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Quantum computing takes a step in from the cold
PHYSICISTS have found a relatively easy way to make a BoseEinstein condensate, the bizarre state of ultra-cold atoms that is helping to shed light on the quantum world.
When cooled to near absolute zero, atoms of certain elements merge into a single quantum state to form a Bose-Einstein condensate (BEC), a form of matter that possesses many of the properties of a quantum wave. Physicists usually have to make these condensates in complex "traps" comprising ultra-high-vacuum chambers surrounded by laser beams and large magnetic coils.
But now scientists report that they can make a BEC much more easily-on the surface of a chip. The advance comes from a team led by Jakob Reichel at the University of Munich and independently by Claus Zimmermann's team at the University of Tiibingen.
The researchers used wires deposited on a chip to create magnetic
fields. They then trapped a cloud of atoms in these fields, a fraction of a millimetre above the surface, and cooled them to form a BEC. The chip does the job 10 times as fast as a normal trap and does not require nearly such a high vacuum, Reichel says.
The chips could one day be used to pluck individual atoms from a BEC, says Jorg Schmiedmayer, a physicist at the University of Heidelberg. Their quantum states could then be entangled, so that a measurement of the state of one immediately affects the state of the other. The atoms could then act as "qubits"-Iuantum-computing bits that can represent 0, I or a mixture of both at the same time. Quantum computers are probably decades away, but Schmiedmayer says the new chips should make possible a wide variety of important experiments on the quantum world: 'You have a magnetic trap that's much, much simpler than any other experiment and that is much more versatile." More at: Nature vol 413, p 498)
Beating the ban Will embryonic stem cells made without embryos keep politicians happy?
COMPANIES in the US and Britain are working on ways to get human embryonic stem cells without destroying viable embryos, New Scientist has learned. Their goal is to create compatible tissue for transplant without falling foul of the legal and ethical objections to getting stem cells from normal or cloned embryos. But will their approach work-and does it really bypass the ethical issues? Stem cells that can grow into all sorts of specialised tissues are thought to have enormous potential in medicine. The most versatile and useful are embryonic stem cells (ESCs), which you get from the ball of cells that forms a few days after fertilisation. The trouble is that to get ESCs you have to destroy an embryo that could become a child. This has led to fierce opposition from some quarters, even though most ESCs come from spare IVF embryos that would otherwise have been discarded. Therapeutic cloning, where you create compatible tissue for transplant by taking ESCs from a cloned embryo, is even more controversial. This will become a crime in the US if the Senate approves an anticloning bill later this yeal Now, however, at least two groups in the US are working with embryos that never have the potential to become a person. But these clumps of cells sometimes develop far enough for researchers to extract the equivalent of embryonic stem cells. The research carried out at Advanced Cell Technology (ACT) in Massachusetts involves a process known as parthenogenesis. Normally, fertilised egg cells get one set of chromosomes from the mother and one from the father. But in parthenogenesis, the egg cell duplicates one set of maternal chromosomes and develops as if it had been fertilised (see above).
Some insects, lizards and even birds can reproduce asexually via parthenogenesis. In mammals, though, having two sets of maternal chromosomes causes such severe problems that parthenogenic embryos never develop into a normal fetus. Despite this, ACT has managed to harvest ESC-like cells from parthenogenic monkey embryos, according to a patent application seen by New Scientist. Like normal ESCS, the cells stayed undifferentiated for four months when kept with mouse cells called feeder cells. When the researchers took away the feeder cells, the parthenogenic cells spontaneously differentiated into what appeared to be more specialised types, such as skin cells and beating heart cells.
The patent is the first report of ESC-like cells being derived from primates via parthenogenesis. If it works in humans too, it means ACT may have found a way to get ESCs without destroying a potential person.
What's more, ACT thinks that this technique could also make therapeutic cloning unnecessary. For example, when creating transplant tissue for a woman, parthenogenic cells derived from one of her egg cells would have half of her genetic material, so any tissue created ftom such stem cells would be a close match. In the case of a man, the patent application suggests that one set of his chromosomes could be transferred to an egg that has had its nucleus removed. Then the egg could be induced to undergo parthenogenesis (this could also be done for a woman). ACT wouldn't comment on the research. "The patent speaks for itself," says chief executive Michael West. "We're not going to be releasing any more information until we publish the data in a peer-reviewed journal." But ACT is not alone. In Los Angeles, Jerry Hall of the Institute of Reproductive Medicine and Genetic Testing is also working on parthenogenesis. He wouldn't reveal any details but was confident it would work in humans. "Not only are we optimistic that parthenogenesis in humans would lead us to the same results, I would be surprised if they didn't." However, much remains to be done to show that the technique is feasible. It is unclear how versatile parthenogenic stem cells will be-or how safe. There is much concern that normal ESC implants could become cancerous, and some human ovarian cancers are formed by parthenogenesis.
But parthenogenesis isn't the only option. Another method that may yield ESC-like cells without using viable embryos is the transfer of cytoplasm from an egg cell into ordinary adult cells. The egg cytoplasm seems to turn the specialised adult cells back into an undifferentiated state.
Several companies are working on this ooplasmic transfer approach, but it is unclear how successful it has been. PPL Therapeutics in Scotland reported this year that cow skin cells injected with cytoplasm from cow eggs dedifferentiated into cells that looked like ESCs, and that the addition of certain growth factors turned them into beating heart cells. The company hopes to try it with human cells, says research director Alan Colman.
For companies that can find a way round the proposed ban on therapeutic cloning in the US, the rewards could be great. To satisfy the ethical concerns of critics, however, they will have to prove that none of their techniques could ever create a viable embryo.
"It's a positive thing if people are working on technologies that would not damage a human embryo,' says Douglas Johnson, legislative director of the National Right to Life Committee. "But we want to see that it's not a word game." Sylvia Pagin Westphat, Boston
Guardian angels Did dancing sprites protect the first stirrings of life on Earth?
RED GLOWS called "sprites" that dance above the clouds safeguard the Earth's atmosphere from huge electric fields generated by thunderstorms, researchers say. This may have been crucial to the survival of early life on Earth. For many years, pilots have reported seeing carrot-shaped red lights that linger for only a split second, several kilometres above thunderstorms. The sprites' vertical red fingers are tens of kilometres long and snake downwards from the Earth's ionosphere.
No one's sure what triggers spritespossibly meteors colliding with the atmospherej,ust above a thunderstorm. But scientists do know that they are made of highly ionised glowing gas. They grow downwards and branch into fractal trees as each patch ionises the patch of air next to it, then fizzle out as the air becomes denser and harder to ionise lower in the atmosphere.
Victor Pasko of Pennsylvania State University in University Park decided to work out how far down a sprite could possibly reach. Using measurements gathered by his sprite-chasing colleagues at Stanford University in California, he modelled their growth using the maths of fractals.
Pasko found that the most powerful sprites could reach down to within 20 kilometres of the Earth's surface, around the height of the tops of thunderclouds. This means they could join up with lightning to form an electrical short circuit from the ionosphere to the ground. 'A sprite is a highly conducting object," says Pasko.
The ionosphere is highly ionised by particles streaming out of the Sun, so sprites could act as a channel for earthing the charge. Pasko thinks this may help to keep the potential difference between the ionosphere and the ground constant-at around 300,000 volts-despite the immense charge thunderstorms generate in the lower atmosphere.
If the ionosphere weren't defused by sprites, Pasko says, its potential difference relative to the ground could be much higher. It's possible that this would alter the numbers and compositions of free radicals in the atmosphere.
"This could affect the chemistry of the atmosphere," Pasko concludes. On the early Earth, this altered chemistry might have hindered the prospects for the evolution of life.
Steve Cummer of Duke University in Durham, North Carolina, says we need to find out more about the frequency of sprites to discover if they safeguarded early life: "If they only happen once a year they don't matter, but ten times a day would." Future studies by sprite-chasers will hopefully resolve this question. Eugenie Samuel, Boston More at: Geophysical Research Letters ivol 28, p 38211
Antibodies on the cob VAST fields of maize could soon be churning out antibodies for preventing sexually transmitted diseases.
Researchers at Epicyte, a biotech company in San Diego, say their technology promises to make the mass production of therapeutic antibodies easier and cheaper (see "Hide and seek", right). At the moment, therapeutic arrtibodies are produced using hamster ovary cells-an expensive method that produces limited amounts. 13ut Epicyte's new "plantibody' technology allows the DNA that codes for antibodies to be introduced into crop plants such as maize. The antibodies are only produced in the maize kernels, making it easy t6 extract them using current maize-processing methods. Epicyte is already well on the way to producing an antibody to prevent herpes infection, says Andrew Hiatt, who helped develop the technology. The antibody, HXB, works by sticking to the virus and blocking its entry into cells, and has proved highly effective in animal tests.
Although condoms provide some protection against herpeps infection, they are not 100 per cent reliable. But HXB can provide protection in the vagina for 24 hours. Epicyte is also developing antibodies that block HIV transmission and the virus that causes genital warts.
The HX8 genes have already been transferred into maize, and Epicyte plans to start clinical trials of the antibody next year. Hiatt hopes plantibodies will be cheap enough for consumers to buy them over the counter. 'That's the ultimate goal,' he says. Claire Ainsworth
Plankton power It's a crazy idea, but ocean minibeasts really can produce the juice
AN UNDERWATER fuel cell that runs on seawater and subsea sediment really can generate small amounts of power indefinitely-just as its inventor hoped. "We've completed 220 days of continuous testing and the power level has been constant throughout," says Leonard Tender of the US Naval Research Laboratory. OSCAR, the Ocean Sediment Carbon Aerobic Reactor, taps into a natural voltage gradient created by a pair of chemical reactions happening on the seabed. Plankton release energy by using oxygen to break down organic matter in seawater or sediment. Deeper down in the sediment there is no oxygen, so the plankton rely on different chemicals. These two reactions set up a potential difference between the seawater and the sediment a few ceiitiiiietres beneath. Tender and colleague Clare Reamers placed one electrode in the sediment and another in the seawater. "Electrons then flow from one electrode to the other and that's how we harvest the power," he says, Until recently, this was just a theory (New Scientist, 5 February 2000, p 10).
The cell's power is more or less unlimited because the sediment is constantly being flushed and replaced as plankton die. Tests in the Atlantic Ocean off the coast of New Jersey have managed to generate 50 milliwatts of power from electrodes covering just 1 square metre. You can see its power output right now on a US Navy website at http.1118.104.22.1681shorestation.html
Tender believes OSCAR will be ideal for powering oceanographic sensors whose batteries now need to be replaced regularly.
Harold Bright, co-inventor of the idea, now manages the programme at the Office of Naval Research. He says that he is funding a number of programmes to tap energy from the sea. "But this is by far the most environmentally benign." Justin Mullins
Secrets of the plague
THE bacterium blamed for the Black Death that swept through medieval Europe, now feared as a potential biological weapon, has had its genes sequenced. Julian Parkhill of the Sanger Centre in
Cambridge says the results show Yersinia pestis is a relative newcomer. It evolved from a bug that causes mild gut upset, by acquiring bacterial and viral genes that allow it to live in flea guts and human blood (Nature, vol 413, p 523).
NEXT time you complain that the weather is dreadful, take comfort from this fact. You're right. but it can't get any worse. Not on a global scale anyhow. According to a controversial new theory, Earth's climate is finely tuned to be as violent as possible. It whips up every last raindrop, wave and gust of wind that it can.
No one knows why the climate should be like this. It's just an observation. and for that reason some meteorologists have dismissed it as a coincidence. But recent evidence suggests that it also holds true for other pianets. If so, it could turn out to be a universal principle that helps us to understand the climate on Earth and other planets now, in the past. and into the future.
Earth's climate is a hugely complex system, with multiple interacting elements. Because they defy simple expression, meteorologists rely on a "bottom up" approach to modelling the climate. They divide the world into boxes, model the forces on the atmosphere and oceans in each box, then guess the overall effect. The approach works- providing you toss in hundreds of "fudge factors" to make the models fit with what we actually see happening. Try to apply the model to a different time in Earth's history, or a different planet. and it falls to pieces.
But beneath this complexity is a single force: the Sun.
Solar energy warms the Earth, and it does so disproportionately, heating the tropics much more than the poles. This creates a temperature gradient which drives the climate. The net effect is to transport heat from low to high latitudes.
Heat flow from the tropics to the poles is an expression , of one of the fundamental rules of nature. This is the second law of thermodynamics, which states that heat always flows spontaneously from warm to cold regions- never the opposite. Put a hot brick next to a cold one in an insulated box and wait, and you get two cool bricks.
One of the consequences of this process is the ability to do mechanical work. You could rig up a contraption to convert the heat flow from brick to brick into electricity. The same is true of Earth's climate. Our atmosphere and oceans are hard at work every moment. They're busy lifting water to drop as rain or snow, whipping up waves and blowing sand into dunes. Wind turbines, sailing boats and hydroelectric power stations all depend on this hard work.
So far so good. But the second law of thermodynamics doesn't tell us how quickly the heat is transported. In other words, it doesn't say how, hard the climate works. It would be nice to know, if only to give us an alternative, "top down" approach to meteorological modelling. We can start by asking how quickly heat flows from the tropics to the poles. One thing we can say without too much difficulty is that it is neither as fast nor as slow as possible. If the heat flow was very vigorous, the temperatures of the zones would be almost the same, since the flows would balance out the different amounts of sunlight. That's obviously not what happens. On the other hand, if heat flow was very slow, the temperature difference would be enormous. Northern Europe, New Zealand and the southern part of South America would be locked in a perpetual ice age.
The reality is somewhere in between. But where? The new idea is that heat flow adjusts itself so that the climate does the maximum amount of mechanical work possible. Work is determined by two factors. First of all there's heat flow. With very weak flows, there is little energy to convert into work. As the heat flow increases, the amount of work also increases-but then it starts to tail off. That's because of the second factor, efficiency. The efficiency with which heat flow is converted into work is proportional to the temperature gradient. Very rapid heat flow flattens out the temperature gradient, so little work gets done.
Like most theories, the idea has been put forward before in other forms. Most scientists credit Garth Partridge, an Australian climatologist now at the University of Tasmania, as the originator of the idea. In the mid-1970s he experimented with a model of the Earth divided into latitude zones with different amounts of sunlight and cloud cover. One of the free parameters of the model was the heat flow between the zones. Paltridge found that if he set this so it maximised the production of the thermodynamic quantity called entropy, the results modelled Earth's climate well. Entropy production is a measure of the generation of disorder, and it is closely related to a system's capacity for mechanical work. Both quantities peak at about the same heat flow. Several other researchers have since confirmed Paltridge's result. Yet most orthodox meteorologists dismiss it as an uninteresting fluke. There's no reason why the climate should maximise entropy, or the work done.
That argument might hold if Earth was the only place the principle works. But new
'THE TEMPERATURE DIFFERENCE ON TITAN IS STRANGE. CONVENTIONAL MODELS SUGGEST IT SHOULD BE MUCH LESS'
research suggests it's not. With Jonathan Lunine and Paul Withers of the University of Arizona and Chris McKay at NASA Ames Research Center in Moffett Field, California, I have shown that Saturn's giant moon Titan seems to follow the principle too. In a way I found that out by accident. In December 1999 1 was supposed to be working on new data from the Mars Polar Lander, but the craft was lost without trace, so I was looking around for another problem. At the time I was also starting to think about what the Cassini mission to Saturn might tell us about Titan. I decided to take a look at Titan's climate. In 1980, the Voyager I spacecraft made some curious measurements of Titan's surface temperature. These showed that the moon was intensely cold-a frigid -179 'C at the equator and 4 'C colder at the poles. No surprise, given how far Titan is from the Sun. But the temperature gradient was strange. Conventional meteorological models suggest the difference should be much less-about one-hundredth of a degree. According to the old models, heat transport depends on three factors: the size of the planet or moon, its rate of rotation, and the thickness of its atmosphere. Judged by all these criteria, Titan should transport heat very quickly. It's small. It has a nitrogen atmosphere thicker than the Earth's. And it rotates very slowly, so the atmosphere isn't whipped into swirling latitudinal weather systems that interfere with heat transfer. To find out more, I wrote a computer model to simulate Titan's climate. When I programmed in rapid heat transfer from the equator to the poles the model refused to settle down. But when I slowed the heat transfer right down the model stabilised. What's more, I could recreate the 4 'C temperature difference seen by Voyager. Then came a crucial discovery. I looked at the entropy production and this peaked just where the temperature difference was 4 'C. In other words, Titan, like the Earth, squeezes as much work out of its climate as it can.
It isn't clear why heat transport on Titan should be so low. Perhaps the atmosphere has strong east-west winds that suppress
heat transport towards the poles. Measurements from the Cassini spacecraft, due to arrive in 2004, should tell us more. But the result raised a wider question. If the principle worked for Earth and Titan, would it work anywhere else? Would it turn out to be a universal feature of planetary climates? Mars was an obvious place to try next. I looked at conventional models of the Martian climate and they all assumed that heat transfer would be very weak, since the atmosphere is so thin. They also did a good job of modelling the planet's climate, which was a problem. For my theory to hold, the atmosphere would have to transport a hundred times more heat. The disagreement looked insoluble. But then I noticed something. None of the studies accounted for the "latent heat" associated with Mars's polar ice caps, huge expanses of frozen carbon dioxide that wax and wane with the seasons. They assumed that the ice was always at -103 'C, the freezing point of C02. Even if the models predicted that the polar temperature would drop below the freezing point, the researchers just fudged it and held it steady. Freezing and thawing, though, involves the exchange of heat. Take a glass of water and cool it at a constant rate. When it hits
0 'C it'll start to freeze, but stay at a steady temperature until all the water has turned to ice. That's because the process of freezing unlocks energy stored in liquid water. The same happens when Mars's ice caps solidify. If you calculate how much heat is exchanged, it is more or less what is predicted by the new theory. In other words, the principle of maximum work-or maximum entropy production, to give it its formal name-holds true for Mars too.
Too good to be true
There are places where the theory won't work. On Mercury the atmosphere is so thin that it can scarcely transport any heat at all. To reach the point of maximum entropy production the wind speed would have to exceed its physical limit. But constrained climates like this are easy to model anyway. If the principle is universal on planets with thicker atmospheres, it could prove very useful. It means we would know something about almost any planetary climate, such as that of early Mars when life may have existed there, or of potentially inhabited planets around other stars. In a way it seems to be too good to be true, and the idea certainly has its opponents. Scientists whose careers are built on
large, sophisticated computer models are reluctant to believe that such a simple theory can be true. Many dismiss the Titan example on the grounds that Voyager's temperature measurements were inaccurate.
They may have a point. The Voyager measurements were made with an infrared detector which could have underestimated the surface temperature at the poles. The problem is that part of the infrared signal came from high up in the moon's stratosphere, where polar temperatures are known to be cooler. True, all the workers who analysed the data included corrections for the stratospheric signal, and believe that the temperature gradient is about right. But we can't be absolutely sure until Cassini measures the gradient with microwaves, which are unaffected by stratospheric interference. There's another serious objection to overcome. No one knows why the theory works. At the moment I'm not sure either. But I'm working on it. In the meantime we might as well play around and see what the theory tells us. One obvious application is the study of the distant past, in particular the idea of a "Snowball Earth" 700 million years ago when the entire globe was covered in ice. Models have shown that once the Earth is sufficiently covered by ice, it will reflect so much sunlight it triggers "runaway glaciation", freezing the whole planet. How could life have survived? Existing climate models cannot agree. We do not know the layout of the continents back then, nor other details that are needed for accurate simulations.
Enter the theory of maximum entropy production. if it is universal, it must have been doing its stuff 700 million years ago. And one of the consequences is that the colder the Earth is as a whole, the weaker the heat transport from the tropics to the poles. Weaker heat transport means there might have been an unfrozen refuge, which may be how life survived. And planetary climatology may not be the only application. There are hints that heat flows inside planets also stick to the principle. The theory may also be useful for studying other dynamic systems such as stars or the protoplanetary discs in which planets form. And if we ever want to design a ship for exploring another world, we'd at least know to build it well. Because it sounds like it's going to be a rough ride out there.
Ralph Lorenz is a planetary scientist at the University of Arizona in Tucson
Further reading: 'Titan, Mars and Earth: entropy production by latitudinal heat transport" by Ralph Lorenz and others, Geophysical Research Letters, vol 28, p 415 (2001)