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NS 20 Jul 2002

Will we be wiped out by a super-eruption?

STOP worrying about Earth-bound asteroids - save your sleepless nights for massive volcanic eruptions. They pose twice as much of a threat to civilisation as impacts from outer space. Cosmic bodies with a diameter of more than 1 kilometre hit the Earth once every ioo,ooo years or so. On today's crowded planet, the dust storms and wildfires created by such impacts would wipe out over a billion people. But super-eruptions capable of wreaking this kind of havoc happen around once every 50,000 years, says earth scientist Michael Rampino of New York University. And while governments are waking up to the asteroid threat and considering defence plans, no one even knows how to predict the next super-eruption. Rampino's warnings are based on investigations of the geological remnants left by previous super-eruptions. Volcanoes in Yellowstone Park and Long Valley in California have erupted three times in the past 2 million years, each time coating the whole of the US with ash. But the biggest and most recent super-eruption happened at Toba, on the island of Sumatra, 73,000 years ago. Toba blasted a crater loo kilometres long, spewing dust and ash 4o kilometres into the air, and releasing 3 billion tonnes of sulphur dioxide - equivalent to a hundred Pinatubos erupting at once. Sulphur dioxide in the atmosphere is converted into aerosols of sulphuric acid, which wash out in rain over several years. In cores taken from Greenland's ice sheet, the largest peak in sulphuric acid coincides with the Toba eruption and lasts for six years. That shows Toba's dense volcanic cloud spread right around the globe, says Rampino. He also suspects that Toba's super-eruption was responsible for the population crash 70,000 years ago, when the number of people fell to no more than io,ooo. The big problem for survivors of the immediate fallout from a super-eruption would be the catastrophic drop in global temperatures, caused by ash and aerosols blocking the Sun. Eruptions at Krakatoa in 1883 and Pinatubo in iggi each pushed temperatures down by several tenths of a degree for a few years. By Rampino's calculations, another Toba would drive them down tO 5'C for a decade or more, and trigger regional cooling of 15'C. "That's going to kill off most of the above ground vegetation in Africa," he says. Globally, the amount of vegetation would be cut by 25 per cent. "It's chilling," agrees palaeontologist Simon Conway Morris of Cambridge University. "All we can do is keep our fingers crossed and hope that by the [next super-eruption] we have a robust social system based on justice and fairness, and the beginnings of an interstellar colonisation programmer" Super-eruptions are also bad news for SETI. The search for alien intelligence relies on the idea that distant civilisations would survive long enough to develop the technology needed to communicate across the vast reaches of space. Volcanoes are a source of essential gases such as carbon dioxide and water vapour, so inhabited planets are likely to be volcanically active, and therefore susceptible to the occasional civilisation-busting super-eruption.

Building block of life found in deep space

ONE of the building blocks of life, an amino acid, has been spotted in deep space. If the find stands up to scrutiny, it means that the sorts of chemistry needed to create life are not unique to Earth - verifying one of astrobiology's cherished theories. That will certainly add oomph to ideas that life exists on other planets, and even that molecules from outer space kick-started life on Earth. Over 13o molecules have been identified in interstellar space so far, including sugars and ethanol. But amino acids are a particularly important find because they link up to form proteins, the molecules that run, and to a large extent make up our cells. Back in 1994, a team led by astronomer Lewis Snyder of the University of Illinois at Urbana- Champaign announced preliminary evidence of the simplest type of amino acid, glycine, but the finding didn't stand up to closer examination (New Scientist, 11 lune 1994, p 4). Now Snyder and Yi-jehng Kuan of the National Taiwan Normal University say they really have found glycine. "We're more confident [this timel," says Kuan. "We have strong evidence that glycine exists in interstellar space."

The researchers monitored radio waves for the spectral lines characteristic of glycine. They studied emissions from more locations than before - giant molecular clouds, huge blobs of gas and dust grains. They have also identified io spectral lines at each location that correspond to the lines created by glycine in the lab; before they had just two. The discovery of glycine supports recent lab-based simulations of deep space, which show that ices containing simple organic matter could form. When researchers bathe those ices in ultraviolet light, amino acids are created. "Glycine is the holy grail," says Jill Tarter, director of the Centre for SETI Research at the SETI Institute in Mountain View. "Let's hope they've got it this time."

Strange Attraction

THE veteran spacecraft hurtles towards the stars. Out at the darkest edge of the Solar System, far beyond Pluto, there should be nothing but the feeble gravity of the receding Sun to slow it down. Yet a mysterious extra force seems to be tugging on the spacecraft. And 24 billion kilometres away in the other direction, the probe's twin is feeling an identical force. Pioneers io and 11 may be old and battered, but in their twilight years they are giving physicists a few sleepless nights. The probes were launched in 1972 and 1973, and Pioneer 10 became the first spacecraft to fly past Jupiter. Pioneer 11 followed it past Jupiter and then became the first to visit Saturn. "After those encounters, we thought that essentially the mission was over," says astronomer John Anderson of NASA's jet Propulsion Laboratory (IPL) in Pasadena. "How wrong we were."

NASA kept track of the probes to see if their trajectories might reveal the influence of a possible tenth planet or the predicted Kuiper asteroid belt. Microwave signals sent to each space probe were bounced back automatically by a transponder, and the Doppler shift in the wavelength of the returning signal revealed the probe's velocity. And with frequent velocity measurements, their positions could be worked out. When the NASA engineers compared their actual positions with the predicted trajectories, things didn't add up. "In the early days, we couldn't be sure of the discrepancy because there was a bigger non- gravitational effect in the inner Solar System - the outward pressure of sunlight," says Anderson. "But by 198o, when Pioneer lo was halfway between Uranus and Neptune, it was clear the probes were not where our calculations said they should be."

Pioneer 10 is experiencing a mysterious deceleration towards the Sun. its magnitude is tiny: less than a nanometre per second per second, a mere ten-billionth of the gravity at Earth's surface - but every year that's passed has simply confirmed the persistence of the effect. And it adds up. Today, Pioneer lo is 80 times as far from the Sun as Earth is, and it's 400,000 kilometres behind schedule, roughly the distance between the Earth and the Moon. And although NASA lost touch with Pioneer ii in 1995, up to that point it was experiencing exactly the same deceleration as its sister probe, also directed roughly towards the Sun. Fora long time, Anderson didn't publicise the Pioneer anomaly. "We were convinced it had to be some mundane effect on board - a leak of fuel, perhaps, or heat," lie says. But in 1994, physicist Michael Martin Nieto from Los Alamos National Laboratory in New Mexico raised another possibility. Nieto noticed that on most cosmic scales we don't know the effects of gravity with any real precision, and he began to wonder how precisely we understand gravity even within the Solar System. Don Yeomans of JPL suggested Nieto should talk to Anderson. He did, and got a big shock. "When John told me the size of the Pioneer discrepancy, I almost fell off my chair," Nieto says. "To me it was huge." To Nieto, the mysterious deceleration sounded like it could be explained by some kind of modification of the laws of gravity. When predicting how the probes ought to move, NASA engineers assume that the only force acting on them is gravity from the Sun and planets. The basic equation they use goes right back to Newton: the strength of gravity falls off with the inverse square of distance. So if gravity behaves just a little bit differently, it could explain the Pioneer anomaly.

Nieto and Anderson began working together along with Slava Turyshev of JPL. Their first task was to rule out any mundane explanation such as a technical fault. Having first ruled out a software error, the team tracked down generation-old data about the space probes'design from retired NASA personnel and painstakingly examined it. One possibility was a heat leak. just 70 watts Of radiation escaping in the direction away from the Sun would be enough to slow the 241-kilogram probes'outward velocities at the observed rate. But do the probes have enough power left to be doing this? Each one carries four radioisotope thermal generators, radioactive plutonium sources that originally generated about 2500 watts of heat. The generators could easily slow the probes by the amount observed if they were located at the front of each craft. But they aren't. Engineers were worried that they would damage the probes' instruments, so they stuck them out to the side of the spacecraft on the end of long booms. So could the power delivered by the generators still end up as heat radiated forwards? Thermoelectric devices convert the original heat to electricity, which is carried to the main body of each spacecraft. But with the decay of the plutonium, and wear and tear on the converters, that only amounts to a measly 70 watts. "To explain the anomaly, all of that would have to be radiated in one [email protected]," says Anderson. And that's all but impossible. The other early idea, a fuel leak, was even easier to dismiss, as the probe's internal sensors would have picked it up. And in any case it's hard to imagine identical accidents producing identical leaks on both probes at the same time. What, then, does that leave? According to Anderson, the most likely explanation is still some unsuspected error to do with the probes or the tracking system. "For the life of me, though, I can't think of what it could be," he says. But Nieta, though he still thinks that an error is most likely, hopes there is more to it. "I admit I want it to be something profoundly important, some entirely new physics." could it really be that the force of gravity breaks the inverse square law in the outer Solar System? Intriguingly, there is evidence that that might happen on even larger, galactic scales. For instance, stars orbiting the centres of spiral galaxies like our own, and galaxies orbiting within galaxy clusters, appear to be moving too fast, as if in the grip of stronger than expected gravity. The standard answer is to postulate huge amounts of invisible "dark matter" exerting its own gravitational influence. But Mordehai Milgrom of the Weizmann Institute in Israel has proposed an alternative explanation, known as modified Newtonian dynamics. In MON D, the inverse square law only applies where the gravity is relatively strong. Where it is very weak, gravity fades more slowly with distance. The physics community is sceptical about MOND because any evidence for it is largely empirical. "There is no fundamental theory beneath it," says Nieto. Yet it could explain the Pioneer results. The only problem is; why doesn't it affect the orbits of the planets too - especially the outer planets, Uranus, Neptune and Pluto? That's the central difficulty facing any explanation of the Pioneer mystery, says Nieto. But here he makes an observation. Unlike the planets, which are gravitationally bound within the Solar System by the Sun, the Pioneer probes are on escape courses. "Could it be that there's something that affects unbound bodies but not gravitationally bound bodies like the planets?"

Look in the mirror

It's always possible that the anomaly is evidence for dark matter after all. If there is dark matter scattered throughout our Galaxy, it could be slowing the Pioneers down. Most of the hypothesised kinds of dark matter couldn't do this strongly enough to explain the anomaly, because they only affect ordinary matter through the feeble force of gravity. But there is one theoretical possibility: "mirror matter". Mirror matter would be like ordinary matter, except at the quantum level it is opposite to matter in the way a right hand is opposite to a left hand. Since it would interact only very weakly with ordinary matter, it could exist all around us without our knowing. However, there may be a small, non-gravitational force between mirror atoms and ordinary atoms (New Scientist, 17 lune 2000, P 36), and according to Robert Foot and Ray Volkas of the University of Melboume, the drag caused by only a few Earth-masses of mirror matter spread throughout the Solar System would be enough to explain the Pioneer measurements. And crucially, it would not have a noticeable effect on the orbits of the planets. Unlike the featherweight Pioneers, the massive planets could plough through a mirror-matter headwind almost completely unperturbed. MOND and mirror matter are just the start of it. Perhaps not surprisingly, proponents of all sorts of speculative theories have latched onto the Pioneer anomaly, suggesting ways that their own theories can explain it. For example, Bernard Haisch of the California Institute for Physics and Astrophysics in Palo Alto has a radical alternative. Rather than unexpectedly strong gravity, the slowing could be due to a reduction in inertial mass. "if the inertia of the Pioneer spacecraft is slightly less that it should be, then the same gravitational force would give an anomalous Sun-directed acceleration," he says. In Haisch's theory, inertia is not an inherent property of an object, but a kind of drag produced by short-lived "virtual" particles predicted by quantum mechanics (New Scientist, 3 February 2001, p 22). But it's not clear why these virtual particles should behave differently as you get further from the Sun. Some other ideas are more tentative still, not so much attempts to explain the anomaly as possible links with other oddities in modern physics. Could the Pioneer puzzle be linked in some way with the acceleration in the expansion of the Universe? A kind of universal energy called quintessence is one theory to explain the accelerated expansion, and Bruce Bassett of the University of Portsmouth wonders whether it could also affect the Pioneers somehow (though it would have to work in the opposite sense from its effects on the Universe, decelerating one while accelerating another). Bassett also suggests that the Pioneer conundrum might just be linked to another big puzzle in physics: the apparent variation of something called the fine-structure constant. This number describes how strong the electromagnetic force is, and thereby how strongly light interacts with matter. By observing light from distant quasars, a team led by John Webb at the University of New South Wales has found evidence that the fine-structure constant was slightly different billions of years ago (New Scientist, ii May, p 28). "It's possible that whatever caused variation of the fine-structure constant in the distant past might have a present-day manifestation," says Bassett. The fact that serious physicists must resort to vague speculations may be telling. Perhaps the Pioneer mystery is something we're not equipped to explain; perhaps we'll only understand it via some as yet unformulated theory that unifies the fundamental forces of nature. A good analogy is general relativity, Einstein's theory of gravity Once it was formulated, all sorts of unexpected things popped out, such as gravitational waves and an explanation for shifts in Mercury's orbit. In the meantime, the only way to whittle down the possibilities is to get better measurements of the effect. Other space probes in the outer Solar System, such as the Voyagers and the Cassini probe, enroute to Saturn and Titan, are no use because their propulsion systems are too sophisticated. The orientations of the Pioneers were stabilised by setting them spinning, whereas the later probes use intermittent rocket boosts to keep them correctly aligned. The unpredictable accelerations caused by these spurts are at least lo times as big as the Pioneer acceleration, and make it impossible to measure the effect. To get to the bottom of the problem, Anderson,NietoandTuryshevareproposinga new mission specifically to test non-standard gravitational effects in the outer Solar System. ideally, says Nieto, the probe should look for acceleration in four specific directions. If the acceleration is along the probe's spin axis, it must be something relatively mundane to do with the spacecraft; if it is along the probe's trajectory, that would imply some kind of drag, perhaps due to mirror matter. if the acceleration is towards the Sun, it might imply a modification of gravity. And finally, if it is towards the Earth, it could indicate that time is running increasingly more slowly on the Pioneers - what you might call an acceleration of time.

Flipping the axis of the spinning spacecraft by 180' would create an opposite acceleration if it were due to something on board. it would also be interesting to see if the anomalous acceleration could be detected by an on-board accelerometer, Weinberg says. If not, it would mean the gravitational field within the Solar System was misbehaving. And if the acceleration were detected, it would have to be some non- gravitational force. Such a probe would cost between $300 million and $5oo million. It could be launched within five years, says Anderson, and reach the zone where the anomaly should be detectable in perhaps another five years. But Nieto admits that neither NASA nor the European Space Agency is likely to fund a probe solely to examine this puzzle. So one possibility is that it could be incorporated into the proposed Pluto/Kuiper Belt mission to the outer reaches of the Solar System. The drawback is the other instruments on board. "That would make it more difficult to discern the effect," says Nieto. "However, we might be able to live with that if we were able to give input into the physical design of the space I craft from the beginning."

So will it happen? Nieto is hopeful. "I'm sure the mission will get studied," he says. Part of the reason for his confidence is that physicists are increasingly interested in the Pioneer anomaly. Already this year, the journal Physics Review D has published a so-page paper by Anderson and his colleagues reviewing all the possible explanations. It's an unusually long paper for the journal. "The anomalous acceleration of Pioneer is indeed extremely interesting," says Bassett. "Anyone aware of the importance of serendipity in scientific discoveries would be unwise to discard the possibility that it really is the footprint of new physics!' * Further Reading: "Study of the anomalous a(celeration of Pioneer 10 and 11" by John Anderson and others, Physical Review D, vol 65, article 082004 (2002) (www.arxiv.orglabsigr-qcl 0104064) "A mission to testthe Pioneer anomaly" byjohn Anderson, Michael Martin Nieto and Slava Turyshev (www.arxivorg/ abs/gr-qc/0205059) "A mirrorworld explanation forthe Pioneer spacecraft anomalies?" by Robert Foot and RayVolkas, Physics Letters B, vol 51, @ 13, (2001) ([email protected] hep-ph/0108051)

Population Crash?

Italian women are terrifying demographers. If other women around the world copytheirbehaviour, wecould be on the brinl( of a population crash. Fred Pearce isalready worried

THE baby boom seems to be turning to baby bust. Not just in the rich world but increasingly in poor countries too. Within 50 years, the world's population could be in free fall. And the only way out could be for men to behave more like women. To see the future in its starkest form, take a look at Italy, where Isabella, Clara and Bianca are less likely to be making babies than young women anywhere else in Europe. With splendid irony, the country that is home to the Catholic Church, noted for its opposition to artificial birth control, is notching up super-low fertility rates way below replacement levels. At jUSt 1.2 children born to each Italian woman, the rate is little more than half the figure needed to prevent the population plummeting. It's a similar picture, if a little less extreme, the world over. A future decline in the world's population sounds like a good thing. Certainly, it will reduce the pressures on global natural resources. But a world of falling population will be very different from the one we know now: a much older world, and perhaps a less innovative, more conservative one. it will be a world in which tabour is in increasingly short supply and the richest countries compete for immigrants who can supply it, not turn them away. And the relationship between the sexes will become it fundamental political issue, as countries seek to revive their child-bearing resources. indeed, to stave off the spectre of demographic decline, some countries may become willing to take ever more radical, even draconian measures, with major implications for the personal liberties of us all. It's quite a turnaround. Ever since the 1970s, forecasters have been scaring us with population figures which appear to wildly outstrip even the most optimistic projections for resources such as food, water and land, while triggering runaway global warming and an even more polluted and paved planet. And so far, the figures have borne them out. During the 20th century, the world's population increased almost fourfold, from i.6 to 6 billion. The baby boom, which peaked in mid-century, has not yet ended. But in the background, fertility has been falling fast. In 1950, worldwide the average woman had five children. Today she has j USt 2.7, and the continued collapse of fertility is set to become the dominant demographic feature of the 2ISt century. Demographers have assumed that during this century most of the world's women would settle down in a conventional Western-style nuclear family with mother, father and two children. That would ensure a stable world population by 2ioo of perhaps @o billion. But nobody told women about the plan, and there is a growing notion that Italy is leading the way to a future of fertility rates far lower than replacement levels. The rule of thumb is that "replacement" fertility requires 2.i children per woman; the extra o.1 compensates for girls who do not live long enough to have families. At 1.2 babies per woman, Italy is clearly falling a long way short. But it's not alone. Its southern European neighbours, Spain and Greece, have similar fertility rates, as do the Czech Republic and former Soviet states such as Russia and Armenia. The amazing bottom line is that new UN population forecasts, to be finalised later this year, are expected to conclude that within two generations four out of five of the world's women will be having two children or fewer. So why is the world still oblivious to this coming demographic shift? It's largely because the children of the greatest population explosion in the planet's history are now of childbearing age. Even with their much-reduced fertility rates, they are bringing many more babies into the world than ever before. This, combined with rising life expectancy, is keeping Europe's population stable, and boosting that of the world as a whole by around 8o million a year. But by the time the 2oth-century baby boomers start to die off and the tide turns, it will be hard to halt a population crash. Almost whatever happens now, the world's population is primed to start diminishing for probably the first time since the Black Death in the 14th century.

Kids in cities are a liability The key questions are why women are choosing to have fewer offspring, and how far their increasing reluctance to take up motherhood will go. Most obviously, the declining death rate, particularly among children, due to better health and medical services, has meant people don't feel they need to have so many children. Other factors have accelerated the process. Urbanisation is certainly one. On a farm, even young children are an asset, minding the animals and helping with the harvest. In cities it's a different story: kids are more likely to be a liability - in purely economic terms, at least. When they are young, they need looking after full-time, and when they are older they need educating to get any sort of job. On top of that, cultural changes have increasingly liberated women from the home and child-rearing. In poor countries with a traditional patriarchal society, the spread of TV has opened many women's eyes to a whole new world, and modern birth control methods have allowed them to turn those aspirations into reality. "Getting married and having children are simply not as important as they used to be," says Tim Dyson of the London School of Economics. Today, more than 6o countries have fertility rates below replacement levels. None shows any sign of sustained recover-y. The club now encompasses much of the Caribbean, japan, Korea and China, the world's most populous nation. This year Thailand, Sri Lanka and Iran are likely to join. Mindful of the "enormous implications" for the future of our species, the UN population division's director Joseph Chamie called a conference of experts in New York in March this year to analyse the phenomenon. A succession of scientists from large countries that have helped drive global population growth in the past half-century told the conference that they expected their own national fertility rates to fall below replacement levels within 20 years. They included India, Brazil, Indonesia, Mexico and Turkey-

No forced contraception

Few of these countries, bar China, have forced contraception on their populations. Rather the reverse. opposition from the Catholic Church has ensured that Brazil has no state family planning programme. Even so, millions of its women have attended sterilisation clinics, and fertility has halved in 20 years to today's 2.3. The case of Iran is even more remarkable. In 1994, the mullahs ruling the country went to a UN population conference in Cairo and declared opposition to much of the international agenda for cutting birth rates. But back home, women were taking charge of their bodies and sending fertility rates crashing from 5.5 children per woman in i988 to jUSt 2.2 in 2000. Prosperity is no longer a necessary passport to reducing national fertility. Bangladesh remains among the half-dozen poorest nations outside Africa. Its girls are among the least educated and marry younger than most. Yet they give birth to just 3.3 children, half the number their mothers had. In Vietnam, which is poorer still, women halved their fertility tO 2.3 children in the decade to the mid-iggos. Rich or poor, socialist or capitalist, Muslim or Catholic, with tough family planning policies or none, most countries tell the same story: women are voting with their wombs. Two major studies have recently failed to find any common factors uniting nations with fast-failing fertility, other than availability of affordable contraception. Dyson is among the demographers who have a new theory for what is happening, "cultural diffusion". Not having children has become a statement of modernity and emancipation, and women are unlikely to give up the new freedoms. They are also taking over from their brothers and husbands the role of shaping their societies. "Go to rural India," says Dyson, "and you find that women are fed up with the men, who seem to be going nowhere. It is the women who are running the farms. It is the women who are getting jobs and taking charge. They don't have time to have children any more." With men no longer in charge, their usefulness to society and the old Indian preference for sons may diminish as a result, he says. That, too, will help reduce fertility as couples see daughters as well as sons as potential heads of a new generation. Where is this all leading? lack Caldwell of the Australian National University in Canberra, doyen of demographers, is among those convinced that "Italy is the future". its super-low fertility arises from female emancipation, or rather ftom its faltering progress, he says. Isabella and her friends are educated as well as or better than their suitors. They have prospects. The last thing they want is to be like their mothers, stuck at home rearing children. French demographer Jean-Claude Chesnais of the National Institute for Demographic Studies in Paris goes further. With poor state childcare provision, and most men unlikely to help in looking after their offspring, "the obstacles to childbearing in countries like Italy are enormous and the economic sacrifices made by mothers are viewed as unbearable". Caldwell's colleague at the Australian National University, Peter McDonald, argues that the southern European phenomenon is a result of the lopsidedness of moves to gender equality. Women have got the freedoms that arise from better education and employment, but not in their relations with their men or in terms of state services for the family. Economic liberalism has clashed with social conservatism. Result: a childbirth strike. Not all of Europe is quite like Italy, however. In Sweden, for instance, Astrid and her friends feel more able to have a family than Isabella's crowd. They have an average of i.6 children. That's not enough to maintain their country's population in the long term, but neither is it demographic meltdown. Indeed, most of northern Europe has maintained higher fertility rates than those around the Mediterranean, with Norway at 1.8 and Britain and Finland at 1.7. Why is Astrid happier to make babies than Isabella? She is just as keen to pursue a career. The difference is that she has more chance of combining a career with motherhood. Her suitors, who are more likely to have set up home on their own before marriage, are better house- trained, and Nordic governments are better at helping couples juggle family and work. About half the jobs held by Swedish women are part- time, [email protected] are near-universal and paid parental leave lasts for a year. All this is unheard of in Italy, where only 12 per cent of employed women have part-time jobs, and in eastern Europe, where fertility rates have plunged since the collapse of communism wrecked state- funded support services for families. So will the rest of the world follow northern or southern Europe? Caldwell thinks the signs are clear: "The Mediterranean patriarchal model is far more common in the world than the northern European model of more helpful husbands."

McDonald says we can already see this in eastern Asia, where conservative family values lie behind the ultra-low fertility rates from Shanghai to Tokyo. Even in Australia, Italian and Greek families are significantly smaller than their Anglo-Saxon counterparts. One part of the world where fast-falling fertility remains extremely patchy is Africa, where the five- or six-child family is still the norm in many countries. But here another factor is keeping the lid on population growth: AIDS. The UN expects 15 million deaths from AIDS in the next five years, the great majority in Africa. Life expectancy in Botswana and Zimbabwe has plunged from 60 years to close to 40 years. In some more economically advanced African countries, birth rates are failing at the same time as death rates are rising. In Kenya, the fertility rate has fallen from 8 to under 5 in two decades, and it could be below 3 within a decade. With AIDS killing off many young children, that could be below the level needed to replace the present population. Kenya could become the first African country with a falling population. Industrialised countries are already complaining about the effects of falling fertility. An ageing population is putting pressure on social services and pensions. In the next few years they will start to see absolute falls in their populations. Japan expects its population to peak in 2Oo6 and then fall by 14 per cent, or almost 20 million people, by 205o. Germany expects a similar drop. Italy and Hungary may lose 25 per cent, and Russia could lose a third by then. if today's low fertility rates continue, then as the current baby boomers die, things will get drastically worse. if each generation's adults continue to produce not much more than half the number of children needed to replace them, McDonald calculates that the population of Italy is set to crash ftom 56 million now to just 8 million by 2ioo. Likewise Spain would lose 85 per cent of its population within the same time frame and Germany 83 per cent. This year, Chamie and his UN colleagues have been redrawing population projections on the assumption that the world will move towards an average level Of 1.85 children per woman. Much depends on how long this move takes, but according to one of his projections it would result in a world population peaking at about 7.5 billion around 2050. it would then begin to implode. By 2150 there would be 5.3 billion people on the planet. Assume lower future fertility levels and the result is an even more drastic die-off. If women settled for a Swedish-style fertility level of i.6 children, we would be down tO 3.2 billion by 2150 - only a fraction over half today's population. Chamie has not yet dared calculate the effect of universal Italian-style fertility. Of course, it may never happen. Some countries seem to have levelled out at above- replacement levels. Argentina and Uruguay, for example, have been stuck at between 2.5 and 3 children per woman for 50 years. Israel and Malaysia stalled at around 3 in the iggos. Many African countries have not begun the demographic transition yet. And some Muslim states such as Afghanistan (6.9), Saudi Arabia (6.i) and Pakistan (5.5) have also bucked the trend. Demographers also suspect that if the downward fertility trend continues for much longer, then deep collective instincts of survival could be unmasked. Politicians will growfearful of the social consequences of declining populations such as the growing burden of supporting the elderly, and an increasing need to find immigrants to augment the tabour force. Such changes will happen in Europe first. "There seems little question that pro-natalist policies will become a central part of the political agenda in the near future," says McDonald. The authoritarian approach would be to try to cut women out of the workforce and keep them at home, to ban abortions, and restrict access to family planning services. But that is unlikely to work, says Dyson. Women wouldn't stand for it. Instead, he argues for a continuation of the "renegotiation" of gender roles under way in much of northern Europe. Paradoxically perhaps, the more feminist attitudes that have helped bring about the dramatic decline in family size in the past So years will need extending rather than dismantling if family sizes are to rise from the worst-case Italian model. But the new agenda may be less about creating new freedoms for women and more about instilling new responsibilities in men and the state. As Dyson puts it, in most of the world today, fertility rates are plunging because women have decided they want to become more like men. Right now that leaves little room for babies. To change that, men must take the plunge and start to become more like women. The future of humanity could depend on it. 0

Is the last hope for certainty gone? JUSTIN MULLINS

GOD really does play dice with the Universe. So claim a group of Italian scientists who say they have disproved the long-standing idea that there is certainty and order beneath the strange fuzziness of the quantum world. Since the probabilistic theory of quantum mechanics was first proposed in the 1920s, many scientists have resisted the counter- intuitive notion that the Universe is govemed by chance. They have argued that quantum theory can't be reconciled with the predictable world of human experience. Einstein was so outraged by the idea that he refuted it with the famous pronouncement that God does not play dice with the Universe. Some scientists have even constructed alternative theories that are based on concrete, predictable rules but reproduce the probabilistic behaviour of quantum theory. Their idea is that even though the actions of particles may appear probabilistic, they are actually governed by strict rules. It's like tossing a coin: even though we can't predict the outcome, its motion is still determined at any instant by the ordinary laws of physics. Since the deterministic rules behind these theories operate unseen, the ideas have become known as "hidden variable" theories. In the ig6os, particle physicist John Bell at CERN, the European centre for particle physics, pinpointed important differences between quantum mechanics and a certain class of hidden-variable theory. This allowed scientists to devise experiments that could determine which framework was correct. In every one of these tests, traditional quantum mechanics has emerged triumphant. But there remained another class of hidden-variable theory with the potential to unseat quantum mechanics. lt was proposed by physicist Louis de Broglie in the 1920s and developed in the 195os by the physicist and philosopher David Bohm. He argued that the wave-like behaviour of quantum particles could be explained if each particle had an associated "guide wave" that steered it into position. In this way, a particle such as an electron has a definite position but can also appear to behave like a wave. By contrast, quantum mechanics says that the electron is a particle and a waveatthesametime. Scientists have been unable to refute Bohm's theory because it matches all the predictions of quantum mechanics, so tests can't tell them apart. But most physicists rejected the idea anyway on the basis that it is just too contrived. Then in 2000, Partha Ghose, a physicist at the S. N. Bose National Centre for Basic Sciences in Calcutta, claimed that under certain simple conditions, Bohm's theory does make different predictions from those of quantum mechanics ( 0103126). And he proposed an experiment that could test which was correct. It consists of passing two identical photons through two closely spaced slits at the same moment. Two detectors on the other side measure when the photons arrive (see Diagram). According to Bohm's theory, says Ghose, if the two detectors are at different distances from the slits, it isn't possible for them to detect both photons at the same time. But in fuzzy quantum mechanics, there's no such constraint. Now, Marco Genovese and his colleagues at the National Institute of Electrical Technology in Turin, Italy, have carried out the experiment and found that the simultaneous detection of the photons that guide-wave theory forbids is indeed possible (, 02o6l96). "We have tested this alternative form of quantum mechanics and found it wanting," says team member Giorgio Brida. Ghose goes even further. "Unless the experiment has a loophole, it should rule out the last of the hidden-variable theories," he says. Not everyone is convinced, however. Antony Valentine, a physicist at Imperial College, London, a proponent of hidden- variable theories (New ScientiSt, 29 lune, P 31), says Ghose is wrong. He says Ghose assumes that the pair of photons are created at a single, deflnable point, when this isn't necessarily the case. Lucien Hardy, a physicist at the University of Oxford, is also unconvinced, pointing out that the Bohm model is set up so that it always agrees with quantum theory. "I am very sceptical about any claim that it must contradict standard quantum theory," he says.

How to make a killer virus

You don't have tostart from scratch to create a deadly bug. There's an easier way SYLVIA PAGAN WESTPHAL, BOSTON

LASTweek scientists alarmed the world by showing for the first time that it is possible to build the polio virus from scratch. The same technique could be used to recreate Ebola or the i9l8 flu strain that killed UP tO 40 million people, experts have told New Scientist. What's even more worrying is that there are easier ways of recreating microbes. You can simply add key genes to a close relative. The key in all cases is knowing the genetic sequence. That raises fundamental questions about the wisdom of publishing the genomes of deadly pathogens on the Internet (see opposite). To recreate polio, the team at Stony Brook University in New York bought bits of its sequence from companies that make any piece of DNA to order. At the moment, only short stretches of DNA can be custom-made, so the team had to assemble the genonie, which is about 75oo base pairs long, by stitching together sequences of about 7o base pairs. When copies of the genome were made into RNA ih a quick lab reaction and put into a vial full of cellular components that mimic a human cell, out came perfectly formed viral particles. Dramatic as it sounds, this was no scientific tour de force. All the steps are routinely followed in thousands of labs worldwide. That means anyone armed with the knowledge of a virus's sequence, some science training and a few common tools could recreate the virus in a test tube. "What is shocking to people is that, suddenly, it's a reality," says Eckard Wimmer, leader of the team. The report won't affect the World Health Organization's campaign to eradicate polio, officials said on Friday. "It's not an'Oh My God' situation," adds Wimmer, who is a member of a WHO committee on the containment of polio samples held around the world. "They know that getting rid of the polio virus in all the freezers in the world will be too hard." But it does mean that vaccine stocks will have to be maintained when vaccination ceases, he says. That's supposed to happen in 15 years, though this is already looking doubtful (see New Scientist, 6 February 2000, p 20). The real worry is that bioterrorists could use the method to recreate viruses such as Ebola and smallpox. Experts have been quick to point out that this would be much harder than making polio. Its genome is relatively easy to assemble because it is so small. And the virus's way of hijacking cells' resources is also simple. An RNA copy of the genome is enough to set off the cascade of events leading to the production of viral particles. That's not how most viruses work: with Ebola or smallpox, for instance, replication requires key viral proteins as well as the genome. But this obstacle has already been overcome. In January, scientists reported they had made Ebola using "reverse genetics". They took the virus's genome plus pieces of DNA coding for the key viral proteins and added them to cells. Once made, those proteins kick- start the replication process. The team got their genome from the virus itself But since the Ebola genome is only slightly larger than polio's, there's no reason why it too can't be assembled from scratch. Or take the 1918 influenza virus, fragments of which have been recovered from preserved tissue samples. The sequences of three out of eight gene segments have already been published, and two more have been sequenced and will probably be published this year, says Jeffery Taubenberger of the Armed Forces Institute of Pathology in Washington DC. It will take a couple of years to finish sequencing the remaining three, but after that anyone could use reverse genetics to bringitback,hesays. Recreatingsmallpoxby painstakingly assembling its genome would be more difficult than polio or Ebola because its sequence, at around 185,ooo base pairs, is much longer. But it could be done, says Lev Sandakhchiev, a smallpox expert and head of Russia's Vector biodefence lab in Siberia. "I am sure scientists may do it sometime," he told New Scientist. Sohowdoyoustopthis happening? Right now, the companies making DNA molecules such as the ones used to recreate the polio virus don't check what their clients are ordering. "We don't care about that," says a technician at a company that ships DNA to more than 40 countries around the world, including some in the Middle East. And even if all orders were monitored to make sure nobody is trying to make, say, smallpox, it wouldn't be enough. Instead of trying to assemble a genome, you could simply take a closely related cousin and change the key portions of its sequence to those of smallpox. Such cousins include camelpox and the easily obtained vaccinia virus. Bacteria resembling anthrax could also made this way. And if you're going to go to all this trouble, why not tweak the virus while you're at it? New Scientist revealed last year (13 January 2001, P 4) that scientists experimenting with mousepox had created a far deadlier strain. All this means that restricting access to dangerous pathogens and certain kinds of equipment won't stop determined bioterrorists. It doesn't even matter if a virus has been totally eradicated. All that is needed to bring it back is knowledge of its sequence and, in some cases, of what it needs to make more copies of itself.

Should the genetic sequences of deadly diseases be kept secret?

THE demonstration that you can create many viruses if you know their genetic sequence (see opposite) comes just as scientists are fighting government attempts to restrict access to this kind of information. Until now, countries'efforts at preventing the proliferation of bioweapons have relied on limiting access to the pathogens themselves. So the 34 members of the Australia Group, which includes many industrialised nations, will not allow some pathogens to be exported without a licence. But if terrorists can produce a bioweapon from genome information alone, the same logic suggests that access to genetic information should be restricted too. Last month, a meeting of the Australia Group in Paris agreed to "control, for the first time, the intangible transfer of information... which could be used for chemical and biological weapons". The group's members have not yet decided what this will cover. But they already restrict the export of critical pieces of DNA. This measure could be extended to posting sequences on the three big genome databases. Scientists have long been aware of the potential dangers. The creation of polio, for instance, was forecast months ago in an essay in Nature immunology pointing out that it would be simple to build an artificial poliovirus. Nevertheless, most scientists still oppose any attempts to restrict access to information. Earlierthis year, the US Department of Defense dropped proposals for checking any research it funds for "sensitive" information before it is published, after scientists protested that this would impede research needed to defend against bioweapons. "We are a good example of this," says Paul Keim of Northern Arizona University, whose lab developed a method of distinguishing between closely related strains of anthrax. "if the Bacillus anthraces genome had not been released, we would not have been able to develop the high-resolution system that is currently so important [to the investigation of last year's attacksl." Anthrax, however, still infects animals and sometimes humans, and samples are held in many labs worldwide. So there's no reason for a terrorist to try to recreate it. The same isn't true of Ebola, smallpox or the 1918 flu virus. Even so, when the American Society for Microbiology considered whether it should publish the smallpox genome, it reasoned that the benefits in terms of understanding the virus and designing drugs outweighed the dsks. Its position remains broadly unchanged even after 11 September. "it is imperative that we win the war against biotermrism without allowing biomedical science to fall victim in the enduring battle against disease," ASM president-elect Ronald Atlas said in December. In 2001, a group of experts at the US National Institutes of Heath came to a similar conclusion when they discussed whether the sequence of the 1918 flu virus should be made public.

"It isimperative that we win the war against bioterrorism without allowing biomedical science tofall victim in the end u ring battleagainst disease"

Not all scientists share such views. Raymond Zilinskas of the Monterey Institute of International Studies in California thinks some limits should be put on the publication of information on organisms such as smallpox. "Right now, there is no consensus," Zilinskas says. "Most scientists feel that anything that is basic research should not be restricted. But where do you cross the line?" He and others have proposed that prof8sional societies and editorial boards at scientific journals should exert more control. If they don't, the decision could be taken out of their hands. In addition to international measures, individual governments are also cracking down. The USA PATRIOTAct passed this year allows the federal government to stop some foreign nationals working in the US from getting access to certain pathogens and toxins. The US cduld extend this to cover access to genetic sequences as well. And Britain's Export Control Bill requires export licences for "intangibles" that might lead to weapons of mass destruction. Despite opposition from scientists, the bill is likely to be passed by parliament next week. Debora MacKenzie and Sylvia Pagin Westphal