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Roger Penrose asks if a cyclic cosmology is lurking in LIGO noise?

Correlated noise in the two LIGO gravitational-wave detectors may provide evidence that the universe is governed by conformal cyclic cosmology (CCC). That is the claim of Roger Penrose of the University of Oxford, who is proposing that the apparent noise is actually a real signal of gravitational waves generated by the decay of hypothetical dark-matter particles predicted by CCC.

Last month, physicists at the Niels Bohr Institute pointed out that some of the noise in the two LIGO detectors appears to be correlated – with a delay that corresponds to the time it takes for a gravitational wave to travel the more than 3000 km between the instruments.

Writing in a preprint on arXiv, Penrose argues that a significant amount of this noise could be a signal of astrophysical or cosmological origin – and specifically CCC.

Infinite aeons

First proposed over a decade ago by Penrose, CCC assumes that the universe consists of a succession of aeons. Each aeon begins with a big bang and proceeds into an unending future in which the universe expands at an accelerating rate. As this expansion becomes infinitely large, Penrose argues that it can be transformed back into the next big bang.

He says that a “reasonably robust implication of CCC” is that dark matter consists of particles called erebons – the name deriving from the Greek god of darkness Erebos. As dark matter goes, erebons are extremely heavy and have masses of about 10^–5 g. This is roughly the Planck mass and on a par with a grain of sand and about 22 orders of magnitude heavier than a proton.

Near-instantaneous impulses

Penrose says that when an erebon decays, it deposits all its energy into a gravitational wave. While such waves have frequencies well above the detection capabilities of LIGO, their arrival at the detectors would be recorded as near-instantaneous impulses that could be mistaken for noise.

Physics societies sign up for unique ORCID identifiers

IOP Publishing, which produces Physics World, and the American Physical Society (APS) have signed an “open letter” committing them to collecting ORCID IDs for all authors submitting papers to their journals.

ORCID – Open Researcher and Contributor ID – is a not-for-profit organization that provides a unique identifier (iD) for every researcher, which means they can clearly distinguish researchers with the same name.

Avoiding confusion

“It is extremely important that researchers are correctly recognized for their work, whether as an author, reviewer or editor, and that the community is able to cite work without confusion,” says Jamie Hutchins, publishing director at IOP Publishing. “ORCID identifiers make this easier, by removing the confusion that can be caused by similarities between researchers’ names, name changes, inconsistencies in abbreviations and cultural differences in how names are presented.”

Matthew Salter, publisher at APS, adds: “With several major funders now requiring ORCID iDs as part of their grant application process, we hope that this will reduce the administrative burden on researchers as well as providing scholarly benefits.”

Easily discoverable

Laurel Haak, executive director of ORCID, says of the announcement: “As two of the main publishers in the physical sciences, use of ORCID iDs by IOP and APS means that authors will be clearly attributed and their body of work more easily discoverable.”

Refugee scientists under the spotlight

The final months of a postdoctoral research contract is a stressful time, but for Syrian climatologist Shifa Mathbout from the University of Barcelona, the stakes are higher. Her European Erasmus Mundus funding for studying Mediterranean rainfall trends runs out at the end of this month but she cannot return to Syria, admitting to having a “very big fear” about the prospect. “It’s very dangerous for me,” she stresses.

Mathbout mainly left Syria in 2013 because her brother is a member of the Syrian National Coalition, which opposes current president Bashar Al-Assad. She says she was “lucky” to get Erasmus funding, which is intended to support global co-operation. And while Mathbout can stay in Spain until 2021, she is clear what displaced scientists need. “The most important thing for us is to have work, to help us and our families back in Syria,” she told Physics World.

The trouble facing refugee scientists often goes unnoticed but recently has been examined by The World Academy of Sciences (TWAS), which is based at the Abdus Salam International Centre for Theoretical Physics (ICTP) in Trieste, Italy. It published a report in May – Refugee Scientists: Transnational Resources – that was based on conclusions drawn up at a workshop in March at the ICTP. At the workshop, where scientists, governments, educational institutions and other national and international organizations discussed the fate of scientists caught in the ongoing refugee crisis. Displaced scientists deserve relevant employment, the report concludes – and institutions must come together to ensure they get it.

“Scientists and science institutions in destination countries can do a great deal now to help identify displaced researchers and help them continue their work or studies,” says TWAS executive director Mohamed Hassan, who adds that some displaced people are highly qualified and so can benefit their destination countries. Finding them relevant work also helps maintain and enhance their knowledge, which can help rebuild their home countries once it is safe to return. The TWAS report notes that Iraq had 500 researchers per million population in 2008. “Countries such as Syria and Iraq previously had quite strong scientific communities – well-educated, published, respected,” says Hassan.

There are now four million Iraqi refugees around the world, with more than 65 million displaced persons globally at the end of 2015. However, there are no comprehensive figures of how many scientists are among this number, and the lack of detailed information is a central issue, according to Hassan. “How do you develop policy and programmatic responses when you don’t know the magnitude of the need?” he notes. Meanwhile, countries have been tackling the sudden recent surge of conflict-driven migration individually, but are now looking further afield. “We’re seeing strong interest in greater knowledge-sharing and co-ordination,” says Hassan. “Countries and programmes want to learn from each other’s experience.”

A work in progress

The UK’s Council for at-Risk Academics (CARA) and the Institute of International Education Scholar Rescue Fund in the US are good examples of how to support affected scientists. These organizations are “well-established and very effective at identifying scientists and other scholars in need and giving them crucial support”, Hassan says. Other efforts include the Philipp Schwartz Initiative, which was recently set up by Germany’s Humboldt Foundation to help universities and research institutions in Germany host “threatened foreign researchers” for a two-year period. Mathbout, for example, is currently applying for support from both CARA and the Philipp Schwartz Initiative.

CARA executive director Stephen Wordsworth stresses that the scientists his organization helps are not refugees, who are defined as people who have been granted asylum. Instead, they are scientists seeking temporary sanctuary who are eligible to enter the UK through the country’s visa system without claiming asylum. More than 110 UK universities, and others elsewhere in the world, help them continue their work until they can safely return home.

With the help of donations, CARA supports 260 displaced academics, up from just 50 in 2013. “In many cases their education has cost the British taxpayer not a penny so far and UK universities are getting the benefit,” says Wordsworth. However, CARA could not afford to support the increased numbers on its own and so relies on universities to provide funding for their research, living costs and accommodation. University contributions have increased from £600,000 in 2013 to £4.2m in 2016 with the money coming in some cases through fundraising directly from alumni.

Given that there is currently no organization similar to CARA in countries such as France and Italy, Wordsworth supports the TWAS call for better co-ordination. While institutional efforts are lagging behind, in other countries there are some grass-roots initiatives. For example, ICTP statistical physicist Matteo Marsili is collaborating with refugee camps near Trieste to offer internships for asylum seekers at ICTP or TWAS. “There is a need to support asylum seekers who have academic backgrounds or ambitions,” says Marsili. “In the time they spend travelling and in the camps this becomes a low priority issue to the point that they abandon their academic career because of more pressing needs.”

However, very few of the residents of the camps near Trieste, who are typically around 25 years old and from Pakistan, Afghanistan and Africa, have an academic background. “Scientists generally have more possibilities to find their way to northern Europe and they typically do so,” says Marsili. Thanks to the contacts established at the TWAS workshop, he is exploring how ICTP could help such physicists in danger. “I realized that helping is not so easy as one would naively think,” adds Marsili. “It’s still a work in progress.”

Limited choices

Ahmed Al-Tabbakh, a physicist at Al-Nahrain University in Baghdad, Iraq, exemplifies the benefit of international support. From 2002 to 2009 – during the worst years of war in Iraq – Al-Tabbakh worked at Pune University in India, gaining his PhD. Though he has not sought asylum, he admits he thought about it several times. “I always believed that my qualifications are my best means to overcome life challenges. As a researcher and a teacher in a country defying and fighting against terrorism on its land, I face many challenges and suffer many problems,” he says. “But I hope I do not get into a situation where I have to be an asylum seeker. I wish for my country to be safe and prosperous.”

Yet Al-Tabbakh, who works on nanomaterials for energy storage and conversion applications, faces difficult living conditions and a lack of research funding in Iraq. Since 2014 scientists have received no financial support to run their laboratories or even take part in international conferences and workshops. “Generally speaking Iraqi scientists live in hardship of resources,” he says. “Despite the circumstances, we are lively and productive.” As a TWAS Young Affiliate, Al-Tabbakh gets some support to attend international meetings and build collaborations. However, less fortunate scientists have fled to neighbouring countries as a result of the difficulties they face, even though it is also not easy to go elsewhere. “Choices are few sometimes,” he adds.

In a situation with similarly limited choices, Mathbout is counting the days until the end of her fellowship. As of late June, she was waiting to hear from the Philipp Schwartz Initiative. To work in the UK, she needs documentary proof of having worked as a paid lecturer or researcher in Syria. However, Mathbout says that she was being paid in cash when working in that position, so cannot just refer to her banking details. Consequently, she had to get written confirmation from the management of the university she worked at, which took more than a month.

“Communications in Syria are very bad,” Mathbout says. “When I think about this matter I feel like I just want to cry, because I have been five years without seeing my parents and my home country, my history, my memories, my books, everything. But I’m still OK. Let’s cross our fingers that this will be good. I hope so, really.”

Gedanken fictions

A soldier returning from war goes to visit the widow and young son of his fallen comrade Ted. During his stay with the family, the soldier takes on Ted’s name, chores and even his hobbies. Despite looking very different in appearance, he has effectively stepped into Ted’s life and his future is mapped out for him.

It doesn’t sound much like science, or even science fiction, but the short story “The tiniest atom” by Sarah Schofield is an exploration of “Laplace’s demon” – a thought experiment that explores the concept of free will in a universe that has to obey the laws of classical mechanics. As the accompanying essay by physicist Rob Appleby explains, Pierre-Simon Laplace theorized that if we know the forces acting on a body, and the precise location and momentum of all of its atoms, we can calculate its past and future. If everything in the universe obeys the same physical laws, then, with the right equations, we can determine the state of the universe – and everything and everyone in it – at any point in the past or the future. All of this suggests there is no free will.

This is just one of 14 pairs of short story and essay in Thought X: Fictions and Hypotheticals, which has at its root the concept that “thought experiments” in science and philosophy effectively tell stories as they build a scenario to prove some point – so why not get fiction authors, as well as scientists and philosophers, to explore them? After all, science and fiction share a capacity for taking the nub of an idea and stretching it far beyond immediate observations. This can lead to great stories and great breakthroughs.

The collection’s co-editors are Appleby, who works at the University of Manchester and the Cockcroft Institute, and Ra Page, who followed up his degree in physics and philosophy with a career in writing and editing short stories. The anthology – part of Comma Press’s ongoing Science-into-Fiction series – received funding from the Institute of Physics (which publishes Physics World) so perhaps it’s not surprising that the majority of the thought experiments here come from physics. From the grandfather paradox of time travel, to Maxwell’s demon, to Olber’s paradox, to Schrödinger’s cat, there is plenty to explore. The other thought experiments come from philosophy, and will perhaps be less familiar to Physics World readers, but no less fruitful.

As the name suggests, a thought experiment makes use of a hypothetical situation to prove or disprove a theory. Although such analyses have been dismissed in favour of practical experiment in modern times, they remain important where practical experiment isn’t (yet) possible – in new areas of physics and mathematics – or where practical experiment might be unethical, such as some strands of psychology. They also remain a very useful teaching tool. Indeed, some of them are simply analogies to explain a concept. Take the oldest example in this book: Galileo’s ship. Galileo knew that the Earth travelled around the Sun. Copernicus had done the maths; but people refused to believe it, because they couldn’t feel or see the Earth moving. So Galileo found a way to explain it that made sense to anyone who’d travelled on a ship.

The fiction in Thought X varies from purely metaphorical to more literal interpretations, but several stories try to do both. In “The tiniest atom”, the deceased Ted’s notebook is filled with jottings from public lectures and his own attempts to isolate the formula that will describe everything. In “Lightspeed” by Adam Marek a space pilot is damaging his marriage by taking longer and longer shuttle runs at near-lightspeed, increasing the physical and mental distance between himself and his wife. Physicist Tara Shears explains the effect of relativity on time and the resultant twin paradox – and why it’s not a paradox at all.

The genre, style, length and pace of the stories varies a lot for such a small collection. It’s unlikely one reader will love all of them, but it would also be a surprise if a reader didn’t find something to love here. The essays are uniformly clear and thorough, the only real variety being how much they interact with the story they follow. The best pairs of story and essay act to help explain each other. In “Tether” by Zoe Gilbert, Hark has built a house for Gertie. All he wants is to make her happy. But Gertie has discovered a shortcut to achieving pure happiness solo, so why would she choose anything else? Philosopher Jonathon Wolff explains that this is about the “experience machine” – a thought experiment by philosopher Robert Nozick that questions utilitarianism and authenticity. If all we value is achieving happiness, then we would all plug into the machine. But by plugging into the machine we cut ourselves off from the rest of the world, which does not give us pure happiness, but which we nevertheless value. Gilbert’s story perfectly illustrates this choice. And Wolff’s essay helps unpack a story filled with complex ideas and motives.

Thought X is a fascinating experiment and by choosing the right roster of writers, it achieves its aim.

2017 Comma Press 272pp £9.99pb

Building bridges with the West

What is the origin of the Kavli Institute of Theoretical Sciences (Kavli ITS)?

In 2006 the Kavli Foundation endowed the Kavli Institute for Theoretical Physics China (KITPC), which was located at the Institute of Theoretical Physics of the Chinese Academy of Sciences (CAS). Last year, the CAS and the Kavli Foundation repurposed the KITPC as Kavli ITS, which is located at the University of CAS (UCAS). The change of the name reflects the broader fields that Kavli ITS will cover, and also avoid the name overlapping with the Kavli Institute for Theoretical Physics at the University of California, Santa Barbara

How will Kavli ITS differ from KITPC?

Kavli ITS has a new vision to establish its own high-quality research team. It will cover all sciences and not just physics. In the first stage we plan to focus on four areas: condensed-matter physics; particle physics; biophysics and quantitative biology; and quantum information and computing. The institute will cover other fields such as quantum chemistry and earth science at a later date.

What attracted you to Kavli ITS?

I carried out a PhD at Virginia Tech in the US in 1983 and spent the next two decades in the US and Europe before moving to the University of Hong Kong in 2003. At that time Hong Kong was considered a window between China and the West and I helped organize visitor programmes and annual workshops in the city that brought top Western and Chinese physicists together. That kind of activity benefited people very much and it is something I want to continue at Kavli ITS. Taking up the post was a good opportunity and challenge for me to do that.

What is the current status of the institute?

I started as director in January and the new institute started from zero. We are now actively recruiting new faculty members. Kavli ITS is temporarily located at UCAS’s Zhong-Guan-Cun campus in Beijing and the institute’s offices have just been finished. Space will gradually increase as we grow as an institute. The institute is planning its permanent building at UCAS’s Huai-Rou campus, which is in a suburb of Beijing. We hope to move there within the next three years.

How is Kavli ITS funded?

Where our faculty have joint appointments with UCAS, their salaries will be paid for by the university. In the first few years, our budget from UCAS will be around ¥6m each year (£670,000). On top of that we also have the endowment from the Kavli Foundation. The Kavli Foundation does not just give us funding – it also provides support.

How many people will work at the institute?

We are aiming for around 30 faculty, whom we will recruit over the next five to 10 years. The institute must be international, so there is a strong desire to hire both Chinese nationals as well as non-Chinese. I will try to have around a third foreigners and the rest Chinese.

How will you attract foreign researchers?

We aim to provide competitive salaries for those researchers from outside China. Indeed, it is a challenge to get foreigners to stay in China for long periods, mostly because of the different culture. Language is also a problem since English is not widely used in China except at the institute or university. We are also competing with other first class institutes in China and the world. For those researchers who do not want to move to China, we have a programme so they can visit for a limited time. Kavli ITS can become a second home to those researchers.

How will you also compete for domestic talent with neighbouring universities such as Peking and Tsinghua?

There is strong competition for us. One benefit is that we offer a salary comparable to that in the US or Europe. Another benefit is the brand name of the Kavli institute. I also hope that I have a good reputation for attracting young people here.

How do you think the Kavli Foundation can help Chinese researchers?

There are 20 Kavli institutes in the world that are all conducting first-class research. The Kavli Foundation can help Kavli ITS to be connected with the other Kavli institutes, and set a fine standard for researchers in China. I hope it can play a positive role as a bridge between China and the West. In the theoretical sciences, we like to exchange ideas, so it is important that we have the methods to exchange ideas with those from outside China and that is why I would like to have high-quality visitor programme.

How has physics in China changes in recent years?

In the past decade, science, and physics in particular, have developed rapidly. When I was in Hong Kong in the early 2000s, it was ahead of China in physics, but now if you compare facilities, in many aspects China is ahead. China is also much more modernized, being close to Western standards.

How would you compare China with the US now?

In terms of speed of advancement, China is ahead. There are a few topics, such as topological insulators and superconductivity, where China is on a par with the US, but overall China is still behind by a distance. Yet it is getting closer each year.

What is your focus for the coming year and beyond?

We need to recruit more physicists, including those from outside China. We are currently designing and building a dedicated building for the institute at UCAS’s new campus, which will be ready in 2020. The campus will also include apartment blocks and schools for employees’ children.

Jocelyn Bell Burnell wins President’s Medal of the Institute of Physics

The astrophysicist Jocelyn Bell Burnell has been awarded the President’s Medal of the Institute of Physics (IOP) “for her outstanding contributions to physics through pioneering research in astronomy, most notably the discovery of the first pulsars, and through her unparalleled record of leadership within the community”. The award, which is given at the discretion of the IOP president, was presented yesterday in Birmingham at the International Conference on Women in Physics.

While presenting the award, IOP president Roy Sambles said: “Jocelyn is a groundbreaking researcher, an inspirational leader within our community and a distinguished ambassador for physics – particularly for widening participation.”

PhD breakthrough

While a PhD student at the University of Cambridge, Bell Burnell discovered the first four pulsars – an achievement that contributed to the awarding of the Nobel Prize for Physics to Antony Hewish and Martin Ryle in 1974. Controversially, Bell Burnell did not share in that prize.

In addition to her distinguished career as a researcher, Bell Burnell has served as president of the IOP and the Royal Astronomical Society, and is currently president of the Royal Society of Edinburgh. She also played an instrumental role in founding the Athena SWAN Charter, which was established in 2005 to advance the careers of women in science, technology, engineering, mathematics and medicine.

Smart glove translates sign language into digital text

A smart glove that translates American Sign Language (ASL) into digital text has been developed by scientists at the University of California, San Diego. Timothy O’Connor, Darren Lipomi and colleagues reckon that their device can be produced for less than $100 and could also find use in virtual-reality and remote-control systems.

Most systems for monitoring body movement involve using a camera or infrared emitters and sensors to capture motion. While such systems are effective, they can be bulky, inflexible and require large amounts of energy. As a result, researchers are keen to develop wearable motion sensors – and gloves offer a natural way of tracking hand motion.

Strain sensors

The new device is based on strain sensors that are made of a piezoresistive composite material comprising carbon particles embedded in a flexible material. To make a sensor, the team begins with a strip of silicone 3 cm long, 0.5 cm wide and 340 μm thick. This is coated with a special paint that contains carbon particles in a fluoroelastomer resin to create a piezoresitive film that is about 50 μm thick. Copper contacts are added at either end and the sensor is then encapsulated in polyurethane.

Nine sensors are placed on the back of a leather athletic glove – two on each finger and one on the thumb. The sensors detect the bending of the knuckles, and the sensor signals are digitized and then sent to an on-board microcontroller that is attached to the back of the glove at the wrist. The glove also contains an accelerometer and a pressure sensor on the thumb. Information from these two devices is used by the microcontroller to differentiate between ASL gestures for different letters that produce similar signals in the strain sensors.

Bluetooth link

The team has shown that the microcontroller can translate the hand gestures associated with all 26 letters of ASL into digital text. These data can then be exchanged with other electronic devices via a wireless Bluetooth link.

What’s more, signals from the glove were also used to control a “virtual hand”. This, the researchers say, could have a wide range of applications including virtual reality, telesurgery and controlling aerial drones or bomb-disarming robots.

The process for making the glove does not involve chemical synthesis or access to a clean room. This, say the researchers, suggests that the system could be used as a test bed for evaluating other stretchable electronic components for human–machine interfaces. “We’ve innovated a low-cost and straightforward design for smart wearable devices using off-the-shelf components,” says Limpomi, adding: “Our work could enable other researchers to develop similar technologies without requiring costly materials or complex fabrication methods.”

Sense of touch

The team is now incorporating a “sense of touch” into the glove to allow a user to better control either a virtual or robotic hand by sending tactile sensations back to the user. O’Connor explains: “Our ultimate goal is to make this a smart glove that in the future will allow people to use their hands in virtual reality, which is much more intuitive than using a joystick and other existing controllers.”

Jim Kyle, emeritus professor of deaf studies at the University of Bristol in the UK says that while the technology looks interesting, the glove’s “application to sign communication is probably limited”. “People (deaf or hearing) who are able to use the manual [ASL] alphabet are also likely to be literate to the extent of being able to write. Use of a glove for communication is not necessarily the highest priority.” Kyle also points out that an individual’s implementation of ASL can be highly idiosyncratic and will often not involve spelling out entire words – making text capture difficult.

The glove is described in PLOS ONE.

DNA origami delivers shape-shifting nanomachines

The survival and behaviour of all organisms is regulated by the highly specialized functions of molecular nanomachines, such as proteins. These nanomachines perform many different functions, including sensing light, smell and heat; generating muscle contraction; and regulating hormones. Scientists are now developing methods to engineer artificial nanomachines that could form the basis of personalized medicine or shape-shifting materials. One of the most promising approaches, called DNA origami, is centred on folding DNA to create elaborate-yet-predictable DNA nanomachines.

A key feature of natural nanomachines is the ability to change into a variety of shapes in response to diverse yet specific stimuli. But previous attempts to mimic this behaviour have been unable to replicate complex multi-step transformations because shape-shifting DNA nanomachines are typically composed of rigid structures connected by a few mobile regions.

Now, however, a research team from Emory University and Purdue University in the US, and Shanghai University in China, is trying to close this gap by creating DNA nanomachines that can shape-shift in a fundamentally new way. “Think of the shape-shifting robots in Transformers,” says Yonggang Ke, senior author on the study and assistant professor at Emory University. “Transforming from a car to a robot occurs in a complex series of movements. Every part of the machine needs to be able to transform. If a machine only has one moving part, it can’t get much more advanced than a light switch.”

The new work, which was reported in Science, is a big step towards creating machines that can undergo similarly complex transformations. The starting point for the researchers was to take a fresh look at a functional unit called the anti-junction, a diamond-shaped intersection of multiple separate strands of DNA.

Anti-junctions have two useful properties. First, they can switch conformations between two energetically equivalent states. This means that the more anti-junctions you can pack into a DNA nanomachine, the more shape-shifting ability the nanomachine will have. Second, when an anti-junction shape-shifts, its neighbouring anti-junctions will shape-shift too. Once the neighbours shape-shift, they will in turn cause their neighbours to shape-shift. A controlled stimulus on one anti-junction can therefore trigger a cascade of transformations throughout the entire nanomachine.

To demonstrate this, the research team created simple DNA nanomachines composed of 20–100 nm-wide rectangular grids of anti-junctions. Using diverse inputs including heat, mechanical stimulation, solvent conditions and a specific DNA “trigger” strand, they showed that they could drive large-scale transformation within the nanomachine. Importantly, they could reverse the transformation using specific inputs (also DNA strands), and lock the nanomachines in partially transformed conformations.

The researchers hope that this technology will one day lead to engineered nanomachines with multiple parallel functions. “Imagine having nanomachines that you can inject into the human bloodstream. Some of them will end up in the kidneys, and based on the kidney biochemical environment will turn into one type of functional machine. Some will end up in the liver and take on a different shape with a different function,” said Ke. This work could also have future applications in DNA computing, cellular signalling and biophysics. While such futuristic technology may be a long way away, we’re now one step closer to engineering molecular machines that rival those formed over the last millions of years.

Graphene bubbles measure shear forces

The force needed to slide sheets of graphene across each other has been measured using a new technique that involves blowing air bubbles made of the material. Developed by Zhong Zhang of the National Center for Nanoscience and Technology in Beijing and colleagues in China and the US, the technique was also used to measure the force needed to slide graphene across a surface of silicon dioxide.

Graphene is a layer of carbon just one atom thick that has a wide range of potentially useful electronic and mechanical properties. Developing practical graphene-based devices will require an understanding of how well graphene layers stick to each other, and also how they stick to popular substrates such as silicon dioxide. This stickiness is expressed as the shear resistance – the minimum force required to slide one layer over another. This quantity is not well known for graphene because it is extremely difficult to measure for a material just one atom thick.

Tiny bubbles

Now, Zhang and colleagues have adapted a technique called the blister test for use on graphene. One of their measurements involves a single layer of graphene on a silicon-dioxide substrate with micron-sized holes in it. The air pressure in the holes is increased, causing the graphene sheet to form tiny bubbles in the regions above the holes. The size of each bubble is monitored using atomic force microscopy (see figure). As the bubble pushes up, the surrounding graphene on the substrate is pulled and stretched towards the hole – creating a zone where shear occurs. Raman spectroscopy is used to measure the stretching of graphene’s carbon bonds in this “shear zone” as the size of the bubble increases. A similar measurement is made on two layers of graphene on the substrate, which has an additional graphene-on-graphene shear zone that can be analysed.

Small resistance

The team found that the shear resistance between layers of graphene was relatively small at 40 kPa. The value for graphene on silicon dioxide was about 40 times greater at 1.64 MPa. Writing in Physical Review Letters, Zhang and colleagues say that their technique could be used to study the interfaces between other 2D materials.

Proton tomography offers better preparation for therapy

Physicists in the US have shown that protons themselves can be used to provide the complex 3D images essential for tailoring proton therapy to individual patients. Researchers in the US built a prototype detector to carry out “proton computed tomography” (proton CT), and found that in around 6 min it could generate maps of proton stopping power – energy lost per unit distance in a material – that were more accurate and required a much lower radiation dose than existing techniques.

Proton therapy is becoming an increasingly popular tool to treat cancer because it can be used to target tumours very precisely. Unlike most other types of radiation, protons deposit a large fraction of their energy at the point where they stop in the body. By tuning the beam so that protons stop where the tumour is located, therapy can be made more effective and safer.

To deliver the radiation to a particular point in a person’s body, medical physicists must first establish the extent to which protons will be slowed down by intermediate layers of tissue. This is currently done by placing the patient in an X-ray CT scanner and creating a 3D plot of X-ray absorption within the relevant part of their body. But the accuracy of this technique is limited by a mismatch between X-ray absorption and proton energy loss within particular types of tissue.

Direct measurement

Proton CT would overcome this problem by measuring proton energy loss directly. Working scanners would employ the same source of protons used to carry out the therapy, but would require a different set-up – a higher-energy but lower-intensity beam and detectors that could be moved relative to the patient to get a 360° view. And as with X-ray CT, the procedure would be carried out ahead of treatment.

The idea of proton CT has been around ever since computer tomography was first proposed in the early 1960s. But the need for a proton beam makes it more expensive than X-ray CT and it also requires a more complex detector as well as more intense computation to transform measurements into an image, according to Robert Johnson, a particle physicist at the University of California, Santa Cruz.

In the latest work, Johnson and colleagues at Santa Cruz, Loma Linda University, Northern Illinois University, University of Wollongong and Baylor University set out to build a prototype proton CT scanner that was large enough to image a fake (phantom) human head and quick enough to be usable clinically. The system uses a couple of spare silicon detectors from NASA’s Fermi Gamma-ray Space Telescope, which Johnson had previously worked on. The idea was to place these detectors in the path of a proton beam, with one positioned in front of the head in question and the other behind, to plot the trajectory of the protons through the head.

Specific trajectories

By subtracting the known energy of the particle beam from the energy dumped in a series of plastic scintillators placed behind the second silicon detector, the researchers could work out how much energy a head absorbs along specific trajectories. By then rotating the head very slightly and repeating the process, they could build up a detailed 3D image of the head’s proton stopping power.

The team tested its device in the proton beam at the Northwestern Medicine Chicago Proton Center using phantom heads made from various materials with different stopping powers. Recording the trajectory and energy of more than a million protons a second using very fast electronics, within about six minutes they were able to generate an image with stopping powers generally within 1% of their known values.

Johnson says he does not have precise figures for the accuracy of proton stopping powers obtained using X-ray CT. But he thinks that proton CT is probably better suited to certain kinds of tissue. “If the tissues are all soft and fairly uniform, then X-ray CT will likely be just as accurate as proton CT,” he says. “If there are layers of dense material, then I think we can do better, but that is what we are currently trying to prove.”

Lower doses

The researchers also found they could significantly reduce the dose needed to do the scanning. Whereas a typical X-ray scan of the head requires between 30–50 mGy, they needed just 1.4 mGy using protons.

Johnson and collaborators are now applying for grants to develop a more compact detector with even faster electronics, to reduce the necessary exposure time to about a minute. Six minutes, he reckons, “isn’t bad”, but is not ideal. Having been in an MRI scanner for longer than six minutes, he says the duration “is doable but not the most pleasant thing”.

The American group is one of several around the world that are developing proton CT, with others located in Italy, Japan and the UK. Leader of the British group, Nigel Allinson of the University of Lincoln, says that Johnson and colleagues “certainly set a standard for others to follow”, but notes that technical success will need to be followed by clinical trials as well as assessments of cost and the extent to which the technique “fits with radiotherapy workflows”.

“Still a long way”

Likewise, Mohammad Naimuddin of the University of Delhi cautions that “there is still a long way” before proton CT can be used clinically. Naimuddin, who has collaborated with some members of Johnson’s group, says that the latest work represents a “significant improvement compared to past efforts”, but argues that better modelling of proton scattering off tissue is needed to improve the technique’s resolution. “In the current paper the spatial resolution of the phantom image is still not comparable to X-ray CT,” he says.

The research is reported on the arXiv server.