Page 1220 – Physics World

Home » Page 1220

When light listens to your every step

Hidden underground beneath international borders and military zones, a new form of stealth technology is taking root. In a technique known as distributed acoustic sensing, the same type of fibre-optic cables that we use for telephony and the Internet are being used as highly sensitive arrays of buried acoustic sensors. Being developed by a number of different firms, these systems can be used to remotely listen out for illegal immigrants crossing international borders, trespassers walking towards a military base, people tampering with oil or gas pipelines, or criminals damaging railway lines.

In the technique, sounds are “heard” by sending pulses of coherent laser light down an optical fibre and analysing the very small signal that is both scattered and reflected back along the fibre. Scattering and reflection occur at every point along the length of the fibre, so the original laser pulse comes back as thousands of very weak overlapping laser pulses. If there are no sounds to detect, the returning signal shows nothing special. But in the presence of a sound it has characteristic features that can be identified as footsteps, for example, or even a drone flying overhead.

The light in the fibre is scattered in all directions – including back along the direction from which it came – because it hits tiny imperfections such as density fluctuations. Known as Rayleigh scattering, this phenomenon is most familiar to us when sunlight passes through the atmosphere and is responsible for the sky appearing blue. “The atmosphere has random fluctuations of density and refractive index from place to place on a small scale due to small particles and molecules in the atmosphere,” says David R Selviah of University College London, who has been involved in commercializing distributed acoustic sensing (DAS). “These random fluctuations cause light to be scattered in all directions.”

But light is also sent back where one piece of cylindrical fibre ends and another begins. Where sound waves come into the picture is that when they pass through the fibre, they cause it to flex ever so slightly, which changes the fibre’s local optical properties. The signal that is scattered and reflected back along the cable is then also altered and it is this pulse modulation that companies are exploiting to develop highly sensitive systems based on DAS. The exterior sound modulates the reflected laser pulses, and once the laser pulses are received, the sound can be extracted from them. As Selviah puts it, such systems essentially work as a distributed underground array of microphones.

The source of a sound can be pinpointed spatially to within tens of metres – which is a good accuracy considering that the optical fibres can be as long as 50 km

Using DAS, the source of a sound can be pinpointed spatially to within tens of metres – which is a good accuracy considering that the optical fibres can be as long as 50 km. “We can tell which point in the fibre reflected which pulse by noting the time that each pulse returns,” explains Selviah, who has in the past worked on the signal processing and pattern-recognition analysis of data from DAS systems for Silixa – a UK-based fibre-optics firm founded in 2007 that operates in the energy, security and industrial sectors. Uses of DAS in these areas include detecting earthquakes and Earth tremors, and hearing the creaking sounds of buildings, bridges or dams to see if they are deteriorating.

Sound interpretation

1 Suspicious sounds

Diagram of using sound to detect criminal activity — (Courtesy: OptaSense)

Using distributed acoustic sensing, sensitive areas such as borders and railway lines can be monitored for warning signs of criminal activity. Shown here, left to right, are the tell-tale signs of gunfire, digging, a vehicle, a person walking and a microlight (a small aeroplane).

Another firm leading the development of DAS systems is OptaSense – a subsidiary of the UK technology group QinetiQ. OptaSense this year won Queen’s awards for both innovation and international trade, and if you visit its headquarters in Farnborough, south-west of London, you’ll see computer screens showing complicated zigzagging lines, similar to those a seismograph would show when picking up Earth tremors. These lines represent ground vibrations, and a car passing above the buried fibre-optic cables generates more lines than footsteps.

These lines are essentially the raw exterior acoustic data, and the control centre’s job is to use high-speed signal-processing algorithms to extract the information relevant to the user’s problem in real time, says David Hill, OptaSense’s chief technology officer. The data are then relayed for integration with other sensors, or sent to users via their mobile devices. Besides detecting footsteps to within roughly 10 m either side of a fibre-optic cable, DAS can even spot a low-flying drone overhead. The sounds can be displayed as shown in figure 1, but they can also be played out loud and made to sound like the original noise, using a bit of signal processing and programming.

Listening for leaks

Oil or gas pipelines — **Safe and sound** DAS can continuously monitor for leaks in oil or gas pipelines. (Courtesy: Shutterstock/Danicek)

One sound that many companies are listening out for using DAS is oil or gas flowing along a pipe or in a well – in particular to check for leaks. In this case, rather than the optical fibres being buried underground, they are inserted into a well bore. Both Hill and Selviah think this is an especially exciting application area for DAS.

That view is echoed by Kari Anne Kjolaas-Holland, business development manager for microseismic services at Schlumberger, which supplies technology services to the oil and gas industry. The company uses DAS to sense an entire well at one time for vibration, temperature, strain and so on. “Without moving the down-hole cable, a snapshot of the properties in the well may be collected instantly, like a photograph,” Kjolaas-Holland explains. “We see both the big picture and fine details in each snapshot, and by combining successive pictures, it becomes very much like watching a movie.”

Before DAS came on the scene in the late 2000s, oil and gas firms had to place arrays of microphones into wells and pipes. Time on the oil or gas rig had to be set aside for this specific activity, and multiple sparse measurements had to be taken by moving the microphones along the length of the well bore. For this purpose, DAS is a game changer. DAS systems can be permanently installed so that conditions in the well are monitored all the time, with the sound being sensed almost continuously along the entire length of the well.

Ships, sharks and submarines

Although DAS is barely able to pick up a conversation, Selviah thinks that might one day change. It is already being buried in roads to listen to cars passing to determine exactly where they are, and – as it develops further – it could also work in buildings. With sufficient future developments, these optical fibres could even, Selviah adds, be inserted into blood vessels to listen to the flow of blood and the heart during operations.

Marine physicist Philippe Blondel of the University of Bath is one researcher who thinks the technology could also be used to make cheap hydrophones – underwater microphones, developed for recording or listening to underwater sounds. Water is a compressible medium, and applying pressure to it will create pressure further away. So any change in water pressure, for example by creating sound, will be picked up by the DAS systems.

Current hydrophones look for a full range of frequencies, and the signal they produce then needs to be processed to extract the frequencies of interest. But with a DAS sensor, it might be possible to design it so that only very specific frequency bands can be detected. It could then be used to listen for ships, submarines or radar instruments on the seabed that “ping” a particular signal – for military, navigation or commercial monitoring purposes.

As an advanced application, these underwater DAS microphones could also be used to identify and monitor fish, sharks or other animals based on the sound they generate under water or on the ground. If these sensors had been implemented under water in the right area, then – who knows – perhaps they could even have been used to look for the Malaysian aeroplane that mysteriously went missing in March this year.

China pursues 52 km collider project

Particle physicists in China have unveiled plans to build a huge 52 km particle collider that would smash electrons and positrons together to study the Higgs boson in unprecedented detail. The so-called “Higgs factory”, if given government approval, would be built by 2028 and put the country at the forefront of international particle physics.

Researchers are currently preparing a proposal to the government to carry out a full R&D study into the machine, which they envisage having an energy of 250 GeV. However, Yifang Wang, director of the Institute of High Energy Physics (IHEP) in Beijing, warns that the project is still in its infancy. “We are still at a very early stage of the discussion and we have a long way to go to get government support,” says Wang.

Vigorous pursuit

Even though the collider is a long way off, Brian Foster from Hamburg and Oxford universities, who is European regional director for the planned International Linear Collider (ILC), which is also designed to study the properties of the Higgs boson, says that Chinese researchers are nevertheless “pursuing it rather vigorously”. Yet he thinks that China would find it hard to build a machine on its own given that the country’s biggest collider – the Beijing Electron Positron Collider at IHEP – is just 240 m in circumference. “They maintain that this will be a Chinese project, although they also admit they don’t have the people to build it themselves, so assistance from the international community would be required,” says Foster.

That view is shared by theorist John Ellis at King’s College London and CERN, who says that China is now beginning to “reach out” to international partners. “They don’t have the expertise at the moment to build something of that size with the technology required,” says Ellis, who adds that China would be much further ahead now if they had been more involved in building CERN’s Large Hadron Collider (LHC).

Encouraging statements

Meanwhile, members of the International Committee on Future Accelerators (ICFA), which met in Valencia in July, reaffirmed in a closing statement their support for the ILC as well as a circular collider like that which China is proposing. “The ICFA continues to encourage international studies of circular colliders, with an ultimate goal of proton–proton collisions at energies much higher than those of the LHC,” the statement said.

Foster points out, however, that the proposed collider will “not interfere” with the ILC proposal, which Japan is currently considering hosting and has even begun discussing at ministerial level with governments in the US and Europe. “The physics output [of a 250 GeV circular electron–positron collider] compared with the ILC is very limited and of course it will need several years before it is in a state that it can be believably costed and proposed to proceed to a proper technical design,” he says.

Cracking the code for single-molecule junctions

Measuring the conductance of single-molecule junctions, which feature in a range of photovoltaic and energy-harvesting devices, is essential to gain better insights into the energy and charge-transfer processes of the molecule, in order to optimize the devices themselves. But accurately calculating the conductance is no mean feat. Now, by applying a correction to a standard modelling method used to make measurements in such systems, an international group of researchers has demonstrated qualitatively and quantitatively correct solutions for a compound known as “porphyrin” for the first time, thereby enabling predictive modelling of complex molecular junctions.

Porphyrins are cyclic organic compounds that are commonly used as a “single molecule junction” wherein – a single organic molecule connected to macroscopic metallic electrodes. Standard methods predict erroneous electron orbitals for transition metals in porphyrin molecules, as well as overestimating the conductance by an order of magnitude. In the new work, the team tackled both inaccuracies using a modified “hybrid” formulation of the “density functional theory” (DFT) – a computational quantum mechanical modelling method that looks at the electronic structure of many-body systems.

“The hybrid functional is the only pragmatically feasible method we can employ to correct both errors at the same time,” says Zhenfei Liu, a postdoctoral researcher at the Molecular Foundry and Materials Sciences Division at the Lawrence Berkeley National Laboratory. “Other approaches need two stages to correct the qualitative and quantitative inaccuracies.”

Metal centres

Porphyrins usually have a transition metal at the centre, which as Liu stresses has an important impact on the molecule’s properties. “For catalysis, for example, the metal centre is key to make the porphyrin work as it’s supposed to,” he says. Calculating the properties of transition metals has some known nuances. To deal with these, chemists developed a mathematical formulation known as the “exact exchange”, which was incorporated into DFT over a decade ago, and is responsible for more accurate calculations of the electronic bandgap, charge transfer and other properties of these systems.

“The functional was already known but had not been applied to molecular junctions,” says Liu. “But we knew you need the exact exchange to make DFT work for transition metals, so we used a combination of standard functional and an exact exchange. This additional component – the exact exchange – gives the qualitatively correct calculations.”

Liu, working with Jeff Neaton at the Molecular Foundry at Lawrence Berkeley Lab and the University of California, Berkeley, along with Latha Venkataraman, Luis M Campos and colleagues at Columbia University in New York and Yonsei University in Korea, incorporated a previously developed modification known as “DFT+Σ”, which corrects inaccuracies that arise due to underestimating the alignment of energy levels in the junction.

Experiment versus theory

The researchers synthesized different types of porphyrin with cobalt, copper, nickel or no metal at the centre. Solutions of the porphyrins were deposited on a gold-on-mica substrate and a gold scanning tunnelling microscopy tip was dipped into the solution and pulled out to create a break junction. Comparisons of measured conductances with calculated values favoured the accuracy of the researchers’ newly modified formulation over standard DFT.

Liu explains that the team’s novel hybrid approach can be applied to other systems as well. “For example, in organic metallic interfaces the junction has two interfaces on each side – probably a more common scenario in nanoscience.”

He also describes previously published work on a study of the conductance of similar junctions but with graphite as one of the electrodes instead of gold. Breaking the symmetry of the system in this way gives rise to rectification in the junction – when the bias is reversed, the conductance value changes, which can be useful for energy-harvesting devices.

The work is published in Nano Letters.

This article first appeared on nanotechweb.org

What can you learn from Descartes?

Taking Descartes back into the physics classroom. (Courtesy: Shutterstock)

By James Dacey in Córdoba, Argentina

What’s the best way to teach tricky physics concepts to students? Naturally, this was one of the questions underpinning many of the talks here at the International Conference of Physics Education (IPCE) in Córdoba. According to a couple of educationalists in Latin America at least, it seems that one approach is to enlist the help of some of the great scientists and philosophers of the past.

Patricia del V. Repossi, a lecturer at the Pontificia Universidad Católica Argentina in Buenos Aires, spoke about how she uses the history of science as a framework for teaching optics. Repossi explained how she had come to realize that some of the students taking her conventional optics course believed that photons are made of the same stuff as “tennis balls”. So, she and her colleagues set about transforming the way they teach the topic – by combining a physics class with a history lesson.

(more…)

Strontium’s nuclear ‘spin symmetry’ revealed

A new measurement, made by an international team of researchers using the world’s most precise clock, shows that the quantum spins of atomic nuclei can help determine an atomic collision’s strength. This phenomenon arises due to a particular type of “spin symmetry” of nuclear spins, the first direct evidence of which has now been found. The finding could help researchers better understand phenomena such as superconductivity and quantum magnetism.

The measurement piggybacks on a recently developed atomic clock based on the element strontium, which has two electrons in its outermost shell. Typically the spins – or magnetic moments – of these electrons point in opposite directions and cancel each other, giving the atoms zero overall electronic spin. This makes electronic spin largely irrelevant for how such atoms interact with each other. Conversely, the strontium nucleus has a non-zero spin – for the isotope strontium-87 this spin can take any of 10 values. But because the nucleus resides in the atom’s centre, inside many layers of electrons, physicists have long thought nuclear spin should not affect how atoms interact when they collide.

Strongly interacting

In 2010, however, theorist Ana Maria Rey at the research institute JILA in Boulder, Colorado in the US, realized that the nuclear spin could have an effect, due to a property called “SU(N) spin symmetry”. According to that symmetry, two colliding atoms with different nuclear spins should interact much more strongly than those colliding with the same nuclear spins, although the particular values of the spins should not matter. No existing device at the time of Rey’s prediction afforded the precision and control needed to measure this effect.

That changed in January 2014, when Rey’s JILA colleague Jun Ye and others reported building a strontium clock that beat all previous clocks in precision and stability. The clock uses a population of weakly interacting strontium atoms cooled to around one-millionth of a degree above absolute zero and trapped in a lattice made by interlacing laser beams. The researchers determined the clock’s tick rate by illuminating the atoms with a red laser at a precise frequency, and averaging the rate of electron transitions between two of the atoms’ quantum-energy states. Rey theorized that this rate should change slightly depending on whether two interacting atoms had the same or different nuclear spins, and that Ye’s newest clock would be sensitive to this change.

Super stability

Ye’s team has now measured the predicted frequency shift. Depending on whether colliding strontium atoms have similar or different nuclear spins, the rate at which Ye’s clock ticked changed by around one part in 10¹⁶, which is like adding or subtracting 14 seconds to the age of the solar system. Detecting this minute effect would be impossible without the clock’s ultra-stable laser, which can maintain a coherent quantum state long enough to average the electronic transition frequency over hundreds or thousands of strontium atoms. “That’s really the secret weapon, that we have a laser that can maintain extremely long coherence times,” says Ye.

Composite image of JILA's strontium clock — Perfect timing: JILA’s experimental atomic clock based on strontium atoms held in a lattice of laser light is the world’s most precise and stable atomic clock. The image is a composite of many photos taken with long exposure times and other techniques to make the lasers more visible. (Courtesy: Ye group and Baxley/JILA)

Understanding and controlling for the spin-symmetry effect will be crucial for building even more precise clocks, one of which will eventually replace the current cesium standard. Beyond better clocks, the team’s methods will also enable physicists to study phenomena that emerge from the quantum behaviour of large ensembles of particles, including superconductivity and quantum magnetism, Ye says. “It’s really fantastic how this clock technology is now used to investigate quantum many-body systems,” says Florian Schreck, a physicist at the University of Amsterdam who calls the result a breakthrough. “I think many people will follow in the steps of this team.”

The work was published in Science.

Thinking about thinking

Shining a light on teaching physics. (Courtesy: iStockphoto/Greyfebruary)

By James Dacey in Córdoba, Argentina

There has been a lot of fancy language flying around here at the International Conference on Physics Education (ICPE), which is taking place in Córdoba. Words such as “pedagogy” and “metacognition” roll off the tongues of education researchers as naturally as a particle physicist at CERN saying the words “Higgs boson”.

At first it seemed a bit like a foreign language to me, but I’ve started to realize that one of the recurring ideas at the conference can be described in more everyday terms: thinking about thinking. Teachers should think about the way they think about learning, and the way their students think about learning.

(more…)

Controlling ferromagnetic domains using light

A variety of magnetic materials can be controlled using only polarized light, according to new work carried out by an international team of researchers. The unexpected and so far unexplained discovery shows that the optical phenomenon, which was previously thought to be possible only in ferrimagnets, is actually much more general. The discovery could potentially have a major impact on data storage, as it could allow magnetic bits to be rapidly switched by optical pulses in state-of-the-art hard drives.

From magnetic tapes to computer hard drives, rewritable data storage has traditionally been achieved using ordering of magnetic domains. Individual bits are stored by setting the magnetization vector of a particular domain to point either up or down. However, as data processing becomes faster in modern computers, data storage needs to speed up too. This presents both practical and theoretical difficulties for magnetic data storage.

The traditional way to flip a bit is to apply a magnetic field. However, speedily setting magnetic domains requires stronger and faster pulsed magnetic fields, and these are difficult to generate in a computer’s hard drive. Furthermore, in 2004 researchers using magnetic fields generated by the Stanford Linear Accelerator showed that the extreme fields needed to switch a domain in less than two picoseconds caused a complete breakdown of the magnetic order of a material, apparently placing an ultimate speed limit on magnetic data storage.

Speedy switching

In 2006 Theo Rasing and colleagues at Radboud University Nijmegen in the Netherlands showed that the magnetization of domains in ferrimagnets – materials that contain two types of magnetic domain oriented in opposite directions – can be controlled by circularly polarized light. This control could be accomplished by a low-power laser pulse with a duration as short as 40 femtoseconds. But while the rare-Earth ferrimagnets used by Rasing and subsequent researchers were popular in magneto-optical drives, they are not used in computer hard drives because they are easily magnetized and demagnetized. The magnetism of very small domains is therefore quite unstable, thus limiting the density at which the materials can store data.

But now, Stéphane Mangin of the University of Lorraine in France, along with colleagues in the US, Germany and Japan, has demonstrated that this type of optical switching can also be achieved in ferromagnetic films made from materials such as cobalt, platinum, nickel and palladium. These materials are of great interest to hard-drive developers but until now most of the theories explaining optical switching were applicable only to ferrimagnets. “Here we are really showing that is not the case – you can have different kinds of ferromagnetic or other magnetic materials that show this behaviour,” says Mangin, but cautions that they still do not know how this occurs.

The researchers tested a selection of ferromagnetic films, varying parameters such as the relative thicknesses, the proportions of different materials and the number of layers, to confirm their finding. Using a standard method, the team viewed each sample under a Faraday microscope, which uses polarized light. A domain polarized in one direction appears black, whereas a domain polarized in the other appears white. The researchers irradiated the samples using 100 femtosecond laser pulses and found that they were able to switch domains as well as introduce polarization to parts with no net polarization.

Frazzled physicists

Both Rasing and Bert Koopmans, a nanomagnetics expert at the Technical University of Eindhoven in the Netherlands, were taken aback. “It gives me a kind of rollercoaster feeling,” says Koopmans. “The interpretation of these [optical-switching] experiments has changed throughout the years. I thought that I understood everything and this experiment has completely frazzled my mind.” Both researchers, however, insist that more needs to be done to demonstrate that the work can be useful in hard drives. “In this paper, there are no results from single pulses – only from accumulated pulses,” says Rasing. “So, it is a bit difficult to tell whether it is really very fast optical switching or heating by the laser – although the helicity is clearly important here – that has been demonstrated.” Mangin and his collaborators are currently working on both a theoretical explanation and practical development of the technique.

The research is published in Science.

Towards a more plausible dragon

Wizards, mermaids, dragons and aliens. Walking, running, flying and space travel. A hi-tech elevator, a computer, a propulsion engine and a black hole. What do all of these things have in common? This might seem like a really hard brainteaser but the answer is simple: they all obey the fundamental laws of our universe. Accordingly, there are certain dos and don’ts, maybes and no ways that should guide our imaginations when we think about them.

That, at least, is the argument that physicist and science-fiction aficionado Charles Adler makes in Wizards, Aliens and Starships: Physics and Math in Fantasy and Science Fiction. In the book, Adler, a professor at St Mary’s College of Maryland, US, offers a grand tour of almost all of the main themes encountered in the -sci-fi literature and attempts to evaluate their plausibility using first principles from physics. From the mythical world of J K Rowling’s Harry Potter series to the many stories that deal with the possibility of space travel, the future of our civilization or the existence of civilizations elsewhere in the universe, he considers a wide variety of topics and scenarios.

Written in a relaxed style, the book is full of enjoyable discussions, and readers who are already familiar with the sci-fi literature will appreciate the numerous references to other works. Those who are less familiar will, most probably, feel tempted to read some of the works referenced and commented on.

Adler’s main tools in exploring the possibility or impossibility of an idea are the “back-of-the-envelope calculations” popularized by Enrico Fermi. As the last great generalist in physics, Fermi combined an exceptional training in theoretical physics with tremendous skill in experimental physics. Equipped with this unique combination of abilities, he was able to ask seemingly impossible questions and, with a few simple arguments, produce fast, approximate answers to them with an accuracy that only detailed and time-consuming computations could surpass.

One topic that Adler examines in the book using this “Fermi problems” approach concerns how size affects various aspects of any living creature, whether real or fantastic. For example, size considerations can explain why humans walk at about 2 m/s, how this speed might change on another planet and how it compares with the speed of species with longer legs. Similarly, in the same chapter, the reader learns why there are no biological flying species beyond a certain mass (roughly that of the California condor). Later, Adler uses the same approach to evaluate the feasibility and cost efficiency of the space elevator – a breathtaking engineering project that has been proposed by some as a way to transport material and people to space. Using conservative estimations, he argues that a space elevator would be risky and its benefits not as great as proponents have insisted.

Having taught a course on physics in Hollywood movies many times using some of the same source material, I must confess that this is the book I have always meant to write if I ever had the time to do so. Reading it, I felt like my notes and my course slides had been magically transformed from their raw form into an engaging book. Although not a textbook in itself, Wizards, Aliens and Starships could serve as a great companion book for courses offered to non-science students, as a means of taking away the students’ boredom and animosity towards science.

The book contains a few typos but, fortunately, they do not create any serious problems for the reader. In some places, however, I found myself questioning a few of the assumptions and/or statements. On occasion, additional research might also have been beneficial. For example, in chapter 11, where Adler discusses speculative propulsion systems, he places a lot of emphasis on the cost, time and efficiency required to produce antimatter. I found his assumptions and calculations unpersuasive because they are based on superficial limitations of the current time. Humans’ engineering efforts have never been focused on the mass production of antimatter, and it is easy to imagine that, with dedication and will, we could create a high–efficiency production factory that would make antiparticles at a much lower cost. The more important issue, I believe, is the separation and storage of antimatter for an extremely long period of time – periods that must be fantastically longer than the lifetimes of the antimatter particles themselves. This issue is not touched on at all.

Similarly, when discussing the possibility of fire-breathing dragons, the author calls the suggestion that dragons might generate large amounts of methane in their digestive systems “ingenious”, but also states that “there may be some reason that this mechanism is fundamentally impossible”. Here, I feel that his intuition might be wrong. Exobiologists study how creatures could have evolved on other planets, and one possibility that has drawn their attention is a “methane world” – that is, one where methane plays a role similar to that of water on Earth. Using scientific principles and imagination, exobiologists have envisioned a very complex, biodiverse environment on such a planet. One could also imagine a milder version where methane is not the dominant compound but nevertheless plays an important role. As Murray Gell-Mann has stated: “anything that is not forbidden [by fundamental laws] is compulsory”. However, in the author’s defence, introducing reasonable assumptions is part of the nature of back-of-the-envelope problems, and hence I cannot be extremely negative on this point. Also, for a book that covers so many topics, adding more details would have made it too bulky.

My other criticism is more important. The book uses a vast number of facts from science and a long list of formulae from physics. As a result, and based on my experience with the general public, the book is not really accessible to people who prefer to read plain text with no calculations, just the facts and the results spelled out explicitly. That is not a problem for readers like myself, who are trained in science and will find much to appreciate here, but the publisher’s declaration that the book “will speak to anyone wanting to know about the correct – and incorrect – science of science fiction and fantasy” is not necessarily valid.

Overall, though, this is an exciting book. To paraphrase John Wheeler, it is about our universe as it really is; a museum of wonder and beauty that often contrasts with the fictional universes of imagination encountered in the sci-fi literature. I would not hesitate to recommend it to anyone who is interested in understanding the relationship between physics and science fiction. Instructors of introductory physics courses, especially, will find it a valuable supplement to dry physics textbooks, and its use may even boost students’ evaluations of the course. I will certainly use it in my classes.

2014 Princeton University Press £19.95/$29.95hb 392pp

Web life: Super Planet Crash

So what is the site about?

Super Planet Crash is a deceptively simple game based on the dynamics of planetary systems. Players start out with a single Earth-sized planet orbiting a Sun-sized star and gain points by adding additional celestial bodies. Bigger objects such as dwarf stars, brown-dwarf planets and gas-giant planets earn more points than smaller ones like ice giants and super-Earths, but be warned: adding the wrong planets at the wrong time or in the wrong places will quickly lead to chaotic behaviour. If your planets crash into each other and/or go rocketing off beyond a certain distance (currently set, somewhat arbitrarily, at 2 AU – twice the distance between the Earth and the Sun), you’ll lose the game and have to start again.

Who is behind it?

The creator of Super Planet Crash, Stefano Meschiari, is a PhD student at the University of Texas at Austin in the US. As part of his “day job”, he maintains a scientific software package called Systemic Console that astronomers use to identify potential exoplanet signals within data sets acquired via Doppler observations of stars. After creating a stripped-down version of Systemic designed for educational use, Meschiari decided that his next project would be even more outreach-oriented. “I had been thinking for a long time about how to use my expertise developing Systemic to create an outreach game that would be appealing to a larger audience,” he told Physics World, adding that he wanted “something that would be fun and easily understandable at a more visceral level than Systemic”.

How much physics is involved?

The science behind the “digital orrery” simulation is pretty basic – these are strictly Newtonian systems, with no adjustments for general relativity. It is also not currently possible to extract numerical data (such as orbital velocities or distances) from games or to “rewind” to the moment just before a crash to study the collision dynamics in more detail. However, Meschiari says that the current game (which has already been played millions of times) is essentially a prototype. He hopes to add more features in the future, and he and some colleagues have applied for funding to develop the game into a full package of “edu-tainment” applications. In the meantime, though, Meschiari hopes that the game will work like a “gateway drug”, stimulating players to learn more about gravity and exoplanets.

What’s it like to play?

Addictive, if sometimes frustrating. To achieve a high score, you really need to chuck a dwarf star into the mix. But while certain binary systems – particularly those where the original star and the added dwarf star are very close together – produce stable orbits, getting there requires “some art and some luck”, Meschiari says. A lot depends on the position of the first planet, which is generated by the game and is therefore random rather than player-determined; hence, in some games, the odds are stacked against you from the start. However, that is perhaps the point, since in real life, as in the game, not all planetary systems are created equal. For example, we do not observe many quadruple-star systems precisely because they are much more likely than binaries to be unstable. And if your reviewer’s efforts are any indication, we shouldn’t expect to see many systems of eight brown dwarf stars either. “Super” planet crash indeed.

Physics World’s futuristic look

Holographic view: The August 2014 issue of *Physics World*, seen through the NPL device. (Courtesy: NPL/Richard Stevens)

Some of you may remember a news story I wrote last month that looked at a new optical gadget that uses a holographic waveguide to augment reality. The device hopes to transform the wearable-display market – it allows users to overlay full-colour, 3D, high-definition images into their normal line of sight, thereby interacting with their surroundings. The waveguide was developed by UK-based company TruLife Optics, along with researchers from the adaptive-optics group at the National Physical Laboratory (NPL) near London.

(more…)

Browse articles by content type

When light listens to your every step

Sound interpretation

1 Suspicious sounds

Listening for leaks

Ships, sharks and submarines