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Voyager – a mission for life

In the summer of 1977, as the strains of David Soul, Abba and Hot Chocolate wafted from their transistor radios, most physics students were busy cramming an extra pair of flares into their rucksacks in preparation for a trip on the highways of North America or the railways of Europe. Few would have been aware that two Voyager spacecraft were about to be launched on journeys of their own, on 20 August and 5 September 1977 respectively, towards the gas giants of our solar system. Yet even avid followers of the development of these pioneering planetary probes could not have possibly guessed that the Voyager mission of exploration would continue, unabated, for the duration of their professional careers.

One of those young physicists was Linda Spilker, who had graduated earlier that year and applied for a job at the Jet Propulsion Laboratory (JPL) in Pasadena, a peaceful and relatively green suburb of the Los Angeles conurbation. “My interview went well and I was offered a job,” recalls Spilker. “I had a choice between working on the Viking extended mission or on a new mission called Voyager. Having never heard of Voyager, I asked ‘Where is Voyager going?’ ” When she heard the craft were going to Jupiter, Saturn and maybe on to Uranus and Neptune, she immediately replied: “Sign me up!”

The JPL project scientist for Voyager at the time – and the man still most likely to be identified with the mission – was Edward Stone. Stone had joined the California Institute of Technology (Caltech), which operates JPL under contract to NASA, in 1964 to help establish a research programme in space physics. Today, he is a professor of physics at Caltech and – as testament to his long-standing contribution to the mission – remains the Voyager chief scientist at JPL. As for Spilker, she worked on Voyager from early 1977 until it flew past Neptune in 1989 and is now the project scientist for the Cassini mission to Saturn, with which she has been involved since 1988.

A cursory glance at the career timelines of these two scientists suggests that long-term NASA missions retain the concept of a “job for life” familiar to workers of the 1970s. But given that planetary missions are years in the planning and the resulting spacecraft then take years to reach their targets, what is it that keeps planetary scientists focused on the task and sufficiently interested to devote a good portion of their careers to what is basically a hi-tech form of data collection?

Grand ambitions

The 1970s were heady days in space exploration for NASA, with the Apollo lunar programme taking most of the limelight at the beginning of the decade. But scientists were already looking further afield. Spacecraft fly-bys of Venus and Mars had been successfully conducted as early as 1962 and 1965, by Mariner 2 and 4 respectively, and preparations were under way for the Mars-Viking landing mission. Next in line were the gas giants.

Large panel display showing multi-coloured text on a black background. — Old timers and young guns

As a “first feel” beyond the asteroid belt, which encloses the inner four planets (Mercury, Venus, Earth and Mars), a pair of 250 kg spacecraft called Pioneer 10 and 11 were launched towards Jupiter in 1972 and 1973, and they conducted fly-by missions in 1973 and 1974, respectively. A combination of powerful rockets, favourable launch windows and relatively lightweight spacecraft produced uncharacteristically fast journey times, which gave the misleading impression that solar system exploration could be done on the timescale of a typical PhD.

Meanwhile, funding was becoming an issue and an extravagant “Grand Tour” mission to Jupiter, Saturn, Uranus, Neptune and beyond – planned as a result of a fortuitous alignment of the planets – was shelved. However, all was not lost. As Stone explains, “When NASA decided in early 1972 that the budget would not support such a mission, JPL proposed ‘Mariner Jupiter Saturn 1977’, a much lower-cost four-year journey to Jupiter and Saturn”. The mission was later renamed Voyager and in 1977 the sister probes were launched to Jupiter and beyond.

Following fly-bys of Saturn in 1980 and 1981, Voyager 1 was targeted out of the plane of the solar system and veered off towards its edge, while Voyager 2 continued the tour to Uranus (in 1986) and Neptune (in 1989, some 12 years after launch). According to Stone, the possibility of exploring beyond the planets was part of the scientific planning for the Grand Tour. By 1970 astronomers had realized that the supersonic expansion of the solar corona, known as the solar wind, creates a giant plasma bubble around the Sun. Called the heliosphere, the bubble envelops all of the planets and serves as a shield against the interstellar wind, impeding the entry of cosmic rays from outside. “A mission to interstellar space would provide the first opportunity to directly determine what is outside pressing to get in,” says Stone, explaining the reasoning for splitting the Voyager mission.

Voyager 1 officially began the Voyager Interstellar Mission on 1 January 1990, passing the long-defunct Pioneer 10 to become mankind’s most distant artefact on 17 February 1998.

Knowing the unknown

At a fundamental level, the motivation behind these planetary missions is part of humankind’s unending quest to know the unknown. Stone characterizes this quest by dividing what Star Trek termed “the final frontier” into three: expanding our knowledge of all that is around us (the knowledge frontier); developing new systems to observe the solar system (the technology frontier); and actually sending those craft to new places (the physical frontier). “Expanding these three frontiers of space often involves long journeys that are not without risk,” says Stone, “but the opportunities to learn about the diversity of objects in the solar system are unequalled.” Stone’s analysis is thoughtful, but fails to explain why some scientists dedicate much of their professional lives to a long-term mission such as Voyager.

Spilker says her 12-year stint on Voyager was fuelled by a sense of “being an explorer, of seeing worlds and vistas that no-one had seen before”. For her, the thrill lay in witnessing volcanoes on Io and geysers on Triton, plotting the structure in Saturn’s rings and being the first to see the Uranian satellites and rings up close. “Each fly-by left me with a sense of wanting to see more,” she says.

Left: Bright orange-and-yellow image showing bird's eye view of a volcano; dark-orange lava steams emanate from a black crater. Right: Beautiful marble-like swirls of blue, white and brown; a large, brown oval dominates at top-right, covering a quarter of the image. — Worlds and vistas

Planetary scientists are apt to use phrases such as “we are now at Jupiter”. Of course, they mean their spacecraft and its scientific instruments are at Jupiter, but there is more to it than that. They seem to transport themselves to the planet in question, vicariously enjoying the view from imaging telescopes on board the spacecraft. However, they have to be patient. Although Spilker stayed with Voyager until the Neptune encounter in 1989, she had already become involved with planning the Cassini mission to Saturn, a liaison that continues to this day. While the pace of Voyager’s progress through the solar system had not exactly been brisk, it had not prepared her for the protracted timescale of Cassini. “One of the negatives of working on Cassini was that I had to wait much longer for, first, the launch in 1997, then the long trip to Saturn, before going into orbit and getting science data in 2004,” she says.

The reason for Cassini’s lengthy seven-year cruise to Saturn is one of basic physics: specifically, the relationship between mass, velocity and inertia. In the two decades since Voyager was launched, spacecraft had developed significantly in terms of complexity, and thus mass, in order to encompass the enhanced expectations of space scientists. Cassini carried the European Space Agency’s Huygens probe, which was dropped onto Saturn’s Moon Titan, bringing the total launch mass to some 5600 kg – eight times that of Voyager. Not only did this mass have to be accelerated out of Earth’s gravity well, but it also had to be decelerated at Saturn, which required additional onboard propellant and hence yet more mass. The solution was to fly a gravitational-assist or “sling-shot” trajectory that exchanged momentum first with Venus and then Jupiter to provide propulsion without expending propellant. In the event, Cassini performed gravity-assist fly-bys of Venus in 1998 and 1999, of Earth in 1999 and of Jupiter in 2000, finally arriving in orbit around Saturn in July 2004. When the Huygens probe eventually parachuted through Titan’s atmosphere to land on its alien surface, it produced some fascinating science (see “Tuning into Titan” February 2006 pp20–23).

All in the family

Despite the wait, Spilker is positive about being involved in long-term missions. “It was exhilarating to be part of designing the next Saturn mission from the ground up, seeing the instrument teams selected, the spacecraft built and then launched,” she says. Equally important, however, is the “sense of family” that develops during the years of working on long-term missions. “This strong connection to the Voyager family (and now the Cassini family) is one of the things that made me want to stay for the long haul with both missions,” says Spilker. “Even though I have moved to Cassini, I still root for Voyager to reach the edge of the solar system and read about its latest accomplishments with a great deal of pride and pleasure.”

This sense of family is further illustrated by a tendency to anthropomorphize the spacecraft. Spilker remembers the time a spacecraft automatically switched itself into “safe mode” because its onboard computer detected an anomaly. “I started rooting for it to recover…I worried about the health of Voyager as though it was a friend or even one of my children.”

White satellite dish in space, with smaller golden objects attached behind it and three long golden arms pointing out from the satellite base. — Far out

But what about keeping up with one’s real family? Balancing a profession and a family can be challenging at the best of times. “Personally, I had to interweave my personal life with my professional life,” admits Spilker, which affected the timing of starting a family. “When I tell my daughters that their births are based on the alignment of the planets, I mean that! There was a five-year window between the Voyager Saturn fly-by in 1981 and the Uranus fly-by in 1986 when I had both of my daughters. Other Voyager moms made similar choices.”

Still, in common with many professions, space science can mean long hours away from the family and outside life. “Working on planetary missions provides a cadence for your life that is driven by the needs of the mission,” says Spilker. “When a mission is active – such as during a planetary fly-by, planetary orbit insertion or roving on the surface of a world – activities outside JPL become secondary for a time and the focus is on the success of the mission.”

On the other hand, Spilker was able to share the excitement of her job during family visits and “take your daughters to work days”. And just like any other group of mothers, the Voyager parents took their children to the same schools, attended the same daycare and often compared notes about raising a family.

Apart from having children, what do space scientists do to occupy themselves during the long hiatus of the cruise phase? According to Stone, for Voyager this was not as much of an issue as first appears. “With six encounters in the first 12 years, the 11 science teams were kept very busy analysing data from each encounter while also planning the observational sequence for the next.” He concedes, however, that when Voyager 1 began heading for interstellar space in 1990, the pace of discovery changed.

In fact, most space scientists are involved with more than one mission at a time, to an extent dependent on the mission phase. For example, during the 40 years of Stone’s association with Voyager, he has also participated in missions such as Galileo (to Jupiter), SAMPEX (studying the Earth’s magnetosphere), ACE (looking at energetic matter from the solar wind and interplanetary space) and STEREO (studying the Sun), the latter two of which are still operational.

Moreover, the results from one mission can continue to be useful long after data collection. According to Spilker, during the long cruise phase of Cassini, “many of us went back to further study the Voyager data to look for clues on how to use our new instruments”. In parallel, she worked on concepts for a mission to Mercury and for orbiters around Uranus and Neptune.

Job for life?

So how do today’s young scientists view the prospect of working on such long-term missions? Understandably, a seven-year cruise must seem like a lifetime when you are 21. Yet Spilker is surprised by how many young people want to work on long-term missions. “On Cassini, we had more than 50 scientists, the vast majority early-career scientists, competing for those spots,” she says.

One of them was Estelle Deau, who arrived in post just as Cassini was about to enter orbit. “At first I thought the timing was perfect,” she says, “but I was not fully prepared for the mission, especially the observation planning.” In retrospect, she admits she would have preferred to have been there from day one, to be able to prepare her own observation plan. Clearly, the sense of “data ownership” is strong, even among the young.

Four people sit on one side of a long table facing an unseen audience. A backdrop to the people shows artist’s impressions of the Voyager spacecraft. — Going strong

As a result of this competitive spirit, the qualification density at JPL is fairly high. Speaking in May at the Spacecraft Technology Expo in Los Angeles, deputy director Eugene Tattini said that 31% of employees have a PhD, 33% a Master’s or similar and 27% a first degree; only 9% have no degree. “Everyone at JPL has a doctorate except the janitor and the deputy director,” he joked. “The difference is the janitor is working on his dissertation!”

In confirmation that young people remain attracted to space science, Tattini painted JPL as a cross between an academic institution and a Silicon Valley start-up. “A discovery is typically made at 3 a.m. in a lab, by a person who probably never wore long pants,” he said, with a nod to their preference for beach shorts and other less formal attire.

To boldly go…

Since the Voyager Interstellar Mission began in 1990, Voyager 1 has been speeding towards the notional edge of the solar system at around 60,000 km/h, or some 3.6 AU per year (where one astronomical unit, or AU, is the Earth–Sun distance of 150 million km). In December 2004 it crossed a feature known as the termination shock, at about 94 AU, and entered the heliosheath (see “Voyagers probe the heliosphere”). It is currently some 18 billion km (122 AU) from Earth, at a point where its radio signals, travelling at the speed of light, take an unbelievable 17 hours and 19 minutes to reach us. (Check where it is right now at http://voyager.jpl.nasa.gov/where/index.html.)

The location of the heliopause – the boundary with interstellar space – is not known, but it has been estimated that Voyager will reach it about 10 years after crossing the termination shock, which could be anytime soon. Indeed, according to Stone, the spacecraft has already been monitoring a gradual increase in galactic cosmic rays for the past three or four years, and from May this year, “cosmic ray hits have increased 5% in a week and 9% in a month”. Although that increase does not prove anything yet, scientists expect a precipitous drop in the number of energetic particles detected from within the heliosphere as the spacecraft crosses the heliopause, and that number has already been falling. A further indication of the breakthrough will be a change in the orientation of magnetic field lines, which are expected to rotate from east–west towards a more north–south direction.

At the middle of this image is a depiction of the solar system, showing the planets and their orbits. This is fully enclosed by a large blue circle, the boundary of which is labelled as the termination shock. This circle sits inside an even larger circle shaded in mottled blue, the boundary of which is labelled as the heliopause. Dashed yellow lines show the curved paths Voyager 1 and Voyager 2 have taken from Earth: Voyager 2 is just past the termination shock but Voyager 1 has travelled further and is almost at the heliopause. Beyond the heliopause is black space speckled with stars. Arrows superimposed across the whole image are labelled as the interstellar medium and travel from top-left to bottom-right; arrows outside the heliopause are straight, while those crossing it are curved as if cupping the circles. — Voyagers probe the heliosphere

“When the Voyagers launched in 1977, the space age was all of 20 years old,” reflects Stone. “Many of us on the team dreamed of reaching interstellar space, but we really had no way of knowing how long a journey it would be – or if these two vehicles that we invested so much time and energy in would operate long enough to reach it.”

Spilker remains optimistic about the prospect: “When I think of the Voyagers today, I think of the Energizer Bunny. They keep going and going…to the very edge of the solar system.”

Most physicists will appreciate the excitement of the search for another “data point” in their own particular field, but probably find it more difficult to appreciate a similar thrill in another field. Stone helps to put Voyager’s continuing mission in context. “Detecting the edge of the heliospheric bubble will be a historic milestone in humankind’s longest journey. Inside the bubble the solar wind and the magnetic field come from the Sun; that’s where the Voyagers have been for the last 35 years and where all of the planets and most of the Kuiper Belt Objects are. Outside, the wind is from the supernova explosions of nearby stars that occurred five to 20 million years ago.”

The only question remaining is how much longer the Voyagers can last. Will they ever be pensioned off – or are they, like the people who devote their careers to them, on a mission for life?

Ancient gas sheds light on universe’s first billion years

For the first time, astronomers have determined the chemical composition of gas from the first billion years of the universe’s life. The gas consists mostly of neutral hydrogen atoms, which means that it may mark the era before stellar radiation began ionizing the universe. Furthermore, the gas shows no signs of the heavy elements that are forged in stars so it may contain only the light elements produced by the Big Bang.

“We are starting to look back to the epoch that is probably when the first stars were turning on,” says Robert Simcoe, an astronomer at the Massachusetts Institute of Technology who built the instrument that acquired the spectrum of the far-off gas. “This is the very first [chemical] measurement that anybody has made in any environment at these early times.”

The Big Bang, which occurred 13.7 billion years ago, showered the cosmos with hydrogen and helium. Aside from a trace of primordial lithium, heavier elements – which astronomers call metals – arose later, after stars formed and exploded, casting oxygen, iron and other metals into space. Furthermore, the first stars radiated extreme ultraviolet light that ionized gas, tearing electrons from the hydrogen nuclei. The universe is still ionized today.

Farthest known quasar

To analyse the ancient gas, Simcoe and his colleagues examined the farthest known quasar. Named ULAS J1120+0641, this quasar resides in the constellation Leo and is so distant that its light travels 12.9 billion light years before reaching Earth. We see the quasar as it was just 770 million years after the Big Bang.

While the quasar’s light races toward Earth, space between the quasar and us expands, stretching the wavelength until it is 708% longer than it was at the start of its journey. Thus, astronomers say the quasar has a redshift of 7.08. Because of the high redshift, what was once far ultraviolet radiation appears to us at infrared wavelengths. To obtain the infrared spectrum, Simcoe’s team used the Magellan telescope in Chile.

The gas itself is much too faint to see. But quasars mark the brilliant centres of some galaxies, so Simcoe’s team searched for wavelengths at which intervening gas absorbed the quasar’s light. The gas is probably a few million light years from the quasar, he says, and so has no connection to it.

Two tantalizing scenarios

Simcoe presents two different scenarios to explain his team’s findings. “Either is interesting,” he says. The more likely: we are seeing the diffuse gas that pervaded space 770 million years after the universe’s birth. If so, then because the gas is neutral, it means astronomers have reached the epoch before reionization had fully occurred. Furthermore, the lack of detectable metals indicates the gas has no more than 1/1000 the metal-to-hydrogen ratio, or metallicity, of the Sun.

Alternatively, the gas could exist in a protogalaxy that happens to lie in front of the quasar. In that case, the metallicity of the gas is less than 1/10,000 solar – comparable to the most primitive star known in the Milky Way’s halo.

“This observation is bringing us directly to where the evolution of the cosmic metallicity started,” says Michele Fumagalli, an astronomer at the Carnegie Observatories in Pasadena, California, who was not part of the research team. “This is very exciting, and I think we are now really entering a new frontier for these types of studies.” Last year, Fumagalli and his colleagues discovered metal-free gas that existed two billion years after the Big Bang – a surprise, since all other gas at that time contains metals. In contrast, the new work reaches much further back in time.

Huge galaxy in the making?

However, Abraham Loeb, chairman of the astronomy department at Harvard University, offers a third explanation for the new observation: the gas is falling onto the galaxy that hosts the quasar. “Usually these very bright quasars reside in massive galaxies, and these massive galaxies grew dramatically during early times through in fall of gas,” Loeb says, noting that the observed gas differs in velocity from the quasar by only 711 km/s. If his idea is right, then we are watching the construction of one of the largest galaxies in the universe.

Whatever the case, Simcoe says, “We have got to find some more of these things. When you look at the first of something, you hope that it is representative, but until you see a few more, you cannot say for sure.”

Although astronomers have glimpsed galaxies at greater distances than this quasar, these galaxies are too faint to use in studies of foreground gas. Instead, such investigations require additional far-off quasars, which serve as brilliant background beacons whose radiation probes the nature of gas from the dawn of time.

Simcoe and his colleagues are publishing their work online today in Nature.

Special report: physics in India

By Matin Durrani

The latest Physics World special report, which examines the challenges for physicists in India, is ready for you to read online now.

The report contains a great mix of news, features and opinion, including a look at the work carried out a top research centres such as the Indian Institute of Science, the Tata Institute of Fundamental Research and the Raman Research Institute.

It also has a great podcast on “India’s physics rebels” – the students who resist the pressure to study engineering and let their passion for physics burn instead.

For the record, here’s a list of the main articles in the report.

Welcome to science city – Why is Bangalore home to so many top science institutes?

Igniting a passion for physics among India’s top students – What the Indian government is doing to get more students turned on to science

New horizons for the Tata institute – How one of Mumbai’s leading research centres has ambitious plans to expand into Hyderabad

Speaking up for women – An interview with Shobhana Narasimhan from the Jawaharlal Nehru Centre for Advanced Scientific Research in Bangalore

India sticks to the thorium trail – Why thorium is still so central to India’s energy plans

India sets its sight on Mars – Opinions are still divided over the country’s bold Martian plans

Digging deep for neutrinos – A look at India’s ambitious plans for a huge underground neutrino detector

Uniting Indian astronomy – An interview with Ajit Kembhavi from the Inter-University Centre for Astronomy and Astrophysics in Pune

Delivering on a promise – Shiraz Minwalla from the Tata Institute of Fundamental Research says that India must urgently reform its education system.

The report reveals that money for India’s top physicists is thankfully not in short supply, but what India currently lacks is a critical concentration of highly capable scientists who can make the country a world leader in research and boost its innovation.

I hope you enjoy reading the report – and do let me have your comments by e-mailing pwld@iop.org.

Love and physics on film

By Matin Durrani

I wrote last week about the imminent launch of a new five-minute online film about particle physics, cosmology and love called The Theory of Everything.

I’d been to the launch in Covent Garden and quite liked the film, but some of my colleagues groaned that it sounded incredibly cheesy and that I might have been brainwashed in my judgement by meeting the cast and crew at the premiere.

Well, the film has just been released on YouTube so it’s now time for you to judge for yourself.

The company that made the film also has a Facebook competition to win a trip to see the Northern Lights.

By the way, the film wasn’t really filmed in Chile as the video suggests, but at an observatory in Mill Hill in London.

India’s physics rebels

For school-leavers in India with a flair for maths and science there is usually only one sensible choice: get an engineering degree, which will almost guarantee a well-paid job in industry. In sharp contrast, natural-science degrees such as physics have become viewed as something you might only do if you failed to get onto an engineering course. To find out more, I recently travelled to India to meet some of these student “rebels” who have rejected the glamour of engineering to instead pursue their passion for the physical sciences. In this documentary, the students talk about their motivations, their ambitions, and the pressures that come with living in one of the most populous – and economically polarized – societies on the planet.

On my journey through the state of Maharashtra I also met some of the nation’s academics and educators to find out whether anything is being done to encourage more students to consider careers in fundamental science. The Indian Prime Minister Manmohan Singh has declared the 2010s a “decade of innovation”, which he believes will be powered by an overhaul of science education – but does the rhetoric match the reality? In addition to this podcast, you can read about the state of science education in India, and about the country’s leading physics labs, in this Physics World special report.

Semiconductor funnel could boost solar cells

Computer simulations by researchers in the US and China could lead to solar cells that work efficiently across a broad range of the solar spectrum. Dubbed a “solar energy funnel”, the new concept offers a way of using strain to modify the band gap of a semiconductor so that it responds to light within a range of different wavelengths. However, the funnels have yet to be made and tested in the lab – some researchers suggest using them in practical devices could prove problematic.

The basic operating principle of a solar cell is that an electron in the valence band of a semiconductor material absorbs a photon and jumps across an energy “band gap” into the conduction band. The result is an electron and a positively charged hole, which do not move separately through the semiconductor but instead form a bound state called an exciton. To extract electrical energy, the electron is collected at one electrode and the hole at another.

Light from the Sun comes in a range of wavelengths and therefore an ideal solar cell should be very efficient at converting this broad spectrum into electricity. Unfortunately, semiconductors with a fixed band gap are not very good at doing this. In particular, longer-wavelength photons do not have enough energy to make an electron to jump the band gap and will not be converted into electrical energy. Photons with energies greater than the band gap will be converted, but regardless of their energy they will only create just one electron–hole pair. Any excess energy will be dissipated in the semiconductor as heat.

Tweaking the band gap

One way of getting around this problem is to create a cascaded solar cell comprising several layers of semiconductor, each with a different band gap. However, these are complex and expensive to produce. Now Ju Li and colleagues at the Massachusetts Institute of Technology, Peking University and Xi’an Jiaotong University say they have come up with a way of tweaking the band gap within a single layer of atoms. The technique is based on elastic strain engineering, a method that has been used by the electronics industry to boost the performance of silicon transistors.

While silicon crystals are not strong enough to sustain the elastic strain necessary for making better solar cells, some other semiconductors are. Molybdenum disulphide, for example, has a layered crystal structure – with individual layers being extremely strong. The material has a relatively large band gap of 1.9 eV at zero strain so most solar energy would pass through without being absorbed. However, when strain is added, the band gap shrinks continuously to 1.1 eV – which is identical to that of silicon.

The team made its calculations using computer models of the mechanical and electronic properties of single layers of molybdenum disulphide. Calculations were made on layers that are periodically indented with the tip of an atomic force microscope and then clamped at the edges to ensure that the tiny indentations stayed put. The calculations suggest that the band gap would vary periodically as a function of position on the structure. Furthermore, the simulations indicate that the wavy band gap would allow the sheet to absorb photons with a variety of different energies and “funnel” the resulting excitons towards the centre where the band gap was smallest. The team believes that this directional drift provides a means for photons with energies between 1.1–2 eV to be harvested efficiently by collecting excitons at multiple points in the funnel.

Driving excitons

Li explains that this gradient in the band gap would allow the excitons to be driven towards the electrodes electromagnetically. This would allow them to be collected more quickly than in a traditional solar cell design, where excitons diffuse towards the electrodes. This could make the solar cell more efficient, explains Li: “You want to absorb different parts of the spectrum and then collect the excitons before they recombine or lose their energy to phonons”. “And having this funnel and this exciton drift instead of a random exciton walk will assist in this exciton collection process,” he adds.

Di Xiao of Carnegie Mellon University in Pittsburgh is very impressed by the team’s prediction of continuous band-gap modification using elastic strain. However, he suggests that the group will encounter a practical difficulty in applying its research to real solar cells, which tend to be much thicker than a single layer in order to maximize light absorption: “In a traditional silicon solar cell…the light has to travel a very long way within that silicon, so it has a much larger chance to be absorbed. But in a monolayer, once it is passed through the material it is gone.”

Another researcher, an expert in solar-cell design who asked not to be named, was more sceptical, doubting that the concept would be of practical use in solar cells because of the difficulty of stopping light with an atomic monolayer. He was also doubtful that it would be feasible to harvest any extra energy by using the solar-funnel method.

The research is published in Nature Photonics.

Virtual phonons get real

An acoustic analogue of the dynamical Casimir effect (DCE) has been demonstrated for the first time. Carried out by physicists in France, the experiment involves converting quantum fluctuations into pairs of quantized sound waves – or phonons – in an ultracold atomic gas. The experimental system could boost our understanding of how radiation emerges spontaneously from a vacuum. Indeed, the team is keen on modifying the set-up so it could be used to simulate Hawking radiation, a type of spontaneous vacuum radiation that is created at the edge of black holes.

One of the more peculiar aspects of quantum mechanics is that the vacuum is never truly empty. Instead it contains a small amount of energy and is buzzing with particles that appear out of nothingness, only to vanish again. One famous consequence of this is the Casimir force, where two parallel mirrors positioned close together in a vacuum experience an attractive force. While the force was first proposed in 1948 by the Dutch physicist Hendrik Casimir, it is so small that it was not measured in the lab until 1997.

Separating virtual particles

In 1970 the American physicist Gerald Moore proposed the dynamical Casimir effect, which builds on Casimir’s original mirror system and shows how these virtual photons could be converted into real photons. The idea is that the phase of an electromagnetic wave goes to zero at the surface of a mirror. However, if the mirror is accelerated to a significant fraction of the speed of light, the electromagnetic field does not have time to adjust. The result is that the mirror can separate the virtual particles before they annihilate – keeping them in existence long enough to be detected.

However, accelerating the mirrors to these speeds in the laboratory has so far proved impossible. To get around this problem, Chris Wilson and colleagues at Chalmers University used a superconducting quantum interference device (SQUID) as an oscillating mirror – and in 2011 they claimed the first demonstration of the DCE in the laboratory.

Now Chris Westbrook and colleagues at the Charles Fabry Laboratory at the University of Paris-Sud say they have created the first acoustic analogue to the DCE – which involves virtual phonons rather than photons. Their experiment was inspired by theoretical work done in 2010 by Iacopo Carusotto of Italy’s University of Trento and colleagues. The Italian physicists argued that an acoustic dynamical Casimir effect should be seen in a Bose–Einstein condensate (BEC) when there is a rapid change in the scattering length that governs how its constituent atoms interact. A BEC is formed when identical bosons – particles with integer spin – are cooled until all particles are in the same quantum state. BECs are a good place to look for quantum effects because their extremely low temperature minimizes the effects of thermal noise.

Changing the speed of sound

The team created its BEC by cooling about 100,000 helium atoms to about 200 nK. Instead of changing the scattering length, the team found it could achieve the DCE by changing the speed of sound within the BEC. This was done by squeezing the BEC through rapidly increasing the intensity of the laser that traps the atoms.

This compression causes virtual phonons to become pairs of real phonons that propagate in opposite directions. These phonons cannot be detected directly. Instead the physicists switch off the laser and then measure the velocity of the atoms as they leave the cloud. This showed that excitations with equal and opposite momenta were moving through the BEC – excitations that were not seen when the BEC was not squeezed.

“Before I started doing this, I had heard of [the dynamic Casimir effect] and it sounded…unfathomably complicated,” says Westbrook. “Doing this shows that it is not. It is a concrete illustration of what can happen. And once you can get your mind around it you can start modifying the conditions and thinking about other things [like] Hawking radiation.”

Gobbling up sound

In 2009 Jeff Steinhauer and colleagues at the Israel Institute of Technology in Haifa produced an acoustic analogue to a black hole, which gobbles up sound instead of light. Westbrook says the team is particularly interested in combining the two systems to eventually create an acoustic analogue to Hawking radiation, a type of spontaneous vacuum radiation that takes place near the edge of black holes.

One potential flaw of the new experiment is that the DCE is seeded by thermal noise in the BEC – not by quantum vacuum fluctuations. This is because even at a chilly 200 nK, thermal effects are significant and therefore it could be argued that this experiment does not demonstrate the “pure” dynamical Casimir effect.

Steinhauer agrees that the goal should be to detect correlated phonons that are seeded by quantum fluctuations. But he says the research is a “good step” toward that goal.

Daniele Faccio at Heriot Watt University in Edinburgh, UK, agrees that the most important next step is for the team to lower the temperature of the BEC. However, Faccio says he feels that the current work is still a demonstration of the physics of the DCE.

“It is still a spontaneous emission of radiation, and it is a spontaneous emission that is being generated by a periodic changing boundary condition. So the physics are there,” says Faccio. “I think it is a beautiful piece of work. It is extremely useful.”

The experiment is described in Physical Review Letters.

Between the lines: Christmas special

Cartoon of poor communication and photo of magpie — The confused and the curious: Getting political about science, plus a nestful of fun science tidbits. (iStockphoto/brett lamb; iStockphoto/Ken Hoehn)

Geeks of the Earth, unite

Are you bothered by misleading science stories in the media? Annoyed when political leaders confuse particle physicists and physicians (ahem, David Cameron)? Irate at ministers who select policies, then cast about for “evidence” to support them? Then according to Mark Henderson, it is time you stopped shouting at the television and started doing something. With The Geek Manifesto, Henderson – a former science editor at The Times newspaper – has produced a rare beast: a polemical book that offers solutions as well as rhetoric. One of the book’s main arguments is that scientists should take it upon themselves to become more engaged in the political process. As one of Henderson’s interviewees puts it in the book, “Scientists tend to feel that politics is something that happens to them, not something they can influence.” In fact, if scientists are prepared to make the first move, they may find politicians more receptive than they had assumed. Henderson does not, however, advocate asking politicians point-blank questions about science and using their replies as a “litmus test” for geek support. In a statement sure to provoke spluttering fury among a few Physics World readers, he suggests that a politician who can answer the question “What have you been wrong about?” may have a better understanding of science than one who can remember Newton’s laws of motion. A minister who reconsiders policies in light of new evidence, and abandons ones that fail, Henderson argues, is following the scientific method. For that, they deserve a bit of geek support, even if they lack formal scientific training. The book has a heavily British slant, and it certainly seems to have struck a chord among the nation’s geeks. Last summer, a campaign to send The Geek Manifesto to MPs raised enough money to buy copies for all 650 members of the House of Commons – though it helped that Henderson’s publisher, recognizing a golden publicity opportunity, stumped up some matching funds. Still, “geek power” remains a limited force, as demonstrated by the fate of Eureka magazine, which Henderson helped to launch as a monthly science supplement to The Times back in 2009. In the book, Henderson often touts Eureka as evidence of growing geek power, noting that the magazine is “chock-full of high-value advertisers who want to reach its readers”. Unfortunately, Eureka was canned in October – apparently due to, erm, poor advertising revenues. The geek movement, it appears, still has a long way to go.

2012 Bantam Press £18.99hb 336pp

Into the magpie’s nest

As its name suggests, The Science Magpie is a panoply of scientific curiosities, plucked from the length and breadth of contemporary science as if by a curious and acquisitive bird. The “bird”, in this case, is a publisher-turned-trainee-teacher called Simon Flynn, who has gathered anecdotes, poems, jokes, facts and the odd diary entry or letter-to-the-editor, and put them together in no particular order to form a delightful little compendium of science oddities. The book includes some fairly well-known science trivia, including tales about Euler’s identity, Occam’s razor, Faraday’s Christmas lectures, Tom Lehrer’s song about “The Elements” and the Large Hadron Collider rap. But Flynn has also found some more obscure gems. A good example is Darwin’s diary entry, written when he was 29 years old, on the pros and cons of marriage. “Less money for books” was one particularly amusing “con”, while “charms of music and female chit-chat” was a “pro”. Another fascinating but gruesome story features the young Isaac Newton suffering for his science. As Flynn writes, during Newton’s studies of colour, “he talks of inserting a bodkin (like a cross between an arrow and a needle) between his eye and his socket as near to the back of the eye as possible. He would then press so as to change the retina’s curvature resulting in his seeing ‘white and darke and coloured circles’ as he continued to vary the pressure and movement”. Also in the book are questions from an 1858 Cambridge science exam for 15 year olds, mnemonics for remembering the geological timescale and the planets of our solar system, a little refresher on determining prime numbers and a handy list of the “10 greatest ever equations”. There is even a list containing the Scrabble scores of some common scientific words. On the whole, The Science Magpie is an easy and enjoyable read, and it will surely give you a host of new jokes and tales for the pub.

2012 Icon Books £12.99hb 278pp

Photo of woman with ponytail and photo of many light bulbs — Make them think: Collected research from the improbable, such as the balance of forces in human ponytails, to the everyday, such as how many lights could be switched on with the energy it takes to fill a hot bath. (iStockphoto/Paffy69; iStockphoto/Galina Peshkova)

Improbable fun

Most readers will have heard of the Ig Nobel Prizes, which are given annually to honour research that, in the words of Ig impresario Marc Abrahams, “first makes people laugh, and then makes them think”. This year’s physics Ig Nobel, for example, honoured an Anglo-American trio of researchers who calculated the balance of forces in human ponytails (March p3). Ceremonies for Ig Nobel winners have been held every year since 1991, and Abrahams has been publishing other examples of semi-silly science in his magazine, the Annals of Improbable Research, since 1995. But oddly enough, he has never grouped these tales together in book form – until now. This is Improbable: Cheese String Theory, Magnetic Chickens, and other WTF Research rehashes a number of past Ig Nobel citations, but delightfully, it also shows that such prize-winning research is the tip of an enormous iceberg. The economics of piracy, the lavatory habits of Antarctic researchers and the anti-skid benefits of wearing socks over shoes – all are described in glorious detail, with copious references to the original papers. As one might expect, many of these papers were published in obscure journals, but there are some surprising exceptions: the aforementioned ponytail research appeared in Physical Review Letters, while the case of a biologist who accidentally incubated more than 70 insects in his left sinus was published in Science. Maybe that one should have been billed as research that “first makes you laugh, and then makes you wince”.

2012 Oneworld £10.99/$15.95pb 320pp

Stand-up science

Actor and comedian Ben Miller is best known for being half of the comic duo “Armstrong and Miller” and for his other roles in film and television. What many do not realize, though, is that Miller was working on a physics PhD at Cambridge when, in his words, he “accidentally became a comedian”. In his new book It’s Not Rocket Science, Miller makes a partial return to his roots by focusing on the exciting bits of science, while avoiding complex maths and calculations. “If you want to build a Large Hadron Collider, you’d better hunker down and get a physics postdoc,” he writes. “If you want to gawp at one and imagine how cool it would be if one blew up…Well, you’ve come to the right place.” In the book, Miller covers a wide, if rather random-seeming, range of contemporary science ideas and subjects. After visiting CERN and whizzing through the Standard Model, he trips along the Milky Way while telling the reader that we are “slowly falling into an enormous black hole” that resides at the centre of our galaxy. He then dips into Darwin and evolution, and follows that thread to the intriguing science of genetics and DNA. A surprising chapter looks into cookery and the chemistry behind it – including the history of food and taste – and also contains a deeper examination of enzymes and molecular interactions that give distinct flavours. The book touches as well on the changing environment of Earth, weather prediction and the complex beast that is climate change, before ending with a look at the dynamics of heavenly bodies, space travel and – of course – aliens. If you are looking for a complete, in-depth view of contemporary science, this is not the book for you, since It’s Not Rocket Science was written mainly to entertain, not to inform. As Miller puts it, “This is not a science lesson. It’s a science orgy.” Still, you might learn something new along the way, without even trying.

2012 Sphere £12.99pb 280pp

The puzzles of daily life

We all know that everyday physical intuition is essentially useless for predicting the workings of the quantum-mechanical world. But as it turns out, intuition is not necessarily reliable even when it is applied to objects on a more familiar, macroscopic scale. Between 2003 and 2011 physicist Jo Hermans explored the complex and often counterintuitive nature of ordinary things via a series of columns in Europhysics News, the magazine of the European Physical Society. Now, these “Physics in Daily Life” columns have been published together in a book of the same name, complete with charming cartoon illustrations by Wiebke Drenckhan. Many of the collected columns begin with an incorrect “layman’s view” of a question. A good example is number 22, in which Hermans asks how many lights could be switched on with the energy required for a nice relaxing hot bath. The answer turns out to be around 1000 – far more than a physics-ignorant guesser might assume, although it doesn’t help that Hermans never specifies how long those lights could stay on. In other chapters, though, the much-abused “layman” turns out to be right. For instance, dark-coloured doors really do get hotter in sunlight than light-painted ones (number 15), even though a more sophisticated (but ultimately incorrect) reasoner might suggest that colour ought not to matter because “a surface that absorbs well must also emit well”. Hermans does a fair job of untangling all these conflicting assumptions and fragments of intuition, and the resulting clear, concise explanations are well worth reading.

2012 EDP Sciences €18pb 112pp

Light bends itself round corners

Five years ago physicists showed that certain kinds of laser beam can follow curved trajectories in free space. Such counterintuitive behaviour could have a number of applications, from manipulating nanoparticles to destroying hard-to-reach tumours. But before this bizarre effect could be put to good use, researchers were faced with the challenge of how to bend the light through large enough angles to be useful. Now, two independent teams have solved this problem – and claim that the bending of sound and other kinds of waves could be next.

The concept of self-bending light was inspired by quantum mechanics and the realization in 1979 by Michael Berry and Nandor Balazs that the Schrödinger equation could support “Airy” wavepackets of particles, which accelerate without an external force. Then in 2007, Demetrios Christodoulides and colleagues at the University of Central Florida created the optical equivalent of an Airy wavepacket. This is possible because the equation describing paraxial beams – beams in which the constituent rays all travel almost parallel to the direction of the beam’s propagation – is mathematically identical to the Schrödinger equation once several parameters are interchanged, such as mass and refractive index.

The Florida team generated a specially shaped laser beam that could self-accelerate, or bend, sideways. The researchers did not bend the laser beam as a whole but rather the high-intensity regions within it. To do this they passed a centimetre-wide ordinary laser beam through a device known as a spatial light modulator that adjusted the phase of the beam at thousands of points across its width. Rather than acting like a lens and focusing all of the beam’s constituent rays to a single point, the modulator instead changed the relative phase of the rays such that their interference produced a region of maximum intensity that curved sideways in the shape of a gentle parabola across the beam as it propagated forward, along with a number of fainter regions on one side.

Intriguing characteristics

In addition to this self-bending, the beam’s intensity pattern also has a couple of other intriguing characteristics. One is that it is non-diffracting, which means that the width of each intensity region does not appreciably increase as the beam travels forwards. This is unlike a normal beam – even a tightly collimated laser beam – which spreads as it propagates. The other unusual property is that of self-healing. This means that if part of the beam is blocked by opaque objects, then any disruptions to the beam’s intensity pattern could gradually recover as the beam travels forward.

A limitation of the Florida work, however, is that Airy beams can only be bent through relatively small angles up to about 15 degrees. This means that they cannot provide the sharp turns needed for manipulation on the micron or nanometre scale.

But then in April this year, Mordechai Segev and colleagues at the Technion-Israel Institute of Technology derived a set of general solutions to Maxwell’s equations showing that a non-diffracting non-paraxial beam should exist and that it should accelerate in a circle. A month later, two teams produced such beams in the lab – each bending light a 60-degree arc. One team was led by Xiang Zhang of the University of California, Berkeley in the US and the other by John Dudley of the University of Franche-Comte in France.

Not just circular motion

Now, two independent teams have shown, both theoretically and experimentally, that non-paraxial acceleration along trajectories other than a circle is possible. One group is led by Berkeley’s Zhang and it studied both elliptical and parabolic motions via analytical and numerical 2D scalar analysis. The other team is led by Florida’s Christodoulides and it considered elliptical motion using numerical 3D vector analysis. In the experiments, both groups used continuous-wave lasers, with a wavelength of 532 nm for Zhang’s group and 633 nm for Christodoulides’ group, shining them through spatial light modulators with phase variation calculated using special computer programs. In both cases, the groups were also able to bend the light through about 60 degrees.

According to Berkeley group member Peng Zhang, these latest studies could lead to a number of practical applications. These include particle manipulation and the burning of curved channels through air to guide plasmas for remote sensing. He also says they could be useful in medicine, allowing doctors, for example, to image or destroy a tumour behind an organ without destroying that organ. “The self-healing of the beam would be very useful,” he adds, “because it would allow you to send energy deep into tissue even with obstacles in the way.”

In addition, Xiang Zhang says that the approach can be generalized to any other kind of wave system, such as matter waves, electron waves or acoustics. In fact, he points out, his group is investigating the bending of sound waves. He believes that it should be possible to transport sound energy around corners by manipulating the phase of acoustic waves with a device equivalent to a spatial light modulator.

Ingenious, but not new?

Jérôme Kasparian of the University of Geneva in Switzerland, who was not involved in the latest work, is enthusiastic, explaining that the two groups have “elaborated a general framework to describe and therefore predict” large-angle bending of light. However, Michael Berry of Bristol University in the UK, is less so. He believes that the authors do not make it clear that in their experiments they are not bending light rays themselves but the rays’ envelopes, or “caustics”. “The technical details in these papers are ingenious and interesting to specialists, and I hope the renewed emphasis will lead to applications,” he says. “But while the papers are technically interesting, they are unsurprising because they contain no fundamental new idea.”

The research is described in two papers published in Physical Review Letters.

Complex 3D nanostructures built using DNA bricks

A new technique to make highly complex 3D nanostructures by assembling together synthetic DNA “bricks” has been developed by researchers at Harvard University in the US. The bricks, which are like tiny pieces of LEGO, can be assembled into a wide variety of shapes and configurations, meaning that they can be used to build elaborately designed nanostructures. The resulting structures might find use in a wide variety of applications, including smart medical devices for targeted drug delivery in the body, programmable imaging probes and even in the manufacture of speedier and more powerful computer-chip circuits.

DNA nanotechnology has now been around for nearly 30 years, but it really took off with the advent of a technique called DNA “origami”. This technique, named after the ancient Japanese art of paper folding and first developed in 2006 by Paul Rothemund at the California Institute of Technology, involves folding long strands of DNA into a wide range of predetermined shapes. The resulting nanostructures can be used as scaffolding or as miniature circuit boards for precisely assembling components such as carbon nanotubes and nanowires.

Powerful though it is for making both 2D and 3D shapes, DNA origami has its limitations. To fold the DNA, several hundred “staples” must be added to the regions surrounding the single DNA strands, and each type of new nanostructure desired requires a new set of staples. Moreover, the DNA structures tend to arrange themselves randomly onto a substrate surface, which makes it difficult to integrate them into electronic circuits afterwards.

Building bricks

A team led by Peng Yin at Harvard first put forward its DNA-brick self-assembly technique earlier this year. Rather than starting with long DNA strands, the researchers succeeded in interlocking short, synthetic strands of DNA together to make larger structures. In fact, they managed to arrange the short strands into a “molecular canvas” by controlling the local interactions between the strands. The technique, like any DNA self-assembly method, works by exploiting the fact that the four base pairs in DNA – adenosine, thymine, cytosine and guanine – are naturally programmed to join up in specific ways: A only binds to T, while C only binds to G. So, the team was able to fabricate a collection of 2D structures using its technique by stacking one DNA brick that was 42 bases long upon another brick.

3D shapes

Now, Yin and colleagues have extended their technique to 3D. The researchers begin with an even smaller DNA-brick strand – only 32 bases long – that contains four regions than can bind to four neighbouring DNA-brick strands. The bricks are connected through 90° and so can be built out in all three directions – up, down, and out – to create a solid “master” DNA molecular-canvas cube containing hundreds of bricks. Compared with hand-assembled LEGO structures, each DNA structure self-assembles thanks to the fact that every brick is encoded with an individual sequence that determines its final position in the nanostructure. Each sequence will only be attracted to one other complementary sequence, which means that specific shapes can be created through the selection of different sequences.

The biggest advantage of the new DNA-brick technique is that any number of structures can effortlessly be made from the same master cube by simply selecting subsets of specific DNA bricks, according to the team. “We have already made more than 100 different shapes in this way (with some containing intricate cavities, surface features and channels), all of which are more complex than any 3D DNA structure constructed in the last decade. What is more, additional DNA bricks can be added, removed or modified independently without affecting other parts of the structure,” says Yin.

Complex structures

The researchers claim that the complex structures that can be made using their DNA-brick assembly technique will help advance existing DNA nanotechnology applications. “We can for example, arrange technologically relevant guest molecules into functional devices that might serve as programmable molecular probes, instruments for biological imaging and drug-delivery vehicles,” Yin tells physicsworld.com. “The structures can also be used to fabricate high-throughput complex inorganic devices for electronics and photonics applications.”

The DNA-brick structures are also entirely synthetic, whereas DNA origami is half biological. This expands the range of potential applications even further, Yin adds. “For instance, by using synthetic polymers rather than the natural form of DNA, we might be able to create functional structures that are stable in a wider variety of different environments.” The team is now busy improving its brick technique by looking more closely at DNA structure and sequence design, enzymatic synthesis for higher-quality strands and optimizing processing conditions. “We would also like to better understand the kinetic pathways involved in DNA assembly,” says Yin.

The work is published in Science.

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