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Cloak could hide ships from flowing water

Ships of the future may be able to move through the water without a creating a wake. That is according to a pair of physicists in the US, who have proposed a new type of material that lets water flow around an object as if it were not there at all. The design, which has yet to be built, could boost the energy efficiency of ships and submarines – and even prevent them from being detected. “The main function of [our] structure is to prevent fluid flowing around an object from ‘feeling’ that object,” says Yaroslav Urzhumov of Duke University.

The past five years have seen a flurry of research into invisibility cloaks. The first functioning cloak, which operated for electromagnetic waves in the microwave range, was demonstrated by a team led by David Smith at Duke University in 2006, and since then researchers have proposed and demonstrated cloaks that work for visible light, sound and even events in time.

Warping fluid flow

The latest design, put forward by Urzhumov and Smith in a paper due to be published in Physical Review Letters, could be called a water cloak, or more accurately a “fluid-flow cloak”. It is based on the same theory that gave us previous cloaks, namely transformation optics. In the same way that the equations of general relativity show how gravity can warp space–time, so the equations of transformation optics can show how materials with unusual properties can warp the path of light – or indeed other waves, such as sound or water. These exotic materials, known as metamaterials, can guide waves around an object, so that from a distance it appears as though the object is not really there.

Researchers have adapted invisibility cloaks to water before. In 2008 physicists at Liverpool University in the UK and the Ecole Centrale Marseille in France showed how a metamaterial could shield an object from surface water waves. But surface waves are different from fluid flow: in waves, the fluid itself does not go anywhere and so no mass is transferred. Urzhumov and Smith are the first to show how an object could be cloaked so it can move through water without leaving a trace.

One problem that the pair faced is how to make water flow around a vessel and meet up neatly at the stern. For this, Urzhumov and Smith suggest that the metamaterial surrounding the vessel would need to be not just porous, but also to have an anisotropic structure that exhibits a different resistance to the flow at different points around the hull. This could be a lattice of blades supported by wires, suggests Urzhumov.

Tiny pumps needed

Even if the metamaterial was able steer water around the vessel, there is a bigger problem: the more the water is steered, the more it will slow down. It is this change in velocity that is responsible for the frothy disturbance at a boat’s wake. Therefore, suggest the researchers, the metamaterial would need to actively pump water to counteract the loss of speed. Since this pumping would have to be done throughout the metamaterial, the pumps would have to be tiny.

Urzhumov has a couple of ideas in mind. One is a piezoelectric pump, which consists of a small crystal that bends when a voltage is applied across it. Another is an electro-osmotic pump, in which a voltage across a membrane creates a pressure difference, forcing water through. “Electro-osmotic micro-pumps have a much lower flow rate, so they may [only] be used to build a proof-of-principle, scaled-down, slow-moving prototype,” Urzhumov says. “Piezoelectric micro-pumps are the most likely candidates.”

If Urzhumov and Smith’s fluid-flow cloak were built, the researchers predict that one advantage would be efficiency. As a vessel moves, it drags nearby water with it, displacing more mass than it strictly has to. On the other hand, if the vessel were propelled only by the active metamaterial, then it would displace only the minimum water necessary.

Evading detection

Another advantage is silence: the turbulent wake of a vessel is where a lot of its acoustic noise is generated. By killing the wake, the metamaterial should make a vessel quieter. “Acoustic noise is definitely used by defence [agencies] for detection purposes,” says Urzhumov.

Sebastien Guenneau, a physicist at Liverpool University who helped develop the water-wave cloak in 2008, says the fluid-flow cloak could have “tremendous potential applications in aeronautics”, reducing the disturbed flow around boats, submarines and even aircraft. “There are obvious applications in civil engineering, but I guess the military would be interested too,” he adds.

Smith’s lab has built several electromagnetic cloaks in the past, but the Duke group is not planning to create the fluid-flow cloak anytime soon. “Our experimental strength is in electromagnetic metamaterials…we do not have a hydrodynamics testing facility,” says Urzhumov. “It would be much more efficient to build a collaboration with an organization that is already set up for such experiments.”

A preprint of the article is available at arXiv:1106.2282.

Elementary curiosity

How much urine do you need to collect if you want to make white phosphorus? A slightly strange question, perhaps, but an understandable one if you are attempting to re-enact Hennig Brand’s discovery of the element – which is exactly what author Hugh Aldersey-Williams tries to do in “Pee for phosphorus”, the 11th chapter of his book Periodic Tales: the Curious Lives of the Elements. Unfortunately, Aldersey-Williams discovers that recreating the scene made famous by Joseph Wright’s iconic 1771 painting The Alchymist, in Search of the Philosopher’s Stone is not as easy as it looks. Aldersey-Williams’ experiment fails not for lack of urine, but because of insufficient heat in the later stages of the process.

Periodic Tales is one of two popular-science books about the periodic table that have recently been published in the UK, presumably to coincide with the 2011 International Year of Chemistry (the other, Sam Kean’s The Disappearing Spoon, is published by Doubleday). It is an eye-catching book, with an amusing dust jacket on which even the author, publisher and the price have been turned into “elements”, with the symbols “Ha-w”, “PvK” and “£”, respectively. Inside, it is similarly well illustrated, although it is somewhat disappointing that the images are embedded in the text without any captions.

The text itself consists of the author’s personal exploration of the periodic table, beginning with a gold sculpture of the model Kate Moss in the British Museum and finishing with an interview with the late Russian experimentalist Yuri Oganessian, creator of several super-heavy elements in the reactor at Dubna in Russia. Inbetween, the reader is entertained with stories – some well known, some new to us – of many, though not all, of the 112 named elements.

The book follows a currently popular genre in which history is mixed with descriptions, almost a travelogue, of the author’s journey when researching the book. For example, the section “Our Lady of Radium” includes an account of the author’s visit to the Curie Museum in Paris. Some readers may find this distracting, and with nearly 400 pages of text, it does result in quite a long book.

For the most part, though, we enjoyed it and found the storytelling engaging. Two examples give a flavour. First, a section on mercury describes how the French film director Jean Cocteau, in a scene for his film Orphée, made it possible for an actor to “walk” through a mirror formed by a horizontal bath of mercury by turning the camera on its side. Perhaps more interesting was the fact that the mercury was only about a centimetre deep, yet still weighed half a tonne. The second example comes in an anecdote about the Manhattan Project. After chemists working on the top-secret project decided to use “copper “(element 49) as a cover name for plutonium (element 94), they needed to find a way of referring to actual copper. Their solution was to call it “honest to God copper”.

At times, though, Aldersey-Williams overreaches himself and allows his own views to intrude on these historical stories. For example, after noting that the Nobel-prize-winning chemist Glen Seaborg said that he chose to name plutonium after the outermost planet of the solar system, the author suggests that Seaborg might have subconsciously meant the name to refer to Pluto, the Greek god of the underworld. But no-one can know what was on Seaborg’s mind, and this speculation applies somewhat unwarranted hindsight to a good story.

So does the book live up to the expectations raised by its colourful cover? Our answer is a qualified “yes”. The author is a chemist turned journalist, which means that his viewpoint is neither that of a professional chemist nor that of a non-specialist; perhaps as a result, he quite often fails to capture the enchantment that chemistry sometimes arouses in the interested layperson. Perhaps we should declare an interest here, for we both contribute to a website, www.periodicvideos.com, that promotes chemistry for a general audience. As such, we have asked some of the same questions as Aldersey-Williams, including “how do you choose the name of an element that you have just synthesized?”. We were told by the scientists at Darmstadt that you write the possible names on the coffee-room blackboard and then you vote – which is a rather more exciting and democratic process than the one described by the author in a bland statement that “claimants are now only permitted to put forward names as suggestions”.

Another of the book’s flaws is that, while it does cover a wide range of chemistry, the author’s desire to make the book “different” has led to a somewhat arbitrary arrangement of the elements. This is a pity, because Mendeleev invented his table precisely to bring some order to the elements. As a result, Periodic Tales lacks the precision of the original table and has more of the character of tossed salad. But even in a salad one can find tasty titbits, and the same is true of this book. For example, “Chromatic revolution” weaves a fascinating thread that begins with a box of oil paints belonging to the author’s father, progresses via chrome-plated American cars, and ends up in an artists’ materials shop in Bloomsbury.

Did we enjoy the book? Yes. Should you read it? Yes, but it is worth remembering that not all the elements have been given their due; in particular, hydrogen is conspicuous by its absence. And should you try to make phosphorus from your urine? Better to pour it on your compost heap.

Messing with your brain

By Louise Mayor

Last night I attended an event at the Royal Society in London celebrating 100 years of superconductivity. Hosted by Oxford Instruments and the Institute of Physics, the evening’s entertainment included talks by top scientists Stephen Blundell, Mark Lythgoe, Steven Cowley and Jonathan Flint.

A take-home message from Blundell was that it took 50 years from the discovery of superconductivity until we got to the point of commercializing the science – something that funding bodies and policy-makers should keep in mind. But as well as such sensible opinions, there were some unusual goings-on that I won’t forget in a hurry.

One such highlight was the video below. Lythgoe showcased what we’ve learned about the human brain through magnetic resonance imaging (MRI), which only has such high resolution due to superconducting magnets. Lythgoe challenged the audience to watch the following video and count how many times the people in white T-shirts pass the ball between each other. Have a go yourself, but try not to be distracted by the people in black T-shirts, who will try to confuse you by running around and throwing a second ball.

So, how many times did the white ball get passed? The answer is 15. Well done if you got that right – it shows you have good attention. However, this was an example of a selective attention test. Did you see the gorilla?

In a particularly curious moment, a group of people stood up and made their way to the front of the room; in hindsight they were conspicuously young and gender-balanced compared with the rest of the crowd. It was explained that we were in for a musical treat – a music/art performance called Brainwaves, one of the composers having been inspired by an MRI scan. The experience was immersive, with visual effects from design studio loop.Ph, and Mira Calix and Anna Meredith’s electronic music sounding menacing and grating next to the more soothing tones of the Aurora string quartet. I’ve never been in an MRI scanner, but watch for yourself and see what you think.

None of the evening’s events would have taken place were it not for that serendipitous discovery of superconductivity 100 years ago. This April, Physics World produced a special issue to celebrate the centenary, a free PDF of which can be downloaded by following this link.

Artificial leaves make fuel from sunlight

Two teams of researchers in the US have taken important steps towards the creation of commercially viable “artificial leaf” – a hypothetical device that can turn sunlight into electrical energy or fuel by mimicking some aspects of photosynthesis.

Earlier this year, the chemist Daniel Nocera at the Massachusetts Institute of Technology (MIT) announced artificial-leaf prototypes at the annual meeting of the American Chemical Society in California. Now, working with two different teams of researchers, he has published two papers on different devices that represent progress towards effective and commercially viable versions of the artificial leaf.

Here comes the Sun

Both teams made their devices from silicon wafers that are coated with catalytic metals and protective layers. The prototype solar cells are about the size of a credit card and can capture sunlight and then use the energy to split water into its constituent oxygen and hydrogen. This is different to conventional photovoltaic cells, which convert light directly into electricity. With these new devices, the ultimate plan is to recombine the two gases in an integrated fuel cell, thus converting the chemical energy to electrical energy. Producing fuel rather than electricity has the advantage that the fuel can be easily stored until it is needed.

Both artificial leaves use a silicon n–p junction: a bilayer of n-type and p-type silicon. An incident photon is absorbed to create an electron–hole pair in the semiconductor. The electrons migrate to the n-side and the holes to the p-side. The holes then drive the splitting of water in a process mediated by the outermost layer of the cell, which is a photocatalyst. Unlike some of the exotic photocatalysts used in earlier devices, the catalyst in these new devices are made of cobalt phosphate, which is an abundant and cheap material.

The main challenge in creating both devices was how to prevent the silicon from reacting with the water. The two teams took different approaches to the problem. One group led by electrical engineer Vladimir Bulovic used the catalyst itself as a protective layer, binding a thin film of pure cobalt firmly to the silicon before converting it to the phosphate form. The other team, led by mechanical engineer Tonio Buonassisi, used a thin film of conductive indium tin oxide in front of the p-type silicon as the protective layer.

Bubbles needed

Buonassisi and colleagues connected two of their cells in series and managed to split water with a solar-to-oxygen conversion efficiency of 0.25%. While this does not sound like much, the efficiency of photosynthesis is only a few per cent. However, the cells make hydrogen ions, and turning this into gas could add considerable cost to the device. “Platinum electrodes are good catalysts for reducing hydrogen ions to hydrogen gas”, says Devens Gust of Arizona State University, who was not involved in the research. “However, the rarity of platinum limits its usefulness.”

Gust describes the MIT work as “very important in that it demonstrates a workable, inexpensive water-oxidation catalyst”. However, he says that the technology is entering a crowded market, pointing out that there is already a production technology for solar fuel that is “pretty much ready to go now”. This system uses photovoltaic cells coupled to an electrolyzer that splits water into oxygen and hydrogen. “Electrolyzer efficiencies can be as high as 70–80%, and currently available photovoltaic efficiencies are as high as 15–20%”, he points out. “None of the artificial photosynthetic systems can compete with this at the moment.”

The MIT technology must also compete with other water-splitting systems based on silicon solar cells coated with photocatalysts. These have been in development since at least 1998 and some have reached solar-to-hydrogen conversion efficiencies of 7% or better.

“Challenges remain”

One of these cells was developed at California Institute of Technology by Nathan Lewis and Harry Atwater. Atwater told physicsworld.com that “Nocera’s work is interesting, but many challenges remain.” It is not clear, for example, whether the catalyst and devices remain stable beyond the few days of operation for which they have so far been tested. Atwater also thinks there is room for improvement in the materials themselves.

Gust agrees, pointing out that while cobalt and other catalysts based on common materials are promising, researchers have yet to develop an inexpensive catalyst that works near the thermodynamic potential for water oxidation/reduction. This property would help to optimize the performance of an artificial-leaf system. Nocera hopes to have a fully working device within about three years, and he has formed a company called SunCatalytix to develop it.

The work by Bulovic’s group is published in Energy & Environmental Science, while the research by Buonassisi’s group is outlined in Proceedings of the National Academy of Sciences USA.

Radioactive decay accounts for half of Earth’s heat

About 50% of the heat given off by the Earth is generated by the radioactive decay of elements such as uranium and thorium, and their decay products. That is the conclusion of an international team of physicists that has used the KamLAND detector in Japan to measure the flux of antineutrinos emanating from deep within the Earth. The result, which agrees with previous calculations of the radioactive heating, should help physicists to improve models of how heat is generated in the Earth.

Geophysicists believe that heat flows from Earth’s interior into space at a rate of about 44 × 1012 W (TW). What is not clear, however, is how much of this heat is primordial – left over from the formation of the Earth – and how much is generated by radioactive decay.

The most popular model of radioactive heating is based on the bulk silicate Earth (BSE) model, which assumes that radioactive materials, such as uranium and thorium, are found in the Earth’s lithosphere and mantle – but not in its iron core. The BSE also says that the abundance of radioactive material can be estimated by studying igneous rocks formed on Earth, as well as the composition of meteorites.

As a result of this model, scientists believe that about 20 TW is generated by radioactive decay – 8 TW from the uranium-238 decay chain; 8 TW from the thorium-232 decay chain and the final 4 TW from potassium-40. Fortunately, these decay chains also produce anti-electron-neutrinos, which travel easily through the Earth and can be detected, thereby giving physicists a way to measure the decay rates and ultimately the heat produced deep underground.

Decay and measure

In 2005 researchers at KamLAND announced that they had detected about 22 such “geoneutrinos”, while last year scientists at the Borexino experiment in Italy said they had detected 10. Now, the KamLAND team has bagged a total of 111 of these tiny almost massless particles. The combined results have allowed the KamLAND team to conclude that the heat flux due to the uranium and thorium decay chains is about 20 TW with an uncertainty of about 8 TW. While the KamLAND experiment cannot detect the lower-energy antineutrinos from potassium-40 decay, the researchers believe that the value predicted by the BSE model of 4 TW is correct.

Although 20 TW from uranium and thorium is more than the 16 TW predicted by the BSE model, it is still within the experimental uncertainty – and is much less than the total flux of 44 TW. “One thing we can say with near certainty is that radioactive decay alone is not enough to account for Earth’s heat energy,” says KamLAND collaborator Stuart Freedman of the Lawrence Berkeley Laboratory in California. “Whether the rest is primordial heat or comes from another source is an unanswered question.”

One possibility that has been mooted in the past is that a natural nuclear reactor exists deep within the Earth and produces heat via a fission chain reaction. Data from KamLAND and Borexino do not rule out the possibility of such an underground reactor but place upper limits on how much heat could be produced by the reactor deep, if it exists. KamLAND sets this limit at about 5 TW, while Borexino puts it at about 3 TW.

Oil-filled balloon

The KamLAND detector is a huge balloon filled with 1000 tonnes of mineral oil that is monitored by more than 1800 photomultiplier tubes. It is located deep underground in a Japanese mine to shield the detector from cosmic rays.

Very occasionally an antineutrino will react with a proton in the oil to create a neutron and a positron. The positron travels a short distance through the oil, giving off a flash of light as it ionizes oil molecules. The positron then annihilates with an electron to create two gamma-ray photons. These two processes happen very quickly and the light can be detected by the photomultiplier tubes. In addition, the energy of the antineutrino can be estimated from the amount of light given off during ionization.

A few hundred milliseconds later, the neutron is captured by a proton to form a deuteron. This results in the emission of a gamma ray, which can also be detected by the photomultiplier tubes. By looking for signals in the photomultiplier tubes that are separated by the appropriate amount of time, KamLAND can discriminate between extremely rare antineutrino events and the much more common signals due to background radiation.

The work is described in Nature Geoscience 10.1038/ngeo1205.

Light propagates as if ‘space is missing’

Researchers in the UK and the US have crafted an optical nanostructure that allows light to pass through without accumulating a phase change – as if the medium were completely missing in space. The device could find applications in optoelectronics, they say, for instance as a way of transporting signals without allowing information to become distorted.

Whenever light travels through a medium it experiences a phase-shift, as individual oscillations become out of phase with each other. In certain optics applications, including interferometers, these phase variations can introduce an unwanted dispersion of frequencies. This effect can lead to phase distortions, which ultimately reduce the quality of signals.

Zero-index material

But in this new study, a team led by Serdar Kocaman, an electrical engineering researcher at Columbia University, has found a way around this issue. Kokaman’s group has designed a way to control the dispersion of light by manufacturing a metamaterial that has with a refractive index of zero.

The device includes photonic crystals, which are materials with a periodic variation of the dielectric constant, resulting in a photonic band gap. Kocaman’s team fabricated photonic crystals with the unusual property of having a negative refractive index. One outcome of this optical property – not found anywhere in nature – is that the phase of light travelling through the photonic crystal flows in the opposite direction to the flow of energy.

The device consists of alternating layers, roughly 2 µm thick, of these photonic crystals along with positive index materials. The result is that the phase of light keeps oscillating but when it emerges from the device is has undergone zero overall phase change, as they explain in a research paper in Nature Photonics.

Missing space

“What we’ve seen is that the light disperses through the material as if the entire space is missing,” said Kocaman. “The oscillatory phase of the electromagnetic wave doesn’t even advance such as in a vacuum – this is what we term a zero-phase delay.”

The new device was fabricated onto a silicon chip, a few microns long. For this reason, the researchers believe that it could be integrated into optoelectronic circuits. Nicolae Panoiu, one of the researchers at University College London, told physicsworld.com that the device could be used as a “perfect mirror” for transporting optical signals within a circuit.

He says that his team has already used the optical structure to fabricate an optical filter that can be used to block photons of given frequencies. This research, he says, will be described in an upcoming paper.

Dark energy spotted in the cosmic microwave background

Astronomers studying the cosmic microwave background (CMB) have uncovered new direct evidence for dark energy – the mysterious substance that appears to be accelerating the expansion of the universe. Their findings could also help map the structure of dark matter on the universe’s largest length scales.

The CMB is the faint afterglow of the universe’s birth in the Big Bang. Around 400,000 years after its creation, the universe had cooled sufficiently to allow electrons to bind to atomic nuclei. This “recombination” set the CMB radiation free from the dense fog of plasma that was containing it. Space telescopes such as WMAP and Planck have charted the CMB and found its presence in all parts of the sky, with a temperature of 2.7 K. However, measurements also show tiny fluctuations in this temperature on the scale of one part in a million. These fluctuations follow a Gaussian distribution.

In the first of two papers, a team of astronomers including Sudeep Das at the University of California, Berkeley, has uncovered fluctuations in the CMB that deviate from this Gaussian distribution. The deviations, observed with the Atacama Cosmology Telescope in Chile, are caused by interactions with large-scale structures in the universe, such as galaxy clusters. “On average, a CMB photon will have encountered around 50 large-scale structures before it reaches our telescope,” Das told physicsworld.com. “The gravitational influence of these structures, which are dominated by massive clumps of dark matter, will each deflect the path of the photon,” he adds. This process, called “lensing”, eventually adds up to a total deflection of around 3 arc minutes – one-20th of a degree.

Dark energy versus structure

In the second paper Das, along with Blake Sherwin of Princeton University and Joanna Dunkley of Oxford University, looks at how lensing could reveal dark energy. Dark energy acts to counter the emergence of structures within the universe. A universe with no dark energy would have a lot of structure. As a result, the CMB photons would undergo greater lensing and the fluctuations would deviate more from the original Gaussian distribution.

However, the opposite was found to be true. “We see too little lensing to account for a universe with no dark energy,” Sherwin told physicsworld.com. “In fact, the amount of lensing we see is consistent with the amount of dark energy we would expect to see from other measurements.”

This is the first time dark energy has been inferred from measurements of the CMB alone. Conventional CMB measurements only reveal details about the very early universe, a time before stars and galaxies. In order to build up a picture of the universe’s evolution, these results had to be combined with an additional measurement such as the Hubble constant. However, the CMB photons observed in this work were deflected by the unfolding evolution of the universe. “That missing information is now built right in,” Sherwin explains.

“Patchwork of evidence”

The fact that this is direct evidence, rather than relying on a second measurement, excites Stephen Boughn, a cosmologist at Haverford College in the US. “We currently only have two pieces of direct evidence for dark energy. Any additional evidence that indicates its existence is very important,” he says. “We want a patchwork of evidence, from many different places, just to make sure the whole picture hangs together. This work helps with that.”

Boughn also believes that the findings could help reveal how dark matter is distributed throughout the universe on large scales. Dark matter has the same gravitational effects as normal matter but does not interact with electromagnetic radiation and so cannot be seen directly. “There are many simulations, but few observations, that suggest how the universe’s dark matter is structured,” he explains. “But because this lensing of the microwave background depends on how the dark matter is clumped, future experiments measuring these distortions in the CMB should be able to get a handle on how large-scale dark matter is distributed.”

The both papers are published in Physical Review Letters.

Physics student awaits espionage trial in Iran

UPDATE July 21, 2011: Omid Kokabee’s trial has been postponed to an unspecified date, according to friends.

A doctoral student who was detained when he tried to leave his native Iran earlier this year will go on trial tomorrow for charges related to espionage, according to sources close to him.

Omid Kokabee has been detained since late January or February this year when he was attempting to fly from Tehran airport to return to his studies at the University of Texas at Austin, US. Physics World understands that he is suspected of leaking Iranian scientific information and working with the CIA.

The trial, apparently based on charges of illegal earnings and communicating with a hostile government, is expected to be headed by Iranian justice Abolghasem Salavati.

Harsh sentence possible

“[Salavati] is known for passing harsh sentences to innocent people,” says Eugene Chudnovsky of the Committee of Concerned Scientists, an international human rights organization. “It is impossible to predict, but I cannot even rule out the death penalty if the purpose is to scare Iranian students abroad. Iran has conducted a record number of executions this year.”

Kokabee, 29, graduated in applied physics and mechanics from the Sharif University of Technology in Tehran in 2005, and went on to receive a master’s degree in optics from the University of Barcelona, Spain, last year. During this period he worked for several companies, including Zener Electronics, based in Iran, and National Iranian Oil. In autumn last year he began a PhD at Austin, studying the interaction of lasers with plasmas.

“Being an applied scientist and having a great taste of engineering from my bachelors, my passion is doing technology-based entrepreneurial business and hi-tech start-ups,” he writes on his LinkedIn page.

Studying optics and photonics

John Keto, the graduate adviser of Austin’s physics department, doesn’t understand why Kokabee’s area of study would be of interest to the Iranian authorities. “Initially the web was full of wrong information about Omid being a world expert in nuclear physics,” he says. “Omid’s presence here was much more innocent than all of that. He was simply a new graduate student taking courses, teaching and beginning his PhD in optics and photonics.”

Other friends and colleagues speculate as to the real reasons behind Kokabee’s arrest. One suspicion is that he has been used as a scapegoat to discourage student participation in Iran’s Green movement, which believes that Iranian president Mahmoud Ahmadinejad was unfairly elected to office in the country’s 2009 election. According to Keto, the Green movement has used Kokabee’s detention to promote its cause. “Perhaps this has further jeopardized his situation there,” Keto says. “Omid was not associated with this reform movement, but mainly concentrated on his science and family.”

One friend, who wanted to remain anonymous, agrees that Kokabee didn’t have much interest in politics: “He was really not a political activist. I shared a room with him for a few years, and he was really not into politics at all. All he cared about was laser physics. I had friends who were into politics, but he was not.”

Kokabee is thought to have been apprehended at Tehran airport while awaiting a flight to Dubai to collect a visa from the US embassy. He was supposed to go on to fly to Austin a week later.

Notorious prison

Close friends of Kokabee say he has spent most of the last few months at the Evin prison in north-west Tehran. Evin has become notorious in recent years for the number of academic and political prisoners detained there as part of Iran’s crackdown on individuals claimed to be involved in espionage for Western states.

Yesterday the Committee of Concerned Scientists sent an open letter requesting clemency to Ali Khamenei, supreme leader of Iran. Meanwhile, Kokabee’s lawyer hopes that pressure from the media and academia will help the student’s case.

Should space missions be privatized?

By James Dacey

With the last space shuttle mission launched last week, would a shift towards privatized space missions be good for science?

Have your say by voting in our Facebook poll.

hands smll.jpg

Last week we asked for your opinion on another astronomy issue. We wanted to know where you think the Square Kilometre Array should be built. This $1.5bn project will be the biggest and most advanced radio telescope ever built. Sites in southern Africa and Australia/New Zealand have been shortlisted to host the central core of the array.

It was a close-run affair as 57% of respondents opted for Australia/New Zealand and 43% went for southern Africa.

The founding board of the SKA project has unveiled the process and timeline for selection of the host site for the telescope. A final decision on the location is expected to be made in early 2012 by the SKA board of directors.

Hands-on physics at the Royal Society

Each year the Royal Society invites the public into its prestigious headquarters in London for its Summer Science Exhibition, where they are wowed by some of the most exciting and innovative research from across the natural sciences. At this year’s event, which ran from 5–10 July, physics featured strongly in a range of exhibits spanning astronomy, optics and condensed matter. Physics World reporter James Dacey went to London armed with a camera and a microphone to capture some of the sights and sounds of the event.

Invisibility science

In the week that the final instalment of the Harry Potter films went on general release, it was no surprise that the exhibit on invisibility science was overrun with excited children and their parents. Dressed in a Hogwarts-style cloak, Ulf Leonhardt from the University of St Andrews in Scotland was on hand to talk people through some of the different approaches to achieving invisibility. For a simple demonstration of transparency, Leonhardt dropped glass beads into a fish tank. As the beads have the same refractive index as water, they appear to disappear. Then, to introduce the concept of bending light around an object, the researchers entrapped Winnie-the-Pooh’s friend Eeyore in a pod made from optical fibres. Peering through a viewing hole at one end of the experiment visitors saw the wall behind, as if the donkey were not there.

Engaging physics

Just as important as the whizz-bang science, however, were the researchers themselves and their enthusiasm for presenting their work to the public in an engaging and accessible way. One exhibitor who was delighted to be given that opportunity was Matthew Dickinson, an engineering designer from the University of Central Lancashire. Dickenson was part of a team presenting some of the technologies used in energy harvesting – the processes for capturing energy that would otherwise be lost to the atmosphere by heat, light and motion. One particularly noisy prop was the “piezoelectric glockenspiel”, which is designed to show that electricity can be generated directly from materials flexing. The instrument-cum-energy-generator was constructed by attaching ferroelectric strips to the ends of metal bars of varying lengths.

The musical theme continued with a fascinating exhibit hosted by mathematicians from the University of Bristol, who arrived at the event with their “singing graphs”. Their display focused on a giant touchscreen connected to a loudspeaker, which were used to create quantum graphs that could be thought of as a network of guitar strings attached to one another. As the position of nodes in the graph were slid around the surface, the movements were captured by cameras beneath. This caused sounds from the speakers to change in pitch. The exhibit was designed to represent “inverse problems” in physics and mathematics. The idea is that the sounds can be used to reconstruct the shape of graphs, in a similar way to how radar and sonar signals can be used to determine the shape of features in nature.

Bee vision

As well as the colourful sounds, there were also plenty of striking sights, not least at the exhibit explaining structural colour in nature.

A team of researchers from the University of Cambridge were showcasing some of the ways in which animals and plants manipulate light to produce eye-catching colours. Ulrich Steiner, a member of the Cambridge team, was particularly enthusiastic when discussing the way these natural structures grow. He believes that the best way to understand the processes is to recreate the colours in the lab. His group is also interested in how organisms in nature interact through colour, but he admits that in many cases it is still far from clear how these interactions occur.

In praise of the wonder material

Another hands-on exhibit offered visitors the chance to produce their very own sheets of graphene – the single-atom-thick layers of carbon dubbed the “wonder material” on account of its outstanding properties. It was for their pioneering work on graphene that Andre Geim and Konstantin Novoselov shared the 2010 Nobel Prize for Physics. This exhibit at the Royal Society was hosted by Geim and Novoselov’s colleagues at the University of Manchester, who were demonstrating the “Scotch tape” method for isolating graphene. This essentially involves ripping a layer off a thin piece of graphite with sticky tape.

Exploring the aurora

No celebration of inspiring science would be complete without reflecting on the achievements of astronomy. But in an interesting exhibit about the Earth’s auroras, part of the display was devoted to a blip in the history of space exploration – when the rocket carrying Cluster satellites crashed in 1996 destroying much of the equipment. The satellites were relaunched in 2000 and they have helped scientists to build a clearer picture of the auroras and the magnetosphere, which helps to shield life on Earth from the Sun’s lethal radiation.

For more photographs from the Royal Society Summer Exhibition, visit the Physics World Flickr page.

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