According to quantum mechanics, our universe is like a Lego set. All matter particles, as well as particles such as light that act as messengers between them, come in discrete blocks of energy. By rearranging these blocks, it is possible to build everything we observe around us.
Well, almost everything. Gravity, a crucial piece of the universe, is missing from the quantum Lego set. But while there is still no quantum theory of gravity, the challenge of detecting its signatures now looks a little more manageable thanks to a proposed experiment that takes inspiration from the photoelectric effect, which Albert Einstein used to prove the quantum nature of light more than a century ago.
History revisited
Quantum mechanics and general relativity each, independently, provide accurate descriptions of our universe – but only at short and long distances, respectively. Bridging the two is one of the deepest problems facing physics, with tentative theories approaching it from different perspectives.
However, all efforts of describing a quantum theory of gravity agree on one thing: if gravity is quantum, then it, too, must have a particle that carries its force in discrete packages, just as other forces do.
In the latest study, which is described in Nature Communications, Germain Tobar and Sreenath K Manikandan of Sweden’s Stockholm University, working with Thomas Beitel and Igor Pikovski of the Stevens Institute of Technology, US, propose a new experiment that could show that gravity does indeed come in these discrete packages, which are known as gravitons.
The principle behind their experiment parallels that of the photoelectric effect, in which light shining on a material causes it to emit discrete packets of energy, one particle at a time, rather than in a continuous spectrum. Similarly, the Stockholm-Stevens team proposes using massive resonant bars that have been cooled and tuned to vibrate if they absorb a graviton from an incoming gravitational wave. When this happens, the column’s quantum state would undergo a transition that can be detected by a quantum sensor.
“We’re playing the same game as photoelectric effect, except instead of photons – quanta of light – energy is exchanged between a graviton and the resonant bar in discrete steps,” Pikovski explains.
“Still hard, but not as hard as we thought”
While the idea of using resonant bars to detect gravitational waves dates back to the 1960s, the possibility of using it to detect quantum transitions is new. “We realized if you change perspectives and instead of measuring change in position, you measure change in energy in the quantum state, you can learn more,” Pikovski says.
A key driver of this perspective shift is the Laser Interferometer Gravitational-wave Observatory, or LIGO, which detects gravitational waves by measuring tiny deviations in the length of the interferometer’s arms as the waves pass through them. Thanks to LIGO, Pikovski says, “We not only know when gravitational waves are detected but also [their] properties such as frequency.”
Data collection: The Laser Interferometer Gravitational-wave Observatory (LIGO) detects gravitational waves produced when massive distant objects, such as neutron stars and black holes, collide. (Courtesy: Caltech/MIT/LIGO Lab)
In their study, Pikovski and colleagues used LIGO’s repository of gravitational-wave data to narrow down the frequency and energy range of typical gravitational waves. This allowed them to calculate the type of resonant bar required to detect gravitons. LIGO could also help them cross-correlate any signals they detect.
“When these three ingredients—resonant bar as a macroscopic quantum detector, detecting quantum transitions using quantum sensors and cross-correlating detection with LIGO— are taken altogether, it turns out detecting a graviton is still hard but not as hard as we thought,” Pikovski says.
Within reach, theoretically
For most known gravitational wave events, the Stockholm-Stevens scientists say that the number of gravitons their proposed device could detect is small. However, for neutron star-neutron star collisions, a quantum transition in reasonably-sized resonant bars could be detected for one in every three collisions, they say.
Carlo Rovelli, a theorist at the University of Aix-Marseille, France who was not involved in the study, agrees that “the goal of quantum gravity observations seem within reach”. He adds that the work “shows that the arguments claiming that it should be impossible to find evidence for single-graviton exchange were wrong”.
Frank Wilczek, a theorist at the Massachusetts Institute of Technology (MIT), US who was also not involved in the study, is similarly positive. For a consistent theory that respects quantum mechanics and general relativity, he says, “it can be interpreted that this experiment would prove the existence of gravitons and that the gravitational field is quantized”.
So when are we going to start detecting?
On paper, the experiment shows promise. But actually building a massive graviton detector with measurable quantum transitions will be anything but easy.
Part of the reason for this is that a typical gravitational wave shower can consist of approximately zillions of gravitons. Just as the pattern of individual raindrops can be heard as they fall on a tin roof, carefully prepared resonant bars should, in principle, be able to detect individual incoming gravitons within these gravitational wave showers.
But for this to happen, the bars must be protected from noise and cooled down to their least energetic state. Otherwise, such tiny energy changes may be impossible to observe.
Vivishek Sudhir, an expert in quantum measurements at MIT who was not part of the research team, describes it as “an enormous practical challenge still, one that we do not currently have the technology for”.
Similarly, quantum sensing has been achieved in resonators, but only at much smaller masses than the tens of kilograms or more required to detect gravitons. The team is, however, working on a potential solution: Tobar, a PhD student at Stockholm and the study’s lead author, is devising a version of the experiment that would send the signal from the bars to smaller masses using transducers – in effect, meeting the quantum sensing challenge in the middle. “It’s not something you can do today, but I would guess we can achieve it within a decade or two,” Pikovski says.
Sudhir agrees that quantum measurements and experiments are rapidly progressing. “Keep in mind that only 15 years ago, nobody imagined that tangibly macroscopic systems would even be prepared in quantum states,” he says. “Now, we can do that.”
Earlier this year, the start of the Paris Olympics was marked by the ceremonial relay of the Olympic torch. You’ll have to wait until 2028 for the next Olympics, but in the meantime there’s the International Year of Quantum (IYQ) in 2025, which also features a torch relay. In keeping with the quantum theme, however, this light source is very, very small.
The light source is currently on tour around 12 different quantum labs around Europe as part of IYQ and last week I visited the Cavendish Laboratory at the University of Cambridge, UK, where it was on stop eight of what’s dubbed QuanTour. It’s a project of the German Physical Society (DPG), organised by Doris Reiter from the Technical University of Dortmund and Tobias Heindel from the Technical University of Berlin.
According to Mete Atatüre, who leads the Quantum Optical Materials and Systems (QOMS) group at Cambridge and in whose lab QuanTour is based, one of the project’s aims is to demystify quantum science. “I think what we need to do, especially in the year of quantum, is to have a change of style.” he says. “So that we focus not on the weirdness of quantum but on what it can actually bring us.”
Indeed, though it requires complex optical apparatus and must be cooled with helium, the Quantour light source itself looks like an ordinary computer chip. It is in fact an array of quantum dots, each emitting single photons when illuminated by a laser. “It’s really meant to show off that you can use quantum dots as a plug in light source” explains Christian Schimpf, a postdoc in the Quantum Engineering Group in Cambridge, who showed me around the lab where QuanTour is spending its time in England.
The light source is right at home in the Cambridge lab, where quantum dots are a key area of research. The team is working on networking applications, where the goal is to transmit quantum information over long distances, preferably using existing fibre-optic networks. In fibre optics, the signal is amplified regularly along the route, but quantum networks can’t do this – the so-called “no-cloning” theorem means it’s impossible to create a copy of an unknown quantum state.
The solution is to create a long-distance communication link from many short-distance entanglements. The challenge for scientists in the Cambridge lab, Schimpf explains, is to build ensembles of entangled qubits that can “store quantum bits on reasonable time scales.” He’s talking about just a few milliseconds, but this is still a significant challenge, requiring cooling close to absolute zero and precise control over the fabrication process.
Elsewhere in the Cavendish Laboratory, scientists in the quantum group are investigating platforms for quantum sensing, where changes to single quantum states are used to measure tiny magnetic fields. Attractive materials for this include diamond and some 2D materials, where quantum spin states trapped at crystal defects can act as qubits. Earlier this year Physics World spoke to Hannah Stern, a former postdoc in Atatüre’s group, who won an award from the Institute of Physics for her research on quantum sensing with hexagonal boron nitride, which she began in Cambridge.
I also spoke to Dorian Gangloff, head of the quantum engineering group, who described his recent work on nonlinear quantum optics. Nonlinear optical effects are generally only observed with high-power light sources such as lasers, but Gangloff’s team is trying to engineer these effects in single photons. Nonlinear quantum optics could be used to shift the frequency of a single photon or even split it into an entangled pair.
When asked about the existing challenges of rolling out quantum technologies, Atatüre points out that when quantum mechanics was first conceived, the belief was: “Of course we’ll never be able to see this effect, but if we did, what would the experimental result look like?” Thanks to decades of work however, it is indeed possible to see quantum science in action, as I did In Cambridge. Atatüre is confident that researchers will be able to take the next step – building useful technologies with quantum phenomena.
At the end of this week, QuanTour’s time in Cambridge will be up. If you missed it, you’ll have to head to University College Cork in Ireland, where it will be spending the next leg of its journey with the group of Emanuele Pelucchi.
LIV.INNO, Liverpool Centre for Doctoral Training for Innovation in Data-Intensive Science, offers students fully-funded PhD studentships across a broad range of research projects from medical physics to quantum computing. All students receive training in high-performance computing, data analysis, and machine learning and artificial intelligence. Students also receive career advice and training in project management, entrepreneurship and communication skills – preparing them for careers outside of academia.
This podcast features the accelerator physicist Carsten Welsch, who is head of the Accelerator Science Cluster at the University of Liverpool and director of LIV.INNO, and the computational astrophysicist Andreea Font who is a deputy director of LIV.INNO.
They chat with Physics World’s Katherine Skipper about how LIV.INNO provides its students with a wide range of skills and experiences – including a six-month industrial placement.
This podcast is sponsored by LIV.INNO, the Liverpool Centre for Doctoral Training for Innovation in Data-Intensive Science.
Magnetic resonance methods, including nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR), are non-invasive, atom-specific, quantitative, and capable of probing liquid and solid-state samples. These features make magnetic resonance ideal tools for operando measurement of an electrochemical device, and for establishing structure-function relationships under realistic condition.
The first part of the talk presents how coupled inline NMR and EPR methods were developed and applied to unravel rich electrochemistry in organic molecule-based redox flow batteries. Case studies performed on low-cost and compact bench-top systems are reviewed, demonstrating that a bench-top NMR has sufficient spectral and temporal resolution for studying degradation reaction mechanisms, monitoring the state of charge, and crossover phenomena in a working RFB. The second part of the talk presents new in situ NMR methods for studying Li-mediated ammonia synthesis, and the direct observation of lithium plating and its concurrent corrosion, nitrogen splitting on lithium metal, and protonolysis of lithium nitride. Based on these insights, potential strategies to optimize the efficiencies and rates of Li-mediated ammonia synthesis are discussed. The goal is to demonstrate that operando NMR and EPR methods are powerful and general and can be applied for understanding the electrochemistry underpinning various applications.
An interactive Q&A session follows the presentation.
Evan Wenbo Zhao is a tenured assistant professor at the Magnetic Resonance Research Center at Radboud Universiteit Nijmegen in the Netherlands. His core research focuses on developing operando/in situ NMR methods for studying electrochemical storage and conversion chemistries, including redox flow batteries, electrochemical ammonia synthesis, carbon-dioxide reduction, and lignin oxidation. He has led projects funded by the Dutch Research Council Open Competition Program, Bruker Collaboration, Radboud-Glasgow Collaboration Grants, the Mitacs Globalink Research Award, and others. After receiving his BS from Nanyang Technological University, he completed a PhD in chemistry with Prof. Clifford Russell Bowers at the University of Florida. Evan’s postdoc was with Prof. Dame Clare Grey at the Yusuf Hamied Department of Chemistry at the University of Cambridge.
A consortium of universities and companies has been awarded the contract to manage and operate Fermilab, the US’s premier particle-physics facility. The US Department of Energy (DOE) announced on 1 October that the new contractor, Fermi Forward Discovery Group, LLC (FFDV), will take over operation of the lab from 1 January 2025.
FFDV consists of Fermilab’s current contractor – the University of Chicago and Universities Research Association (URA), a consortium of research universities – as well as the industrial firms Amentum Environment & Energy, Inc. and Longenecker & Associates. The conglomerate’s initial contract will last for five years but “exemplary performance” running the lab could extend that by a further decade.
“We are honoured that the Department of Energy has selected FermiForward to manage Fermilab after a rigorous contract process,” University of Chicago president Paul Alivisatos told Physics World. “FermiForward represents a new approach that brings together the best parts of Fermilab with two new industry partners, who bring broad expertise from a deep bench from across the DOE complex.”
Alivisatos notes that the inclusion of Amentum and Longenecker will strengthen the management capability of the consortium given the companies’ “exemplary record of accomplishment in project management, operations, and safety.” Longenecker, a female-led company based in Las Vegas, is part of the managerial teams currently running Sandia, Los Alamos, and Savannah River national laboratories. Virginia-based Amentum, meanwhile, has a connection to Fermilab through Greg Stephens, its former vice president, who is now Fermilab’s chief operating officer.
The choice of the new contractor comes after Fermilab has faced a series of operating and budget challenges. In 2021, the institution scored low marks on a DOE assessment of its operations. A year later, complaints emerged that the lab’s leadership was restricting access to its campus despite reduced concern about the spread of COVID-19. In July, a group of Fermilab staff whistleblowers claimed that a series of problems indicated that the lab was “doomed” without a change of management. And in late August, the lab underwent a period of limited operations to reduce a budgetary shortfall.
The Fermilab staff whistleblowers, however, see little change in the DOE’s selection of FFDV. Indeed, the key members of FFDV – the University of Chicago and URA – made up Fermi Research Alliance, the previous contractor that has overseen Fermilab’s operations since 2007.
“We understand that the only reaction by DOE to our investigative report is that of coaching the University of Chicago’s teams that steward the university’s relationships with the national labs,” the group wrote in a letter to Geraldine Richmond, DOE’s Undersecretary for Science and Innovation, which has been seen by Physics World. “By doing so, the DOE is once again showing that it is for the status-quo.”
The DOE hasn’t revealed how many bids it received or other details about the contract award. In a statement to Physics World it noted that it “cannot discuss the contract at the current time because of business sensitive information”. Fermilab declined to comment for the story.
Research done at a mountaintop cosmic-ray observatory in Armenia has shed new light on how thunderstorms can create flashes of gamma rays by accelerating electrons. Further study of the phenomenon could answer important questions about the origins of lightning.
This accelerating process is called thunderstorm ground enhancement (TGE), whereby thunderstorms create strong electric fields that accelerate atmospheric free electrons to high energies. These electrons then collide with air molecules, creating a cascade of secondary charged particles. When charged particles are deflected in these collisions they emit gamma rays in a process called bremsstrahlung.
The flashes of gamma rays are called “gamma-ray glows” and are some of the strongest natural sources of high-energy radiation on Earth.
Physicist Joseph Dwyer at the University of New Hampshire, who was not involved in the Armenian study says, “When you think of gamma rays, you usually think of black holes or solar flares. You don’t think of inside the Earth’s troposphere as being a source of gamma rays, and we’re still trying to understand this.”
Century-old mystery
Indeed, the effect was first predicted a century ago by Nobel laureate Charles Wilson, who is best known for his invention of the cloud chamber radiation detector. However, despite numerous attempts over the decades, early researchers were unable to detect this acceleration.
This latest research was led by Ashot Chiliangrian, who is director of the Cosmic Ray Division of Armenia’s Yerevan Physics Institute. The measurements were made at a research station located 3200 m above sea level on Armenia’s Mount Aragats.
Chiliangrian says, “There were some people that were convinced that there was no such effect. But now, on Aragats, we can measure electrons and gamma rays directly from thunderclouds.”
In the summer of 2023, Chiliangrian and colleagues detected gamma rays, electrons, neutrons and other particles from intense TGE events. By analysing 56 of those events, the team has now concluded that the electric fields involved were close to Earth’s surface.
Though Aragats is not the first facility to confirm the existence of these gamma-ray glows, it is uniquely well-situated, sitting at a high altitude in an active storm region. This allows measurements to be made very close to thunderclouds.
Energy spectra
Instead of measuring the electric field directly, the team inferred its strength by analysing the energy spectra of electrons and gamma rays detected during TGE events.
By comparing the detected radiation to well-understood simulations of electron acceleration, the team deduced the strength of the electric field responsible for the particle showers as 2.1 kV/cm.
This field strength is substantially higher than what has been observed in most previous studies of thunderstorms, which typically use weather balloons to take direct field measurements.
The fact that such a high field can exist near the ground during a thunderstorm challenges previous assumptions about the limits of electric fields in the atmosphere.
Moreover, this discovery could help solve one of the biggest mysteries in atmospheric science: how lightning is initiated. Despite decades of research, scientists have been unable to measure electric fields strong enough to break down the air and create the initial spark of lightning.
“These are nice measurements and they’re one piece of the puzzle,” says Dwyer, “What these are telling us is that these gamma ray glows are so powerful and they’re producing so much ionizing radiation that they’re partially discharging the thunderstorm.”
“As the thunderstorms try to charge up, these gamma rays turn on and cause the field to kind of collapse,” Dwyer explains, comparing it to stepping on bump in a carpet. “You collapse it in one place but it pops up in another, so this enhancement may be enough to help the lightning get started.”
Spider silk is among the toughest of all biological materials, and scientists have long been puzzled by how spiders manage to cut it. Do they break it down by chemical means, using enzymes? Or do they do it mechanically, using their fangs? Researchers at the University of Trento in Italy have now come down firmly on the side of fangs, resolving a longstanding debate and perhaps also advancing the development of spider-fang-inspired cutting tools.
For spiders – especially those that spin webs – the ability to cut silk lines quickly and efficiently is a crucial skill. Previously, the main theory of how they do it involved enzymes that they produce in their mouths, and that can break silk down. This mechanism, however, cannot explain how spiders cut silk so quickly. Mechanical cutting is faster, but spiders’ fangs are not shaped like scissors or other common cutting tools, so this was considered less likely.
In the new work, researchers led by Nicola Pugno and Gabriele Greco studied two species of spiders (Nuctenea umbratica and Steatoda triangulosa) collected from around the campus in Trento. In one set of experiments, they allowed the spiders to interact with artificial webs made from Kevlar, a synthetic carbon-fibre material. To weave their own webs, the spiders needed to remove the Kevlar threads and replace them with silk ones. They did this by first cutting the key structural threads in the artificial webs, then spinning a silken framework in between to build up the web structure. Any discarded fibres became support for the web.
Pugno, Greco and colleagues also allowed the spiders to build webs naturally (that is, without any artificial materials present). They then removed some of the silken threads and substituted them with carbon fibre ones so they could study how the spiders cut them.
Revealing images
One of the researchers’ first observations was that the spiders found it harder to cut fibres made from Kevlar than those made from silk. While cutting silk took them just a fraction of a second, they needed more than 10 s to cut Kevlar. This implies that much more effort was required.
A further clue came from scanning electron microscope (SEM) images of the spider-cut silk and carbon fibres. These images showed that the fracture surfaces of both were similar to those of samples that were broken with scissors or during tensile tests.
Meanwhile, images of the spider fangs themselves revealed micro-structured serrations similar to those found in animals such as crocodiles and sharks. The advantage of serrated edges is that they minimize the force required to cut a material at the point of contact – something humans have long exploited by making serrated blades that quickly cut through tough materials like wood and steel (not to mention foods like bread and steak).
In spider fangs, however, the serrations are not evenly spaced. Instead, Pugno and Greco found that the gap between them is narrowest at the tip of a fang and widest nearest the base. This, they say, suggests that when spiders want to cut a fibre, their fangs slide inwards across it until it becomes trapped in a serration of the same size. At the contact point between fibre and serration, the required cutting force is at a minimum, thereby maximizing the efficiency of cutting.
“We conducted specific experiments to prove that the fang of a spider is a ‘smart’ tool with graded serrations for cutting fibres of different dimensions naturally placed in the best place for maximizing cutting efficiency,” Pugno explains. “This makes it more efficient than a razor blade to cut these fibres,” Greco adds.
The researchers, who report their work in Advanced Science, also conducted analytical and finite-element numerical analyses to back up their observations. These revealed that when a fibre presses onto a fang, the stress on the fibre becomes concentrated thanks to the two bulges at the top of the serration. This concentration initiates the propagation of cracks through the fibre, leading to its failure, they say.
The researchers note that serration had previously been observed in 48 families of modern spiders (araneomorphs) as well as at least three families of older species (mygalomorphs). They speculate that it may have been important for functions other than cutting silk, such as chewing and mashing prey, with the araneomorphae possibly later evolving it to cut silk. But their findings are also relevant in fields other than evolutionary biology, they say.
“By explaining how spiders cut, we reveal a basic engineering principle that could inspire the design of highly efficient, sharper and more performing cutting tools that could be of interest for high-tech applications,” Pugno tells Physics World. “For example, for cutting wood, metal, stone, food or hair.”
Every day the International Space Station (ISS) orbits the Earth 16 times. Every day its occupants could (if they aren’t otherwise occupied) observe each one of our planet’s terrains and seasons. For almost a quarter of a century the ISS has been continuously inhabited by humans, a few at a time, hailing from – at the latest count – 21 countries. This impressive feat of science, engineering and international co-operation may no longer be noteworthy or news fodder, yet it still has the power to astonish and inspire.
This makes it an excellent setting for a novel that’s quietly philosophical, tackling some of the biggest questions humanity has ever asked. Orbital by British author Samantha Harvey follows four astronauts and two cosmonauts through one day on the ISS. It is an ordinary, unremarkable day and yet their location makes every moment remarkable.
We meet our characters – four men and two women, from five countries – as they are waking up during orbit 1 and leave them fast asleep in orbit 16. Harvey has clearly read astronaut accounts and studied information available from NASA and the European Space Agency. She includes as much detail about life on the ISS as a typical popular-science book on the subject.
These minutiae of astronaut tasks are interspersed with descriptions of Earth during each of the 16 orbits, as well as long passages deliberating everything from whether there is a God and climate catastrophe to global politics and the futility of trying to understand another human being.
The characters going about their tightly scheduled day in Orbital are individual people, each with their own preoccupations, past and present. While they exercise and perform maintenance tasks, science experiments and self-assessments, their thoughts roam to give us an insight that feels as true as any astronaut memoir. One character muses on the difficulty of sending messages to her loved ones, feeling that everything she has to say is either hopelessly mundane or so grandiose as to be ridiculous. I don’t know if an astronaut on the ISS has ever thought that, but for me, it perfectly encapsulates their situation.
The ISS’s orbit 400 km above Earth is close enough to see the topography and colours that pass beneath, but far enough that signs of humanity can only be inferred – at least in daylight. This doesn’t stop the characters from learning to see the traces of humans: algal blooms in oceans warmer than they once were; retreated glaciers; mountains bare of snow that were once renowned for their white caps; absent rainforest; reclaimed land covered by acres of greenhouses.
It’s a curious choice to set a book on the ISS that isn’t science fiction. It’s fiction, yes, and certainly based in the world of science, but the science it depicts isn’t futuristic or even particularly cutting-edge. The ISS is now quite old technology, nearing the end of its remarkable life, as Harvey points out in an insightful essay for LitHub. Its occupants still do experiments to further our scientific knowledge, but even there what Harvey describes is sci-fact, not sci-fi.
In her LitHub essay, Harvey says it was precisely this “slow death” of the ISS that appealed to her. The ISS is almost a time capsule, hearkening back to the end of the Cold War. It now looks likely that Russia will pull out – or be ejected – from the mission before its projected end date of 2030.
Viewed from the ISS, no borders are visible, and the crew joke comfortably about their national differences. However, their lives are nevertheless dictated by strict and sometimes petty rules governing, for example, which toilet and which exercise equipment to use. These regulations are just one more banal reality of life on the ISS, like muscle atrophy, blocked sinuses or packing up waste to go in the next resupply craft.
Just consider the real-life NASA astronauts Suni Williams and Butch Wilmore, whose stay on the ISS has been extended following problems with the Boeing craft that was supposed to bring them home in August. Having two extra people living on the space station for several months longer than planned is an intensely practical matter, made easier by such innovations as the recycling of their urine and sweat into drinking water, or that astronauts must swallow toothpaste rather than spit it out.
Harvey manages to convey that these details are quotidian. But she also imbues them with beauty. During one conversation in Orbital, a character sheds four tears. He and a crew mate then chase down each floating water droplet because loose liquids must be avoided. It’s a small moment that says so much with few words.
Orbital has been shortlisted for both the 2024 Booker Prize (winner to be announced on 12 November) and the 2024 Ursula K Le Guin Prize for Fiction (the winner of which will be announced on 21 October). The recognition reflects the book’s combination of literary prose and unusual globe-spanning (indeed, beyond global) perspective. Harvey’s writing has been compared to Virginia Woolf – a comparison that is well warranted. And yet Orbital is as accessible and educational as the best of popular science. It’s a feat almost as astonishing as the existence of the ISS.
The semiconductor physicist Richard Friend from the University of Cambridge has won the 2024 Isaac Newton Medal and Prize “for pioneering and enduring work on the fundamental electronic properties of molecular semiconductors and in their engineering development”. Presented by the Institute of Physics (IOP), which publishes Physics World, the international award is given annually for “world-leading contributions to physics”.
Friend was born in 1953 in London, UK. He completed a PhD at the University of Cambridge in 1979 under the supervision of Abe Yoffe and remained at Cambridge becoming a full professor in 1995. Friend’s research has led to a deeper understanding of the electronic properties of molecular semiconductors having in the 1980s pioneered the fabrication of thin-film molecular semiconductor devices that were later developed to support field-effect transistor circuits.
When it was discovered that semiconducting polymers could be used for light-emitting diodes (LEDs), Friend founded Cambridge Display Technology in 1992 to develop polymer LED displays. In 2000 he also co-founded Plastic Logic to advance polymer transistor circuits for e-paper displays.
As well as the 2024 Newton Medal and Prize, Friend’s other honours include the IOP’s Katherine Burr Blodgett Medal and Prize in 2009 and in 2010 he shared the Millennium Technology Prize for the development of plastic electronics. He was also knighted for services to physics in the 2003 Queen’s Birthday Honours list.
“I am immensely proud of this award and the recognition of our work,” notes Friend. “Our Cambridge group helped set the framework for the field of molecular semiconductors, showing new ways to improve how these materials can separate charges and emit light.”
Friend notes that he is “not done just yet” and is currently working on molecular semiconductors to improve the efficiency of LEDs.
Innovating and inspiring
Friend’s honour formed part of the IOP’s wider 2024 awards, which recognize everyone from early-career scientists and teachers to technicians and subject specialists.
Other winners include Laura Herz from the University of Oxford, who receives the Faraday Prize “for pioneering advances in the photophysics of next-generation semiconductors, accomplished through innovative spectroscopic experiments”. Rebecca Dewey from the University of Nottingham, meanwhile, receives the Phillips Award “for contributions to equality, diversity and inclusion in Institute of Physics activities, including promoting, updating and improving the accessibility of the I am a Physicist Girlguiding Badge, and engaging with British Sign Language users”.
In a statement, IOP president Keith Burnett congratulated all the winners, adding that they represent “some of the most innovative and inspiring” work that is happening in physics.
“Today’s world faces many challenges which physics will play an absolutely fundamental part in addressing, whether its securing the future of our economy or the transition to sustainable energy production and net zero,” adds Burnett. “Our award winners are in the vanguard of that work and each one has made a significant and positive impact in their profession. Whether as a researcher, teacher, industrialist, technician or apprentice, I hope they are incredibly proud of their achievements.”
For the first time, physicists in the US have done lab-based experiments that show how an asteroid could be deflected by powerful bursts of X-rays. With the help of the world’s largest high frequency electromagnetic wave generator, Nathan Moore and colleagues at Sandia National Laboratories showed how an asteroid-mimicking target could be freely suspended in space while being accelerated by ultra-short X-ray bursts.
While most asteroid impacts occur far from populated areas, they still hold the potential to cause devastation. In 2013, for example, over 1600 people were injured when a meteor exploded above the Russian city of Chelyabinsk. To better defend ourselves against these threats, planetary scientists have investigated how the paths of asteroids could be deflected before they reach Earth.
In 2022, NASA successfully demonstrated a small deflection with the DART mission, which sent a spacecraft to collide with the rocky asteroid Dimorphos at a speed of 24,000 km/h. After the impact, the period of Dimorphos’ orbit around the larger asteroid, Didymos, shortened by some 33 min.
However, this approach would not be sufficient to deflect larger objects such as the famous Chicxulub asteroid. This was roughly 10 km in diameter and triggered a mass extinction event when it impacted Earth about 66 million years ago.
Powerful X-ray burst
Fortunately, as Moore explains, there is an alternative approach to a DART-like impact. “It’s been known for decades that the only way to prevent the largest asteroids from hitting the earth is to use a powerful X-ray burst from a nuclear device,” he says. “But there has never been a safe way to test that idea. Nor would testing in space be practical.”
So far, X-ray deflection techniques have only been explored in computer simulations. But now, Moore’s team has tested a much smaller scale version of a deflection in the lab.
To generate energetic bursts of X-rays, the team used a powerful facility at Sandia National Laboratories called the Z Pulsed Power Facility – or Z Machine. Currently the largest pulsed power facility in the world, the Z Machine is essentially a giant battery that releases vast amounts of stored electrical energy in powerful, ultra-short pulses, funnelled down to a centimetre-sized target.
Few millionths of a second
In this case, the researchers used the Z Machine to compress a cylinder of argon gas into a hot, dense plasma. Afterwards, the plasma radiated X-rays in nanosecond pulses, which were fired at mock asteroid targets made from discs of fused silica. Using an optical setup behind the target, the team could measure the deflection of the targets.
“These ‘practice missions’ are miniaturized – our mock asteroids are only roughly a centimetre in size – and the flight is short-lived – just a few millionths of a second,” Moore explains. “But that’s just enough to let us test the deflection models accurately.”
Because the experiment was done here on Earth, rather than in space, the team also had to ensure that the targets were in freefall when struck by the X-rays. This was done by detaching the mock asteroid from a holder about a nanosecond before it was struck.
X-ray scissors
They achieved this by suspending the sample from a support made from thin metal foil, itself attached to a cylindrical fixture. To detach the sample, they used a technique Moore calls “X-ray scissors”, which almost instantly cut the sample away from the cylindrical fixture.
When illuminated by the X-ray burst, the supporting foil rapidly heated up and vaporized, well before the motion of the deflecting target could be affected by the fixture. For a brief moment, this left the target in freefall.
In the team’s initial experiments, the X-ray scissors worked just as they intended. Simultaneously, the X-ray pulse vaporized the target surface and deflected what remained at velocities close to 70 m/s.
The team hopes that its success will be a first step towards measuring how real asteroid materials are vaporized and deflected by more powerful X-ray bursts. This could lead to the development of a vital new line of defence against devastating asteroid impacts.
“Developing a scientific understanding of how different asteroid materials will respond is critically important for designing an intercept mission and being confident that mission would work,” Moore says. “You don’t want to take chances on the next big impact.”
In this webinar, we will discuss that patient specific quality assurance (PSQA) is an essential component of the radiation treatment process. This control allows us to ensure that the planned dose will be delivered to the patient. The increasing number of patients with indications for modulated treatments requiring PSQA has significantly increased the workload of the medical physics departments, and the need to find more efficient ways to perform it has arisen.
In recent years, there has been an increasing evolution of measurement systems. However, the experimental process involved imposes a limit on the time savings. The 3D dose calculation systems are presented as a solution to this problem, allowing the reduction of the time needed for the initiation of treatments.
The use of 3D dose calculation systems, as stated in international recommendations (TG219), requires a process of commissioning and adjustment of dose calculation parameters.
This presentation will show the implementation of PSQA based on independent 3D dose calculation for VMAT treatments in breast cancer using DICOM information from the plan and LOG files. Comparative results with measurement-based PSQA systems will also be presented.
An interactive Q&A session follows the presentation.
Daniel Venencia
Dr Daniel Venencia is the chief of the medical physics department at Instituto Zunino – Fundación Marie Curie in Cordoba, Argentina. He holds a BSc in physics and a PhD from the Universidad Nacional de Córdoba (UNC), Daniel has completed postgraduate studies in radiotherapy and nuclear medicine. With extensive experience in the field, Daniel has directed more than 20 MSc and BSc theses and three doctoral theses. He has delivered more than 400 presentations at national and international congresses. He has published in prestigious journals, including the Journal of Applied Clinical Medical Physics and the International Journal of Radiation Oncology, Biology and Physics. His work continues to make significant contributions to the advancement of medical physics.
Carlos Bohorquez
Carlos Bohorquez, MS, DABR, is the product manager for RadCalc at LifeLine Software Inc., a part of the LAP Group. An experienced board-certified clinical physicist with a proven history of working in the clinic and medical device industry, Carlos’ passion for clinical quality assurance is demonstrated in the research and development of RadCalc into the future.
A new sensor can detect mechanical strains that are more than an order of magnitude weaker than was possible with previously reported devices. Developed at Nanjing University, China, the sensor works by detecting changes that take place in single-crystal vanadium oxide materials as they undergo a transition from a conducting to an insulating phase. The new device could have applications in electronics engineering as well as materials science.
To detect tiny deformations in materials, you ideally want a sensor that undergoes a seamless and easily measurable transition whenever a strain – even a very weak one – is applied to it. Phase transitions, such as the shift from a metal to an insulator, fit the bill because they produce a significant change in the material’s resistance, making it possible to generate large electrical signals. These signals can then be measured and used to quantify the strain that triggered them.
Traditional strain sensors, however, are based on metal and semiconductor compounds, which have resistances that don’t change much under strain. This makes it hard to detect weak strains caused by, for example, the movement of microscopic water droplets around a surface.
A research team co-led by Feng Miao and Shi-Jun Liang has now got around this problem by developing a sensor based on the bronze phase of vanadium oxide, VO2(B). The team initially chose to study this material purely to understand the mechanisms behind its temperature-induced phase transitions. Along the way, though, they noticed something unusual. “As our research progressed, we discovered that this material exhibits a unique response to strain,” Liang recalls. “This prompted us to shift the project’s focus.”
A fabrication challenge
Because the structure of vanadium oxide is not simple, fabricating a sensor from this quantum material was among the team’s biggest challenges. To make their device, the Nanjing researchers used a specially-adapted hydrogen-assisted chemical vapour deposition micro-nano fabrication process. This enabled them to produce high-quality, smooth single crystals of the material, which they characterized using a combination of electrical and spectroscopic techniques, including high-resolution transmission electron microscopy (HRTEM). They then needed to transfer this crystal from the SiO2/Si wafer on which it was grown to a flexible substrate (a smooth and insulating polyimide), which posed further experimental challenges, Liang says.
Once they had accomplished this, the researchers loaded the polyimide substrate/VO2(B) into a customized strain setup. They bonded the device to a homemade socket and induced uniaxial tensile strain in the material by vertically pushing a nanopositioner-controlled needle through it. This bends the flexible substrate and curves the upper surface of the sample.
They then measured how the current-voltage characteristics of the mechanical sensor changed as they applied strain to it. Under no strain, the channel current of the device registers 165 μA at a bias of 0.5 V, indicating that it is conducting. When the strain increases to 0.95%, however, the current drops to just 0.50 μA, suggesting a shift into an insulating state.
A strikingly large variation
The researchers also measured the response of the device to intermediate strains. As they increased the applied strain, they found that at first, the device’s resistance increased only slightly. When the uniaxial tensile strain hit a value of 0.33%, though, the resistance jumped, and afterwards it increased exponentially with applied strain. By the time they reached 0.78% strain, the resistance was more than 2600 times greater than it was in the strain-free state.
This strikingly large variation is due to a strain-induced metal-insulator transition in the single-crystal VO2(B) flake, Miao explains. “As the strain increases, the entire material transitions to an insulator, resulting in a significant increase in its resistance that we can measure,” he says. This resistance change is durable, he adds, and can be measured with the same precision even after 700 cycles, proving that the technique is reliable.
Detecting airflows and vibrations
To test their device, the Nanjing University team used it to sense the slight mechanical deformation caused by placing a micron-sized piece of plastic on it. As well as detecting the slight mechanical pressure of small objects like this, they found that the device can also monitor gentle airflows and sense tiny vibrations such as those produced when tiny water droplets (about 9 μL in volume) move on flexible substrates.
“Our work shows that quantum materials like vanadium oxide show much potential for strain detection applications,” Miao tells Physics World. “This may motivate researchers in materials science and electronic engineering to study such compounds in this context.”
This work, which is detailed in Chinese Physics Letters, was a proof-of-concept validation, Liang adds. Future studies will involve growing large-area samples and exploring how to integrate them into flexible devices. “These will allow us to make ultra-sensitive quantum material sensing chips,” he says.
Surgical sutures are strong, flexible fibres used to close wounds caused by trauma or surgery. But could these stitches do more than just hold wounds closed? Could they, for example, be designed to accelerate the healing process?
A research team headed up at Donghua University in Shanghai has now developed sutures that can generate electricity at the wound site. They demonstrated that the electrical stimulation produced by these sutures can speed the healing of muscle wounds in rats and reduce the risk of infection.
“Our research group has been working on fibre electronics for almost 10 years, and has developed a series of new fibre materials with electrical powering, sensing and interaction functions,” says co-project leader Chengyi Hou. “But this is our first attempt to apply fibre electronics in the biomedical field, as we believe the electricity produced by these fibres might have an effect on living organisms and influence their bioelectricity.”
The idea is that the suture will generate electricity via a triboelectric mechanism, in which movement caused by muscles contracting and relaxing generates an electric field at the wound site. The resulting electrical stimulation should accelerate wound repair by encouraging cell proliferation and migration to the affected area. It’s also essential that the suture material is biocompatible and biodegradable, eliminating the need for surgical stitch removal.
To meet these requirements, Hou and colleagues created a bioabsorbable electrical stimulation suture (BioES-suture). The BioES-suture is made from a resorbable magnesium (Mg) filament electrode, wrapped with a layer of bioabsorbable PLGA (poly(lactic-co-glycolic acid)) nanofibres, and coated with a sheath made of the biodegradable thermoplastic polycaprolactone (PCL).
Fibre design Illustration showing the structure of the BioES-suture and the mechano-electrical conversion mechanism. (Courtesy: Zhouquan Sun and Chengyi Hou)
After the BioES-suture is used to stitch a wound, any subsequent tissue movement results in repeated contact and separation between the PLGA and PCL layers. This generates an electric field at the wound site, the Mg electrode then harvests this electrical energy to provide stimulation and enhance wound healing.
Clinical compatibility
The researchers measured the strength of the BioES-suture, finding that it had comparable sewing strength to commercial sutures. They also tested its biocompatibility by culturing fibroblasts (cells that play a crucial role in wound healing) on Mg filaments, PLGA-coated Mg and BioES-sutures. After a week, the viability of these cells was similar to that of control cells grown in standard petri dishes.
To examine the biodegradability, the researchers immersed the BioES-suture in saline. The core (Mg electrode and nanofibre assembly) completely degraded within 14 days (the muscle recovery period). The PCL layer remained intact for up to 24 weeks, after which, no obvious BioES-suture could be seen.
Next, the researchers investigated the suture’s ability to generate electricity. They wound the BioES-suture onto an artificial muscle fibre and stretched it underwater to simulate muscle deformation. The BioES-suture’s electrical output was 7.32 V in air and 8.71 V in water, enough to light up an LCD screen.
They also monitored the BioES-suture’s power generation capacity in vivo, by stitching it into the leg muscle of rats. During normal exercise, the output voltage was about 2.3 V, showing that the BioES-suture can effectively convert natural body movements into stable electrical impulses.
Healing ability
To assess the BioES-suture’s ability to promote wound healing, the researchers first examined an in vitro wound model. Wounds receiving electrical stimulation from the BioES-suture exhibited faster migration of fibroblasts than a non-stimulated control group, as well as increased cell proliferation and expression of growth factors. The original wound area of approximately 69% was reduced to 10.8% after 24 h exposure to the BioES-sutures, compared with 32.6% for traditional sutures.
The team also assessed the material’s antibacterial capabilities by immersing a standard suture, BioES-suture and electricity-producing BioES-suture in S. aureus and E. coli cultures for 24 h. The electricity-producing BioES-suture significantly inhibited bacterial growth compared with the other two, suggesting that this electrical stimulation could provide an antimicrobial effect during wound healing.
Finally, the researchers evaluated the therapeutic effect in vivo, by using BioES-sutures to treat bleeding muscle incisions in rats. Two other groups of rats were treated with standard surgical sutures and no stitches. Electromyographic (EMG) measurements showed that the BioES-suture significantly increased EMG signal intensity, confirming its ability to generate electricity from mechanical movements.
After 10 days, they examined extracted muscle tissue from the three groups of rats. Compared with the other groups, the BioES-suture improved tissue migration from the wound bed and accelerated wound regeneration, achieving near-complete (96.5%) wound healing. Tissue staining indicated significantly enhanced secretion of key growth factors in the BioES-suture group compared with the other groups.
The researchers suggest that electrical stimulation from the BioES-suture promotes wound healing via a two-fold mechanism: the stimulation enhances the secretion of growth factors at the wound; these growth factors then promote cell migration, proliferation and deposition of extracellular matrix to accelerate wound healing.
In an infected rat wound, stitching with BioES-suture led to better healing and significantly lower bacterial count than wounds stitched with ordinary surgical sutures. The bacterial count remained low even without daily wound disinfection, indicating that the BioES-suture could potentially reduce post-operative infections.
The next step will be to test the potential of the BioES-suture in humans. The team has now started clinical trials, Hou tells Physics World.
More than 90 papers from China have been recognized with a top-cited paper award for 2024 from IOP Publishing, which publishes Physics World. The prize is given to corresponding authors who have papers published in both IOP Publishing and its partners’ journals from 2021 to 2023 that are in the top 1% of the most cited papers.
Among them are quantum physicist Xin Wang from Xi’an Jiaotong University and environmental scientist Huijuan Cui from the Institute of Geographic Sciences and Natural Resources Research.
Cui, who carries out research into climate change, says that China’s carbon neutrality goal has attracted attention all over the world, which may be a reason why the paper, published in Environmental Research Letters, garnered so many citations. “As the Chinese government pays more attention on sustainability issues like climate change…we see growing activities and influence from Chinese researchers,” she says.
A similar impact can be seen in Wang’s work in “chiral quantum networks”, which is published in Quantum Science and Technology, and is equally seen as an area that is quickly gaining traction.
Citations have an important role in Chinese research, and they can also highlight a research topic’s growing impact. “They indicate that what we are studying is a mainstream research field,” Wang says. “Our peers agree with our results and judgement of the field’s future.” Cui, meanwhile, says that citations reflect a “a positive acceptance and recognition of the quality of the research”.
Wang, however, notes that citations and impact doesn’t necessarily happen overnight and that researchers must not base their work’s impact on instantly generating citations.
He adds that some pioneering papers are not well-cited initially with researchers only beginning to realize their value after several years. “If we are confident that our findings are important, we should not be upset with its bad citation but keep on working,” he says. “It is the role of the researcher to stick with their gut to uncover their key research questions. Citations will come afterwards.”
Language barriers
When it comes to Chinese researchers getting their research cited internationally, Wang says that the language barrier is one of the greatest challenges. “The readability of a paper has a close relation with its citation,” adds Wang. “Most highly cited papers not only have an insight into scientific problems, but also are well-written.”
He adds that non-native speakers tend to avoid using “snappy” expressions, which often leads to a conservative and uninspiring tone. “These expressions are grammatically correct but awkward to native speakers,” Wang states.
Despite the potential difficulties with slow citations and language barriers, Cui says that success can be achieved through determination and focussing on important research questions. “Constant effort yields success,” adds Cui. “Keep digging into interesting questions and keep writing high-quality papers.”
That view is backed by Wang. “If your research is well-cited, congratulations,” adds Wang. “However, please do not be upset with a paper with few citations – it still might be pioneering work in its field.”
For the full list of top-cited papers from China for 2024, see here. Xin Wang’s and Huijuan Cui’s award-winning research can be read here and here, respectively
Glioblastoma, the most common primary brain cancer, is treated with surgical resection where possible followed by chemoradiotherapy. Researchers at the University of Miami’s Sylvester Comprehensive Cancer Center have now demonstrated that delivering the radiotherapy on an MRI-linac could provide an early warning of tumour growth, potentially enabling rapid adaptation during the course of treatment.
The Sylvester Comprehensive Cancer Center has been treating glioblastoma patients with MRI-guided radiotherapy since 2017. While standard clinical practice employs MRI scans before and after treatment (roughly three months apart) to monitor a patient’s response, the MRI-linac enables daily imaging. The research team, led by radiation oncologist Eric Mellon, proposed that such daily scans could reveal any changes in the tumour volume or resection cavity far earlier than the standard approach.
To investigate this idea, Mellon and colleagues studied 36 patients with glioblastoma undergoing chemoradiotherapy on a 0.35 T MRI-linac. During 30 radiotherapy fractions, delivered over six weeks, they imaged patients daily on the MRI-linac to assess the volumes of lesions and surgical resection cavities (the site where the tumour was removed).
The researchers then compared the non-contrast MRI-linac images to images recorded pre- (one week before) and post- (one month after) treatment using a standalone 3T MRI with gadolinium contrast. Detailing their findings in the International Journal of Radiation Oncology – Biology – Physics, they report that in general, lesion and cavity volumes seen on non-contrast MRI-linac scans correlated strongly with volumes measured using standalone contrast MRI.
Of the patients in this study, eight had a cavity in the brain, 12 had a lesion and 16 had both cavity and lesion. From pre- to post-radiotherapy, 18 patients exhibited lesion growth, while 11 had cavity shrinkage. In 74% of the cases, changes in lesion volume (growth, shrinkage or no change) assessed on the MR-linac matched those seen on contrast MRI.
“If MRI-linac lesion growth did occur, which was in 60% of our patients [with lesions], there is a 57% chance that it will correspond with tumour growth on standalone post-contrast imaging,” said first author Kaylie Cullison, who shared the study findings at the recent ASTRO Annual Meeting.
In the other 26% of cases, contrast MRI suggested lesion shrinkage while the MRI-linac scans showed lesion growth. Cullison suggested that this may be partly due to radiation-induced oedema, which is difficult to distinguish from tumour on the non-contrast MRI-linac images.
The significant anatomic changes seen during daily imaging of glioblastoma patients suggest that adaptation could play an important role in improving their treatment. In cases where lesions or surgical resection cavities shrink, for example, treatment margins could be reduced to spare normal brain tissue from irradiation. Conversely, for patients with growing lesions, radiotherapy margins could be expanded to ensure complete tumour coverage.
Importantly, there were no cases in this study where patients showed a decrease in their MRI-linac lesion volumes and an increase in their standalone MRI volumes from pre- to post-treatment. In other words, the MR-linac did not miss any cases of true tumour growth. “You can use the MRI-linac non-contrast imaging as an early warning system for potential tumour growth,” said Cullison.
Based on their findings, the researchers propose an adaptive workflow for glioblastoma radiotherapy. For resection cavities, which are clearly visible on non-contrast MRI-linac images, adaptation to shrinkage seen on weekly (standalone or MRI-linac) non-contrast MR images is feasible. Alongside, if an MRI-linac scan shows lesion progression during treatment, gadolinium contrast could be administered (for standalone MRI or MRI-linac scans) to confirm this growth and define adaptive target volumes.
An additional advantage of this workflow is it reduces the use of contrast. Glioblastoma evolution is typically evaluated using contrast-enhanced MRI. However, potential gadolinium deposition with repeated contrast scans is a concern among patients, and the US Food & Drug Administration advises that gadolinium contrast studies should be minimized where possible. This new adaptive approach meets this requirement by only requiring contrast when non-contrast MRI shows an increase in lesion size.
Cullison tells Physics World that the team will next conduct an adaptive radiation therapy trial using the proposed workflow, to determine whether it improves patient outcomes. “We also plan further exploration and analysis of our data, including multiparametric MRI from the MRI-linac, in a larger patient cohort to try to predict patient outcomes (tumour growth; true progression versus pseudo-progression; survival times, etc) earlier than current methods allow,” she explains.
With a rapidly growing interest in magnetic materials for unconventional computing, data storage, and sensor applications, active research is needed not only on material synthesis but also characterization of their properties. In addition to structural and integral magnetic characterizations, imaging of magnetization patterns, current distributions and magnetic fields at nano- and microscale is of major importance to understand the material responses and qualify them for specific applications.
In this webinar, four experts will present on some of the key magnetic imaging technologies for the upcoming decade:
Scanning SQUID microscopy
Nanoscale magnetic resonance imaging
Coherent X-ray magnetic imaging
Scanning electron microscopy with polarization analysis
The webinar will run for two hours, with time for audience Q&A after each speaker.
Left to right: Katja Nowack, Christian Degen, Claire Donnelly, Mathias Kläui
Katja Nowack received her PhD in physics at Delft University of Technology in 2009, focussing on controlling and reading out the spin of single electrons in electrostatically defined quantum dots for spin-based quantum information processing. During her postdoc at Stanford University, she shifted to low-temperature magnetic imaging using scanning superconducting quantum interference devices (SQUIDs). In 2015, she joined the Department of Physics at Cornell University, where her lab develops magnetic imaging techniques to study quantum materials and devices, including topological material, unconventional superconductors and superconducting circuits.
Christian Degen joined ETH Zurich in 2011 after positions at MIT, Leiden University and IBM Research, Almaden. His background includes a PhD in magnetic resonance (Beat Meier) and postdoctoral training in scanning force microscopy (Dan Rugar). Since 2009, he has led a research group on quantum sensing and nanomechanics. He is a co-founder of the microscopy start-up QZabre.
Claire Donnelly. Following her MPhys at the University of Oxford, Claire went to Switzerland to carry out her PhD studies at the Paul Scherrer Institute and ETH Zurich. She was awarded her PhD in 2017 for her work on 3D systems, in which she developed X-ray magnetic tomography, work that was recognized by a number of awards. After a postdoc at the ETH Zurich, she moved to the University of Cambridge and the Cavendish Laboratory as a Leverhulme Early Career Research Fellow, where she focused on the behaviour of three-dimensional magnetic nanostructures. Since September 2021 she is a Lise Meitner Group Leader of Spin3D at the Max Planck Institute for Chemical Physics of Solids in Dresden, Germany. Her group focuses on the physics of three-dimensional magnetic and superconducting systems, and developing synchrotron X-ray-based methods to resolve their structure in 3D.
Mathias Kläui is professor of physics at Johannes Gutenberg-University Mainz and adjunct professor at the Norwegian University of Science and Technology. He received his PhD at the University of Cambridge, after which he joined the IBM Research Labs in Zürich. He was a junior group leader at the University of Konstanz and then became associate professor in a joint appointment between the EPFL and the PSI in Switzerland before moving to Mainz. He has published more than 400 articles and given more than 250 invited talks, is a Fellow of the IEEE, IOP and APS and has been awarded a number of prizes and scholarships.
About this journal
JPhys Materials is a new open access journal highlighting the most significant and exciting advances in materials science.
Editor-in-chief: Stephan Roche is ICREA professor at the Catalan Institute of Nanosciences and Nanotechnology (ICN2) and the Barcelona Institute of Science and Technology.
It came as a bolt from the blue for many Nobel watchers. This year’s Nobel Prize for Physics went to John Hopfield and Geoffrey Hinton for their “foundational discoveries and inventions that enable machine learning and artificial neural networks”.
In this podcast I explore the connections between artificial intelligence (AI) and physics with the author Anil Ananthaswamy – who has written the book Why Machines Learn: The Elegant Maths Behind Modern AI. We delve into the careers of Hinton and Hopfield and explain how they laid much of the groundwork for today’s AI systems.
We also look at why Hinton has spoken out about the dangers of AI and chat about the connection between this year’s physics and chemistry Nobel prizes.
SmarAct proudly supports Physics World‘s Nobel Prize coverage, advancing breakthroughs in science and technology through high-precision positioning, metrology and automation. Discover how SmarAct shapes the future of innovation at smaract.com.
Determining the surface structure of an insulating material is a difficult task, but it is important for understanding its chemical and physical properties. A team of researchers in Austria has now succeeded in doing just this for the technologically important insulator aluminium oxide (Al2O3). The team’s new images – obtained using non-contact atomic force microscopy (AFM) – not only reveal the material’s surface structure but also explain why a simple cut through a crystal is not energetically favourable for the material and leads to a complex rearrangement of the surface.
Al2O3 is an excellent insulator and is routinely employed in many applications, for example as a support material for catalysts, as a chemically resistant ceramic and in electronic components. Characterizing how the surface atoms arrange themselves in this material is important for understanding, among other things, how chemical reactions occur on it.
A technique that works for all materials
Atoms in the bulk of a material arrange themselves in an ordered crystal lattice, but the situation is very different on the surface. The more insulating a material is, the more difficult it is to analyse its surface structure using conventional experimental techniques, which typically require conductivity.
Researchers led by Jan Balajka and Johanna Hütner at TU Wien have now used non-contact AFM to study the basal (0001) plane of Al2O3. This technique works – even for completely insulating materials – by scanning a sharp tip mounted on a quartz tuning fork at a distance of just 0.1 nm above a sample’s surface. The frequency of the fork varies as the tip interacts with the surface atoms and by measuring these changes, an image of the surface structure can be generated.
The problem is that while non-contact AFM can identify where the atoms are located, it cannot distinguish between the different elements making up a compound. Balajka, Hütner and colleagues overcame this problem by modifying the tip and attaching a single oxygen atom to it. The oxygen atoms on the surface of the sample being studied repel this oxygen atom, while its aluminium atoms attract it.
“Mapping the local repulsion or attraction enabled us to visualize the chemical identity of each surface atom directly,” explains Hütner. “The complex three-dimensional structure of the subsurface layers was then determined computationally with novel machine learning algorithms using the experimental images as input,” adds Balajka.
Surface restructuring
According to their analyses, which are detailed in Science, when a cut is made on the Al2O3 surface, it restructures so that the aluminium in the topmost layer is able to penetrate deeper into the material and chemically bond with the oxygen atoms therein. This reconstruction energetically stabilizes the structure, but it remains stoichiometrically the same.
“The atomic structure is a foundational attribute of any material and is reflected in its macroscopic properties,” says Balajka. “The surface structure governs any surface chemistry, such as chemical reactions in catalytic processes.”
Balajka says that the challenges the team had to overcome in this work were threefold: “The first was the strongly insulating character of the material; the second, the lack of chemical sensitivity in (conventional) scanning probe microscopy; and the third, the structural complexity of the alumina surface, which leads to a large configuration of possible structures.”
As an enigmatic insulator, alumina has posed significant challenges for experimental studies and its surface structure has evaded precise determination since 1960s, Balajka tells Physics World. Indeed, it was listed as one of the “three mysteries in surface science” in the late 1990s.
The new findings provide a fundamental piece of knowledge: the detailed surface structure of an important material, and pave the way for advancement in catalysis, materials science and many other fields, he adds. “The experimental and computational approaches we employed in this study can be applied to study other materials that have been too complex or inaccessible to conventional techniques.”
The enigmatic Ξ(2030) particle, once thought to consist of three quarks, may actually be a molecular pentaquark – an exotic hadron comprising five quarks. That is the conclusion of Chinese physicists Cai Cheng and Jing-wen Feng at Sichuan Normal University and Yin Huang at Southwest Jiaotong University. They employed a simplified strong interaction theory to calculate the decay rate of the exotic hadron, concluding that it comprises five quarks.
This composition aligns more closely with experimental data than does the traditional three-quark model for Ξ(2030). While other pentaquarks have been identified in accelerator experiments to date, these particles are still considered exotic and are poorly understood compared to two-quark mesons and three-quark baryons. As a result, this latest work is a significant step towards understanding pentaquarks.
The Ξ(2030) is named for its mass in megaelectronvolts and was first discovered at Fermilab in 1977. At that time, the idea of exotic hadrons that did not fit into the conventional meson–baryon classification was not widely accepted. Conventionally, a meson comprises a quark and an antiquark and a baryon contains three quarks.
Deviation from three-quark model
Consequently, based on its properties, the scientific community classified the particle as a baryon, similar to protons and neutrons. However, further investigations at CERN, SLAC, and Fermilab revealed that the particle’s interaction properties deviated significantly from what the three-quark model predicted, leading scientists to question its three-quark nature.
To address this issue earlier this year, Yin Huang and colleague Hao Hei proposed that the Ξ(2030) could be a molecular pentaquark, suggesting that it consists of a meson and a baryon loosely bound together by the strong nuclear force. In the present study, Cheng, Feng, and Huang elaborated on this idea, analysing a model where the particle is composed of a K meson, which contains a strange antiquark and a light quark (either up or down), alongside a Σ baryon that comprises a strange quark and two light quarks.
To do the study, the team had to use a simplified approach to calculating strong interactions. This is because quantum chromodynamics, the comprehensive theory describing such interactions, is too complex for detailed calculations of hadronic properties. Their approach focuses on hadrons rather than the fundamental quarks and gluons that make up hadrons. They calculated the probabilities of the Ξ(2030) decaying into various strongly interacting particles, including π and K mesons, as well as Σ and Λ baryons.
“It is confirmed that this particle is a hadron molecular state, and its core is primarily composed of K and Σ components,” explains Feng. “The main decay channels are K+Σ and K+Λ, which are consistent with the experimental results. This conclusion not only deepens our understanding of the internal structure of the Ξ(2030), but also further supports the applicability of the concept of hadronic molecular state in particle physics.”
Extremely short lifetime
The Ξ(2030) particle has an extremely short lifetime of about 10-23 s , making it challenging to study experimentally. As a result, measuring its properties can be imprecise. The uncertainty surrounding these measurements means that comparisons with theoretical results are not always conclusive, indicating that further experimental work is essential to validate the team’s claims regarding the interaction between the meson and baryons that make up the Ξ(2030).
“However, experimental verification still needs time, involving multi-party cooperation and detailed planning, and may also require technological innovation or experimental equipment improvement,” said Huang.
Despite the challenges, the researchers are not pausing their theoretical investigations. They plan to delve deeper into the structure of the Ξ(2030) because the particle’s complex nature could provide valuable insights into the subatomic strong interaction, which remains poorly understood due to the intricacies of quantum chromodynamics.
“Current studies have shown that although the theoretically calculated total decay rate of Ξ(2030) is basically consistent with the experimental data, the slight difference reveals the complexity of the particle’s internal structure,” concluded Feng. “This important discovery not only reinforces the hypothesis of Ξ(2030) as a meson–baryon molecular state, but also suggests that the particle may contain additional components, such as a possible triquark configuration.”
Moreover, the very conclusion regarding the molecular pentaquark structure of Ξ(2030) warrants further scrutiny. The effective theory employed by the authors draws on data from other experiments with strongly interacting particles and includes a fitting parameter not derived from the foundational principles of quantum chromodynamics. This raises the possibility of alternative structures for Ξ(2030).
“Maybe Ξ(2030) is a molecular state, but that means explaining why K and Σ should stick together – [Cheng and colleagues] do provide an explanation but their mechanism is not validated against other observations so it is impossible to evaluate its plausibility,” said Eric Swanson at University of Pittsburgh, who was not involved in the study.
Baker bagged half the prize “for computational protein design” and Hassibis and Jumper share the other half for “for protein structure prediction”.
Baker is a biochemist based at the University of Washington in Seattle. Hassibis did a PhD in cognitive neuroscience at University College London and is CEO and co-founder of UK-based Google DeepMind. Also based at Google DeepMind, Jumper studied physics at Vanderbilt University and the University of Cambridge before doing a PhD in chemistry at the University of Chicago.
Entirely new protein
In 2003 Baker was the first to create an entirely new protein from its constituent amino acids – and his research group has since created many more new proteins. Some of these molecules have found use in sensors, nanomaterials, vaccines and pharmaceuticals.
In 2020 Jumper and Hassibis created AlphaFold2, which is an artificial-intelligence model that can predict the structure of a protein based on its amino-acid sequence. A protein begins as a linear chain of amino acids that folds itself to create a complicated 3D structure.
These structures can be determined experimentally using techniques including X-ray crystallography, electron microscopy and nuclear magnetic resonance. However this is time-consuming and expensive.
Used by millions
AlphaFold2 was trained using many different protein structures and went on to successfully predict the structures of nearly all of the 200,000 known proteins. It has been used by millions of people around the world and could boost our understanding of a wide range of biological and chemical processes including bacterial resistance to antibiotics and the decomposition of plastics.
SmarAct proudly supports Physics World‘s Nobel Prize coverage, advancing breakthroughs in science and technology through high-precision positioning, metrology and automation. Discover how SmarAct shapes the future of innovation at smaract.com.
A sensible crew cut, a chic bob, an outrageous mullet. You can infer a lot about a person by how they choose to style their hair. But it might surprise you to know that it is possible to learn more about some objects in the natural world from their “hair” – be it the “quantum hair” that can reveal the deepest darkest secrets of what happens within a black hole, or glassy hair that emerges from the depths of our planet, via a volcano.
In December 2017 University of Oxford volcanologist Tamsin Mather travelled to Nicaragua to visit an “old friend”: the Masaya volcano, some 20 km south of the country’s capital of Managua. Recent activity had created a small, churning lava lake in the centre of the volcano’s active crater, one whose “mesmerising” glow at night attracted a stream of enchanted tourists.
For those who could draw their eyes away from the roiling lava, however, another treat awaited: a gossamer carpet of yellow fibres strung across the downwind crater’s edge. Known to geologists as “Pele’s hair”, Mather describes these beautiful deposits as like “glistening spiders’ webs”, shiny and glass-like, looking like “fresh cut grass after some dew”.
These glassy strands, often blown along by the wind, have been found in the vicinity of volcanoes across the globe – not only Masaya, but also Mount Etna in Italy, Erta Ale in Ethiopia, and across Iceland, where they are instead dubbed nornahár, or “witches’ hair”. They have even been found produced by underwater volcanoes at depths of up to 4.5 km below sea level. However, Pele’s hair is arguably most associated with Hawaii, from whose religion (not the footballer) the deposits take their name (see box “The legend of Pele”).
Lava fountains and candy floss
Although you might hardly guess it from its fine nature, Pele’s hair has quite the violent birth. It forms when droplets of molten rock are flung into the air from lava fountains, cascades, particularly vigorous flows or even bursting gas bubbles. This material is then stretched out into long threads as the air (or, in some cases, water) quenches them into a volcanic glass. Pele’s hair can be both thicker and finer than its human counterpart, ranging from around 1 to 300 µm thick (Jour. Research US Geol. Survey5 93). While the strands are typically around 5–15 cm in length, some have been recorded to reach a whopping 2 m long.
Fringe science A handful of Pele’s hair – collected in Hawaii in 2022 – viewed under a microscope. The golden-brown glass strands are intertwined, whereas the thicker dark glass is rigid. The shape of the darker glass reflects the shape of the lava droplets as they quenched. (Courtesy: USGS)
Katryn Wiese – an earth scientist at the College of San Mateo in California – explains that the hairs form in the same way that glass blowers craft their wares. “Melt a silica-rich material like beach sand and as it cools down, blow air through it to elongate it and stretch it out,” she says. Key to the formation of Pele’s hair, Wiese notes, is that the molten lava does not have time to crystallize as it cools. “Pele’s hair is really no different than ash. Ash is basically small beads of microscopic glass, whereas Pele’s hair is a strung-out thin line of glass.”
Go to a funfair and you’ll see this same process at play at the candy floss stall. “Sugar is melted by a heat coil in the centre of a cotton candy machine and then the liquid melted sugar is blown outwards while the device spins,” Wiese explains, to produce “thin threads of liquid that freeze into non-crystalline sugar or glass”.
Just as there is a fine art to spinning cotton candy, so too does the formation of Pele’s hair require very specific conditions to be met. First, the lava has to cool slowly enough so it can stretch out into thin strands. Second, the lava must be sufficiently fluid, rather than being more viscous. That’s why Pele’s hair is only formed by so-called basaltic eruptions, where the magma has a relatively low silica content of around 45–52%.
The composition of the initial lava is also a factor in the colour of the hairs, which can range from a golden yellow to a dark brown. “Hawaiian glasses are classically amber coloured,” notes Wiese. She explains that basalts from Hawaii are primarily made up of silica and aluminium oxides (a mix of iron, magnesium and calcium oxides), as well as trace amounts of other elements and gases. “The gases often contribute to oxidation of the elements and can also lead to different colours in the glass – the same process as blown glass in the art world.”
The legend of Pele
Elemental spirits According to legend, the Halema‘uma‘u pit crater of the volcano Kīlauea is the final resting place of Pele, the Hawaiian goddess of fire and volcanoes. (CC BY-SA 3.0 Tim Bray)
Both Pele’s hair and Pele’s tears take their name from the Hawaiian goddess of volcanoes and fire: Pelehonuamea, “She who shapes the sacred land”, who is believed to reside beneath the summit of the volcano Kīlauea on the Big Island – the current eruptive centre of the Hawaiian hotspot.
Many ancient legends of Pele depict the deity as having a fiery personality. According to one account, it was this temperament that brought her to Hawaii in the first place, having been born on the island of Tahiti. As the story goes, Pele seduced the husband of her sister Nāmaka, the water goddess. This led to a fight between the siblings that proved the final straw for their father, who sent Pele into exile.
Accepting a great canoe from her brother, the king of the sharks, Pele voyaged across the seas – trying to light her fires on every island she reached – pursued by the vengeful Nāmaka. Mirroring how the Hawaiian islands were erupted in sequence as the Earth’s crust moved relative to the underlying hotspot, Pele moved along the chain repeatedly trying to dig a fiery crater in which to live, only for each to be extinguished by Nāmaka.
The pair had their final confrontation on Maui, with Nāmaka defeating Pele and tearing her apart at the hill known today as Ka Iwi o Pele – “the bones of Pele”. Her spirit, meanwhile, flew to Kīlauea, finding its eternal home in the Halema‘uma‘u pit crater.
Tears and hairs – volcanic insights
Another important factor in the formation of Pele’s hair is the velocity at which magma is “spurted” out during an eruption, according to Japanese volcanologist Daisuke Shimozuru, who was studying Pele’s hair and tears in the 1990s.
Based on experiments involving jets of ink released from a nozzle at different speeds, Shimozuru concluded that thread-like expulsions like Pele’s hair are only formed when the eruption velocity is sufficiently high (Bulletin of Volcanology56 217). At lower speeds, the molten material is instead quenched without being stretched, forming glassy droplets, referred to as Pele’s tears, sometimes with a hair or two attached.
Glassy droplets Some Pele’s tears from Volcanoes National Park in Hawaii. (Courtesy: Shutterstock/Cata Hula)
According to Kenna Rubin – a volcanologist at the University of Rhode Island – studying the shape of these black globules can shine a light on the properties of the lava that formed them. They can provide information not only about the ejection speed, but also related parameters such as the temperature, viscosity and the distance they travelled in the atmosphere before solidifying.
Furthermore, the tears can preserve tiny bubbles of volcanic gases within themselves, trapped in cavities known as “vesicles”. Analysing these gases can reveal many details of the chemical composition of the magma that released them. These can be a useful tool to shine a light on the exact nature of the hazard posed by such eruptions.
In a similar fashion, Pele’s hair can also offer valuable insights to volcanologists about the nature of the eruptions that formed them – thereby helping to inform models of the hazards that future volcanoes may pose to nearby life and property.
Window within, and to the past
“Pele’s hair and tears are a subset of the pantheon of particles ejected by a volcano when they erupt,” notes Rubin. By examining the particles that come out over time, as well as studying the geophysical activity at a volcano, such as seismicity and gas ejection, researchers “can then make inferences about the conditions that were extant in past eruptions”. In turn, she adds, “This allows us to look at old eruption deposits that we didn’t witness erupting, and infer the same kinds of conditions.”
While Pele’s hair and tears are both relatively rare volcanic products, when they do exist they can help to constrain the eruption conditions – offering a window into not only recent but also past eruptions when so-called “fossil” samples have been preserved.
Blown away The Masaya volcano just outside of Managua, Nicaragua, is frequently active. Collecting samples of volcanic material such as Pele’s hair produced in different eruptions allows scientists to analyse how the volcano’s interior is changing over time. (Courtesy: Shutterstock/Christian Hartmann)
Alongside the composition of the glasses (and any trapped gases within such), the shape of hairs and tears can shine a light on the various forces that affected them as they were flying through the air cooling. In fact, the presence of the hair around a volcano is itself a sign that the lava is of the least viscous type, and is undergoing some form of fountaining or bubbling.
There are, of course, many other types of material or fragments of rock that get ejected into the air when volcanoes erupt. But the great thing about Pele’s hair is that, having cooled from lava to a glass, it represents the lava’s bulk composition. As Wiese notes, “We can quickly determine the composition of the lavas that are erupting from just a single sample.”
For example, Mather collected samples of Pele’s hair from Masaya during a 2001 return visit to her cherished Nicaraguan haunt, enabling Mather and her colleagues to determine the composition of the lava erupting from Masaya’s vent in terms of both major elements and lead isotopes (Journal of Atmospheric Chemistry46 207; Atmospheric Environment37 4453). As Mather says, “With other measurements we can think about how this composition changes with time and also compare it with the gas and particles that are dispersed in the plume.”
Pele’s curse
Hairy hazard Prevailing winds blow much of the Pele’s hair from Kīlauea Volcano’s summit lava lake into the Ka‘ū Desert, where it accumulates against gully walls. (Courtesy: USGS)
There is an urban legend on the islands that anything native to Hawaii – whether it be sand, rock or even volcanic glass – cannot be removed without being cursed by Pele herself. Despite invoking Hawaii’s ancient volcano goddess, the myth is believed to actually be quite recent in origin. According to one account, it was dreamt up by a frustrated park ranger who were frustrated by tourists taking rocks from the island as souvenirs. Another attributes it to tour drivers, who tired of tourists bringing said rocks onto their buses, and leaving dirt behind.
Either way, the story has taken hold as if it were an ancient Hawaiian taboo, one that some take extremely seriously. Volcanologist Kenna Rubin, for one, often receives returned rocks at her office at the University of Hawaii. “Tourists and visitors find my contact details online and return the lava rocks, or Pele’s hair,” she explains. “They apologise for taking the items as they feel they have been cursed by the goddess.”
The legend of Pele’s curse may be fictitious, but the hazards presented by Pele’s hair are very real, both to the unwitting visitor to Hawaii, and also the state’s permanent residents. Like fibreglass – which the hairs closely resemble – broken slivers of the hair can gain sharp ends that easily puncture the skin (or, worse, the eye) and break into smaller pieces as people try to remove them.
Not only can an active lava lake produce enough of the hair to carpet the surrounding area, but strands are easily picked up by the wind. From Kīlauea Volcano, for example, the US Geological Survey notes that prevailing winds tend to blow much of the Pele’s hair that is produced south to the Ka‘ū Desert, where it builds up in drifts against gully walls (see photo). In fact, hairs have been known to be carried up to tens of kilometres from the originating volcanic vent – and it is not uncommon on Hawaii to find Pele’s hair snagged on trees, utility poles and the like.
Hair in the catchment
Wind-blown Pele’s hair also poses a threat to the many locals who collect rainwater for drinking. “As ash, laze [“lava haze” – a mix of glass shards and acid released when basaltic lava enters the ocean] and Pele’s hair have been found to contain various metals and are hazardous to ingest, catchment users should avoid accumulating it in their water tanks,” the Hawaii State Department of Health advises in the event of volcanic activity.
However, even though Pele’s hair has the potential to harm humans, there are some residents of Hawaii who do benefit from it – birds. Collecting the strands like the bits of straw they resemble, our avian friends have been known to use the volcanic deposits to feather their nests; in fact, one made entirely from Pele’s hair has been preserved for posterity in the collections of the Hawaii Volcanoes National Park.
Pele’s tears can also serve as a proxy for the severity of eruptions. In a study published this March, geologist Scott Moyer and environmental scientist Dork Sahagian showed that the diameter of vesicles preserved in Pele’s tears from Hawaii is related to the height of the lava fountains that formed them (Frontiers in Earth Science12 10.3389/feart.2024.1379985). Fountain height, in turn, is constrained by the separated gas content of the source magma, which controls eruption intensity.
It’s clear that Pele’s hair and tears are far more than a beautiful natural curiosity. Thanks to the tools and techniques of geoscience, we can use them to unravel the mysteries of Earth’s hidden interior.
John Hopfield and Geoffrey Hinton share the 2024 Nobel Prize for Physics for their “foundational discoveries and inventions that enable machine learning and artificial neural networks”. Known to some as the “godfather of artificial intelligence (AI)”, Hinton, 76, is currently based at the University of Toronto in Canada. Hopfield, 91, is at Princeton University in the US.
Ellen Moons from Karlstad University, who chairs the Nobel Committee for Physics, said at today’s announcement in Stockholm: “This year’s laureates used fundamental concepts from statistical physics to design artificial neural networks that function as associative memories and find patterns in large data sets. These artificial neural networks have been used to advance research across physics topics as diverse as particle physics, materials science and astrophysics.”
Speaking on the telephone after the prize was announced, Hinton said, “I’m flabbergasted. I had no idea this would happen. I’m very surprised”. He added that machine learning and artificial intelligence will have a huge influence on society that will be comparable to the industrial revolution. However, he pointed out that there could be danger ahead because “we have no experience dealing with things that are smarter than us.”
“Two kinds of regret”
Hinton admitted that he does have some regrets about his work in the field. “There’s two kinds of regret. There’s regrets where you feel guilty because you did something you knew you shouldn’t have done. And then there are regrets where you did something that you would do again in the same circumstance but it may in the end not turn out well. That second kind of regret I have. I am worried the overall consequence of this might be systems more intelligent than us that eventually take control.”
Hinton spoke to the Nobel press conference from the West Coast of the US, where it was about 3 a.m. “I’m in a cheap hotel in California that doesn’t have a very good Internet connection. I was going to get an MRI scan today but I think I’ll have to cancel it.”
Hopfield began his career as a condensed-matter physicist before making the shift to neuroscience. In a 2014 perspective article for the journal Physical Biology called “Two cultures? Experiences at the physics–biology interface”, Hopfield wrote, “Mathematical theory had great predictive power in physics, but very little in biology. As a result, mathematics is considered the language of the physics paradigm, a language in which most biologists could remain illiterate.” Hopfield saw this as an opportunity because the physics paradigm “brings refreshing attitudes and a different choice of problems to the interface”. However, he was not without his critics in the biology community and wrote that one must have “have a thick skin”.
In the early 1980s, Hopfield developed his eponymous network, which can be used to store patterns and then retrieve them using incomplete information. This is called associative memory and an analogue in human cognition would be recalling a word when you only know the context and maybe the first letter or two.
A Hopfield network is layer of neurons (or nodes) that are all connected together such that the state, 0 or 1, of each node is affected by the states of its neighbours (see above). This is similar to how magnetic materials are modelled by physicists – and a Hopfield network is reminiscent of a spin glass.
When an image is fed into the network, the strengths of the connections between nodes are adjusted and the image is stored in a low-energy state. This minimization process is essentially learning. When an imperfect version of the same image is input, it is subject to an energy-minimization process that will flip the values of some of the nodes until the two images resemble each other. What is more, several images can be stored in a Hopfield network, which can usually differentiate between all of them. Later networks used nodes that could take on more than two values, allowing more complex images to be stored and retrieved. As the networks improved, evermore subtle differences between images could be detected.
A little later on in the 1980s, Hinton was exploring how algorithms could be used to process patterns in the same way as the human brain. Using a simple Hopfield network as a starting point, he and a colleague borrowed ideas from statistical physics to develop a Boltzmann machine. It is so named because it works in analogy to the Boltzmann equation, which says that some states are more probable than others based on the energy of a system.
A Boltzmann machine typically has two connected layers of nodes – a visible layer that is the interface for inputting and outputting information, and a hidden layer. A Boltzmann machine can be generative – if it is trained on a set of similar images, it can produce a new and original image that is similar. The machine can also learn to categorise images. It was realized that the performance of a Boltzmann machine could be enhanced by eliminating connections between some nodes, creating “restricted Boltzmann machines”.
Hopfield networks and Boltzmann machines laid the foundations for the development of later machine learning and artificial-intelligence technologies – some of which we use today.
Born on 6 December 1947 in London, UK, Hinton graduated with a degree in experimental psychology in 1970 from Cambridge University before doing a PhD on AI at the University of Edinburgh, which he completed in 1975. After a spell at the University of Sussex, Hinton moved to the University of California, San Diego, in 1978, before going toCarnegie-Mellon University in 1982 and Toronto in 1987.
After becoming a founding director of the Gatsby Computational Neuroscience Unit at University College London in 1998, Hinton returned to Toronto in 2001 where he has remained since. From 2014, Hinton divided his time between Toronto and Google but then resigned from Google in 2023 “to freely speak out about the risks of AI.”
Elected as a Fellow of the Royal Society in 1998, Hinton has won many other awards including the inaugural David E Rumelhart Prize in 2001 for the application of the backpropagation algorithm and Boltzmann machines. He also won the Royal Society’s James Clerk Maxwell Medal in 2016 and the Turing Award from the Association for Computing Machinery in 2018.
Hopfield was born on 15 July 1933 in Chicago, Illinois. After receiving a degree in 1954 from Swarthmore College in 1958 he completed a PhD in physics at Cornell University. Hopfield then spent two years at Bell Labs before moving to the University of California, Berkeley, in 1961.
In 1964 Hopfield went to Princeton University and then in 1980 moved to the California Institute of Technology. He returned to Princeton in 1997 where he remained for the rest of his career.
As well as the Nobel prize, Hopfield won the 2001 Dirac Medal and Prize from the International Center for Theoretical Physics as well the Albert Einstein World Award of Science in 2005. He also served as president of the American Physical Society in 2006.
Two papers written by this year’s physics laureates in journals published by IOP Publishing, which publishes Physics World, can be read here.
The Institute of Physics, which publishes Physics World, is running a survey gauging the views of the physics community on AI and physics till the end of this month. Click here to take part.
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