Skip to main content

Reset your password

Please enter the e-mail address you used to register to reset your password

Note: The verification e-mail to change your password should arrive immediately. However, in some cases it takes longer. Don't forget to check your spam folder.

If you haven't received the e-mail in 24 hours, please contact customerservices@ioppublishing.org

Registration complete

Thank you for registering with Physics World
If you'd like to change your details at any time, please visit My account

Close
Home » Page 1409

Transparent material opens a new window on solar energy

Researchers in the US have developed a new kind of organic solar cell that converts a small but significant fraction of the sunlight that falls onto it into electricity, while still allowing most of the visible part of that light to pass through. Thanks to this transparency, the team says that the cell could be mounted onto windows in buildings or cars in order to tap a currently under-exploited source of energy.

Most of today’s commercial solar cells are made from the semiconductor silicon. When photons with sufficient energy strike the silicon they create pairs of electrons and holes. An electric field created by adding impurities to the silicon splits the electron–hole pairs apart, which results in an electric current. However, the costs involved in processing the silicon mean that photovoltaic cells remain very expensive compared to other forms of electricity generation.

One alternative is plastic – or organic – semiconductors, which are much cheaper to work with and are also flexible and lightweight. However, in plastic solar cells the liberated electrons and holes bind strongly to one another, forming particle-like entities known as excitons. These excitons only break apart when they reach a “heterojunction”, which is the interface formed by making cells from two different organic materials. But because excitons tend to travel only very short distances before they self-annihilate, cells must be very thin for significant numbers of excitons to reach the heterojunction and generate a measurable current. This need for thinness makes the cells inefficient.

Exploiting excitons

Now, Richard Lunt and Vladimir Bulovic of Massachusetts Institute of Technology have turned the exciton problem on its head. They exploit the fact that the formation of excitons alters a material’s absorption properties. So rather than absorbing wavelengths more or less equally across a broad spectrum, as is the case in silicon, their prototype cell instead displays distinct absorption peaks. By combining the organic molecules chloroaluminium phthalocyanine and carbon-60, their cell absorbs light at infrared and ultraviolet wavelengths but has limited absorption at visible wavelengths. In other words, it is able to extract energy from the non-visible parts of the spectrum while leaving most of the visible light free to propagate.

The fact that the new device does not absorb appreciably at visible wavelengths makes it less efficient than opaque organic cells. However, say Lunt and Bulovic, it is more efficient that other kinds of transparent cell that absorb across the spectrum. As they point out, these other cells must be made very thin if they are not to become opaque and as a result have efficiencies of less than 1% when at least partially transparent. In contrast, they achieved efficiencies of up to 1.3% when transmitting at least 65% of the incident visible light and up to 1.7% for transparencies greater than 55%.

While these efficiencies are very low compared to the 22% of the best commercial silicon cells, the MIT researchers claim that they should be able to up the efficiency of their cell to around 12% by increasing the length of the heterojunction interface. They will do this by blending the two organic materials, and also by stacking a series of cells together, each absorbing at a slightly different position within the infrared spectrum (ultraviolet absorption provides a very small fraction of the cell’s output).

Rolling onto existing windows

They say that their cell could be coated directly onto the glass in new windows or onto a flexible substrate that is then rolled onto existing windows, pointing out that exploitation of existing window structures would lower installation costs compared with conventional solar cells. They estimate that they will need between five and ten years to commercialize their technology because uncertainties remain regarding the durability of organic cells.

Lunt acknowledges that while the new cell could allow private individuals and companies to exploit more of the sunlight that falls on their buildings, it will not avoid the need for other energy sources. “It will be one of the tools in the clean-energy tool box,” he says.

Martin Green, of the University of New South Wales in Australia, believes that organic photovoltaic cells will be used in niche applications, but he does not think that they can compete with mainstream cell technologies even if they are cheaper to manufacture. He argues that the savings obtained in the manufacturing process will be nullified by the extra costs, common to all photovoltaic technologies, needed to “field-safe, professional systems with a long field life.”

Green’s position is backed up by a recent report from US technology analysts Lux Research, which concluded that the low efficiencies and short lifetimes of organic photovoltaic cells will make them uncompetitive with crystalline silicon and inorganic thin-film technologies over the next decade.

The work is described in App. Phys. Lett. 98 113305.

Cleaning oil spills drop by drop

By Matin Durrani

If you want to get your research results noticed by us here at Physics World headquarters, you can always try e-mailing us a copy of your paper, preferably well before it’s about to be published.

But Burak Eral from the physics of complex fluids group at the University of Twente in the Netherlands has taken a novel approach in flagging his research to us — he’s sent us a three-minute Youtube video consisting of a series of Powerpoint slides put to music.

As you can see, his tactic has worked. The video describes how Eral and his pals have studied the morphology of a drop clinging to a cylindrical fibre — a problem first studied by Joseph Fourier in the late 19th century.

If you can bear Eral’s rather soporiphic choice of music, you’ll find that the drops can either surround the fibre symmetrically, like a barrel, or attach themselves to one side of the fibre, rather like a clam-shell. By using the technique of “electrowetting”, Eral’s team was then able to reversibly change which form the drops adopt — with what they claim is “previously unachieved precision”.

The work has its practical side too as it could potentially lead to a way to clean oil spills in the world’s oceans. Eral envisages creating special fibres that could be dropped into the affected, oil-damaged region. Although the oil would naturally tend to form barrell-shaped drops around the fibre, the drops could be forced into adopting the clam-shell shape, which are much easier to wash off from the fibre. The result: cleaner oceans with the oil drained safely away.

Eral is not, of course, the first physicist to find the lure of creating an educational video about their work. In fact, you can find plenty of these “video abstracts” at the New Journal of Physics — an open-access journal published by the Institute of Physics, which also publishes physicsworld.com.

Eral’s full paper about his work appears in the journal Soft Matter

Between the lines

Apocalypse eventually

The list of disasters that threaten life on Earth is long and varied. The list of books that have been written about such disasters, however, is even longer. With what is, in retrospect, spectacularly bad timing, we picked this month to review a trio of recent books that explores the science of disasters. Of the three, Armageddon Science: the Science of Mass Destruction is the most conventional. In it, the science writer Brian Clegg presents a tour of the science and history behind numerous possible doomsday scenarios, ranging from the unlikely (antimatter bombs and planet-eating black holes) to the all too real (climate change). Not all of them are covered in the same depth. For example, tsunamis, earthquakes, asteroid impacts, supervolcano eruptions, alien invasions and irradiation by interstellar gamma-ray bursts are all crammed into a mere 26 pages. In contrast, the chapter on nuclear weapons takes up almost a quarter of the book, and sections on nanotechnology and climate change are also relatively meaty. One reason for this emphasis may be the author’s own background: Clegg is a physicist by training, and he seems more at home with physics-related disasters than he does with geological ones. However, as the book’s thoughtful introduction and conclusion make clear, Clegg is also primarily interested in disasters that are in some sense caused by science, not merely explained by it. Noting that Marie Curie died of radiation-induced leukaemia, he observes that “scientists don’t always have a great track record in keeping themselves and others safe”. Apparently callous attitudes such as these – which Clegg links, tenuously, to the fact that many scientists exhibit mild symptoms of autism – have a detrimental effect on the way outsiders perceive the scientific community.

• 2010 St Martins Press £18.99/$25.99hb 304pp

A scientific conspiracy?

Large-scale US government support of scientific research was born in the Second World War. To keep federal dollars flowing in peacetime, scientists have repeatedly spread alarms about natural disasters such as asteroid impacts and climate change – the solutions to which, inevitably, involve more government-funded research. This, at least, is the argument put forward by James Bennett in The Doomsday Lobby: Hype and Panic from Sputniks, Martians, and Marauding Meteors. As this synopsis indicates, Bennett, a political scientist at George Mason University in Virginia, is actively hostile to government support of scientific research – or, as he terms it, “the federal appropriation dole”. However, readers who are thick-skinned enough to withstand repeated insults will find a few atoms of truth inside Bennett’s layers of anti-government ideology. As he points out, state-funded science is not always a benign matter: it has also meant despoiling large swathes of the American West with dams, subsidized mining and weapons testing. Moreover, it is true that in former times, science functioned tolerably well without state support. As Bennett describes in the book’s opening chapters, the rise of US astronomy in the early 20th century was funded almost entirely by philanthropists. Yet his privately funded scientific utopia has a fundamental flaw. One of the anecdotes he uses to describe it concerns a 19th-century “Society for the Diffusion of Useful Knowledge”, which built itself an observatory after selling more than 300 memberships at $25 each. That may sound commendably egalitarian, but it is worth noting (as Bennett does not) that when the observatory opened in 1845, $25 was worth as much to the average person as $12,300 is today, as measured by per capita GDP. The fact is that before the late 19th century, scientists were, overwhelmingly, either aristocrats or people who could persuade aristocrats to back them financially. Is that really a better system?

• 2010 Springer £22.99/$24.95pb 200pp

Things fall apart

What do the Tay Bridge disaster, a tense family game of Monopoly and the loss of vegetation in the Sahara have in common? According to Bristol University physicist Len Fisher, who uses each of them as examples in his book Crashes, Crises and Calamities, they all have something to tell us about “critical transitions”, which occur when a system “abruptly, without apparent warning…jump[s] to a very different state”. Sometimes, such transitions are obvious, as in the 1879 collapse of the rail bridge across Scotland’s Tay estuary, or a player overturning a Monopoly board in frustration. Others, such as desertification, are more subtle, and are preceded by characteristic signs that can – if properly interpreted – alert observers to impending change. The key point, Fisher writes, is that “to anticipate and deal with such disasters, we need to be able to predict the changeover point”. His book outlines three overlapping approaches for doing this. One of them, catastrophe theory, classifies transition-prone systems into distinct mathematical types – including one, the “cusp catastrophe”, that has variously been used to explain love–hate relationships and the behaviour of cornered dogs. The second approach, computer modelling, is useful for predicting the outcome of complex situations, while the third focuses on early-warning signs such as fluctuations in the population of an animal species. It is all fascinating stuff, even if the threads that bind Fisher’s examples together sometimes seem weak.

• 2011 Basic Books £13.99/$23.95hb 256pp

The dark-energy game

The universe is not like a clock, where well-understood parts tick in predictable ways, nor like a balloon expanding or contracting. It is in fact pushing itself apart with a strange kind of energy, and 96% of it is made of an unknown kind of matter. How we discovered this is the subject of The 4% Universe, which condenses the complex, messy and startling tale – people, science, instruments, events – into an easily digestible, fast-paced 243 pages. That is a startling achievement in itself. To the connoisseur of popular science, indeed, the way author Richard Panek tells the tale is as interesting as the events: half drama, half detective story.

The prologue begins with a one-page “wow!” moment. On 5 November 2009 scientists at 16 institutions around the world dropped their collective jaws as they seemed to catch a first-ever glimpse of an entirely new structure of the universe. Two pages follow explaining its significance. Referring to the year when Galileo first used the telescope to reveal entire new worlds previously unknown to humankind, Panek writes “It’s 1610 all over again.”

What follows in Act One is the story of how cosmology went from speculation to science: how astronomers discovered that the furniture of the universe was more than planets and stars, and was on the move to boot. The universe “had a story to tell”, Panek writes. “Instead of a still life, it was a movie,” he says. We learn how scientists uncovered this movie’s plot by peering over the shoulders of Act One’s two main characters: theoretical physicist Jim Peebles, author of the classic textbook Physical Cosmology on the physics of the early universe; and astronomer Vera Rubin, whose work on the galaxy-rotation problem pointed the way to the idea that the universe contains some amount of “dark” matter, invisible to present-day instruments.

Act Two introduces more characters and “the game”, in which two different teams of scientists vie to unravel the plot by finding distant “Type 1a” supernovae. The game is played with telescopes equipped with charge-coupled devices, which revolutionized astronomical photography, and with the Hubble Space Telescope, which peered into hitherto invisible corners of the universe, among other equipment. The first team, the Supernova Cosmology Project (SCP), was led by Saul Perlmutter and Carl Pennypacker, particle physicists at the Lawrence Berkeley National Laboratory who applied the tools of their trade to astronomy. In doing so, Panek observes, “[T]hey weren’t drifting towards a new discipline. The discipline was drifting towards them.”

The second team was known as High-Z, where Z is a term for redshift. Highly redshifted objects are among the oldest and most distant in the universe, meaning that they would bear the clearest traces of any expansion or contraction. High-Z‘s main members were Adam Reiss and Brian Schmidt, who hailed from Harvard University and viewed supernovae as their area of expertise. They saw the Berkeley group as being out to “beat them at their own game”. While SCP had a six-year head start, High-Z recruited the “old-boy network” to, in effect, beat the Berkeley group at beating them at their own game.

In 1997 the two teams converged – simultaneously, yet reluctantly – on two wild, toothfairy-like ideas: that the universe contained “dark matter they couldn’t see and [a] new force they couldn’t imagine”. In Act Three, all the main characters introduced so far in the drama gather at a meeting where the SCP’s results (picked up by discerning newspaper reporters) suggest that “SCP was beating [High-Z] at beating the SCP at beating [High-Z] at their own game”. Then High-Z outdid that by securing full credit in the media. The discovery of this new force – soon dubbed “dark energy” – became Science magazine’s “breakthrough of the year” in 1998.

The new idea – that the universe’s expansion is accelerating – both simplifies things, by explaining a lot of puzzling data, and makes them more complex, by raising a lot of questions.

In Act Four, SCP and High-Z make plans to hunt for answers to one question – dark matter – while struggling over credit for the other, dark energy. The existing picture of the universe turns “preposterous”. But as Perlmutter remarks on the final page of the book, what usually attracts physicists to their field is “not the desire to understand what we already know but the desire to catch the universe in the act of doing really bizarre things”. And so, at the book’s conclusion, while one chapter in astronomy ends, another begins.

Panek tells the story briskly yet warmly, capturing personalities and not overlooking controversies. He chooses characters carefully. Through Rubin, for instance, we not only learn about dark matter, but also what it is like to be a woman in science, literally balancing child and career: textbook in one hand, pram in the other. Panek also has a knack for summarizing developments concisely and efficiently, such as in the following passage about how astronomy became more specialized over time:

You couldn’t just study the heavens anymore; you studied planets, or stars, or galaxies, or the Sun. But you didn’t study just stars anymore, either; you studied only the stars that explode. And you didn’t study just supernovae; you studied only one type. And you didn’t study just Type 1a; you specialized in the mechanism leading to the thermonuclear explosion, or you specialized in what metals the explosion creates, or you specialized in how to use the light from the explosion to measure the deceleration of the expansion of the universe – how to perform the photometry or do the spectroscopy or write the code.

Inevitably, Panek makes some compromises, and the seams of his crisp storytelling occasionally show. Galileo is mentioned once too often, and Panek’s apothegmatic style can ring precious, as in this remark about the signal from a radio antenna: “[T]his time the source wasn’t a radio broadcast from the West Coast. It was the birth of the universe.” The reader sometimes feels manipulated, too. That “wow!” moment that kicks things off so dramatically in the prologue? You don’t find out until page 197 that it was phoney – not a discovery after all.

Another author might have explored why it initially seemed to be a discovery, why its announcement was hyped even after problems were uncovered, and what this says about science and scientists. But by this time, you are so absorbed in the story that you do not care that much. And the book does convey a good picture of scientists in the act of catching the universe doing really bizarre things – while also showing that this is why they took the job. Give this book to your non-scientist friends to show them what it is all about – and to fellow scientists as a model of how to write popular science.

Web life

So what is the site about?

STAR-LITE is a game designed to teach basic laboratory safety to researchers at the start of their careers; the name is an acronym of Safe Techniques Advance Research – Laboratory Interactive Training Environment (whew!). To play, you must guide an on-screen avatar through 15 safety-related “quests”, helped (and sometimes hindered) by your computer-controlled lab mates. The game is free to download and is available for PCs and Macs.

What are some of the quests?

After you have designed your avatar (warning: if you try to wear dangly jewellery or flip-flops, you’ll get told off), you begin the real game with a tour of the virtual laboratory environment. This includes an equipment storage room and a tissue-culture area, as well as a large multipurpose lab, library and staff room. Once you complete this orientation, your avatar’s next task is a “scavenger hunt”, where you must find and identify pieces of lab kit such as fume hoods and centrifuges, as well as warning signs for biohazards, flammable materials and the like. As the game progresses, you come across problems such as broken equipment and chemical spills that you have to deal with safely. One helpful feature is that before you can begin a particular task, your avatar needs to be wearing the correct protective gear. For example, if you try to handle liquid nitrogen with latex gloves instead of insulated ones, your lab mates get annoyed and you lose “health points”.

Who is behind it?

The game was developed by the Division of Occupation Health and Safety within the US National Institutes of Health (NIH) to make safety training fun and engaging. The idea is that by playing an interactive video game, trainees will retain more information than they would if they just listened to a safety officer drone on about improperly stored gas cylinders for an hour and a half. (Not that we speak from personal experience.)

Who is it aimed at?

High-school students and undergraduates are the game’s main audience, but some quests could also be part of a refresher course for postgraduates or other new lab users. Annoyingly, there are no menus within the game that would allow you to choose which quests to complete, so you cannot skip irrelevant or too-simple ones once you have started playing. However, it is possible to tinker with the game files to delete quests before you begin; see the site’s FAQs page for details.

How useful is it for physicists?

Moderately. As you would expect from an NIH initiative, the game is primarily designed with microbiologists and biochemists in mind. Consequently, a few of the quests – such as operating a centrifuge and disposing of Petri dishes – will probably only interest the biophysicists in Physics World‘s readership. The game environment does include a laser room and a radiation lab, but unfortunately both are “dummy” areas that your avatar is not trained to access. This is a pity, because both the idea and the execution of STAR-LITE are excellent, and if these specialized rooms were made “live” (perhaps as an advanced game level), then it would be a great improvement. That said, the game’s designers have obviously tried to be as inclusive as possible, and quests such as storing chemicals, looking up information in material safety data sheets and identifying trip hazards are pretty much universal.

  • In this audio clip, Margaret Harris speaks to NIH representative Kerstin Haskell about lab safety and STAR-LITE.
Safety in the lab
Safety in the lab

 

Things that go bump in the night

By Tushna Commissariat

Looks like the Large Hadron Collider (LHC) at the CERN particle physics lab had an interesting few days last week, just before everybody left for Easter, and the Internet is now abuzz with rumours of an impending discovery.

But before we get into any of the highly interesting and debatable stuff, let's look at one thing that has definitely happened at the LHC.

Around midnight on Friday 22 April, the LHC set a new world record for beam intensity when it collided beams with a luminosity of 4.67 × 1032 cm–2s–1. This was significantly more than the previous luminosity record of 4.024 × 1032 cm–2s–1 held by the US Fermi National Accelerator Laboratory's Tevatron collider in 2010.

This new beam intensity was achieved after two weeks of planning and readying the collider. The machine is now moving into a phase of continuous physics scheduled to last until the end of the year when, after a short technical stop, the machine will resume running for 2012.

"Beam intensity is key to the success of the LHC, so this is a very important step," said CERN director Rolf-Dieter Heuer in a statement. "Higher intensity means more data, and more data means greater discovery potential."

But didn't I read all about some record being broken by the LHC last year, you ask? Yes, but that was the LHC accelerating its proton beams to 3.5 TeV each, leading to later collisions at 7 TeV. Now it is the beam intensity or the "luminosity" that is record breaking. Luminosity gives a measure of how many collisions are happening in a particle accelerator. So the higher the luminosity, the more particles are likely to collide which is necessary while looking for rare particles like the infamous Higgs boson.

"There's a great deal of excitement at CERN today," said CERN's director for research and scientific computing, Sergio Bertolucci, "and a tangible feeling that we're on the threshold of new discovery."

Well, it looks like Bertolucci spoke a tad too soon, as on the same day a leaked memo posted by an anonymous commenter on mathematician Peter Woit's blog, claimed that certain researchers at the ATLAS experiment at CERN had seen firm evidence for the Higgs particle in recent data.

The memo, though not official by any means, was authored by four ATLAS members who claimed to have seen an excess number of photons produced at energy of 115 GeV that could be caused by the decay of the Higgs particle into photons.

Surprisingly, only a few websites and blogs mentioned the news for the first day or so, before slowly more people seemed to notice this juicy story of physics, Higgs and betrayal!

On 25 April, Nature reported on its blog, an official statement from ATLAS spokeswoman Fabiola Gianotti. Gianotti said "Only official ATLAS results, i.e. results that have undergone all the necessary scientific checks by the collaboration, should be taken seriously." She went on to say that signals of the kind reported in the memo show up often during data analysis and are later falsified after more detailed scrutiny.

But the damage had already been done as physicists and others began to comment on the legitimacy of the claim made in the memo and the ethics of such an internal memo being posted and talked about online.

As people began to look deeper into the memo, interesting facts began to creep up.

Tommaso Dorigo, from the University of Padova in Italy wrote an initial post on his blog A Quantum Diaries Survivor that turned into a debate and eventually a bet! His post was sceptical from the start and he gave his reasons for why he was sure it as nothing more than a blip in the data, then went on to explain in more detail what other data already exists.

After that, a regular reader of his blog pointed out that the authors of the ATLAS study are actually physicists from Wisconsin, and include a Professor Wu, "who was among those less happy of the decommissioning of LEP [the Large Electron-Positron Collider] at the time when they were claiming a possible Higgs signal at 115 GeV. So maybe these guys have been looking for some confirmation of the 115 GeV Higgs all along".

Woit too was quick to distance himself from the memo saying that "it should be made clear that, while members of ATLAS work here at Columbia, I have no connection at all to them, and they had nothing to do with this. The source of the abstract posted here anonymously as a comment is completely unknown to me."

As more people debated and commented over the memo, Dorigo came back to say that he would bet anyone who "has a name and a reputation in particle physics (this is a necessary specification, because I need to be sure that the person taking the bet will honour it) that the signal is not due to Higgs boson decays" and then updated that comment by saying that if he is wrong he would pay $1000 but that if he is right he would be given only $500.

Meanwhile, Channel 4 conducted an interview with Jon Butterworth, a particle physics professor at University College London, who also works at ATLAS. He went on to say the same thing; that nothing would be definitive until it was scrutinized by CERN officially (look above).

So at the end of the day, it looks like the world is going to have to wait a while longer before Higgs boson gets its official post in the Standard Model hall of fame.

Does dark matter link gamma rays to galactic haze?

Annihilating dark matter at the heart of the Milky Way could account for signals detected by two space telescopes, according to a pair of US physicists. If true, the theory provides a new indirect measurement of one of astronomy's most elusive entities. However, some physicists believe that we don't know enough about the galactic core – or dark matter – to come to this conclusion.

In recent years the Fermi Gamma-ray Space Telescope has detected a flux of gamma-rays from the centre of our galaxy and the Wilkinson Microwave Anisotropy Probe (WMAP) has revealed a "haze" that also seems to envelop the Milky Way's core. Now, Fermilab's Dan Hooper and Tim Linden believe that dark matter can tie these two observations together.

Although dark matter is thought to make up more than 80% of all matter in the universe, it does not interact with electromagnetic radiation and its existence has been inferred from its gravitational effects on visible matter. The precise nature of dark matter is unknown and over the years many contenders have been put forward to account for it. The option currently favoured by most physicists, including Hooper, is as yet undiscovered weakly interacting massive particles (WIMPs).

Annihilating WIMPs

"If you get two WIMPS in the same place at the same time they interact in such a way that both get destroyed and liberate normal energy," Hooper explains. He believes that this process supplies the gamma-rays observed by Fermi. The process would also create large numbers of electrons and positrons at the core of the Milky Way. Hooper and Linden believe that these particles interact with the galaxy's magnetic field to produce a haze of radiation – and have modelled what they expect this radiation profile to look like. "Our model falls right on top of the WMAP data; they are in remarkably good agreement," says Hooper.

Some researchers, however, believe it isn't quite that simple. "The galactic centre is very hard to model; we don't know much about it. There are also controversies about the nature of the Milky Way's magnetic field," explains Julien Lavalle, from the Institute of Theoretical Physics in Madrid. "This makes it very difficult to claim whether dark matter is responsible or not," he adds.

There might also be problems with the mass of WIMPs used by Hooper and Linden. Their WIMPs weigh in at 7–9 GeV, which is in line with possible dark-matter signals seen in the ground-based CoGeNT and DAMA/LIBRA detectors. However, a heated debate has been raging over the veracity of these detections, as physicsworld.com reported last year ("Dark matter 'no result' comes under fire"). Indeed, the debate's latest instalment has seen the CoGeNT and DAMA/LIBRA signals coming under increased fire from the XENON100 experiment (WIMP no-show casts a shadow over dark matter").

Fighting his corner

However, Hooper is prepared to fight his corner. "I am still not entirely convinced by the XENON100 argument. There are reasons to think that due to the atomic properties of xenon [the element used in the detector], such a low energy WIMP interaction would leave no observable trace," he argues.

But recent work (arXiv:1104.0679) by Martin Winkler and Rolf Kappl of the Technical University of Munich has put the matter under further scrutiny. If Hooper's flavour of WIMPS exists then they should be efficiently captured by stars, including the Sun. Their annihilation in the Sun would produce energetic neutrinos detectable from Earth. Winkler and Kappl used the Super-Kamiokande neutrino detector to search for such high energy solar neutrinos. So far the search has yielded nothing. "We can constrain Hooper's model substantially, but not rule it out," Winkler told physicsworld.com.

Hooper still believes he is on the right track though. "Neither my paper nor anyone else's is claiming a smoking gun for dark matter. But certainly, of all the signals that have come and gone over the years, it is this collection [Fermi, WMAP, CoGeNT, DAMA/LIBRA] that I find by far the most compelling," he says.

Hooper and Linden's findings are described in Phys. Rev. D 83 083517 and arXiv:1011.4520.

The planet hunter

In a special audio interview (below), Michael Banks catches up with astronomer Alan Boss from the Carnegie Institution in Washington to talk about the hunt for a second Earth. Indeed, Boss thinks that Kepler may have already spotted a planet in the 1235 candidates with a similar mass to Earth, which orbits at a distance from its star where life could flourish. "Kepler has a couple of candidates. But we have to wait three or four years to see enough transits to make it believable," says Boss. "The enticing thing about Kepler is that we know it has the precision to find Earth-like planets."

Kepler is also teaching us more about the myriad of stars in the universe and Boss says that the mission may even spot the first moon outside our solar system. "Finding a moon would be a bit difficult although one would hope that Kepler could do it", says the 60-year-old astronomer. "As Kepler can see a lot of gas giant planets, if one of those has an Earth-sized moon to it then it could potentially be seen in the Kepler data."

Not resting on his laurels, Boss is already planning for the next mission after Kepler to start examining our nearest Earth-like planet and calls for funding to start building such a probe. "Once Kepler shows us that Earth-sized planets are common then there will be more pressure from scientists and the public to say 'why don't we go out there and study those that are close'," says Boss. "Scientists know how to do this and they just need the money."

Planet hunting
Planet hunting

Boss is the author of two books – Looking for Earths: The Race to Find New Solar Systems and The Crowded Universe: The Search for Living Planets. He also wrote a feature on extrasolar planets for the March 2009 issue of Physics World, which marked the 2009 International Year of Astronomy.

Once a physicist: Rob Cook

Why did you decide to study physics?

I became interested in it in high school when I read a book on relativity. I thought it was the most fascinating thing around, and I was hooked.

How did you get into computer graphics?

After I graduated from Duke University in 1973, I was not sure what I wanted to do. However, I had learned to program computers as part of a lab course, so I found a job at the Digital Equipment Corporation in Massachusetts. There was one person there who was doing computer graphics, but he was actually more interested in medical databases, so I said I would do graphics instead. After I got into it, I thought "This is great, this is what I want to do", so I went to Cornell University to get a Master's in computer graphics.

How did you get involved in film?

At that time, images that were made using computer graphics looked really artificial, like plastic, and nobody knew why. It turned out that the model they were using for light reflecting off surfaces was just something someone had made up – it was not based on physics at all. So for my thesis, I used a different model that included the physics of how light reflects off surfaces. The results looked really good: I was able to simulate particular types of materials and really get control over the appearance of the surface. That caught the attention of Lucasfilm, which was just setting up a computer-graphics division, and it hired me.

What inspired you to develop RenderMan?

When you look around, you notice that most things are not just made of one material such as bronze or ivory. They are more complex than that: they have multiple materials, they are beaten up, they have scratches. We needed to give artists control over those surface appearances, so I worked on something called programmable shading that uses equations to describe how a surface looks, but also builds a framework over them to allow artists to make really complex, rich surfaces. That is at the heart of what we do with RenderMan, and over the last 16 years, every film nominated for visual effects at the Academy Awards has used it.

How has your training in physics helped you?

Aside from my thesis work, it also helped when we were developing RenderMan. In computer graphics, you have a virtual camera looking at a virtual world, and for special effects you want to match this with live-action footage. But for it to look convincingly real, you have to get the characteristics of your virtual camera to match those of the physical camera. That turns out to be hard for a number of reasons. One is something called "motion blur": when a physical camera takes a picture, it opens the shutter and a certain amount of time goes by before it closes. During that time, things move, and this causes the image to blur. This blur turns out to be really important for making the motion look smooth, so you have to simulate it in the renderer.

Another thing you have to simulate is the aperture of the lens – the light is not entering the camera in one spot, but all over the lens, and that gives you depth of field. You need to simulate both blur and lens effects, but that means that not only are you integrating the scene around each pixel, you also have to integrate that pixel over time and over the lens and over other things. You end up with this incredibly complex integral, and it turns out that there is a technique in physics called Monte Carlo integration that is perfectly suited to dealing with it.

However, none of this stuff was in the undergraduate curriculum – I had to learn it on my own later. What physics really taught me was how to think about things in a creative and rigorous way. It taught me how to think about hard problems.

Are there other ways that physics is used in animation?

At Pixar, we use physics a lot to simulate the motion of complex things like clothes or hair. But you have to remember that in the animated world, animators do wacky things. They exaggerate enormously in order to tell the story, so the physics has to be reworked so that it can apply to this very non-physical cartoon world. For example, in Monsters Inc there is a scene where Mike is training Sulley to scare twins in a bunk bed. As Sulley is popping from the top to the bottom, there are several g's worth of force on his hair! In the animated world, you can do that, because it looks better. These are the sorts of effects that make us love animation.

What would you like to be able to do with computer graphics that you cannot do now?

One thing that remains a challenge is a simulated human. We have come a long way, but it is hard because our brains are wired to perceive other humans. If the simulated person looks cartoony, we are willing to accept it not being perfect, but as things get more realistic, a part of our brain that responds to people gets involved and then we are very picky about what we see: if things look a little off, they look creepy. That is a challenge, but I think we are almost there.

Any advice for today's physics students?

I always advise people to do something they really love because you are likely to be better at it and you are going to spend a lot of time doing it, so it should be something you genuinely enjoy. I think it is a mistake to decide "I'm going to go into this even though I don't really like it that much because I think it's going to be a good career". It is your life, and you want to spend it doing something you love.

Plants like we have never seen them before

100 supercon.jpg
Gliese 667 is one of two multiple star systems known to host planets below 10 Earth masses. (Courtesy: ESO/L Calçada)

By Tushna Commissariat

If you have thought about planets with two or more suns ever since you saw the dual suns of Tatooine in the first Star Wars film, looks like you are on the same wavelength as some astrobiologists. Jack O'Malley-James, a PhD student at the University of St Andrews, Scotland, has been studying what kind of habitats would exist on Earth-like planets orbiting binary or multiple star systems. He shared his results with peers at the RAS National Astronomy Meeting in Llandudno, Wales on Tuesday 19th April.

O'Malley-James and his team have been running simulations for planets that would orbit multiple star systems and trying to understand the kind of vegetation that might flourish there, depending on the type of stars in the system. Energy via photosynthesis is the foundation for majority of life on Earth, and so it is natural to look for the possibility of photosynthetic processes occurring elsewhere.

With different types of stars occurring in the same system, there would be different spectral sources of light shining on the same planet. Because of this plants may evolve that photosynthesize all types of light, or different plants may choose specific spectral types. The latter would seem more plausible for plants exposed to one particular star for long periods, say the researchers.

Their simulations suggest that planets in multi-star systems may host exotic forms of the plants we see on Earth. “Plants with dim red dwarf suns for example, may appear black to our eyes, absorbing across the entire visible wavelength range in order to use as much of the available light as possible," says O'Malley-James. He also believes the plants may be able to use infrared or ultraviolet radiation to drive photosynthesis.

The team simulated combinations of G-type stars (yellow stars like our Sun) and M-type stars (red-dwarf stars), with a planet identical to Earth, in a stable orbit around the system, within its habitable “Goldilocks zone”. This was because Sun-like stars are known to host exoplanets and red dwarfs are the most common type of star in our galaxy, often found in multi-star systems, and are old and stable enough for life to have evolved.

While the binary systems were not exact copies of any particular observed systems, plenty of M-G star binary systems exist within our own galaxy. O’Malley-James calculated the maximum amount of light per unit area- referred to as the “peak photon flux density” from each of the stars as seen on the planets for each set of simulations. This was compared to the peak photon flux density on Earth to determine whether Earth-like photosynthesis would occur.

Factors like star separation were taken into consideration, to give the best possible scenario for photosynthesis. “We kept the stars as close to the planet as we could, so that there would be a useful photon flux from each one [star] on the planet's surface while still maintaining a stable planetary orbit and a habitable surface temperature,” says O’Malley James.

Copyright © 2026 by IOP Publishing Ltd and individual contributors