds of this (in the case of quarks). But if normal electrons interact with mirror particles via the heavy intermediaries that Holdom conjectured, they effectively have a small mirror electric charge. Similarly, mirror particles would have a small normal charge-perhaps only a tenthousandth or a hundred-thousandth of that carried by an electron. Physicists have searched for such "milli-charged" particles, both in oil-drop experiments and in bubble chambers attached to particle accelerators, but so far in vain. If Gninenko does find mirror matter, it will surely tell physicists something fundamental about the Universe-perhaps even providing them with the ultimate theory. "A mirror world is predicted by some versions of superstring theory," says John Cramer of the University of Washington in Seattle. "It is therefore pleasing to see that experimental tests of the ideas are possible, since the predictions of superstring theories are usually untestable." If it exists, mirror matter wouldn't just turn up in esoteric particle experiments. Large amounts of it could fill our Universe. After all, astronomers are crying out for a new type of matter: at least 90 per cent of the Universe's mass appears to be mysterious "dark matter", which reveals itself only by the gravitational pull it exerts on stars and galaxies. "Mirror matter could certainly be the dark matter," says Foot. Cramer disagrees: "The mirror matter would only boost the gravitational matter density around galactic clusters by a factor of two-we need about a factor of seven." Foot counters that Cramer is making an unjustified assumption, namely that there should be only as much mirror matter as ordinary matter. We know little about the initial conditions of the Universe, he asserts. Maybe the big bang created more mirror than ordinary matter? If there is indeed a mirror universe sharing our space, it might hold mirror galaxies, mirror stars and mirror planets. They could even host mirror life-real Tweedledums and Tweedledees. According to Foot, mirror stars might be observable, but only when they explode as supernovae. These would look very different from ordinary supernovae. Because of the very weak coupling of mirror matter to ordinary photons, mirror supernovae would be difficult to spot by looking for ordinary light. But in a mirror supernova, oscillations of mirror neutrinos into ordinary ones should produce a huge burst of observable neutrinos. "These could reveal a mirror supemova to underground experiments, just as a burst of neutrinos revealed an ordinary supernova in 1987," says Foot.

Other worlds

So much for stars. What about planets? Whole mirror solar systems would be impossible to detect, because even their parent stars would be almost invisible. But mirror planets might orbit ordinary stars. For example, some of the 40 or so extrasolar planets discovered in the past few years might be mirror planets, as hardly any have yet been observed directly. With the next generation of telescopes it should be easy to tell, because if the extrasolar planets are mirror worlds then they should, unlike normal planets, reflect none of the ordinary light from their stars. "The idea that the extrasolar planets are made of mirror matter can be tested in the near future as searches for reflected light improve in sensitivity," says Foot. Foot thinks it unlikely that our own Solar System contains mirror planets, unless they are either so small or so far from the Sun that their gravitational influence is negligible. "However, they may orbit in a different plane to the ordinary planets, making them harder to detect," he says. Such mirror rogues may have a sinister side. In 1984, David Raup and John Sepkoski of the University of Chicago suggested that the Sun might have a distant stellar companion, dubbed Nemesis, which periodically stirred up the Oort Cloud of comets, sending icy bodies on a collision course with the Earth. This would explain an apparently regular pattern of species extinctions in the geological record. So far no one has spotted Nemesis, so Zurab Silagadze of the Budker Institute of Nuclear Physics in Novosibirsk, Russia, has proposed that it could be an invisible mirror star. "It's very hard to prove," he says, "but fun to speculate." Whether there's a mirror star out there or not, the rocks that rain down on Earth might be mirror missiles. Foot speculates that the object that devastated the Tunguska region of Siberia in 1908, flattening 2000 square kilometres of forest but leaving no crater, could have been a mirror asteroid or comet. With a big enough body, even the tiny forces between mirror and ordinary matter could have released enough energy to flatten the trees. Once it ploughed into Earth, it should have melted a huge volume of rock. 'The idea could be tested by taking samples under the Tunguska site to look for evidence of a large release of energy underground," says Foot. "Of course, I have a vivid imagination, and Tunguska may have nothing to do with mirror matter." The mirror world provokes a lot of speculation. Cramer has even written a novel about mirror matter. Published in 1986, Twistor speculates on the existence of a mirror Earth occupying the same volume of space as the ordinary Earth. Tidal forces lock the spins of the two Earths together, and their cores are fairly light, but in combination they add up to the Earth's observed mass. The hero even devises a way to escape into the mirror world. Not surprisingly, Foot finds this unlikely. Unfortunately, unless there is a parallel Earth to support you, stepping through the mirror might be perilous. "You would probably fall straight to the centre of the Earth," says Foot. At least it should be painless, and you might see some interesting things on the way. Rather like falling down a rabbit hole... Fl