Quantum electrodynamics – competition for precision

Quantum electrodynamics (QED) is the fundamental theory describing all electromagnetic phenomena including light (photons). At the same time, it is the most precisely tested theory in physics at all. It has been stringently tested in various ways up to 0.1 parts per billion. But it is just the very strength of this theory that drives physicists to test it even more rigorously and to explore its possible limits. Any significant deviation would be a hint for new physics.

QED understands the electromagnetic interaction among charged particles as the exchange of “virtual” photons – the way electrons in an atom “talk” to each other and to the nucleus – and with themselves via emission and reabsorption of a photon, a QED effect called “self energy.” Furthermore, it turned out that the physical vacuum is not empty but filled with virtual particles such as electron-positron pairs which appear all the time “out of nothing” but have to disappear within the limits set by the uncertainty principle of quantum physics. Though this might sound spooky, it is just the way to explain the underlying physics of experiments performed in atomic physics already in the 1940s.

A state-of-the-art access to QED phenomena is the so-called g-factor of the electron which describes the relation of its mechanic (intrinsic angular momentum: spin) and magnetic properties. According to Dirac’s theory (relativistic quantum mechanics), the g-factor of the free electron should be exactly 2. However, various QED interactions change the g-factor and lead to a small but precisely measurable deviation from the value 2. QED effects depend in a strong nonlinear way on external fields. Electrons experience the extremely high electric field due to the high nuclear charge in heavy elements. The simplest systems are hydrogen-like highly charged ions which have been investigated both theoretically and experimentally with great success [1].

In a joint collaborative experimental-theoretical work, researchers at the Max Planck Institute for Nuclear Physics in Heidelberg have now investigated the g-factor of the outermost bound electron in lithium-like tin. This system is similar to hydrogen but adds the interaction with the two tightly-bound electrons of the inner atomic shell.

Theory: ab initio QED calculations

An ab initio calculation takes into account all electromagnetic interactions among the constituents – here of a Lithium-like ion – on a fundamental level including QED effects up to a certain degree. Electron structure effects where the electrons exchange photons are incorporated into the calculations, as well as QED screening effects, where the electron interacts both with the other electrons and with itself or with the vacuum. The ab initio prediction has been further improved by using the two-loop QED contribution extracted from the recent measurement in hydrogen-like tin [33] scaled to the lithium-like electron case. This yields an “experimentally enhanced” theoretical prediction of

g_th = 1.980 354 797(12)

with the uncertainty given in parentheses. Compared to the hydrogen-like case, this is overall a 25-fold improvement.

Experiment: counting spin flips

The measurement of the g factor of the bound electron was performed using the cryogenic Penning trap ALPHATRAP at MPIK. The strong magnetic field inside the trap leads to a characteristic motion of the ion confined by the field as well as to a precession of the spin of the outer electron like a tiny magnetic spinning top. The g factor can be extracted from the ratio of the ion’s motional frequency and the precession frequency while the magnetic field is eliminated from this calculation. The ion motion can be detected directly from small induced electric signals in the trap electrodes of the “precision trap.” To determine the precession frequency, microwave radiation is sent into the trap which can induce a spin flip, a change of the orientation of the spin (due to quantisation there are only two measurable spin states “up” and “down”). The rate of spin flips reaches a maximum when the microwave matches resonantly the precession frequency.

Results and outlook

The experimental value for the g factor of the lithium-like tin ion is

g_exp = 1.980 354 799 750(84)_stat(54)_sys(944)_ext

with the statistical, systematic and external uncertainties given in parentheses. The external uncertainties are dominated by the ion mass uncertainty, currently limiting the experimental accuracy. The overall accuracy is 0.5 parts per billion. The experimental result agrees well with the theoretical prediction given above within the uncertainty of the calculation. On the experimental side it is feasible to improve the precision of the mass value by more than an order of magnitude and consequently enhance the precision of the g factor if motivated by advancements in theory. In the future, measurements of heavier lithium-like systems such as ²⁰⁸Pb⁷⁹⁺ and the expected progress in two-loop QED calculations will provide even better tests in the strong electric field regime using highly charged ions. The advanced theoretical methods developed here for interelectronic QED effects can be applied to g-factor calculations of more complex ions (boron- or carbon-like), parity non-conserving transitions in neutral atoms and other effects.

In the new study, an international team of 21 scientists from ten countries used eight glacier models to calculate the potential ice loss from more than 200,000 glaciers outside of Greenland and Antarctica. The team evaluated a wide range of global temperature scenarios, assuming that temperatures would remain constant for thousands of years in each scenario.

“The choices we make today will resonate for centuries, determining how much of our glaciers can be preserved,” says Harry Zekollari, co-lead author from Vrije Universiteit Brussel, who began this research as a postdoctoral fellow at the Chair of Glaciology in the Department of Civil, Environmental and Geomatic Engineering (D-BAUG) at ETH Zurich.

Looking beyond 2100 reveals new insights

In all scenarios, the glaciers lose mass rapidly over decades and then continue to melt at a slower pace for centuries — even without additional warming. This long-term response means glaciers will continue to feel the effects of today’s heat far into the future, gradually retreating to higher altitudes before reaching a new equilibrium.

“One of the key strengths of our study is that we were able, for the first time, to project global glacier evolution over multi-centennial timescales, and did so using eight models instead of one or two,” explains Harry Zekollari. “Most glacier studies stop at 2100, which is problematic when simulating the long-term impact of today’s climate policies, given the long-term response of glaciers over time.”

For example, while studies limited to the year 2100 estimate that around 20 per cent of today’s glacier mass will be lost regardless of future warming, the new study reveals that nearly twice as much would vanish under present-day conditions when multi-centennial timescales are considered. “We find that around 40 per cent of glacier mass is effectively ‘doomed’ to disappear,” says co-lead author Harry Zekollari.

Melting glaciers reveal the reality of global warming

“Glaciers are good indicators of climate change because their retreat allows us to see with our own eyes how climate is changing. However, since they adjust over longer timescales, their current size vastly understates the magnitude of climate change that has already happened. The situation for glaciers is actually far worse than visible in the mountains today,” says co-lead author Lilian Schuster from the University of Innsbruck.

Beyond contributing to sea-level rise, glacier loss has far-reaching consequences. It threatens freshwater availability, increases the risk of glacier-related hazards such as floods and landslides, and jeopardizes glacier-fed tourism economies. These cascading impacts will be felt across regions and generations.

“These effects underscore the critical importance of present-day climate policies,” says Harry Zekollari. “Our study makes it painfully clear that every fraction of a degree matters. If we manage to limit global warming to +1.5°C instead of +2.7°C, we could still save twice as much glacier ice.”

Current policies are projected to lead to an average global warming of around +2.7°C. As Zekollari emphasizes, the degree of warming between +1.5°C and +3.0°C plays a decisive role in glacier loss. Put simply: for every additional 0.1°C of warming, the world risks losing approximately 2per cent more of its glacier ice.

Contributing to the UN-Year of Glaciers’ Preservation

“This study is a major contribution to the United Nations International Year of Glaciers’ Preservation, emphasizing the urgent need for global climate action to protect the world’s glaciers,” says Daniel Farinotti, Professor of Glaciology at ETH Zurich and the Swiss Federal Institute for Forest, Snow and Landscape Research WSL.

His research group at the Laboratory of Hydraulics, Hydrology and Glaciology (VAW) played a central role in producing the new findings. The entire study led by Zekollari and Schuster was conducted as part of the Glacier Model Intercomparison Project (GlacierMIP) and coordinated by the Climate and Cryosphere (CliC) Project of the World Climate Research Programme (WCRP).

Farinotti notes that the release of the Science study coincides with the opening of the High-Level International Conference on Glaciers’ Preservation, initiated by the President of Tajikistan through the United Nations (UN) Resolution that established both the UN Year of Glaciers’ Preservation and, later, the UN Decade of Action for the Cryospheric Sciences 2025-2034.

On the Swiss side, the Federal Department of Foreign Affairs (FDFA) was invited to support the organisation of the event, particularly in drafting what is intended to become the “Dushanbe Glacier Declaration.” Daniel Farinotti, for his part, is acting as an advisor to the FDFA in the preparation of the declaration.

The same was true 73 million years ago, according to a paper featured on the cover of this week’s edition of the journal Science. The paper documents the earliest-known example of birds nesting in the polar regions.

“Birds have existed for 150 million years,” said lead author Lauren Wilson, a doctoral student at Princeton University who earned her master’s degree at the University of Alaska Fairbanks. “For half of the time they have existed, they have been nesting in the Arctic.”

The paper is the result of Wilson’s master’s thesis research at UAF. Using dozens of tiny fossilized bones and teeth from an Alaska excavation site, she and her colleagues identified multiple types of birds — diving birds that resembled loons, gull-like birds, and several kinds of birds similar to modern ducks and geese — that were breeding in the Arctic while dinosaurs roamed the same lands.

Prior to this study, the earliest known evidence of birds reproducing in either the Arctic or Antarctic was about 47 million years ago, well after an asteroid killed 75% of the animals on Earth.

“This pushes back the record of birds breeding in the polar regions by 25 to 30 million years,” said Pat Druckenmiller, the paper’s senior author, director of the University of Alaska Museum of the North and Wilson’s advisor for her master’s degree work. The bird fossils are part of the museum’s collections.

“The Arctic is considered the nursery for modern birds,” he said. “It’s kind of cool when you go to Creamer’s Field [a Fairbanks-area stopover for migrating geese, ducks and cranes], to know that they have been doing this for 73 million years.”

The mere existence of the large collection of ancient bird fossils is remarkable, Wilson said, given how delicate bird bones are. That is doubly true for baby bird bones, which are porous and easily destroyed.

“Finding bird bones from the Cretaceous is already a very rare thing,” she said. “To find baby bird bones is almost unheard of. That is why these fossils are significant.”

The fossils were collected from the Prince Creek Formation, an area along the Colville River on Alaska’s North Slope known for its dinosaur fossils. Scientists identified more than 50 bird bones and bone fragments.

“We put Alaska on the map for fossil birds,” Druckenmiller said. “It wasn’t on anyone’s radar.”

The collection is a testament to the value of an uncommon excavation and research approach at the Prince Creek Formation. Much of vertebrate paleontology focuses on recovering large bones.

The scientists who work in the Prince Creek Formation make sure to get every bone and tooth they can, from the visible to the microscopic, Druckenmiller said. The technique, which involves hauling tubs of screened sediment back to the lab for examination under a microscope, has yielded numerous new species and unprecedented insights into the behavior and physiology of the dinosaurs, birds and mammals that lived in the Arctic during the Cretaceous Period.

“We are now one of the best places in the nation for bird fossils from the age of the dinosaurs,” Druckenmiller said. “In terms of information content, these little bones and teeth are fascinating and provide an incredible depth of understanding of the animals of this time.”

It remains to be seen whether the bones found on the Colville River are the earliest-known members of Neornithes, the group that includes all modern birds. Some of the new bones have skeletal features only found in this group. And, like modern birds, some of these birds had no true teeth.

“If they are part of the modern bird group, they would be the oldest such fossils ever found,” Druckenmiller said. Currently, the oldest such fossils are from about 69 million years ago. “But it would take us finding a partial or full skeleton to say for sure.”

Other collaborators on the paper include Daniel Ksepka from the Bruce Museum, John Wilson from Princeton University, Jacob Gardner from the University of Reading, Gregory Erickson from Florida State University, Donald Brinkman and Caleb Brown from the Royal Tyrrell Museum of Palaeontology and the University of Alberta (Brown is also affiliated with the University of Manitoba), Jaelyn Eberle from the University of Colorado Boulder and Chris Organ from Montana State University.

Leprosy is a neglected disease, mainly caused by the bacterium Mycobacterium leprae, affecting thousands of people worldwide: approximately 200,000 new cases of leprosy are reported each year. Although M. leprae remains the primary cause, this study focused on another species, Mycobacterium lepromatosis, discovered in the United States in 2008 in a Mexican patient, and later in 2016 in red squirrels in the British Isles. Led by scientists from the Laboratory of Microbial Paleogenomics at the Institut Pasteur, also associated with the CNRS, and the University of Colorado, in collaboration with Indigenous communities and over 40 scientists from international institutions including archaeologists, this study analyzed DNA from nearly 800 samples, including ancient human remains (from archaeological excavations) and recent clinical cases presenting symptoms of leprosy. The results confirm that M. lepromatosis was already widespread in North and South America long before European colonization and provide insights into the current genetic diversity of pathogenic Mycobacteria.

“This discovery transforms our understanding of the history of leprosy in America,” said Dr. Maria Lopopolo, the first author of the study and researcher at the Laboratory of Microbial Paleogenomics at the Institut Pasteur. “It shows that a form of the disease was already endemic among Indigenous populations well before the Europeans arrived.”

The team used advanced genetic techniques to reconstruct the genomes of M. lepromatosis from ancient individuals found in Canada and Argentina. Despite the geographic distance of several thousand kilometers, these ancient strains dating from similar periods (approximately 1,000 years ago) were found to be surprisingly genetically close. Although they belong to two distinct branches in the evolutionary tree of the genus Mycobacterium, these branches are genetically closer to each other than to any other known branch. This genetic proximity, combined with their geographical distance, necessarily implies a rapid spread of the pathogen across the continent, likely within just a few centuries.

The scientists also identified several new lineages, including an ancestral branch that despite having diverged from the rest of the known species’ diversity over 9,000 years ago, it continues to infect humans today in North America — a discovery suggesting an ancient and long-lasting diversification on the continent, as well as a largely unexplored diversity that likely remains to be found.

Notably, the analyses also suggest that the strains found in red squirrels in the UK in 2016 are part of an American lineage that was introduced to the British Isles in the 19th century, where it subsequently spread. This discovery highlights the recent ability of the pathogen to cross continents, likely through human or commercial exchanges.

“We are just beginning to uncover the diversity and global movements of this recently identified pathogen. The study allows us to hypothesize that there might be unknown animal reservoirs,” said Nicolás Rascovan, the lead author of the study and head of the Laboratory of Microbial Paleogenomics at the Institut Pasteur. “This study clearly illustrates how ancient and modern DNA can rewrite the history of a human pathogen and help us better understand the epidemiology of contemporary infectious diseases.”

The project was conducted in close collaboration with Indigenous communities, which were involved in decisions regarding the use of ancestral remains and the interpretation of results. Ancient DNA and remaining materials were returned when requested, and the generated data was shared via ethical and adaptable platforms designed to allow data sharing that meets the specific expectations of Indigenous communities.

The new robot, dubbed ATMO (aerially transforming morphobot), uses four thrusters to fly, but the shrouds that protect them become the system’s wheels in an alternative driving configuration. The whole transformation relies on a single motor to move a central joint that lifts ATMO’s thrusters up into drone mode or down into drive mode.

The researchers describe the robot and the sophisticated control system that drives it in a paper recently published in the journal Communications Engineering.

“We designed and built a new robotic system that is inspired by nature — by the way that animals can use their bodies in different ways to achieve different types of locomotion,” says Ioannis Mandralis (MS ’22), a graduate student in aerospace at Caltech and lead author of the new paper. For example, he says, birds fly and then change their body morphology to slow themselves down and avoid obstacles. “Having the ability to transform in the air unlocks a lot of possibilities for improved autonomy and robustness,” Mandralis says.

But midair transformation also poses challenges. Complex aerodynamic forces come into play both because the robot is close to the ground and because it is changing its shape as it morphs.

“Even though it seems simple when you watch a bird land and then run, in reality this is a problem that the aerospace industry has been struggling to deal with for probably more than 50 years,” says Mory Gharib (PhD ’83), the Hans W. Liepmann Professor of Aeronautics and Medical Engineering, director and Booth-Kresa Leadership Chair of Caltech’s Center for Autonomous Systems and Technologies (CAST), and director of the Graduate Aerospace Laboratories of the California Institute of Technology (GALCIT). All flying vehicles experience complicated forces close to the ground. Think of a helicopter, as an example. As it comes in for a landing, its thrusters push lots of air downward. When that air hits the ground, some portion of it bounces back up; if the helicopter comes in too quickly, it can get sucked into a vortex formed by that reflected air, causing the vehicle to lose its lift.

In ATMO’s case, the level of difficulty is even greater. Not only does the robot have to contend with complex near-ground forces, but it also has four jets that are constantly altering the extent to which they are shooting toward each other, creating additional turbulence and instability.

To better understand these complex aerodynamic forces, the researchers ran tests in CAST’s drone lab. They used what are called load cell experiments to see how changing the robot’s configuration as it came in for landing affected its thrust force. They also conducted smoke visualization experiments to reveal the underlying phenomena that lead to such changes in the dynamics.

The researchers then fed those insights into the algorithm behind a new control system they created for ATMO. The system uses an advanced control method called model predictive control, which works by continuously predicting how the system will behave in the near future and adjusting its actions to stay on course.

“The control algorithm is the biggest innovation in this paper,” Mandralis says. “Quadrotors use particular controllers because of how their thrusters are placed and how they fly. Here we introduce a dynamic system that hasn’t been studied before. As soon as the robot starts morphing, you get different dynamic couplings — different forces interacting with one another. And the control system has to be able to respond quickly to all of that.”

In a study published today (May 28, 2025) in PLOS One, an international team of archaeologists documents how monuments changed as the climate transitioned from a humid environment to, eventually, an arid desert.

Early monuments were built by larger groups at one time. But as people dispersed with the increasingly drier climate, smaller groups began constructing monuments and eventually built many of them in several visits.

“The findings show that monuments are a flexible technology that reflect the resilience of desert pastoralists in the face of a changing climate,” said Joy McCorriston, lead author of the study and professor of anthropology at The Ohio State University.

But the key role that these monuments played in people’s lives remained a constant.

“These monuments are touchstones for human social belonging,” McCorriston said.

“As these groups became smaller and more spread out in the desert, people’s interactions with the monuments consolidates a sense of being part of a larger society.”

The research team analyzed 371 archaeological monuments in the arid Dhofar region of Oman. The earliest monuments studied were created from 7500 to 6200 BP (years Before Present) in the Holocene Humid Period. This period was characterized by higher-than-modern rainfall in southern Arabia.

The most recent monuments studied were created from 1100-750 BP, during the Late Antiquity when the area had become a desert.

While examples of most of the monuments and archaeological sites had previously been studied and classified, that research was generally very time- and place-specific, McCorriston said.

“What we’ve done is take a holistic look and show how all these individual monuments were part of a larger story of how the monuments changed as the lives of the people changed over thousands of years,” she said.

The researchers did this by looking at a standard set of observations for all the monuments and developing a model that could be used in other contexts and places around the world.

For example, the model may be applicable and adaptable to assess social resilience in regions such as Saharan, Mongolian, or the high Andes.

One of the key measurements the researchers made was the volume and size of stones used in construction of the monuments. The earliest-built monuments in the study were Neolithic platforms, which contained larger stones. They were the largest monuments studied and were built at one time.

“The significance of the larger stones is that it takes more people to lift them. We know that it took at least seven strong men to lift the largest stones,” McCorriston said.

“These large monuments that were built in one episode could only be built early on, before the region became arid. This is when large groups of people could still come together at one time.”

Some of these larger monuments could serve large gatherings of people, where they could converge with multiple herds of cattle, and have animal sacrifices and feasts.

As the region became more arid and could no longer support large numbers of people nor their coming together, small groups traveled widely, going to where they could find water and places for their animals to graze.

They still had to build monuments in one episode, such as for burials, but by this time they tended to be smaller and use smaller stones, the researchers found.

What became more common were what are called accretive monuments, which people built over time — sometimes many years — rather than in one episode, like the earlier platform monuments.

One example of such monuments is accretive triliths. The higher number of triliths, along with the smaller stone volumes with few heavy stones, are consistent with monuments built over time by smaller, dispersed groups in an era of hyper-aridity.

These accretive monuments functioned as touchstones, allowing pastoralists to maintain connections and social resilience even as their movements and populations became more dispersed.

“In many cases, they were building a memory. They come to a monument and add their piece, which was a replicated element of the whole. It helped people maintain a community, even with those they may rarely see,” she said.

It is impossible to say what were the precise messages the monuments were meant to convey, according to McCorriston. “What we can say is that the monuments conveyed readable meanings to others who shared the same cultural context.”

It is possible, though, that some monuments were built to assure others in a social network access to important environmental information as they came by later.

“People would need to know, did it rain here last year? Did the goats eat all the grass? Pastoralists used this technology to help absorb the risk of being in an inherently variable and risky environment,” she said. And they would need to depend on social networks for livestock exchanges, marriage partners, and rare materials, like sea shells, carnelian and agate and metal.

“That is one of the key points of what we found. Our model highlights a reliance on monuments to preserve connections and adapt socially in a changing world.”

Other Ohio State co-authors on the study were Lawrence Ball, Ian Hamilton, Matthew Senn and Abigail Buffington. Other co-authors were Michael Harrower of Johns Hopkins University; Sarah Ivory of Penn State University; Tara Steimer-Herbet of the Université de Genève, Geneva, Switzerland; and ‘Ali Ahmad Al-Kathiri and ‘Ali Musalam Al-Mahriof the Ministry of Heritage and Tourism, Salalah, Sultanate of Oman.

The phenomenon is described in a new study that uses on-site field work and airborne imagery to track the health of the reef in Hōnaunau Bay, Hawaii. Overfishing is the main culprit behind the explosion in sea-urchin numbers, said Kelly van Woesik, Ph.D. student in the North Carolina State University Center for Geospatial Analytics and first author of the study.

“Fishing in these areas has greatly reduced the number of fishes that feed on these urchins, and so urchin populations have grown significantly,” van Woesik said. “We are seeing areas where you have about 51 urchins per square meter, which is among the highest population density for sea urchins anywhere in the world.”

Those urchins eat the reef, which is already not growing at a healthy rate, van Woesik said. Water pollution and overheated water created by climate change result in a poor environment for the coral to reproduce and grow, leaving the reef even less able to keep up with the pace of erosion caused by the urchins.

Reef growth is generally measured in terms of net carbonate production, which refers to the amount of calcium carbonate produced in a square meter over a year. Prior research in the 1980s found areas in Hawaii with carbonate production around 15 kilograms per square meter, which would signal a healthy, growing reef, van Woesik said. The reef in Hōnaunau Bay today, however, showed an average net carbonate production of only 0.5 kg per square meter, indicating that the reef is growing very slowly.

By combining data gathered through on-site scuba diving with images taken from the air, van Woesik determined that the reef would need to maintain an average of 26% coral cover to break even with the pace of urchin erosion, and a higher cover in order to grow. The average coral cover across all depths was 28%, she said, but areas in shallow depths with more erosion would still need nearly 40% cover to break even.

For the islands they surround, coral reefs like those in Hōnaunau Bay provide important coastal protection against erosion from waves, absorbing up to 97% of incoming wave energy. They are also often vital to the economies of those areas, which rely on the reefs and the fishes that live there. Van Woesik said the study highlights the need for more robust fisheries management in the area to bolster the populations of carnivorous fishes that eat the urchins.

“The reefs cannot keep up with erosion without the help of those natural predators, and these reefs are essential to protecting the islands they surround,” she said. “Without action taken now, we risk allowing these reefs to erode past the point of no return.”