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Muon g-2 achieves record precision, but theoretical tensions remain

In 2018, the Muon g-2 Experiment at Fermilab near Chicago, set out to measure the muon’s anomalous magnetic moment to a precision of 140 parts per billion (ppb). This component of the muon’s magnetic moment is the result of several subtle quantum effects and is also known as the muon g-2 – which reflects how the gyromagnetic ratio of the muon deviates from the simple value of two.

After six years of producing, storing, and measuring more than a trillion muons, the collaboration released its long-anticipated final result in June, achieving an unprecedented precision of 127 ppb. This landmark measurement not only solidifies confidence in the experimental value of muon g-2 but also sets a new benchmark as the most precise accelerator-based measurement of a fundamental particle to date.

Studies of the muon g-2 have served as a rigorous test of the Standard Model – physicist’s leading theory describing known particles and forces – for much of the last century. Theoretically, the muon’s anomalous magnetic moment can be predicted from the Standard Model to a similar precision as the experiment. For decades, a persistent discrepancy between prediction and measurement hinted at the possibility of new physics, with experimental results favouring a higher value than the theory. Such a difference, if confirmed, could point to phenomena not accounted for in the Standard Model – potentially explaining unresolved mysteries like the existence of dark matter.

However, extraordinary claims require extraordinary scrutiny. To address the experimental side, Fermilab launched the Muon g-2 Experiment. On the theoretical side, the Muon g-2 Theory Initiative was established as a global collaboration of theorists working to refine the Standard Model prediction using state-of-the-art methods, techniques, and input data.

Problematic contribution

One of the most problematic contributions to the theoretical value is the hadronic vacuum polarization (HVP), historically determined using experimental data as input to complex calculations. While the Theory Initiative has improved these methods, progress has remained limited due to discrepancies in the available experimental data. Crucially, a recent input from the CMD-3 Experiment diverged significantly from previous results, suggesting a larger HVP contribution (see figure below). This, in turn, yields a Standard Model prediction that aligns with the new Fermilab measurement – apparently eliminating the discrepancy and, with it, any evidence of new physics.

Muon g-2 values
Evolving results Summary of the four values of the anomalous magnetic moment of the muon aμ that have been obtained from different experiments and models. The most recent (2025) theory and experiment values are in agreement. (Courtesy: Alex Keshavarzi)

Despite years of investigation, the origin of the CMD-3 tension remains unknown. Its result stands in contrast to a vast catalogue of earlier data from multiple experiments over decades. As a result, the traditional, data-driven approach to estimating the HVP is deemed currently unable to produce a reliable estimate .

Thanks to the efforts of the Theory Initiative, however, the HVP can now also be calculated using lattice QCD (quantum chromodynamics) simulations on supercomputers, reaching a precision comparable to that of the data-driven methods. Multiple independent lattice QCD groups have arrived at consistent values, which also agree with the Fermilab measurement, indicating no discrepancy and thus no sign of new physics. This computational feat, once considered out of reach, marks a major breakthrough. Yet, the tension remains unresolved: Why do lattice QCD and CMD-3 agree, while both conflict with decades of experimental data?

No physics beyond the Standard Model

Given the improved control in lattice QCD, the Theory Initiative has also recently updated its recommended Standard Model prediction with the HVP fully based on lattice results. The resulting value agrees with the Fermilab measurement and currently implies no evidence for physics beyond the Standard Model. However, the Initiative has emphasized that this is far from conclusive. Future predictions are intended to incorporate data-driven estimates again – once the inconsistencies in the experimental input are resolved.

The field now faces two possibilities. One is that the CMD-3 result and lattice QCD are correct. In this case, there is no new physics – but an impressive validation of the Standard Model. The other scenario is that new experimental HVP input data align with the older results, supporting a smaller HVP contribution. This would reintroduce the discrepancy with the Fermilab result, reviving the exciting possibility of new physics. In either case, the inconsistencies between CMD-3, lattice QCD, and the existing data must be explained.

So, is there new physics or not? We know there must be. The Standard Model cannot not explain dark matter, the accelerating expansion of the universe, the absence of antimatter, or the quantum nature of gravity. Precision tests like muon g-2 offer a window into this unknown. That window has not closed – for now it’s propped open.

Where we’ll be in five years is uncertain. The Muon g-2 Theory Initiative will continue to refine predictions and resolve open questions. For now, one thing is clear: the Muon g-2 Experiment at Fermilab has delivered an historic achievement and its legacy will continue to contribute to our understanding of fundamental physics for decades to come.

 

Terahertz optoacoustics allows real-time monitoring of blood sodium levels

An imbalance in sodium ions in the blood causes a number of physiological problems, but so far it has not been possible to measure these ion concentrations in vivo. Now researchers have successfully applied their terahertz optoacoustic technology to measure blood ion concentrations non-invasively, overcoming the challenges posed by previous approaches. They report their findings in Optica.

The idea to combine terahertz spectroscopy with optoacoustic detection came about during a recruitment trip when Zhen Tian from the School of Precision Instrument and Optoelectronics Engineering at Tianjin University in China got chatting with colleague Jiao Li – co-author of this latest study. At the time, Tian’s work was focused primarily on terahertz technology while Li had been working on optoacoustics, but the more they talked, the more interested they became in each other’s fields, and took “every available opportunity to discuss these topics in depth” during the trip.

Putting their heads together on their return, in 2021 they successfully demonstrated terahertz optoacoustic detection of ions in water, despite the challenges of the pandemic. “We thought things would progress smoothly from there, but deeper investigations revealed a series of technical challenges,” Tian tells Physics World. “What began as a fortunate opportunity soon turned into a demanding endeavour.”

The sodium focus

Since ions are strongly polar, they absorb highly in the terahertz range, making them easy to detect. As such, Tian and Li were keen to find a scenario where the tracking of ions might be useful. Another colleague at Tianjin University (also a co-author on this new study) pointed out that ion imbalances in the blood can cause kidney disease and serious neurological conditions. The most abundant ion in the blood is sodium, and as Li explains, not only do imbalances in sodium ions need prompt correction, but the lack of means for monitoring sodium ions in vivo poses risks of neural demyelination and brain damage during sodium ion supplementation.

One of the key challenges was the high water content of body tissues, because water absorbs terahertz radiation so strongly. The researchers turned this to an advantage by using the water to detect emitted terahertz radiation from the sample, exploiting the fact that the optoacoustic response is temperature dependent. At cold temperatures, absorbing terahertz radiation emitted from the sample heats up the water, which detectably impacts its optoacoustic signal. Therefore, comparing the sample’s optoacoustic response to terahertz radiation with values for pure water gives a quantitative indication of the absorption by the sample and thus the concentration of ions present.

Although the researchers demonstrated a proof-of-principle for this approach in 2021, they then had to battle with several other issues. They improved the stability of the light source by reducing thermal fluctuations and making other optimizations to the experimental environment; they used higher-intensity light sources and enhanced detectors to increase the detection sensitivity; and they used spectral filtering to achieve molecular specificity in the optoacoustic detection. Tian expresses his gratitude to Yixin Yao, a co-first author of the paper, as well as to the students involved. “It was their commitment and perseverance that helped us overcome each hurdle,” he says.

The team demonstrated that the enhanced system could detect sodium ions in human blood flowing through a microfluid chip and measure increases in blood sodium levels in living mice. The operating temperature for the technique was 8 °C, cold enough to cause damage to many parts of the body. However, the researchers noted that the ear is particularly resilient to temperature, so they cooled and monitored just the animal’s ear, limiting the experiment duration to 30 min. This way they were able to complete their measurements without incurring any tissue damage.

Although the numerous previous in vitro experiments had left the researchers full of “anticipation” for the success of the attempts in vivo, Tian tells Physics World, “when we saw the terahertz optoacoustic signal enhance after sodium ion injection, all of us, including the students conducting the experiment, cheered with excitement”.

“It is very nice to see [that] fundamental studies on dielectric response of aqueous salt solutions may result in a sensor for human health,” says Andrea Markelz from the University at Buffalo, whose research focuses on biomolecular dynamics and terahertz time domain spectroscopy, although she was not directly involved in this study. She notes that tagless terahertz-based biomonitoring is challenging, due to both the strong aqueous background and the lack of narrowband signatures. “It will be very interesting to see if the sensitivity remains robust for different organisms under different conditions,” she adds.

Next, Tian and his collaborators plan to apply the approach to detect neural ion activity without the need for labelling. “It’s admittedly a bold and ambitious idea – but one that has truly excited our team,” he says.

Hyperdisorder appears in pigment patterns on squid skin

Researchers at the Okinawa Institute of Science and Technology (OIST) in Japan have identified the first known example of hyperdisorder occurring in a biological system. This phenomenon combines order at the microscopic scale with disorder at the macro level, and it is often present in systems studied in statistical physics. However, the researchers were surprised to observe it while monitoring the development of pigment cells in squid skin. As the hyperdisorder is directly linked to the squid’s growth, the researchers say the discovery could shed light on the physics of growing structures.

In inanimate objects, the emergence of disordered patterns is relatively well understood in physical terms. Living creatures are different, however, as they can display unexpected phenomena as they grow and develop.

To better understand how growth impacts the formation of patterns, a team led by Robert Ross, Simone Pigolotti and Sam Reiter at OIST studied how pigment cells known as chromatophores arrange themselves on the skin of squid as the animal grows and its skin expands. “These pigment cells are important because they play an essential role in camouflage and communication for these animals,” Reiter explains.

Highly unusual statistical patterns

The researchers took a series of 3D optical images of the squid over a period of three months. These observations revealed that the chromatophores behave very differently from other disordered structures. “The chromatophores appear at fixed positions in relation to one another, in a specific pattern,” Reiter explains.

It is this pattern that met the technical criteria for hyperdisorder, which is defined as occurring when the variation in the number of points within a particular measured space increases more rapidly than the volume of that space.

In the squid he and his colleagues studied, Ross explains that new chromatophores appear only at a minimum exclusion distance from pre-existing ones as the animal grows. “We found that this rule coupled with tissue growth leads to the highly unusual statistical patterns we observe,” he says. “Simply put, when you observe a tiny area in a system, it may appear quite ordered, but when viewed at larger scales, it becomes more disordered.”

To explain this finding, the researchers modelled squid development as static circle packing on a growing surface and showed how the hyperdisordered behaviour emerges. “The result is exciting because it highlights the importance of growth on physical properties,” Ross says.

A general feature of many biological structures?

The researchers note that other growing systems, such as the cells in chicken retinas, often display the exact opposite property, which is known as hyperuniformity. In these systems, there is long-range order and patterning despite randomness at a close scale, Ross explains. Such behaviour is thought to provide optimal retinal coverage properties for vision. “This is what we thought we would see in the squid, but what we actually observed was quite different and we have not yet seen any other instances of this packing behaviour in biology,” he says.

The mechanisms described in this work, which is detailed in Physical Review X, may be common in growing, dense natural systems, says Ross: “Indeed, this simple type of growth combined with distance-limited cell insertion might be a general feature of many biological structures.”

Spurred on by their findings, the researchers plan to continue working on a variety of theoretical and experimental systems related to the physics of growing structures. “These include both growing brains and pattern formation in fish,” says Ross.  “We hope these systems will provide further examples of the novel physics of growing systems,” he tells Physics World.

Physics meets fashion as bioluminescent dress debuts at Paris Haute Couture Week

Fashion designer Iris van Herpen has unveiled a bioluminescent dress that features 125 million living algae. The garment involved Herpen collaborating with designer Chris Bellamy as well as biophysicists Nico Schramma and Mazi Jalaal from the University of Amsterdam.

bioluminescent dress
Dress to impress: the “living” garment was part of van Herpen’s new fashion collection – Sympoiesis (courtesy: Molly SJ Lowe, for Iris van Herpen)

Bioluminescence is the production of light by a living organism, caused by a chemical reaction such as the molecule luciferin reacting with oxygen to release light.

The bioluminescent dress is composed of a gel material that incorporates millions of single celled bioluminescent algae of the species Pyrocystis lunula, named after their moon-like shape.

In the wild, the bioluminescent algae emit light as a defence mechanism. The flash serves as a warning signal that attracts secondary predators, which hunt the main predator of the cells.

In 2019, Jalaal, Schramma and colleagues began to study how the cells respond to mechanical stresses. By combining microscopy and mechanical tests, they were able to measure the light-emission of the cells and how it depended on deformation, which led to a mathematical model that described the light-production mechanism (Phys. Rev. Lett. 125 028102).

The researchers then worked with Chenghai Li and Shengqiang Cai at the University of California San Diego and bioluminescence researcher Michael Latz from the Scripps Institution of Oceanography in San Diego.

They incorporated the cells in a gel matrix to create a flexible yet resistant substance that emits light upon deformation and movement while at the same time keeping the cells alive.

Bellamy and van Herpen developed and refined the bioluminescent material and incorporated it into a spectacular “living” garment, which on Monday was part of van Herpen’s new fashion collection – Sympoiesis – that was unveiled at Paris Haute Couture Week.

Astronomers observe the biggest booms since the Big Bang

Astronomers at the University of Hawai’i’s Institute for Astronomy (IfA) in the US have detected what they say are the most energetic cosmic explosions known to have occurred since the the universe began. These colossal events, dubbed extreme nuclear transients (ENTs), emit at least 10 times as much energy as the previous record holders, and studying them could open a new window into physical processes that take place at very high energies.

ENTs occur when stars that are at least three times as massive as the Sun pass so close to a supermassive black hole that its colossal gravity shreds them to pieces. The resulting string of matter then spirals into the black hole in a phenomenon known as accretion.

Such events are extremely rare, occurring a few hundred times less frequently than supernovae. However, when they do happen, they release huge amounts of energy, producing long-lasting flares that can then be detected on Earth.

Optical transient surveys have spotted several classes of accretion-powered flares over the past decade or so, explains Jason Hinkle, who led the study as part of his PhD research at the IfA. Examples include tidal disruption events, rapid turn-on active galactic nuclei and ambiguous nuclear transients.

The new ENTs are a different kettle of fish, however. They release between 0.5 × 1053 and 2.5 × 1053 erg (0.5‒2.5 × 1046 J) making them at least twice as energetic as any other known transient. “They are also 10 times as bright (emitting 2 × 1045 to 7 × 1045 erg per second) and remain luminous for years, far surpassing the energy output of even the brightest known supernova explosions,” Hinkle says.

Looking for smooth, high-amplitude and long-lived signals

Hinkle began searching for ENTs at the beginning of his PhD studies by sifting through data from the European Space Agency’s Gaia mission. Gaia is ideal for such a search as it has been observing the full sky since late 2014. As a space-based mission, it also typically has shorter seasonal breaks than ground-based surveys.

Hinkle’s search for smooth, high-amplitude, long-lived signals revealed two possible sources. Designated Gaia16aaw (AT2016dbs) and Gaia18cdj (AT2018fbb), each comes from the centre of a distant galaxy. For Gaia16aaw, that galaxy bears the catchy name WISEA J041157.03-420530.8. Gaia18cdj, for its part, lies within the equally memorable WISEA J020948.15-420437.1

In 2020, astronomers began observing these sources with space-based UV/X-ray missions and ground-based facilities, including the University of Hawai’i’s Asteroid Terrestrial-impact Last Alert System and the W M Keck Observatory. “These gave us the first indication that we were seeing something special,” Hinkle says. “When the Zwicky Transient Facility [a wide-field optical survey] published data on a third similar event, ZTF20abrbeie, also sometimes called ‘Scary Barbie’ (AT2021lwx), in 2023, it gave us additional confidence that we had found a rare, new class of transient phenomena.”

These data show that the brightness of the light emitted from ENTs increases for more than 100 days, peaks, and then slowly declines over a period of more than 150 days. ENTs also produce infrared light, which suggests that circumnuclear dust is being heated up and reemitted at longer wavelengths, Hinkle says.

The fact that Gaia16aaw and Gaia18cdj are located relatively close to the centres of their host galaxies (within 0.68 and 0.25 kpc, respectively) confirms their status as nuclear transients, he adds. Their long timescales and high peak luminosities also suggest that they originate from accretion onto a supermassive black hole. “The way they accrete is very different from normal black hole accretion, however, which typically shows irregular and unpredictable changes in brightness,” Hinkle explains. “Instead, the smooth and long-lived flares of ENTs imply a distinct physical process – the gradual accretion of a tidally disrupted star by a supermassive black hole.”

Several ENTs could be detected per year

According to IfA team member Benjamin Shappee, ENTs provide a valuable new tool for studying massive black holes in distant galaxies. Since they are so bright, they can be seen across vast cosmic distances, equivalent to redshifts between z = 4 and 6. This means they could give astronomers new information about black hole growth when the universe was less than half its present age, during a period when galaxies were forming stars and feeding their supermassive black holes up to 10 times more vigorously than they are today.

Now that astronomers know what to look for, Hinkle says that new survey instruments such as the Vera C Rubin Observatory and NASA’s Roman Space Telescope should turn up several ENTs per year. “From a physics perspective, building a sample of ENTs will give us the best look yet at massive black holes in the early universe, especially the large majority of those that are not otherwise accreting,” he says. “This will serve as an excellent complement to studies of accreting black holes in the early universe with the James Webb Space Telescope, for example.

“We have a great starting point, but as with many things in observational astronomy, we need larger samples to gain a fuller understanding of how these events work and how we can best use them to test fundamental physics.”

The present study is detailed in Science Advances.

Acoustic rainbows emerge from novel sound-scattering structure

Researchers in Denmark have produced the acoustic equivalent of a rainbow, creating a structure that spatially decomposes sound into its component frequencies in free space. Developing such a structure had proven difficult due to the complexity required, but the team at Danmarks Tekniske Universitet (DTU) managed it thanks to an advanced structural design technique. The new architecture could be used to make devices tailored to emit or receive certain frequencies of sound.

Optical rainbows occur when white light is split into its different spectral components, for example by passing through dispersive media such as prisms or droplets. Although acoustic rainbows are less well-known, they follow the same principle, being the spatial decomposition of sound in free space where waves oscillating at different frequencies propagate in different directions. They have previously been created in confined media using arrays of resonant structures that “trap” sound at different positions in space depending on their frequency. Examples include waveguides, solid and/or fluid mixtures and devices known as acoustic circulators.

Acoustic spectral decomposition also occurs in several natural structures, including the outer ear structures, or pinnae, of mammals such as bats, cetaceans and primates. Indeed, the pinnae of primates (including humans) have an intricate geometry that generates complex interference phenomena via scattering of sound waves, thereby enabling the animals to localize external sources of sound.

An acoustic scattering structure

While researchers have previously attempted to imitate such biological designs, these efforts were largely unsuccessful. The new work, which was co-led by Rasmus Ellebæk Christiansen and Efren Fernandez-Grande at the DTU, succeeded in part thanks to a new technique known as computational morphogenesis, or topology optimization.

This technique, which the researchers describe in Science Advances, builds on an earlier morphogenetic design framework for tailoring passive acoustic scattering structures with dimensions on the order of a few wavelengths. Using an iterative process, the team spatially redistributed sound-reflecting material in an air background inside a specified region of space. This enabled them to tailor the sound field emitted from the created structure to match a predefined target emission pattern across a specified frequency band, mimicking naturally-occurring “sound shaping” structures.

“Such a technique is possible today thanks to the rapid growth in computational power in recent years that has allowed us to model and synthesize sound on the large scale,” Ellebæk Christiansen explains. When combined with advanced production techniques like additive manufacturing (also known as 3D printing), he adds that the team benefitted from “nearly unlimited design freedom”, with the new technique enabling the design of metamaterials and nonintuitive structures hitherto deemed unrealizable.

“Our approach to designing the structures is to re-formulate the device design problem carefully and meticulously as a mathematical optimization problem and to use topology optimization to solve this problem,” he explains. “In this way, we do not rely on simplified design rules derived from underlying physics models, on design intuition or on prior design experience to come up with our device geometry. Instead, we use rigorous mathematical modelling and simulation coupled with advanced numerical algorithms.”

Towards new and very different structures/geometries

The geometry and topology of the metamaterial the team created has several features reminiscent of structures present in the pinnae that spatio-spectrally decompose sound, Ellebæk Christiansen says. However, he tells Physics World that the technique may also enable them to develop structures/geometries that offer new possibilities never realized in nature.

One option, Fernandez-Grande suggests, would be to design acoustic materials that reflect different frequencies of sound in different ways – for example, by scattering high frequencies diffusely and redirecting low frequencies towards an absorbing surface. “It might also help in the development of acoustic lenses – that is, sound sources (such as loudspeakers) that control how different frequencies are radiated in space,” Fernandez-Grande adds.

In the future, the researchers would like to transition from their current two-dimensional design to one that is fully three-dimensional. “This would offer significantly more design freedom, and acoustic field complexity, which might allow for even better/more elaborate spatio-spectral sound field control,” Ellebæk Christiansen says.

Inside ATLAS: Sara Alderweireldt explains how the CERN experiment homes in on new physics

This podcast features an interview with Sara Alderweireldt, who is a physicist working on the ATLAS experiment at CERN – the world-famous physics lab that straddles the Swiss-French border and is home to the Large Hadron Collider (LHC).

Based at the UK’s University of Edinburgh, Alderweireldt is in conversation with Physics World’s Margaret Harris and explains how physicists sift through the vast amount of information produced by ATLAS’ myriad detectors in search of new physics.

They also chat about the ongoing high-luminosity upgrade to the LHC and its experiments – which will be finished in 2030 – and the challenges and rewards of working a very long term project.

Construction begins on new £93m European weather-forecasting headquarters

Construction has begun on the new headquarters of the European Centre for Medium-Range Weather Forecasts (ECMWF). Yesterday, senior officials marked the start of construction on the new £93m centre at the University of Reading, which will provide cutting-edge meteorological research and forecasting.

The ECMWF is an independent intergovernmental organization with 35 member and cooperating states. Established in 1975, the centre employs around 500 staff from more than 30 countries at its existing headquarters at Shinfield Park in Reading, UK, and sites in Bologna, Italy, and Bonn, Germany.

As a research institute and 24/7 operational service, the ECMWF produces global numerical weather predictions four times per day and other data for its member/cooperating states and the broader meteorological community.

The new centre at the University of Reading, built by construction firm Mace, is funded by the UK’s Department for Science, Innovation and Technology. When it opens in 2027, it will accommodate up to 300 scientists and staff who will relocate from Shinfield Park.

The centre will carry out work on all aspects of weather prediction, forecast production and research into climate change.

“This state-of-the-art facility places the UK at the heart of international efforts that are helping us to make better sense of our weather and climate,” notes UK science minister Patrick Vallance. “By improving our weather predictions we can optimise our energy consumption estimates, adjust transport schedules effectively and give our farmers time to prepare for extreme weather – helping people and businesses to save money, cut energy use and stay safe.”

Diversity in the UK tech sector must improve, says report

The UK technology industry is struggling with persistent challenges around diversity and inclusion. That is according to a new report by the Department for Science, Innovation and Technology, which concludes that despite some modest recent progress, all minority groups still remain significantly underrepresented in the technology sector.

The tech startup ecosystem is valued at over $1.1 trillion worldwide with the technology sector employing more than 1.8 million people in the UK. Women and people from ethnic-minority groups, however, account for only around a quarter of the technology workforce. People from ethnic-minority groups also only hold 14% of senior roles.

Based on surveys of UK technology industries and a review of existing research on the sector, the report finds that recent diversity gains diminish at mid-career and leadership levels. In the last year, female representation in senior technology positions increased by only 1%, while one in three women are planning to quit their jobs due to a lack of career progression, poor work-life balance and an unsupportive culture.

This persistent “leaky pipeline” is linked to structural and cultural barriers that result in poor retention and promotion of underrepresented people. Cultural attitudes reinforce this gender bias, the report says, with one recent study finding that 20% of men in technology believe that women are “naturally less suited” to technical work. Indeed, a previous national study found that underrepresented minorities were nearly twice as likely to leave a technology job because of unfair treatment than for a better role.

Underrepresentation is particularly stark for Black technologists, who make up only 5% of workers, while just 0.07% of technology employees are Black women. Socio-economic diversity is also poor with only 9% of technology employees coming from poorer backgrounds, compared with 29% in finance and 23% in law. Data also shows that individuals from working-class backgrounds in technology earn, on average, almost £5000 less per year than their peers from more affluent backgrounds.

’Lack of progress’

There is also a lack of diversity when it comes to technology funding, with the report showing that 92% of angel investments in 2022 went to all-white teams, while female and ethnic minority-led startups secured just 2% of venture capital funding. On average female-founded technology businesses receive £1.1m, figures show, while male-owned startups receive £6.2m.

The report also points to one analysis that found that about 14% of technologists identify as disabled, while another put the figure as low as 6%, suggesting a reluctance to disclose disabilities. The later survey also suggests that 53% of technology employees identify as neurodivergent, yet employers claim that just 3% of their staff are neurodivergent.

To improve diversity and inclusion in the technology sector, the report calls for improvements in flexible-working options; diversity, equity and inclusion reporting; improved governance structures; and socio-economic mobility initiatives.

Sarah Bakewell, head of diversity and inclusion at the Institute of Physics, which publishes Physics World, describes the report’s conclusions as “concerning” as “it reveals the lack of diversity in the sector and who funding is allocated to”. Even more worrying, she says, is the lack of progress in boosting the diversity of people in UK tech. “To unleash a new wave of UK innovation, we must attract, develop and retain people from all backgrounds in inclusive work environments where everyone can realise their full potential.”

Black holes could act as cosmic supercolliders

As they approach a black hole’s event horizon, particles of accreting gas can take on opposing orbital trajectories – remarkably similar to the paths produced in manmade particle colliders. Using advanced new models, Andrew Mummery at the University of Oxford, together with Joseph Silk at Sorbonne University, showed how such particles could collide at colossal energies, with detectable collision products that could offer valuable new insights for particle physics.

Within a black hole’s accretion disk, gas particles travel in circular orbits that gradually shrink under its immense gravity. Once an orbit contracts beneath a critical radius, it becomes unstable, and the particles it carries will suddenly plunge toward the black hole.

“Long ago, Roger Penrose showed that these particles could extract energy from the spin of massive black holes in the region where they decay,” Silk explains. “This happens in the ergosphere – the region just outside the event horizon where debris can gain energy from the black hole’s intense gravitational and rotational fields.”

In the theory described by Penrose, a particle approaching a black hole splits into two fragments – possibly through a collision or spontaneous decay. After the split, one fragment falls into the event horizon, while the other gains enough energy from the black hole’s spin to escape its gravity – exiting the ergosphere with more energy than the original particle.

Building on this idea, Silk and two of his previous collaborators – Maximo Bañados and Stephen West – proposed an alternative escape mechanism. Their idea involves gas particles in retrograde orbits (moving opposite to the black hole’s spin) within the accretion disk. Since a retrograde orbit becomes unstable at larger radii than a prograde orbit (movement in the same direction as the black hole’s rotation), these particles fall farther before reaching the ergosphere, allowing them to gain more energy through gravitational acceleration.

Within the ergosphere, Bañados, Silk and West considered how these now highly energetic particles could collide with those originating from prograde orbits, travelling in opposite directions. If this occurred, the relative velocity between the two would be enormous – imparting extreme relativistic energies to their collision products. The trio proposed that some of these products could escape the ergosphere with more energy than either of the original particles.

In their latest study, reported in Physical Review Letters, Silk and Mummery explored this possibility in greater detail. They used models recently developed by Mummery to simulate the flow of particles accreting onto rapidly spinning supermassive black holes.

“We showed that the infalling gas would develop a pattern of turbulent rotating and counter-rotating vortices as it plunged into the black hole’s ergosphere,” Silk explains. The rotation direction of each vortex depends on whether the particles originated from prograde or retrograde orbits within the accretion disk.

When particles travelling in opposite directions collide in the ergosphere, their circular paths resemble the magnetically guided trajectories of protons and heavy ions in manmade particle colliders, such as CERN’s Large Hadron Collider – only on a vastly larger scale. “We found that the collisions occurred at hundreds of times higher energies than those reached in any existing collider, and would approach or even exceed the energies expected for the proposed Future Circular Collider,” Silk notes.

At such colossal energies, Mummery and Silk predict that the collision products could include gamma rays and ultrahigh-energy neutrinos, which might be detectable from nearby supermassive black holes – such as Sagittarius A* at the centre of our own galaxy. As a result, the process could offer an entirely new approach to observations in particle physics.

“Our predicted signatures would complement those of the next generation of giant particle supercolliders planned by CERN and in China, helping to provide evidence of new particle physics beyond the Standard Model,” says Silk. In particular, the duo suggest that these signatures could lead to a highly sensitive probe of dark matter – potentially offering more robust tests for candidates such as weakly interacting massive particles.

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