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Home » Page 1345

Further doubts cast over lunar formation models

An international team of researchers claims that nearly all of the material that makes up the Moon came from the early Earth. These findings contradict astronomical models of the formation of the Moon by the impact of a Mars-sized object with the early Earth that suggests more than 40% of the Moon-forming material came from the Mars-sized impactor known as “Theia”.

The study, published in Nature Geoscience, is based on the popular Moon-formation scenario that suggests that Theia collided with a young proto-Earth and that this giant impact threw up a disc of material that then orbited the Earth. The disc is thought to have condensed within a few centuries and then rapidly accreted to form the Moon as we now know it. Many numerical simulations of this scenario have suggested that a maximum of 60% of the disc-forming material would have come from the Earth’s mantle, with the other 40% being Theia material. This means that the distinct geochemical compositions of the Earth and Theia should be found in lunar material, as Theia matter should be easily distinguishable from Earth matter.

Tricky titanium

But Junjun Zhang from the University of Chicago in the US and other colleagues in Switzerland found that when they compared the relative abundance of titanium isotopes in samples of rocks from the Earth and the Moon – after correcting the lunar signature for any “noise” that might be caused by cosmic rays – they found both signatures identical to about four parts per million. According to the researchers, this isotopic homogeneity of titanium suggests that the Moon is composed almost entirely of material from the mantle of Earth. This result seems to contradict most numerical giant-impact models or suggests that Theia’s composition was surprisingly similar to that of the Earth’s.

In the past, other researchers have looked at the isotopic signatures of lunar material and have found that silicon, chromium, tungsten and oxygen isotopes for Earth and lunar material are very similar. Silicon, chromium and tungsten work in accordance with the giant-impact numerical calculations, working on the assumption that Theia’s composition is quite similar to that of Mars. But, as it is with the titanium isotopes, the oxygen isotopic signatures of both terrestrial and lunar rocks are so similar that Theia cannot have contributed more than a few per cent of material to the Moon-forming disc.

Earth’s twin?

The researchers do consider the possibility of Theia having a surprisingly similar composition to that of the Earth; but they also point out in the paper that “this idea seems to be contrived and requires special circumstances for an embryo [Theia] to have the same titanium, oxygen and tungsten isotopic compositions as a growing planet [Earth]”. An alternative scenario that the researchers suggest is that isotopic differences between Earth and Theia matter could have been erased during the volatile exchange of matter in the aftermath of the impact. But for this to happen with titanium, unrealistic scenarios – such as extremely slow cooling of the Moon-forming disc or large-scale turbulent mixing would be needed to be invoked. Other possibilities, including the fact that the lunar material may have been created by an exchange of material between the Earth’s magma ocean and the proto-lunar disc, or by being thrown off from a rapidly rotating post-impact Earth are now being considered.

The work is published in Nature Geoscience.

Fermilab told to rein in planned neutrino experiment

Physicists in the US working on a major new experiment to study the properties of neutrinos have suffered a major setback with the announcement that the US Department of Energy (DOE) will not fund their current plan. Known as the Long-Baseline Neutrino Experiment (LBNE), it would involve creating a beam of neutrinos at Fermilab and detecting them 1300 km away in a new detector to be built deep underground in South Dakota’s Homestake mine. But writing to Fermilab director Pier Oddone last week, William Brinkman, who is director of the DOE’s Office of Science, says that the DOE “cannot support the LBNE project as it is currently configured”.

Brinkman adds that the decision has not been based on the scientific merits of the LBNE proposal, but rather because the full cost of the project (expected to be more than $1bn) cannot be accommodated by the current US budget or by projected budgets over the next decade. The LBNE would also involve building a new neutrino detector at Fermilab, which is located just outside Chicago, and physicists had hoped to have the experiment running by 2020. The LBNE is supposed to be the successor to the NOvA experiment, which should start sending neutrinos 810 km from Fermilab to an underground detector in Minnesota in 2014.

In a separate letter to Fermilab staff, Oddone insists that the laboratory remains committed to achieving the scientific goals of the LBNE. “We will work closely with the DOE and the particle-physics community over the coming months to outline options for a phased approach to long-baseline neutrino experiments,” he says. Fermilab now plans to work with the DOE and neutrino physicists to develop a new strategy for building the LBNE.

Phased approach

Neutrinos come in three “flavours” – electron, muon and tau – with the neutrinos able to change, or “oscillate”, between the different types. However, physicists also believe that neutrinos can be described in terms of combinations of three mass states – m1, m2 and m3. Interference between these mass states gives rise to the observed oscillations of neutrino flavour – a muon neutrino could change into an electron or tau neutrino, for example, as it travels through the Earth.

Under Fermilab’s plans for the LBNE, a pure beam of muon neutrinos would be created at Fermilab, and then when reached the Homestake neutrino detector it would be a combination of all three flavours. Measuring the rates at which these oscillations occur would therefore provide new information about neutrino mixing angles. The LBNE would also allow physicists to compare the oscillation rates of neutrinos and antineutrinos, and look for differences. Any such differences would violate the Standard Model of particle physics, which incorporates a fundamental symmetry between particles and antiparticles known as CP (“charge–parity”) symmetry.

Neutrinos from elsewhere

Fermilab physicists also hope to use the underground detector to study neutrinos produced naturally in a number of places, including the Earth’s atmosphere and distant supernovae. Brinkman now suggests that, instead of building the LBNE all at once, Fermilab should take “an affordable and phased approach that will enable important science results at each phase”. He also says that the plans for Homestake facility should be revised to include the option of co-locating other physicists experiments – such as a facility for doing dark-matter experiments.

Multiverses in the movies

By Matin Durrani

Far from being an arcane concept in theoretical physics, the idea of “parallel worlds” and “parallel universes” has for many years served as a source of inspiration for numerous artists, movie-makers and writers, as the Stony Brook University philosopher and historian Robert P Crease discussed in his column for Physics World last December.

The latest – and probably not the last – example of multiple universes in popular culture comes with a new German film released earlier this month entitled Schilf, which means “Reeds” in English.

The film’s based on the bestselling book of the same name by German author Juli Zeh, the English translation of which, rather confusingly, was entitled Dark Matter in the UK and In Free Fall in the US.

Reviewing the book for Physics World in 2010, US science writer Jennifer Ouellette called it “a compelling intellectual thriller”, which she commended for its “meticulous plotting…and lyrical turns of phrase”. You can read her review here.

Anyway, the film version, like the book, features – unusually in the movie world – not one, but two bona fide physicists, in the form of a professor at the University of Jena called Sebastian Wittich (played by Mark Waschke) and an old pal from his student days called Oskar Hoyer (Stipe Erceg), who’s now based at CERN.

According to Ouellette’s review of the book, Sebastian and Oskar are passionate rivals when it comes to physics, and the story begins with the pair discussing the philosophical implications of the possibility of parallel worlds, before quickly veering off to include a kidnapping, a ransom, a grisly death and the unravelling of Sebastian’s life.

“Eventually, an unorthodox detective with a love of physics and an inoperable brain tumour steps in to solve his final case by connecting these seemingly random events,” Ouellette writes.

I’ve only watched the trailer for the film – directed by Claudia Lehmann – so I can’t comment on how closely it follows the novel or if the movie is worth watching.

But the trailer itself looks okay, with realistic-looking shots of a physics lecture hall and a scene inside Sebastian’s home, where his son starts going on about Schrödinger’s cat. Then the cheery (cheesy?) accordion music turns predictably sinister, various mobile phones go off, assorted trains/cars/bikes come and go, an old, beardy guy with dark glasses and a scarf stumbles into view, before a character, with his back to us, admits “I’ve killed someone – but not in this world.” There’s also a glimpse of a place that might, or might not, be CERN.

By the way, a gripe of mine: why is it that mobile phones in movies never have silly ring tones?

More details of the film can be found at IMDb.com.

Electron tomography peers into the nanoworld

Electron tomography is a powerful way to image materials with extremely high resolution but the method cannot provide 3D images on the atomic scale for several reasons. Now, researchers in the US are saying that they may have overcome some of the technique’s limitations by taking a new approach to the problem. The advance could be a boon for those imaging nanomaterials, including biological samples.

Being able to visualize how atoms are arranged in materials has played a crucial role in the evolution of modern science and technology. Crystallography techniques have long been employed to reveal 3D atomic structures and scanning probe microscopes can determine surface structures on the atomic scale. Electron microscopes, for their part, can routinely resolve atoms in 2D projections of 3D crystalline samples.

However, currently there is no direct way to determine the 3D structure of samples on the atomic scale without first knowing the material’s lattice structure and assuming that the atoms fit rigidly on that lattice. These conditions do not apply to a nanoparticle, which can have a different lattice structure than a bulk sample of the same material.

Now, Jianwei Miao and colleagues at the University of California, Los Angeles (UCLA) and the Lawrence Berkeley National Laboratory say that they have performed the first experiments in which they can directly image 3D local structures without relying on a priori structural information. “We have succeeded in observing individual atoms inside some regions of a gold nanoparticle 10 nm in size and identifying several grains therein at a resolution of just 2.4 Å in three dimensions,” Miao says. “This has never been done before.”

Overcoming old challenges

One major drawback of conventional tomography is that samples have to be tilted a number of times and images taken at each tilt to build up the overall structure. But the problem is that prolonged exposure to the electron beams used in the microscope damages the sample and therefore limits the number of projections possible from a single object. Another disadvantage of the technique is that it is technically challenging to align the sample along a common axis and then tilting it with respect to this axis with atomic-level precision. Finally, samples cannot normally be tilted beyond ±79°, which means that data cannot be acquired from the “missing wedge”.

Miao and colleagues have now overcome all of these problems by combining a new alignment method with an iterative tomographic reconstruction technique that uses an annular dark-field scanning transmission electron microscope (ADF-STEM).

The team says that it has been working towards this goal since 2005, when it decided to improve on conventional tomography by developing an “equally sloped tomography” (EST) technique. This method allows the researchers to tilt their samples by small, and as the technique’s name suggests, equally sloped, increments and perform 3D reconstructions on them using a Fourier-based iterative algorithm. The EST helped to significantly reduce the required number of projections from a given sample, which automatically meant less beam damage, while maintaining the same image resolution, image contrast and image quality.

“In our new work, we have combined this EST method with another new projection alignment approach, which relies on the centre of mass of a particle, to image a 10 nm gold nanoparticle in 3D at the atomic scale using only 69 projections,” explains Miao.

According to the UCLA researchers, their general method could, in principle, be used to determine the 3D local structure of crystalline, polycrystalline and even disordered nanomaterials at the atomic scale. It might also help to improve the resolution and image quality when performing electron tomography of biological samples.

The work is presented in Nature.

How to hide from a magnetic field

Researchers in Europe have built a magnetic cloak that, in theory, is reasonably practical to manufacture. An object concealed by the new cloak, the researchers claim, is magnetically undetectable, while the cloak itself is made from materials available in many physics labs the world over. This means that it is, in principle, the first cloak that should be reasonably practical to manufacture.

Cloaks and shields

In 2011 Alvaro Sanchez and colleagues at Universitat Autònoma de Barcelona, Spain, developed a theory for a type of magnetic cloak they called an “antimagnet” that would have two crucial properties. One is that any magnetic field created within the cloak would not leak outside the cloaked region and the other is that the cloak and the cloaked region would be undetectable by an external magnetic field; that is, the field would not be distorted by the cloak. Now, Sanchez along with Fedor Gömöry and colleagues from the Slovak Academy of Sciences, has designed and demonstrated a modified version of the cloak proposed last year.

The new cloak is a simple bi-layer cloak made up of two common materials – an inner superconducting layer made up of a high-temperature superconducting tape and an outer ferromagnetic layer composed of a few turns of a thick FeNiCr commercial alloy sheet. “The cloak we proposed last year was more of an ideal cloak,” explains Sanchez. “But it was complicated with 10 layers and included superconducting plates. This new cloak, while not perfect, is a much simpler design for achieving similar results using a static uniform magnetic field.” He adds that it is fair to say that this is the first cloak that is an exact cloak that can be feasibly implemented in practice.

The superconducting layer on its own repels the magnetic field, while a ferromagnetic layer on its own attracts the magnetic field lines; so both independent layers distort the field. The cloak is the accurate combination of the two layers, determined by a specific radius, which adjusts for the permittivity (μ) such that there is no external field distortion at all. This radius is calculated using Maxwell equations. “It is quite amazing that almost 160 years after Maxwell equations were first developed, we are still finding new solutions based solely on them!” says Sanchez.

Perfection problems

Sanchez tells physicsworld.com that the entire team is highly inspired by the initial work on building invisibility cloaks using transformation optics carried out by John Pendry and colleagues at Imperial College London since 2006. “There are generally two ways of achieving a cloak – either using transformation optics or using plasmonics. The problem with the first is that, while it is theoretically the perfect cloak, it is nearly impossible to physically create. With plasmonics, while the materials are available, you get a slight shadowing or scattering effect, not a complete cloak at all. This is the first time that you get both using commercially viable materials,” Sanchez explains.

Sanchez points out that an advantage in developing a cloak for a static magnetic field is that, for such a field, the magnetic and electric effects decouple and the researchers only have to consider the magnetic permeability. The team tested its cloak using a static field of 40 mT – which is greater than the Earth’s magnetic field. Currently, the cloak has been built on a small but reasonable scale – 12.5 × 12 mm. Sanchez explains that another advantage is that, for a static magnetic field, the cloak can work on any length scale – from microns to metres – as there is no intrinsic cut-off, unlike other cloaks that work at fixed wavelengths.

Because the cloak is capable of running under relatively strong magnetic fields and relatively warm liquid-nitrogen temperatures, and as it is made from commercially available materials, it could be readily put to practical use, the researchers say. The team is also looking at other methods to manipulate and control magnetic fields into different “shapes”, for purposes other than cloaking, in the coming months.

The research is published in Science 335 1466.

Proteins wander through whorls and vortices

A multidisciplinary team of physicists and biologists has discovered a new system of proteins that organizes itself spontaneously into a spectacular pattern of whorls and vortices. But unlike similar systems studied in the past, the individual protein molecules move freely throughout the pattern, while the pattern itself remains fixed. The scientists believe that further study of the system could shed light on the collective behaviour of living organisms.

The spontaneous, self-organized collective motion of entities as diverse as flocks of birds and colonies of bacteria have long fascinated humans. Scientists, however, have struggled to understand the underlying mechanisms that allow hundreds, thousands or even millions of individuals to act as one. It is often the case that the dynamics of a group cannot be predicted in a straightforward manner from observing the behaviour of individual members.

Difficult to disentangle

While this collective behaviour in living organisms is fascinating, it is also extremely difficult to model mathematically because of the complicated ways in which living things can interact. These range from one bird hearing another’s distress call to a person being influenced by a friend’s political ideas. It is often difficult to identify all these interactions, let alone to disentangle their influences on a system’s behaviour. Furthermore, conducting interventional experiments can be problematic, as many of the parameters may not be under the experimenter’s control.

Fortunately for scientists, self-organization is not limited to living organisms. By studying often simpler inanimate systems, important insights into collective behaviour can be gleaned. In 2010, two research groups turned their attention to the formation of patterns by protein filaments made from actin, which are driven by motor proteins. These filaments are used by cells to convert chemical energy into kinetic energy by propelling the tiny protein filaments like the outboard motor of a speedboat. Above a certain critical concentration, the researchers discovered that protein filaments on a glass slide organized themselves into various ordered structures such as waves and spirals.

Whorls and vortices

Now, scientists in Japan have looked at a different combination of filament and motor, and found that the proteins then organize themselves into an elaborate pattern of whorls and vortices when placed on a glass plate. The research provides an important new demonstration of complex behaviour arising in a simple system.

The new research is a collaboration involving Yutaka Sumino at the Aichi University of Education, Ken Nagai at the University of Tokyo, Kazuhiro Oiwa at the Advanced ICT Research Centre in Kobe, Hugues Chaté at CEA-Saclay in France and their respective teams. The researchers used a different kind of protein filament called a microtubule and a different type of motor protein called dynein. As with actin, they found that when the density of motor proteins was high enough, the filaments organized themselves into patterns. However, unlike the previous examples of relatively simple patterns, the pattern here was a far more elaborate, repeating “lattice” of adjacent vortices.

Crucially, however, while the established pattern remained stable, a single microtubule was not stuck in one vortex. Individual molecules wandered across the surface of the substrate from vortex to vortex, sometimes rotating clockwise and sometimes anticlockwise, all without disturbing the stability of the overall pattern. Although the researchers were able to construct a computer model of the behaviour, it is not yet clear whether the behaviour is the result of a specific property of the microtubule molecule or whether there may be a more general explanation.

Not frozen-in

Biophysicist Andreas Bausch of the Technical University of Munich in Germany. leader of one of the research groups that originally discovered pattern formation in protein filaments, is fascinated. “These are not frozen-in structures as, for example, we observed last year,” he explains. “You’ve got the constant exchange of material between the vortices and yet they’re stable over very long timescales.”

Tamas Vicsek of Eotvos University in Hungary, an expert on complexity theory and author of an accompanying commentary on the paper describing the work, believes the research marks a significant step forward in the study of complex behaviour in protein filaments. “They just observed a new phase in the study of collective motion that has not been seen before,” he says. “It seemed that searching for new patterns was over, but this shows that the whole system of self-propelled particles is a much richer world than we had assumed.”

Sumino hopes that the work may one day provide a basis for understanding complexity theory more widely. “In our model all you have is a short-range collisional interaction,” he explains. “Depending on the system, sometimes there is more detailed interaction. So if you think about the collective motion of fish, fish have eyesight in one direction only. Such detailed information can be taken into the model later.”

The research is published in Nature.

Should we engineer the climate to counter the effect of global warming?

By James Dacey

Geoengineering is the idea of controlling the weather and climate by the large-scale engineering of the environment. The idea has come to prominence in recent years as concerns about man-made global warming have increased and governments have faltered on negotiations to restrict carbon-dioxide emissions.

hands smll.jpg

One of the more radical proposals is to intervene with the Earth’s solar-energy balance by deploying technologies to reflect sunlight. Suggestions include painting buildings white to make them more reflective, injecting reflective aerosols into the atmosphere, or even deploying a fleet of shields into the Earth’s orbit to directly intercept incoming sunlight.

The other main approach to geoengineering is to try to directly remove carbon dioxide from the atmosphere. One area already being developed is carbon capture and storage (CCS), a three-stage process that involves harvesting, transporting and then storing the carbon dioxide in suitable underground locations such as vast saline aquifers. A more radical approach is to fertilize the ocean with a limiting nutrient such as iron to promote more marine flora, which will draw more carbon out of the atmosphere during photosynthesis.

Earlier this week we published an interview with the high-profile geophysicist Ken Caldeira of the Carnegie Institution for Science in the US. Caldeira has some severe reservations about geoengineering, specifically concerning: its environmental impact; how the presence of a “plan B” that may prove unreliable could affect efforts to cut carbon emissions; and who on the global stage should regulate use of the technology, particularly when it may reduce rainfall in some areas.

We want to know your opinion on this issue, via this week’s Physics World Facebook poll.

Should we engineer the climate to counter the effect of global warming?

Let’s do it!
We should prepare to do it as a “plan B” if carbon emissions continue to rise
No way! The environmental risks are too high
No, because it won’t work anyway

Have your say by casting your vote on our Facebook page. As always, please feel free to explain your response by posting a comment.

In last week’s poll we looked at the issue of university ranking exercises. The issue was on our minds because the Times Higher Education (THE) had just released its annual list of the top 100 universities, which was dominated by institutions in English-speaking countries. We asked whether you think these university ranking exercises are inherently biased. The outcome was highly conclusive, with 96% of respondents opting for “yes”.

Thank you for your participation and we look forward to hearing from you in this week’s poll.

Physics for students, not poets

My immediate response to the title of Quantum Physics for Poets is “I am not worthy.” Although I have written a couple of limericks and a particularly dire sonnet, I am hardly a poet. Luckily, the book’s target audience is not actually so limited. Instead, the authors’ stated aim is to introduce quantum physics in a way that enables arts students – and presumably poets are regarded as the ultimate of that ilk – to get their heads around this truly mind-bending subject.

With this principle in mind, I was a little disappointed with the verbose introduction, in which drawn-out parallels are made with revolutions in the arts and politics – as if to prove that quantum physics is particularly suited to the intellectual rebel and make it more palatable to arty types. This seems rather condescending. The introduction also reflects a problem that reoccurs throughout the book. Although their intent is to present information in a non-technical way, authors Leon Lederman and Christopher Hill struggle to detach themselves from their jargon. Theoretical physicist Hill and Nobel-prize-winning particle physicist Lederman have had a long involvement in the public understanding of science, but demonstrate here how difficult it is for science professionals to understand the worldview of the non-scientist.

As an example, I find it difficult to believe that anyone with a non-scientific background would be comfortable with this sentence from the introduction: “Since the location of June can be deduced without measuring the electron Molly, whose properties are correlated by the initial quantum state of the radioactive parent particle, the properties of the particle arriving at Alpha Centauri must seemingly have an objective reality.” I can imagine an awful lot of poets (and other people) going “Huh?”.

After the introduction, we are eased into the quantum world with a brief historical exploration of classical physics. Galileo and Newton feature heavily here, providing a good mix of historical context and basic science. Occasionally, though, the history is something of a caricature; for example, we are told that Galileo dropped balls off the leaning tower of Pisa, an event that most historians of science consider unlikely. The exploration then moves on to cover light, which introduces the reader to the “ultraviolet crisis” – the prediction from 19th-century electromagnetic theory that all atoms should emit vast quantities of high-energy light – and the origins of quantum physics.

As the book’s scientific side comes to the fore, the historical context is downplayed, though we do get occasional snippets. I found it particularly delightful to discover that Max Born was Olivia Newton-John’s grandfather. But again, there is something of a tendency to tiptoe around historical accuracy. So, for example, we hear that in 1685 the Danish astronomer Ole Rømer’s calculations “yielded the first precise measurement of the speed of light, a whopping 300,000,000 m s–1“. In reality, Rømer’s value was closer to 220,000,000 m s–1. Suggesting otherwise condenses history a little too much.

Once we enter the 20th century, the science is given considerably more opportunity to develop, so the reader is taken with some care through Planck’s idea that radiation should be split up into “bunches, or quanta”. An interesting revelation in this section is that Planck did not really see this as an observation about light itself, but rather a description of the action of the atoms in a black body that is radiating light. Soon, Einstein enters the picture, and from this point on, a key part of the book’s message is the “shock of the new”. Looking back, it is hard to imagine just how much of a departure from classical thinking was required to begin to grasp quantum theory, and Lederman and Hill make sure that we really understand that the culture shock among physicists was immense. Indeed, some – Einstein and Schrödinger being two obvious examples – were never comfortable with its implications.

To get this far has taken only around one-third of the book. Now we plunge into the structure of the atom, matrix mechanics, the uncertainty principle and the Schrödinger equation. A whole chapter is dedicated to quantum entanglement and its implications, with an unusually detailed exploration of Bell’s theorem – a topic that is often considered too confusing for the general reader, as the authors demonstrate here. After exploring Dirac’s relativistic expansion of the Schrödinger equation and a quick tour of Feynman’s sum-over-paths approach, the book concludes with a rapid crescendo of supersymmetry, holographic universes, quantum gravity and string theory, climaxing with a brief introduction to some of the new quantum technologies of quantum cryptography and quantum computing.

Throughout the book, I get the impression that it is essentially a collection of physics lectures for arts students, generated by simplifying standard introductory physics lectures. This is acceptable for an actual course, for students who are prepared to sit through it to get their credits, but it does not work as well as a science book for the general reader. Such “science-for-the-arts” courses are quite common at US universities, but even if this were the target audience of this book, the authors could take lessons in how to go about it from Richard Muller’s superb Physics for Future Presidents, while a more general audience would benefit much more from the approach of a title such as Marcus Chown’s Quantum Theory Cannot Hurt You. Unfortunately, poets have not been well served here.

This is, nonetheless, a good book. Lederman and Hill provide the reader with plenty of introductory meat on the development of quantum physics and they really bring out the startling surprises at the heart of it. But the approach they take is not for poets. It would be much better targeted at high-school physics students to help prepare them for university physics. Rather than quantum physics for poets, this is quantum physics 101 lite. That is a useful book, and in that role I would heartily recommend it. But it doesn’t do what it says on the tin.

Robot jellyfish fuelled by hydrogen

 

A robotic swimmer that mimics the motion of a jellyfish has been built by researchers in the US. Dubbed “Robojelly”, the swimmer propels itself using an “artificial muscle” built in part from carbon nanotubes and powered by hydrogen. Such robots could be used in a number of scientific, military and commercial marine applications, the researchers say.

Robojelly has been built by a team led by Yonas Tadesse from Virginia Tech and the University of Texas at Dallas that is claiming “the first successful powering of an underwater robot using external hydrogen as a fuel source”. As well as only producing water as a waste product, hydrogen is attractive for remote marine vehicles because, in principle, the fuel could be obtained from seawater using energy from the Sun.

Deform and reform

At Robojelly’s heart is a commercially available nickel-titanium shape-memory alloy (SMA) – a deformable material that returns to its original shape when heated. The SMA is wrapped in a sheet of carbon nanotubes that itself is coated in titanium particles, which catalyse the reaction between hydrogen and oxygen. The heat produced as a result of this oxidation then allows the SMA to revert to its original shape.

Robojelly uses carbon nanotubes because they are highly porous, which allows the hydrogen and oxygen to reach the catalyst. And apart from being lightweight and very robust, nanotubes also conduct heat well, which is good because heat must be transferred quickly to and from the SMA during operation.

The robot has an umbrella-like structure that mimics the propulsion strategy of the common jellyfish Aurelia aurita. The “bell” of the robot is made from silicone and is supported by eight springy steel ribs, with a string running alongside each rib from the edge of the bell to a pulley at the centre. Each string then goes down into a tube that holds the SMA actuator.

In one design, all the strings are attached to one central SMA actuator, while in another design there are eight different actuators. One benefit of the latter design is that, in principle, different segments of the bell can be controlled individually. “This should allow the robot to be controlled and moved in different directions,” explains Tadesse.

Flapping bell

To demonstrate Robojelly in action, the team placed it in a tank of water. A fixed amount of hydrogen and oxygen is then introduced to the tube, which warms up the SMA and makes it change shape. As it deforms, the alloy pulls on the strings, causing the bell to flap in one direction. As the SMA cools, the restoring force of the steel ribs makes the bell flap in the opposite direction. According to the team, an entire cycle can occur in less than 10 s.

The team measured the deformation of the bell – defined as the distance moved by the edge of the bell divided by the length of its curve – and got a value of about 14%. This is smaller than the 29% achieved when the robot was electrically powered and the 42% that is typical of a living jellyfish. Although the researchers have so far only operated the robot jellyfish clamped at the bottom of a water tank, they are looking at ways of boosting the performance and efficiency of the system.

The research is described in Smart Materials and Structures 21 045013 and a video of Robojelly can be viewed below.

Geoengineering: the pitfalls and politics

Geoengineering is the idea of controlling the weather and climate by the large-scale engineering of the environment. The idea has come to prominence in recent years as concerns about man-made global warming have increased and governments have faltered on negotiations to restrict carbon-dioxide emissions. But many people are concerned about geoengineering: its environmental impact; how the presence of a “plan B” that may prove unreliable could affect efforts to cut carbon emissions; and who on the global stage should regulate use of the technology, particularly when it may reduce rainfall in some areas.

One high-profile geophysicist who has reservations about geoengineering is Ken Caldeira of the Carnegie Institution for Science in the US. Caldeira believes that geoengineering efforts should focus on existing areas of science and technology research. This includes the removal of carbon dioxide from smokestacks in power plants, no-till agriculture and other soil amendments, and stratospheric particle and chemistry research based on volcanoes and the ozone layer. Activities such as whitening clouds over the ocean, meanwhile, could be carried out by those who are already studying marine clouds.

Caldeira is in favour of environmental-science studies into the option of sunlight reflection by distributing reflective particles into the atmosphere. But he is against work that strays into the engineering development of implementation techniques.

Take the engineering out of geoengineering
Take the engineering out of geoengineering

While geoengineering is not yet on most people’s agendas, future events could cause a radical shift in public opinion in the US, says Caldeira. In turn, this would put pressure on politicians to implement the technology. But Caldeira is aware that what is best from a political point of view is not necessarily best for the environment, given the variations in timescales between political and environmental cycles.

Politics versus environment
Politics versus environment

As well as altering the environment, geoengineering will affect human systems such as agriculture. This gives poor people in tropical countries perhaps the biggest incentive of all to implement geoengineering, says Caldeira, as crop yields in the tropics are more likely to be badly affected by heat stress than those in northern climes, where yields may even improve.

Developing gains
Developing gains

But are countries likely to go it alone when it comes to geoengineering, without complying with any international agreements that may be set up? On this point, at least, Caldeira is sure that when citizens and environmental resources are under threat, politicians will not refrain from acting in their national interests.

Taking sides
Taking sides
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