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Dark matter claims disputed

Physicists remain sceptical about claims made last week that dark matter has been detected at a laboratory in Italy, insisting that the data need to be backed up by further experiments. The Italian team, which made the claims at a meeting in Venice, says it has strong evidence that dark matter takes the form of weakly interacting particles.

Dark matter was originally put forward to explain why galaxies appear to contain just a fraction of the mass they need to rotate at their observed speeds. Nobody has yet managed to see it directly, although astrophysical measurements imply that it should make up around 95% of the total matter in the universe, mostly in “haloes” around the centre of galaxies.

No-one disputes that the DAMA team observes an annual modulation, but the community needs to be persuaded that dark matter has anything to do with it Henrique Araujo, ZEPLIN-III experiment

In 2000 physicists working at the DAMA experiment at the Gran Sasso laboratory in Italy said they had discovered what many believe is the most promising candidate for dark matter: a weakly interacting massive particle, or “WIMP”. Such a particle would not interact with light, although it would interact with normal matter through the weak nuclear force. In fact, it might interact enough to occasionally collide with a nucleus inside some 100 kg of sodium-iodide, which the DAMA team kept in an array of detectors buried more than a kilometre underground.

The DAMA team found that flashes of light produced by recoiling nuclei inside their detectors tended to be more frequent in July than in December. This seasonal modulation fitted with the prediction that the Earth should switch its movement to be with or against the prevailing “wind” of WIMPs as it orbits the Sun, which is itself moving through our galaxy’s dark-matter halo. After analysing their results, they claimed this modulation pointed to a WIMP in the mass range 44 to 62 GeV — at the lighter end of the scale of those predicted by theory — and at a confidence level of 6.3 σ (or 99.9999998% certainty).

At the time many physicists suspected that the seasonal modulation could be the result of something more mundane, such as a change in ambient temperature. So, over the past eight years, the team have upgraded the amount of sodium iodide in their detector array to 250 kg. Last week they claimed that this upgrade, branded as “DAMA/LIBRA”, has enabled them to confirm the existence of the WIMP with an improved confidence of 8.2 σ.

Even with this level of certainty in the modulation, however, many remain unconvinced. “The DAMA announcement has had a mixed reception,” says Henrique Araujo of ZEPLIN-III, another WIMP experiment. “No-one disputes that they observe an annual modulation, but the community needs to be persuaded that dark matter has anything to do with it.”

DAMA experimental data
High confidence

Agrees with predictions

The DAMA team insist that their signature matches the dark-matter signature that was predicted in the mid 1980s by theorist Katherine Freese. Rita Bernabei, a spokesperson for the DAMA team, says: “The former DAMA experiment over seven annual cycles and the new DAMA/LIBRA experiment over four annual cycles show, with high confidence level, agreement with all the features expected for the presence of dark-matter particles in the galactic halo.”

Other physicists point out other potential causes for the modulation. Any neutral particle can scatter off a nucleus to make it recoil, so it is possible that the modulation signature is caused by neutrons from cosmic rays. However, several independent experiments have failed to find the same rate of bombardments by cosmic-ray neutrons in the DAMA experiment’s energy range (2–6 keV), which goes against this interpretation.

Another possibility is that the DAMA team are actually observing flashes of light produced by particles scattering off atomic electrons, rather than nuclei. “ZEPLIN-III might be able to detect an electron-recoil modulation at the rates and energies reported by DAMA, but for that we need a longer run,” says Araujo.

A third possibility is that the recoils are being produced by a hypothetical particle known as an axion. These light particles, which were conjured to resolve the “strong-CP” problem in the theory of the strong force in the late 1970s, have been suggested as candidates for dark matter before, but have so far evaded all attempts to be found. An axion fit to explain the modulation in the DAMA experiment would need a mass around 3 keV.

The DAMA experiments show agreement with all the features expected for the presence of dark-matter particles in the galactic halo Rita Bernabei, DAMA experiment

New physics?

Even if none of these alternative interactions is producing the seasonal modulation seen at the DAMA experiment, there is still a question as to why other direct WIMP searches — such as ZEPLIN-III in the UK, CDMS-II and COUPP in the US, and XENON10 in Italy — have not recorded similar results. In the DAMA experiment’s energy range, these experiments have found no evidence for coherent scattering of nuclei, which is the WIMP-scattering process favoured by theory. “For the DAMA/LIBRA result be a true dark-matter signal requires a new interaction process, or other new physics,” says Alexander Murphy, also at the ZEPLIN-III experiment. “This would be extraordinary! The particle-physics theorists have predicted many scenarios, but not this.”

“If someone has detected dark matter, it’s clearly a Nobel-prize-winning experiment,” says Richard Gaitskell, who works on several direct WIMP searches. Gaitskell also notes that other interactions could be producing the signature seen at DAMA, but feels that the DAMA team has not yet been rigorous enough in calibrating their detector system. “What would be best would be if they could switch off their beam, like you can in a particle detector,” he explains. If such a thing were possible they would be able to see if the seasonal modulation is caused by anything else. “Unfortunately, you can’t switch off WIMPs,” he adds.

Bernabei told physicsworld.com that she and the rest of the DAMA team are confident they have performed all the necessary calibrations. But Gaitskell says it would have been better if the DAMA team performed a running calibration while they were taking experimental data by creating their own light flashes and looking for signs of seasonal modulation. “If I could have one thing for Christmas it would be to see clear calibration data showing the necessary stability,” he says.

• Preprints including the DAMA/LIBRA results can be found at arXiv:0804.2738 and arXiv:0804.2741.

Graphene transistors cut from ribbons into dots

Researchers at Manchester University in the UK have made the first transistors from graphene quantum dots, suggesting that graphene is a promising replacement for silicon in the next-generation of electronic devices.

Graphene is a two-dimensional sheet of carbon just one atom thick, and is usually made by cleaving small crystals of graphite. At a molecular level it looks like a sheet of chicken wire — a continuous spread of joined-up benzene rings.

Because of its unusual physical properties, graphene is often touted to replace silicon as the electronic material of choice. These include the fact that electrons in the material behave like relativistic particles, which have no rest mass and can therefore travel at speeds of around 106 m/s. “The good thing is that these properties remain when we scale graphene devices down to a few benzene rings, which is what one needs for top-down molecular electronics,” team member Kostya Novoselov told physicsworld.com.

Until now, researchers have only been able to make transistors from ribbons of graphene. But such a long shape does not maximize the conductivity, which is why Novoselov and colleagues cut the ribbons back into sizes that can quantum confine electrons. They do this using a combination of electron beam lithography and reactive plasma etching to carve small islands out of large graphene sheets (Science 320 356). “We have demonstrated a proof of concept — that it is possible to create a transistor based on graphene quantum dots by standard technological procedures,” says Novoselov. “Furthermore, the device will be able to operate at room temperature.”

Andre Geim, another member of the team, says they can now make reproducible transistors with features as small as 10 nm, which should reduce to 1 nm in the future. “It is molecular electronics using the top-down approach. No other material allows this approach for making structures smaller than 100 nm, which is the dimension needed for operating single-electron transistors at room temperature.”

Jie Chen of the University of Alberta in Canada, whose team also works on making electronic devices from graphene, is impressed at how quickly Novoselov, Geim and colleagues are making developments with graphene. “They are the global leaders in this field,” he says.

Quasar tests general relativity to the limit

Astronomers have obtained the most compelling evidence yet that massive objects dramatically warp space–time, as predicted by Einstein’s general theory of relativity. Although the geometric nature of gravity was first demonstrated in 1919, when Arthur Eddington famously detected the subtle warping effect of the Sun on the light from distant stars, the new results provide the first test of Einstein’s theory in much stronger gravitational fields.

In fact, team leader Mauri Valtonen of Tuorla Observatory in Finland claims the work provides the first hard evidence for black holes, which are so massive that space–time is predicted to completely curve in on itself: “People refer to the concept of black holes all the time, but strictly speaking one first has to prove that general relativity holds in strong gravitational fields before we can be sure that black holes exist,” he told physicsworld.com.

Binary system

The new test of general relativity concerns a distant galactic core or quasar called OJ287, which is known to emit a pair of bright optical bursts every 12 years or so. In 1988, Valtonen and others suggested that this emission is powered by a primary black hole 18 billion times more massive than the Sun, around which orbits a second black hole some 200 times lighter. In such a binary system, the lighter object passes through matter in the accretion disk of the primary black hole twice per orbit, releasing a burst of energy each time it does so.

By modelling such a system, researchers could then put general relativity to the test by predicting when the next burst should occur. At the time, the next major bursts (which were due in the mid 1990s) could only be predicted with an accuracy of a few weeks, which was too vague to test general relativistic effects. But early last year, based on refined models and years spent monitoring OJ287, Valtonen and others were able to predict the date on which the next bright pulse should appear: 13 September 2007, give or take a day or two.

To have any hope of detecting the pulse, more than 25 astronomers from 10 countries had to work together. This is because in September OJ287 rises in the east just before sunrise, and is therefore only visible at any one location on Earth for about 30 minutes before the sky becomes too bright. By starting observations in Japan, followed by China, Europe and ending in the Canary Islands, observers were able to follow the sunrise westward around the globe and maximise observing time. In total, about 100 measurements were made between 4 September and 20 October, some of which by amateur astronomers.

Successful campaign

The quasar pulse occurred right on schedule, strongly suggesting that OJ287 is a binary black hole system (Nature 452 851). In addition to verifying the enormous mass of the primary black hole, the result shows that the orbit of the secondary black hole precesses at a rate of 39 degrees per period. For comparison, the distorting effect of the Sun on the local space–time causes Mercury’s orbit to precess by little more than 0.1 degrees per century.

Furthermore, the work suggests that the binary system is losing energy by emitting gravitational waves — a key prediction of Einstein’s theory that is yet to be verified directly. When this emission is not included in the model, the quasar outburst is predicted to occur 20 days later, providing indirect support for gravitational waves. Indeed, according to Valtonen, the rate of emission observed in OJ287 make it the brightest known source of gravitational waves in the universe, and therefore a good target for the Laser Interformeter Space Antenna (LISA) — especially in the period 2016–2019 when the next big outbursts are due.

US renews call for science debate

Scientists in the US have sent out a fresh call for the three main US presidential candidates to take part in a public debate on science.

Organizers of ScienceDebate 2008, a petition for a science debate that has so far been signed by some 37,000 people, had invited the candidates to a debate on Friday 18 April in Philadelphia, but were forced to cancel last week because of a poor response. Barack Obama, one of the two Democratic candidates, declined to attend, while both Hillary Clinton, the other Democratic candidate, and John McCain, the stronger Republican candidate, gave no reply.

The candidates have now been sent new invitations for a debate in Oregon for three possible dates in early May.

One of the organizers, Lawrence Krauss, a physicist from Case Western Reserve University in Cleveland, Ohio, told physicsworld.com he is disappointed that the candidates did not react, although he is not particularly surprised. “When I first publicly called for a debate I never imagined it would be a realistic possibility, and the avalanche of support from all sectors of the country has amazed me,” he says. “Of course, the candidates have to be convinced it is in their best interest, and also that they will not come off as looking ignorant. It is an uphill battle, but a battle that I think is worth continuing to fight.”

Gathering momentum

The idea of ScienceDebate 2008 was formed towards the end of last year by a small group of six people including Krauss and screenwriter Matthew Chapman. Although initially there was little attention from the media, awareness grew quickly via internet blogs, forums and a page on the social networking site Facebook.

Among those to have signed the petition are 80 university presidents, more than 100 representatives from institutions such as the American Institute of Physics, and over 20 Nobel laureates, including the likes of David Gross, Steve Chu and John Mather.

Backers of the debate proposal want the presidential candidates to discuss a number of key issues, including:

  • Innaccurate media coverage of science
  • Poor science eductation
  • Public illiteracy
  • Either flat funding or funding cutbacks for research
  • Insufficient public policy response to climate change and other environmental issues
  • Government suppression of science information

“Such a debate will do a tremendous amount to raise public awareness of the vital connection between science, technology and public policy,” says Krauss. “As I have often stated, science and technology form a fundamental component of almost every major issue the next president will have to deal with, from the environment, to national defence, from energy to economic competitiveness.

“Many people in the public simply do not realize this deep connection, nor do they have regular access to important information that can help them make sensible decisions in the voting booth. As far as the candidates are concerned, I would like to see them forced to focus, even for a day or two, on precisely what the key issues are, and what their positions should be on them, in advance of being thrust into the Oval office and playing catch-up.”

Hidden policies

So far the candidates have only mentioned science in passing. Clinton has an “innovation agenda”, which includes promises to increase basic research funding by 50%, triple the number of National Science Foundation fellowships, encourage more women into science and “restore integrity to science policy”. Obama plans to double basic research funding, increase the number of scientists from ethnic minorities in science and listen more carefully to the opinions of scientists when tackling political science issues such as climate change. McCain has devoted less time to science issues, though he has also vowed to tackle climate change.

“I watched most of the debates and I was surprised to see how little science and technology were being mentioned,” says Chapman. “It seemed to me that although polls suggest the American public is not interested in science, that is probably only because it does not make the connection between the benefits it has received from science, and the solutions science could provide in the future, and science itself. People are concerned about the environment, for example, but don’t seem to understand that science is what brings us the information that we either use or abuse.”

Chapman refuses to consider that the candidates might not respond to the new invites, which are for May 2, May 9 or May 16. “It has been suggested that the candidates may be worried that they will not be scientifically equipped to discuss these issues. They should not have this concern — this is not a pop quiz. What we are looking for is a debate in which they explain some of their proposed approaches to these important issues. Most importantly, we want them to demonstrate, by attending our debate, that they are aware of the incredible importance of science and technology in modern life.”

John Wheeler: 1911-2008

John Wheeler, the legendary physicist who helped to develop the theory of nuclear fission and coined the term “black hole”, died yesterday at the age of 96. Wheeler, who spent most of his academic career at Princeton University, was supervisor to a roll call of great names in physics, including Richard Feynman, cosmologist Kip Thorne and “many-worlds” theorist Hugh Everett.

Born on 9 July 1911 in Jacksonville, Florida, Wheeler went to Johns Hopkins University in Baltimore, Maryland, in 1927, aged just 16. He originally enrolled to study engineering but switched to physics after a being inspired by his physics lecturer John C Hubbard. Wheeler eventually combined his undergraduate and doctoral studies, graduating with a PhD in 1933 on the properties of the helium atom.

The following year saw Wheeler travel to Copenhagen, where he did a post-doc with Niels Bohr, with whom he was to have many discussions concerning the foundations of quantum mechanics. In his application to the US National Research Council for a fellowship to go to Denmark, Wheeler said Bohr was “the best man under whom to investigate the nucleus”. The pair later collaborated on the theory of nuclear fission when Bohr arrived in the US in 1941 as a refugee from Denmark, including the “liquid-drop” model.

In 1943 Wheeler joined the Manhattan atomic-bomb project at Los Alamos in New Mexico, taking a break from his academic career at Princeton University, which had given him a professorship in 1938. Wheeler returned to Princeton after the Second World War, remaining there until his retirement in 1976.

Wheeler coined the name “black hole” in 1967 to describe what is created when a star of sufficient mass collapses under its own weight. The gravitational pull of the object then becomes so strong that no light can escape. Wheeler originally did not like the idea because it would lead to a singularity, where space is infinitely curved and matter is infinitely dense. He also was a pioneer of quantum gravity, developing the “Wheeler-deWitt” equation with Bryce deWitt.

After retiring from Princeton in 1976, which he established as one of the top centres for the study of general relativity, Wheeler moved to the University of Texas. In 1998 he published an autobiography “Geons, Black Holes and Quantum Foam: A Life in Physics”, which he co-authored with Kenneth Ford. Wheeler died at home in Hightstown, New Jersey, on Sunday 13 April.

Astronomers pinpoint source of slow solar wind

An international team of astronomers claims to have pinpointed the source of the “slow solar wind” — a stream of charged particles that is expelled by the Sun. The team believes the particles come from areas on the surface of the star where strong magnetic fields collide.

The discovery was made using the Japanese Space Agency’s Hinode solar observatory and could help scientists gain a better understanding of the solar wind, which can sometimes disrupt satellite communications.

The solar wind consists of mostly electrons and protons that are ejected from the Sun’s upper atmosphere in all directions. It has two components: a “fast” wind moving at about 800 km/s and a “slow” wind at about 400 km/s.

When these particles interact with Earth’s magnetic field they create the beautiful auroras seen at higher latitudes in the northern and southern hemispheres. However, the wind can also interfere with electronic systems on orbiting satellites and even overload electrical power grids on Earth.

Matter of debate

While the fast wind has been studied extensively and is well understood, the origin of the slow wind is a matter of some debate. However, Earth is often immersed in the slow solar wind, so understanding its origins could help mitigate its interference with communications and other technologies.

The Sun’s surface (photosphere) is a plasma of charged particles that is shaped by magnetic fields, which create “active regions” — areas of enhanced brightness and magnetic field strength that are often associated with sunspots. Previously, it had been difficult to determine whether significant amounts of plasma flowed from these regions into the corona — the region just beyond the surface of the sun — a process that was thought to be related to the slow solar wind.

Now Peter Young of the Rutherford Appleton Laboratory in the UK and other members of the Hinode team have used the observatory to study active regions and understand how the plasma escapes the surface to become the slow solar wind.

The team used Hinode’s Extreme Ultraviolet Imaging Spectrometer (EIS) to measure the speed at which material flows out from the Sun. They discovered that plasma is ejected at high speeds at the edges of these active regions, where strong magnetic fields are believed to collide.

“It is fantastic to finally be able to pinpoint the source of the solar wind – it has been debated for many years and now we have the final piece of the jigsaw,” says Louise Harra, another team member at the UCL-Mullard Space Science Laboratory in London. “All the planets in the solar system sit in the solar wind, so understanding it is important for us.”

Fundamental process

Young agrees. “We have identified a fundamental process operating on the Sun that is responsible for accelerating material out into space,” he adds. “Winds are seen in a wide range of astronomical bodies, including many different types of stars, black holes and galaxies, and our result will influence studies of all these other objects.”

The team is now working to confirm their findings and understand the mysterious phenomenon of solar flares — massive explosions on the Sun’s surface that send huge amounts of high energy radiation and particles into space and which are a major threat to space exploration and satellite technology.

Prototype gravitational-wave detector uses squeezed light

Physicists in the US and Australia have used the quantum nature of light to make an important step towards improving the sensitivity of kilometre-sized interferometers used to search for gravitational waves. By using light in a “squeezed state” they achieved a 44% improvement in sensitivity of a prototype gravitational-wave detector.

This figure could reach 300% in a full-scale detector and the team believes that squeezed-light sources could be tested in working detectors such as LIGO within 1-3 years. More sensitive detectors would be able to search larger volumes of the universe for sources of gravitational waves, making their detection more likely.

Gravitational waves are ripples in the fabric of space-time that are produced when massive bodies accelerate through space. Predicted by Einstein’s general theory of relativity, the waves are very weak — even for events as extreme as supernova explosions or collisions between neutron stars and black holes — and have therefore not been detected directly.

Physicists have built a number of very large interferometers to try to look for such waves, including two LIGO facilities in the US, the Virgo detector in Italy, the GEO 600 facility in Germany, and the TAMA project in Japan. These devices split a laser beam into two components at 90° to one another, sending each component down a separate “arm” that can be several kilometers long. The beams bounce off “test masses” with highly reflective mirror surfaces at the end of each arm. The beams are then recombined on a beamsplitter before being detected by a photodetector.

Tiny changes

A gravitational wave passing through the device stretches one of the arms and compresses the other, therefore changing the interference pattern at the photodetector. However, the change to the length of the arms will be tiny — about 10-18 m — and the associated change in the interference pattern will therefore also be extremely small. Indeed, existing detectors have so far not detected any gravitational waves.

The next generation of detectors should begin operation in the early 2010s and they will be so sensitive that their performance will be limited by noise that arises from the quantum nature of the laser light. Quantum theory requires the product of the uncertainties in two quantities that are associated with the phase and amplitude of the light to be greater than a minimum value. In an interferometer that measures the phase of light, this means that there will always be some “quantum” noise in the detector.

Fortunately, the uncertainty principle does not prevent physicists from minimizing this noise by “squeezing” the quantum state of the light such that the uncertainty in amplitude becomes large, while the uncertainty in phase becomes small. Such squeezed states were first demonstrated experimentally in 1985, a few short years after they were proposed for use in gravitational wave detectors. Now a team of LIGO physicists led by Nergis Mavalvala and Keisuke Goda of the Massachusetts Institute of Technology have proven experimentally that a squeezed source can be used to improve the sensitivity of a prototype detector (Nature Physics doi:10.1038/nphys920) .

Squeezing light

The squeezed state was created by passing infrared laser light through two devices that contain special “non-linear” optical materials. In such materials the index of refraction (which affects the phase of the light) changes according to the amplitude of the light.

The source is based on a neodymium-doped yttrium aluminium garnet (Nd:YAG) laser, which fires a continuous beam of 1064-nm wavelength light at a “second harmonic generator”. This creates a beam of light with exactly one-half the wavelength (532 nm) of the laser beam. This shorter-wavelength light is then passed through an “optical parametric oscillator” (OPO), which converts individual 532-nm photons into two 1064-nm photons. The photons emerging from the OPO are in squeezed states such that the quantum noise in the phase of the light is reduced by 9.3 dB compared with the input laser light.

These photons are then introduced into a prototype gravitational wave detector with 40-m long arms at Caltech near Los Angeles. Ideally, the team should have seen about a 300% improvement in the sensitivity of the interferometer, but instead they only managed to get 44%. According to Mavalvala, this shortcoming was expected because of optical losses, which were larger in the prototype than in a working gravitational wave detector.

According to Mavalvala, a 200% improvement in sensitivity would allow LIGO to detect gravitational waves from within a volume of the universe eight times larger than currently possible — boosting the probability that such waves would be detected. Even a modest 44% increase in sensitivity would increase this volume by about a factor of three. Mavalvala told physicsworld.com that their squeezed source could be tested on gravitational wave detectors in the next 1-3 years.

‘Very important step’

James Hough at the University of Glasgow, UK, described the work of the LIGO team as a “very important step towards the implementation of squeezed sources in gravitational wave detectors”. Hough is principal investigator for the UK on the GEO 600 gravitational wave detector, in Hanover, Germany.

There are two other research groups also working on the use of squeezed light in gravitational wave detectors. One group at the University of Hanover is planning to implement a squeezed source on the GEO 600 later this year, according to Hough. The other group is at the Australian National University in Canberra and was the first to show that light at 1064 nm could be squeezed. This was an important breakthrough because all gravitational wave detectors use 1064-nm light from Nd:YAG lasers because they are extremely stable.

‘Racetrack’ memory demonstrated

Physicists in the US have demonstrated for the first time that data can accessed from forest-like arrays of 3D nanowires or “racetracks”. The demonstration indicates that so-called racetrack memory, which should be faster and cheaper than other forms of information storage, is on its way to becoming a commercial reality.

Stuart Parkin and colleagues at IBM’s Almaden research centre in San Jose, US first came up with the concept of racetrack memory back in 2004. The idea is that arrays of U-shaped nanowire racetracks are planted on a bed of silicon wafer. Along the length of each racetrack are domains that are magnetized in one of two directions, with each domain boundary or “wall” acting as a single bit — either a 1 or 0 — of information. By sending a current of spin-polarized electrons into one end of a racetrack, these domain walls can be shifted to and fro.

In theory, both reading and writing information should be possible at the base — in other words, the curve of the “U” — on the silicon wafer. For example, a read head could sense the configuration of a domain wall by measuring how the wall’s resistance changes in the magnetic field (a property known as magnetoresistance). A write head could consist of a perpendicular nanowire, which would switch the configuration of the domain walls as its own domains are shifted.

Three bits

Putting this theory into practice has been tricky, but Parkin’s team have now shown that it is possible, at least when the racetracks are positioned flat on the silicon wafer rather than upright. They have been able to write and read three bits in a matter of nanoseconds (Science 320 209).

“It looks like very good progress,” says Yongbing Xu at the Univeristy of York, UK. “In principle there are no moving parts, and that’s a great advantage.” Xu is also impressed that the reading and writing can be perfomed so quickly — at a conference last year the IBM team suggested it would take microseconds. Still, he notes that manipulating many well defined domain walls will be challenging, which might mean a commercial prototype is three to five years away.

When racetrack memory does hit the shelves, it seems likely that it will quickly supplant existing memory types. Unlike random-access memory (RAM), which is based on 2D arrays of transistors, it should be cheap to mass produce. And unlike hard disk storage, which contains magnetic bits spread over a rotating metal disk, it should be fast.

“It’s certainly a clever idea and has been executed very elegantly,” says Mark Blamire at the University of Cambridge, UK. “However, the substantial gap between this and any sort of commercialization is whether an appropriate technology can be developed to turn what is a linear device occupying a large amount of wafer area into a vertical structure, which occupies much less. The paper doesn’t address that question but it would definitely be extremely challenging.”

Nanotube ‘monorail’ moves cargo

Researchers in Europe have built a “monorail” from carbon nanotubes that can transport a fleck of metal over a distance of about 800  nm. The metal cargo sits on a 5-nm long nanotube “sleeve” surrounding a much longer nanotube “rail” that is stretched across a trench in a silicon chip. Surprisingly, the team believes the sleeve is driven by lattice vibrations called phonons — rather than electrical interactions, which they originally thought would propel the device.

Carbon nanotubes are sheets of carbon one atom thick that are rolled up into tubes that are only several nanometres in diameter. Adrian Bachtold and colleagues at the Autonomous University of Barcelona along with collaborators at the University of Vienna and the Swiss Federal Institute of Technology in Lausanne built their device using a multiwalled nanotube, which comprises several concentric nanotubes (Science DOI: 10.1126/science.1155559) .

The team began by attaching a 1500-nm long multiwalled tube across the trench with metal electrodes. They then used an electrical-breakdown technique to remove several outer layers from most of the nanotube, leaving a short sleeve that could rotate freely and move to and fro along the inner rail.

Hotter in the middle

The team operate the motor by passing an electrical current through the rail, which causes it to heat up. However, the region of the rail in the middle of the trench becomes much hotter than the ends — because the electrodes act as heat sinks. If the sleeve and its cargo — a tiny piece of gold — are placed in the middle of the trench, they will move to one side of the trench at speeds of up to 1  µm/s.

This is a beautiful experimental resultRamin Golestanian, University of Sheffield

Bachtold told physicsworld.com that the team had originally hoped that they could apply a voltage between the electrodes to encourage atomic interactions between the rail and sleeve, causing the sleeve to move in a helical manner in one direction — and then in the other direction when they reversed the voltage. Such motion was expected, according to Bachtold, because the atoms on the inner and outer nanotubes would both be arranged in slightly different spiral configurations.

Instead, the team found that the sleeve always moved away from the centre of the trench. According to Bachtold, the first clue that heat was driving the motion was that the gold cargo particle changed shape by partially melting. The team confirmed the role of heating by doing computer simulations of the system.

Phonon propulsion

Heat is transported through carbon nanotubes in the form of quantized lattice vibrations called phonons, which behave much like particles. Copious numbers of phonons are created at the hot centre of the rail and move towards both electrodes, striking the outer sleeve and dragging it along with them.

Ramin Golestanian of the University of Sheffield describes the work as “a beautiful experimental result”. However Golestanian, who studies the physics of moving nanoparticles and nanomechanical devices, told physicsworld.com that much more investigation is required to understand the mechanism responsible for the motion and the role of phonons in it.

Bachtold and colleagues have now turned their attention to making more practical motors based on the effect. They are currently working on reversible devices in which one end of the rail is heated and the other is not, which should cause the sleeve to move from the hot end to the cold end. The direction of travel could be reversed by simply switching which end is heated.

In the longer term, Bactold believes that such motors could be used to drive nanometre-sized machines, such as those that perform drug delivery or other medical functions in the body.

Global spending on space increases

Amid the gloomy reports of meagre budgets and spiralling project costs at NASA, US scientists may find some recompense in the fact that the global space economy is apparently booming. The Space Report 2008, released on Tuesday by the Space Foundation in the US, reveals that over $251bn was spent on worldwide space activity last year.

According to the report, which was compiled using global space budget and revenue data, the total spending for 2007 increased 11% from 2006.

“It shows that overall the space economy is very healthy,” Kendra Horn, a spokesperson for the Space Foundation, told physicsworld.com. “When various other areas are suffering recession, the space economy is continuing to grow.”

One reason for strong economy is commercial sector growth. Digital satellite television, otherwise known as direct-to-home television, and global positioning system (GPS) equipment and chipsets — the two largest segments of the commercial space industry — have grown by 19% and 20%, respectively. Other growth areas include emerging space programmes in Asian counties such as China, which tripled its spending between 2006 and 2007 (although Japan’s spending slipped by 14%).

The report also gives a favourable outlook for those in the US space industry. Employment is growing, and the average wage stands at $88,200 — more than double the broader average private sector wage.

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