As well as a high-end hotel on the Amalfi coast, a wildlife lodge in Guyana, and a “bamboo sanctuary” in Indonesia, Time magazine’s slightly pretentious list of the “world’s greatest places” for 2024 includes one destination physicists might actually want to visit.
We’re talking about CERN’s “Science Gateway”, an outreach centre designed by the “master of hi-tech architecture” Renzo Piano, which features a transparent skywalk between two raised tubular buildings.
Time calls the gateway a “family-friendly, admission-free offshoot” of CERN that “bridges the gap between the general public and the people in lab coats”.
The Science Gateway took some three years to build and opened in October 2023. It includes exhibitions, labs, a 900-seat auditorium as well as a shop and a Big Bang café. Aimed at those aged five and above, the centre is expected to welcome half a million visitors each year.
To compile the list, Time selected from nominations made via an application process as well as suggestions from its international network of correspondents and contributors.
If you have been watching skateboarding at the Olympics, you may be wondering how the skaters manage to keep going up and down ramps long after friction should have consumed their initial gravitational potential energy.
That process is called pumping, and most skaters will learn how to do it by going back and forth on a half-pipe. If you are not familiar with the lingo, a half-pipe comprises two ramps that are connected by a lower (sometimes flat) middle section. A good skateboarder can skate up the side of a ramp, turn around and do the same on the other side – and continue to oscillate back and forth in the half-pipe.
What’s obvious about the physics of this scenario is that the gravitational potential energy of the skater while at the top of the half-pipe will be quickly lost to friction. So how does a skater keep going? How do they pump kinetic energy into the system?
Variable pendulum
It turns out that the process is similar to an obscure way that you can keep a playground swing going – by standing on the seat and shifting your centre of mass by squatting down in the centre of a swing and rising up at both ends of a swing (see video below). This can be understood in terms of a pendulum with a length that varies in a regular way – and that is how Florian Kogelbauer at ETH Zurich and colleagues in Japan have modelled pumping in a skateboard half-pipe.
Their model considers how a skilled skater modulates their centre of mass relative to the surface of the half-pipe. Essentially this involves crouching down as the skateboard travels across the flat bit of the halfpipe, then pushing up from the board during the curved ascent of the ramp. Pushing up reduces the moment of inertia of the system, and conservation of angular momentum dictates that the skater must speed up.
The team compared their model to video data of experienced and inexperienced skaters pumping a half-pipe. They found that experienced skaters did indeed adhere to their model of pumping. They now plan to extend their model to include other movements done by skaters during pumping. They also say that their model could be used to better understand the physics of other sports such as ski jumping.
On 10 April 2019, physicist Barbora Špačková was peering through an optical microscope in her lab at Chalmers University of Technology in Sweden when she saw a short DNA segment – a biological object so small it was thought nearly unimaginable for conventional microscopy to reveal it. She remembers the exact date, because, in a poetic coincidence, it was the day that the first image of a black hole was released.
While everyone on campus was talking about the iconic astronomical image, Špačková was in the lab witnessing her first successful experiment on the path to a new microscopy technique that today enables other scientists to see molecules just a nanometre in size. “That was a perfect day,” she recalls. “Science is often about 95% failure, troubleshooting and wondering why experiments don’t go as planned. So having that moment of success was beautiful.”
But her path to that moment had not been linear. As an undergraduate studying physics at the Czech Technical University in Prague, Špačková took a two-year break from her studies, during which she worked as a freelance graphic designer. “It was a period when I was not exactly sure what to do with my life. But one night, I woke up with a clear realization that my heart was in science,” she says. “Coming back to the university, I felt more determined than ever.”
After defending her Master’s thesis in physical engineering, which focused on theory and simulations, Špačková felt drawn towards experimental work – particularly technologies with applications in the life sciences. So she started a PhD studying plasmonic biosensors, which use metal nanostructures to detect biological molecules. These sensors exploit surface plasmon resonance, in which electrons on a metal surface oscillate in response to light. When a biological molecule binds to the sensor’s surface, it causes a measurable shift in the resonant frequency, thus signalling the molecule’s presence.
Špačková’s PhD thesis on detecting extremely low concentrations of molecules won her the 2015 Werner von Siemens Award for Excellence. One of the concepts she’d worked on led to her developing a working prototype of a sensor able to detect cancer biomarkers. “I was really happy that I went with my dream,” says Špačková, adding that she “moved from the realm of theory to hands-on experimentation and eventually built a box with a functional button designed to serve a greater good.”
A serendipitous discovery
After her PhD, Špačková wanted to broaden her perspective by working abroad. Fortunately, she found a postdoc matching her interests in the group led by Christoph Langhammer at Chalmers so she and her young family moved there.
The project focused on nano-plasmonic biosensing again, but in a novel configuration, by combining it with nanofluidics. This involves studying fluids confined in nanoscale structures – a technology Špačková had not worked with before. “I had this playground of new toys that I had never seen in my life,” she says. “It was exciting. I learned a lot. I was experiencing a new part of the universe.”
Early on in her project, a colleague showed her a strange optical effect he was seeing in his devices. Špačková decided to investigate, developing a theory of how biomolecules inside nanofluids interact with light. To her surprise, her calculations suggested it should be possible to see even an individual biomolecule.
Into the nano-universe Barbora Špačková’s technique enables scientists to observe single biomolecules in their natural state moving freely in real time. (Courtesy: Daniel Spacek)
These nanometre-sized objects had never been seen using traditional optical microscopy, but repeated calculations convinced Špačková she was onto something. With support from her supervisor and help from other team members, she equipped the lab with instruments needed to pursue the theory.
The trick was to put a biomolecule inside a nanochannel on a chip. Although biomolecules scatter too little light to be seen directly, interference with the light scattered by the nanochannel creates a much higher contrast. By subtracting an image of the empty channel from one with the biomolecule inside, the resulting interfered light reveals the presence of the molecule.
While other optical microscopy methods have enabled scientists to see single molecules before, they usually involve labelling these objects with fluorescent markers or fixing them to a surface – both of which can affect their properties and behaviours. The unique advantage of Špačková’s technique – named “nanofluidic scattering microscopy” (NSM) – is that it unveils single molecules in their natural state, moving freely in real time.
Looking at life
When Špačková succeeded in getting the microscope to work in 2019, it was not just a remarkable technological achievement; it was also an exciting step towards potential applications. Her invention could deepen biologists’ understanding of living processes, by showing how biomolecules move around and interact. It could also accelerate drug development, by illuminating how drug candidates interact with cell components.
Recognizing these possibilities, Špačková and her colleagues founded a start-up called Envue Technologies, with support from the Chalmers Ventures incubator, to develop and commercialize NSM instruments. Although Špačková returned to Czechia in 2022, after receiving a grant from the Czech Science Foundation and Marie-Curie Fellowship, she remains a scientific adviser to the company. “This is a super exciting experience for academics,” she says. “You get in touch with the real world and potential end users of your technology.”
Earlier this year, it was announced that Špačková will be setting up one of three new Dioscuri Centres of Scientific Excellence in the Czech Republic – an initiative of the Max Planck Society to support outstanding scientists establishing research groups.
In this programme, Špačková will be working in partnership with a German research group studying molecular transport in cells. The ultimate goal is to develop imaging tools based on NSM, for this application. “We would really like to dive inside this biological nano-universe and observe it in ways that were not possible before,” says Špačková.
It is also the first time that Špačková will be a team leader, and she is looking forward to embracing the challenge of this new role. Reflecting on her career so far, she emphasizes that everyone has a unique path and must tune in to what is right for them. “I think there’s a subtle art of deciding whether to give up or keep going,” she says. “You have to be very careful with the decision. In my case, following the heart worked.”
A new mechanical computer made from an array of rigid, interconnected plastic cubes can store, retrieve and erase data simply by stretching the array and manipulating the position of the cubes. The device’s construction is inspired by the ancient Japanese art of paper cutting, or kirigami, and its designers at North Carolina State University in the US say that more advanced versions could be used in stable, high-density memory and logic computing; in information encryption and decryption; and to create displays based on three-dimensional units called voxels.
Mechanical computers were first developed in the 19th century and do not contain any electronic components. Instead, they perform calculations with levers and gears. We don’t often hear about such contraptions these days, but researchers led by NC State mechanical and aerospace engineer Jie Yin are attempting to bring them back due to their stability and their capacity for storing complex information.
A periodic array of computing cubes
The NC State team’s computer comprises a periodic array, or metastructure, of 64 interconnected polymer cubes, each measuring 1 cm on a side and grouped into blocks. These cubes are connected by thin hinges of elastic tape that can be used to move the cubes either physically or by means of a magnetic plate attached to the cubes’ top surfaces. When the array is stretched in one direction, it enters a multi-stable state in which cubes can be pushed up or down, representing a binary 1 and 0 respectively. Thus, while the unit cells are interconnected, each cell acts as an independent switch with two possible states – in other words, as a “bit” familiar from electronic computing.
If the array is then compressed, the cubes lock in place, fixing them in the 0 or 1 state and allowing information to be stored in a stable way. To change these stored bits, the three-dimensional structure can be re-stretched, returning it to the multi-stable state in which each unit cell becomes editable.
Ancient inspiration
The new device was inspired by Yin and colleagues’ previous work, which saw them apply kirigami principles to shape-morphing matter. “We cut a thick plate of plastic into connected cubes,” Yin explains. “The cubes can be connected in multiple ways in a closed loop so that they can transform from 2D plates to versatile 3D voxelated structures.”
These transformations, he continues, were based on rigid rotations – that is, ones in which neither the cubes nor the hinges deform. “We were originally thinking of storing elastic energy in the hinges so that they could lead to different shape changes,” he says. “With this came the bistable unit cell idea.”
Yin says that one of the main challenges involved in turning the earlier shape-morphing system into a mechanical computer was to work out how to construct and connect the unit cells. “In our previous work, we made use of an ad-hoc design, but we could not directly extend this to this new work,” he tells Physics World. “We finally came up with the solution of using four cubes as a base unit and [assembling] them in a hierarchical way.”
While the platform has several possible applications, Yin says one of the most interesting would be in three-dimensional displays. “Each pop-up cube acts as a voxel with a certain volume and can independently be pushed up and remain stable,” he says. “These properties are useful for interactive displays or haptic devices for virtual reality.”
Computing beyond binary code
The current version of the device is still far from being a working mechanical computer, with many improvements needed to perform even simple mathematical operations. However, team member Yanbin Li, a postdoctoral researcher at NC State and first author of a Science Advances paper on the work, points out that the density of information it can store is relatively high. “Using a binary framework – where cubes are either up or down – a simple metastructure of nine functional units has more than 362 000 possible configurations,” he explains.
A further advantage is that a functional unit of 64 cubes can take on a wide variety of architectures, with up to five cubes stacked on top of each other. These novel configurations would allow for the development of computing that goes well beyond binary code, Li says.
In the nearer term, Li suggests that the cube array could allow users to create three-dimensional versions of mechanical encryption or decryption. “For example, a specific configuration of functional units could serve as a 3D password,” he says.
This podcast explores the extraordinary life of the Pakistani physicist Abdus Salam, who is celebrated for his ground-breaking theoretical work and for his championing of physics and physicists in developing countries.
In 1964, he founded the Abdus Salam International Centre for Theoretical Physics (ICTP) in Trieste, Italy – which supports research excellence worldwide with a focus on physicists in the developing world. In 1979 Salam shared the Nobel Prize for Physics for his work on the unification of the weak and electromagnetic interactions.
Salam spent most of his career at Imperial College London and the university is gearing up to celebrate the centenary of his birth in January 2026. In this episode of the Physics World Weekly podcast, Imperial physicists Claudia de Rham and Ian Walmsley look back on the extraordinary life of Salam – who died in 1996. They also talk about the celebrations at Imperial College.
Physicists in the US have detected the nuclear decay of individual helium nuclei by embedding radioactive atoms in a micron-sized object and measuring the object’s recoil as a particle escapes from it. The technique, which is an entirely new way of studying particles emitted via nuclear decay, relies on the principle of momentum conservation. It might also be used to detect other neutral decay products, such as neutrinos and particles that could be related to dark matter and might escape detection by other means.
The conservation of momentum is a fundamental concept in physics, together with the conservation of energy and the conservation of mass. The principle is that as momentum (mass multiplied by velocity) can be neither created nor destroyed, the total amount of it must remain constant – as described by Newton’s laws of motion.
Outgoing decay product will exert a backreaction
Physicists led by David Moore of Yale University have now used this principle to determine when a radioactive atom emits a single helium nucleus (or alpha particle) as it decays. The idea is as follows: if the radioactive atom is embedded in a larger object, the outgoing decay product will exert a back-reaction on the object, making it recoil in the opposite direction. “This is similar to throwing a ball while on a skateboard,” explains team member Jiaxiang Wang. “After the ball has been thrown, the skateboard will slowly roll backward.”
The backreaction on a large object from just a single nucleus inside it would normally be too tiny to detect, but the Yale researchers managed to do it by precisely measuring the object’s motion using the light scattered from it. Such a technique can gauge forces as small as 10-20 N and accelerations as tiny as 10-7g, where g is the local acceleration due to the Earth’s gravitational pull.
“Our technique allows us to determine when a radioactive decay has occurred and how large the momentum the decaying particle exerted on the object,” Wang says. “Momentum conservation ensures that the momentum carried by the object and the emitted alpha particle are the same. This means that measuring the object’s recoil provides us with information on the decay products.”
Optical tweezer technique
In their experiment, Moore, Wang and colleagues embedded several tens of radioactive lead-212 atoms in microspheres made of silica. They then levitated one microsphere at a time using the forces generated by a focused laser beam. This technique is known as an optical tweezer and is routinely employed to hold and move nano-sized objects. The researchers recorded recoil measurements over a period of two to three days as the lead-212 (which has a half-life of 10.6 hours) decayed to the stable isotope lead-208 through the emissions of alpha and beta particles (electrons).
According to Wang, the study is an important proof of principle, demonstrating conclusively that a single nuclear decay can be detected when it occurs in a much larger object. But the team also hopes to put it to good use for other applications. “We undertook this work as a first step towards directly measuring neutrinos as decay products,” Wang explains. “Neutrinos are central to many open questions in fundamental physics but are extremely difficult to detect. The technique we have developed could be a completely new way to study them.”
As well as detecting neutrinos, the new method, which is detailed in Physical Review Letters, could also be of interest for nuclear forensics. For example, it could be used to test whether dust particles captured from the environment contain potentially harmful radioactive isotopes.
The Yale researchers now plan to extend their technique to smaller silica spheres, which have better momentum sensitivity. “These smaller objects will allow us to sense the momentum kick from a single neutrino,” Wang tells Physics World. “Eventually, an approach like ours might also be applied to large arrays of spheres to sense other types of previously undetected, rare decays.”
Nuclear reactors – whether operational or undergoing decommissioning – create radioactive waste. Management of this waste is a critical task and this practice has been optimized over the past few decades. Nevertheless, strategies for nuclear waste disposal employed back in the 1960s and 70s were far from ideal, and the consequences remain for today’s scientists and engineers to deal with.
In the UK, spent nuclear fuel is typically stored in ponds or water-filled silos. The water provides radiation shielding, as well as a source of cooling for the heat generated by this material. In England and Wales, the long-term disposal strategy involves ultimately transferring the waste to a deep geological disposal facility, while in Scotland, near-surface disposal is considered appropriate.
The problem, however, is that some of the legacy storage sites are many decades old and some are at risk of leaking. And when this radioactive waste leaks it can contaminate surrounding land and groundwater. The potential for radioactive contamination to get into the wet environment is an ongoing problem, particularly at legacy nuclear reactor sites.
“The strategy for waste storage 50 years ago was different to that used now. There wasn’t the same consideration for where this waste would be disposed of long term,” explains Malcolm Joyce, distinguished professor of nuclear engineering at Lancaster University. “A common assumption might have been ‘well it’s going to go in the ground at some point’ whereas actually, disposal is a necessarily rigorous, regulated and complicated programme.”
In one example, explains Joyce, radioactive waste was stored temporarily in drums and sited in near-surface spaces. “But the drums have corroded over time and they’ve started to deteriorate, putting containment at risk and requiring secondary containment protection,” he says. “Elsewhere, some of the larger ponds in which spent nuclear fuel was stored are also deteriorating and risking loss of containment.”
Problematic radioisotopes
The process of nuclear fission generates a range of radioactive products with a variety of half-lives and levels of radiotoxicity – a complex factor governed by their chemistry and radioactivity behaviours. One contaminant of particular concern is strontium-90 (Sr-90), a relatively high-yield fission product found in significant amounts in spent nuclear fuel and other radioactive waste.
Sr-90 emits relatively high-energy (0.6 MeV) beta radiation, has a relatively short half-life (about 30 years) and is water soluble, enabling it to migrate with groundwater. The major hazard, however, is its potential for uptake into biological systems. As a group 2 element similar to calcium, Sr-90 is a “bone seeker” that’s taken up by the bones and remains there, increasing the risk of leukaemia and bone cancer.
“The other challenge with strontium is that its daughter is even worse in radiotoxicity terms,” explains Joyce. Sr-90 decays into yttrium-90 (Y-90), which emits very high-energy beta radiation (2.2 MeV) that can penetrate up to 3.5 mm into aluminium. “The engineering challenge associated with Y-90 was first encountered at Three Mile Island, when they realised that the energy of the beta particles from it was sufficiently high to penetrate their personal protective equipment,” he notes.
Do not disturb
These potential biological hazards make it imperative to monitor potential radioactive contamination and address any leakages, and they also provide a basis for in situ monitoring of such leaks. One approach is to extract water or earth samples, often via boreholes, for offsite analysis in a laboratory. Unfortunately, what’s measured in the lab could be completely different to the radiological environment that you’re trying to understand. “This is an example that highlights the fact that trying to measure something actually changes the thing you’re trying to measure,” notes Joyce.
When undisturbed, Sr-90 and Y-90 reach secular equilibrium, a quiescent state in which Y-90 is produced at the same rate as its decay. Y-90 can tend to react with oxygen in the environment, dependent on pH, to form insoluble products such as yttrium oxide, known as yttria, and colloidal carbonate complexes that precipitate out of the surrounding water environment and can combine with calcium and silicon in the surrounding geology.
“There’s a steady-state radioactivity environment because it’s in secular equilibrium, and also a steady-state geochemistry environment associated with how much yttria is in suspension, settled out or stuck in the geology around it,” says Joyce. “But should it be disturbed by manual intervention this might lift plumes of material, redistributing the radioactivity in the area you’re working in. The risk associated with that is different to the risk assessments associated with the quiescent environment.”
PhD opportunity A PhD studentship is available in Lancaster University’s engineering department to study Sr-90 and Y-90 contamination in land and aqueous environments, under Joyce’s supervision. See “Apply now” below for more details. (Courtesy: Lancaster University)
Joyce and his team are taking a different approach, by developing a method to monitor radioactive contamination in situ. The technique exploits the bremsstrahlung radiation generated when high-energy beta particles emitted by Sr-90 and Y-90 interact with their surrounding environment and slow down. And while beta particles only travel a few millimetres before they can no longer be detected, bremsstrahlung radiation comprises far more penetrating X-ray photons that can be measured at much greater distances.
The researchers are also using an astrophysical technique to determine the distribution of the measured radioactivity. The approach uses the Moffat point spread function – developed back in 1969 to find the distribution of galaxies – to analyse the depth and spread of the contamination and, importantly, how it is changing over time.
“If the depth of these radioactive features changes, that tells you whether things are getting worse or better,” Joyce explains. “Put simply, if they’re getting nearer to the surface, that’s probably not something that you want.”
The PhD project
The team has now demonstrated that bremsstrahlung measurements can discriminate the combined Sr-90/Y-90 beta emission from gamma radiation emitted by caesium-137 (another high-yield fission product) during in situ groundwater monitoring. The next goal is to distinguish emissions from the two beta emitters.
As such, Joyce has secured funding from the UK’s Nuclear Decommissioning Authority for a PhD studentship to develop methods to detect and quantify levels of Sr-90 and Y-90 in contaminated land and aqueous environments. The project, based at Lancaster University, also aims to understand the accuracy with which the two radioisotopes can be separated and investigate their respective kinetics.
The first task will be to determine whether bremsstrahlung emissions can discriminate between these two sources of radioactive contamination. Bremsstrahlung is produced in a continuous energy spectrum, with a maximum corresponding to the maximum energy of the beta particles (which also have a continuous energy distribution). Joyce points out that, while it is quite difficult to pinpoint this maximum, it could enable deconvolution of the contributions from Sr-90 and Y-90 to the bremsstrahlung spectrum.
It may also be possible to distinguish the two radioisotopes via direct detection of the beta particles, or a completely different solution may emerge. “Never say never with a PhD,” says Joyce. “There may be a better way of doing it that we’re not aware of yet.”
Joyce emphasizes the key role that such radiation monitoring techniques could play in nuclear decommissioning projects, such as the clean-up of the Dounreay shaft, for example. The 65-m deep shaft and silo were historically used to store radioactive waste from the Dounreay nuclear reactor in Scotland. This waste now needs to be retrieved, repackaged and stored somewhere isolated from people, animals and plants.
As the facility is emptied of radioactive material, the radiological environment will change. Ideally, it will become safer, and uncertainty reduced, with any changes potentially able to inform planning. “With this new technology we’ll be able to monitor radiation levels as the programme progresses, to understand exactly what’s happening in the environment as things are being cleaned up,” explains Joyce.
“The world would be a better place as a result of the ability to make these measurements, and they could inform how similar challenges are dealt with the world over,” Joyce tells Physics World. “If you asked me ‘why should somebody do this PhD?’, altruistically, it’s about taking us closer to the point where our grandchildren don’t have to worry about these things – that’s what’s important.”
Apply now
To find out more about the PhD studentship, which is fully funded for eligible UK students, contact Malcolm Joyce at m.joyce@lancaster.ac.uk. Candidates interested in applying should send a copy of their CV together with a personal statement or covering letter addressing their background and suitability for this project before the closing date of 31 August 2024.
Science can be a messy business. Scientists caught in the storm of a scientific revolution will try to react with calm logic and reasoning. But in a revolution the stakes are high, the atmosphere charged. Cherished concepts are abandoned as troubling new notions are cautiously embraced. And, as the paradigm shifts, the practice of science is overlaid with passionate advocacy and open hostility in near-equal measure. So it was – and, to a large extent, still is – with the quantum revolution.
Niels Bohr insisted that quantum theory is the result of efforts to describe a fundamentally statistical quantum world using concepts stolen from classical physics, which must therefore be interpreted “symbolically”. The calculation of probabilities, with no reference to any underlying causal mechanism that might explain how they arise, is the best we can hope for.
In the heat of the quantum revolution, Bohr’s “Copenhagen interpretation” was accused of positivism, the philosophy that valid knowledge of the physical world is derived only from direct experience. Albert Einstein famously disagreed, taking the time to explore alternatives more in keeping with a realist metaphysics, with a “trust in the rational character of reality and in its being accessible, to some extent, to human reason”, that had served science for centuries. Lest there be any doubt, Adam Forrest Kay’s Escape from Shadow Physics: the Quest to End the Dark Ages of Quantum Theory demonstrates that the Bohr–Einstein debate remains unresolved, at least to anybody’s satisfaction, and continues to this day.
Escape from Shadow Physics is a singular addition to the popular literature on quantum interpretations. Kay holds PhDs in both literature and mathematics and is currently a mathematics postdoc at the Massachusetts Institute of Technology. He stands firmly in Einstein’s corner, and his plea for a return to a realist programme is liberally sprinkled with passionate advocacy and open hostility in near-equal measure. He writes with the zeal of a true quantum reactionary.
Like many others before him, in arguing his case Kay needs first to build a monstrous, positivist Goliath that can be slain with the slingshot of realist logic and reasoning. This means embracing some enduring historical myths. These run as follows. The Bohr–Einstein debate was a direct confrontation between the subjectivism of the positivist and the objectivism of the realist. Bohr won the debate by browbeating the stubborn, senile and increasingly isolated Einstein into submission. Acting like some fanatical priesthood, physicists of Bohr’s church – such as Wolfgang Pauli, Werner Heisenberg and Léon Rosenfeld – shouted down all dissent, establishing the Copenhagen interpretation as a dogmatic orthodoxy.
Historical scientific myths are not entirely wrong, and typically hold some grains of truth. Rivals to the Copenhagen view were indeed given short shrift by the “Copenhagen hegemony”. Pauli sought to dismantle Louis de Broglie’s “pilot wave” interpretation soon after it was presented in 1927. He went on to dismiss its rediscovery by David Bohm in 1952 as “shadow physics beer-idea wish dreams”, and “not even new nonsense”. Rosenfeld dismissed Hugh Everett III’s “many worlds” interpretation of 1957 as “hopelessly wrong ideas”.
But Kay is not content with the myth as it is familiarly told, and so seeks to deepen it. He confers on Bohr “the charisma of the hypnotist, the charisma of the cult leader”, adding that “the Copenhagen group was, in a very real sense, a personality cult, centred on the special and wise Bohr”. Prosecuting such a case requires a selective reading of science history, snatching quotations where they fit the narrative, ignoring others where they don’t. In fact, Bohr did not deny objective reality, or the reality of electrons and atoms. In interviews conducted shortly before his death in 1962, Bohr reaffirmed that his core principle of “complementarity” (of waves and particles, for example) was “the only possible objective description”. Heisenberg, in contrast, was much less cautious in his use of language and makes an easier target for anti-positivist ire.
It can be argued that the orthodoxy, such as it is, is not actually based on philosophical pre-commitments. The post-war Americanization of physics drove what were judged to be pointless philosophical questions about the meaning of quantum theory to the fringes. Aside from those few physicists and philosophers who continued to nag at the problem, the majority of physicists just got on with their calculations, completely unconcerned about what the theory was supposed to mean. They just didn’t care.
As Bohm explained: “Everybody plays lip service to Bohr, but nobody knows what he says. People then get brainwashed into saying Bohr is right, but when the time comes to do their physics, they are doing something different.” Many who might claim to follow Bohr’s “dogma” satisfy their physical intuitions by continuing to think like Einstein.
Anton Zeilinger, who shared the 2022 Nobel Prize for Physics for his work on quantum entanglement and quantum information science, confessed that even physicists working in this new field consider foundations to be a bit dodgy: “We don’t understand the reason why. Must be psychological reasons, something like that, something very deep.” Kay admits this much when he writes: “Yes, many people think the debate is over and Bohr won, but that is actually a social phenomenon.” In other words, the orthodoxy is not philosophical, it is sociological. It has very little to do with Bohr and the Copenhagen interpretation. In truth, Kay is fighting for attention against the apathy and indifference characteristic of an orthodox mainstream physics, or what Thomas Kuhn called “normal science”.
As to how a modern-day realist programme might be pursued, Kay treats us to some visually suggestive experiments in which oil droplets follow trajectories determined by wave disturbances on the surface of the oil bath on which they move. He argues that such “quantum hydrodynamic analogues” show us that the pilot-wave interpretation merits much more attention than it has so far received. But while these analogues are intuitively appealing, the so-called “quantization” involved is as familiarly classical as musical notes generated by string or wind instruments. And, although such analogues may conjure surprising trajectories and patterns, they cannot conjure Planck’s constant. Or quantum entanglement.
But the pilot-wave interpretation demands a hefty trade-off. It features precisely the non-local, “peculiar mechanism of action at a distance” of the kind that Einstein abhorred, and which discouraged his own exploration of pilot waves in 1927. In an attempt to rescue the possibility that reality may yet be local, Kay reaches for a loophole in John Bell’s famous theorem and inequality. Yet he overlooks the enormous volume and variety of experiments that have been performed since the early 1980s, including tests of an inequality devised by the Nobel-prize-winning theorist Anthony Leggett that explicitly close the loophole he seeks to exploit.
Escape from Shadow Physics is a curate’s egg. Those readers who would condemn Bohr and the Copenhagen interpretation, for whatever reasons of their own, will likely cheer it on. Those looking for balanced arguments more reasoned than diatribe will likely be disappointed. Despite an extensive bibliography, Kay commits some curious sins of omission. But, while the journey that Kay takes may be flawed, there is yet sympathy for his destination. The debate does remain unresolved. Faced with the mystery of entanglement and non-locality, Bohr’s philosophy offers no solace. Kay (quoting a popular textbook) asks that we consider future generations in possession of a more sophisticated theory, who wonder how we could have been so gullible.
Spatially fractionated radiotherapy is a novel cancer treatment that uses a pattern of alternating high-dose peaks and low-dose valleys to deliver a nonuniform dose distribution. Numerous preclinical investigations have demonstrated that by shrinking the peaks and valleys to submillimetre dimensions, the resulting microbeams confer extreme normal tissue tolerance, enabling delivery of extremely high peak doses and providing excellent tumour control.
The technique has not yet, however, been used to treat patients. Most preclinical studies employed synchrotron X-ray sources, which deliver microbeams at ultrahigh dose rates but are not widely accessible. Another obstacle is that these extremely narrow beams (100 µm or less) are highly sensitive to any motion during irradiation, which can blur the pattern of peak and valley doses.
Instead, a team at the Mayo Clinic in Rochester, Minnesota, is investigating the clinical potential of minibeam radiation therapy (MBRT), which employs slightly wider beams (500 µm or more) spaced by more than 1000 µm. Such minibeams still provide high normal tissue sparing and tumour control, but their larger size and spacing makes them less sensitive to motion. Importantly, minibeams can also be generated by conventional X-ray sources with lower dose rates.
To perform MBRT, the researchers adapted the Xstrahl 300, a clinical orthovoltage unit with 180 kVp output. “Because minibeam radiotherapy uses very narrow beams of radiation spaced very closely together, it requires low-energy orthovoltage X-rays,” Grams explains. “Higher-energy X-rays from linear accelerators would scatter too much and blur the peaks and valleys together.”
The team used cones with diameters between 3 and 10 cm to define the field size and create homogeneous circular fields. This output was then split into minibeams using tungsten collimators with 0.5 mm wide slits spaced 1.1 mm apart.
Commissioning measurements showed that the percentage depth dose decreased gradually with depth, reaching 50% somewhere between 3.5 and 4 cm. Peak-to-valley ratios were highest at the surface and inversely related to cone size. Peak dose rates at 1 cm depth ranged from 110 to 120 cGy/min.
The low dose rate of the orthovoltage system means that treatment times can be quite long and patient motion may be an issue. To mitigate motion effects, the researchers created 3D printed collimator holders that conform to the patient’s anatomy. These holders are fixed to the patient, such that any motion causes the patient and collimator to move together, maintaining the spatial separation of the peak and valley doses.
“This treatment had never been delivered to a human before, so we had to figure out all of the necessary steps in order to do it safely and effectively,” says Grams. “The main challenge is patient motion, which we solved by attaching the collimator directly to the patient.”
First-in-human treatments
The team treated two patients with MBRT. The first had a large (14x14x11 cm) axillary tumour that was causing severe pain and restricted arm motion, prompting the decision to use MBRT to shrink the tumour and preserve normal tissue tolerance for future treatments. He was also most comfortable sitting up, a treatment position that’s only possible using the orthovoltage unit.
The second patient had a 7x6x3 cm ear tumour that completely blocked his external auditory canal, causing hearing loss, shooting pain and bleeding. He was unable to undergo surgery due to a fear of general anaesthesia and instead was recommended MBRT to urgently reduce pain and bleeding without compromising future therapies.
“These patients had very few treatment options that the attending physician felt would actually help mitigate their symptoms,” explains Grams. “Based on what we learned from our preclinical research, they were felt to be good candidates for MBRT.”
Both patients received two daily MBRT fractions with a peak dose of 1500 cGy at 1 cm depth, using the 10 cm cone for patient 1 and the 5 cm cone for patient 2. The radiation delivery time was 11.5 or 12 min per fraction, with the second fraction delivered after rotating the collimator by 90°.
Treatment response (A) The tumour of patient 2 on the day of MBRT. (B) Treatment photo showing the patient with the collimator holder attached, the 5 cm cone flush and in contact with the tungsten collimator. Tumour response 16 days (C) and 34 days (D) after MBRT. (Courtesy: Int. J. Radiat. Oncol. Biol. Phys. 10.1016/j.ijrobp.2024.06.035)
Prior to treatment, the collimator was attached to the patient and a small piece of Gafchromic film was placed directly on the tumour for in vivo dosimetry. For both patients, the films confirmed the pattern of peak and valley doses, with no evidence of dose blurring.
For patient 1, the measured peak and valley doses were 1900 and 230 cGy, respectively. The expected doses (based on commissioning measurements) were 2017 and 258 cGy, respectively. Patient 2 had measured peak and valley doses of 1800 and 180 cGy, compared with expected values of 1938 and 248 cGy.
Both patients exhibited positive clinical responses to MBRT. Six days after his second treatment, patient 1 reported resolution of pain and improved arm motion. Three weeks later, the tumour continued to shrink and his full range of motion was restored. Despite the 10 cm cone not fully encompassing the large tumour, a uniform decrease in volume was still observed.
After one treatment, patient 2 had much reduced fluid leakage, and six days later, his pain and bleeding had completely abated and his hearing improved. At 34 days after MBRT, he continued to be asymptomatic and the lesion had completely flattened. Pleased with the outcome, the patient was willing to reconsider the recommended standard-of-care resection.
“The next step is a formal phase 1 trial to determine the maximum tolerated dose of minibeam radiotherapy,” Grams tells Physics World. “We are also continuing our preclinical work aimed at combinations of MBRT and systemic therapies like immunotherapy and chemotherapy drugs.”
A new scanning transmission electron microscope (STEM) technique that modulates the electron beam in response to the scattering rate allows images to be formed with the fewest electrons possible. The researchers hope their “event-responsive electron microscopy“ could be used on fragile samples that are easily damaged by electron beams. The team is now working to implement their imaging paradigm with other microscopy techniques.
First developed in the 1930s, transmission electron microscopes have been invaluable for exploring almost all branches of science at tiny scales. These instruments rely on the fact that electrons can have far shorter de Broglie wavelengths than optical photons and hence can observe much finer details. Visible light microscopes cannot normally resolve features smaller than about 200 nm, but electron imaging can often achieve resolutions well below 0.1 nm. However, the higher energy of these electrons makes them more damaging to samples than light. Researchers must therefore keep the number of electrons scattered from fragile sample to the absolute minimum needed to build up a clear image.
In a STEM, an image is created by rapidly scanning a focused beam of electrons across a sample in a grid of pixels. Most of these electrons pass straight through the sample, but a small percentage are scattered sharply by collisions. Detectors that surround the beam path record these scattering events. The electron scattering rate from a particular point tells microscopists the density around that point, and thereby allows them to reconstruct an image of the sample.
Unnecessary radiation damage
Normally, the same number of incident electrons is fired at each pixel and the number of scattered electrons is counted. To create enough collisions at weakly scattering regions to resolve them properly, strongly scattering regions are exposed to far more incident electrons than necessary. As a result, samples may suffer unnecessary radiation damage.
In the new work, electron microscopists led by Jonathan Peters and Lewys Jones at Trinity College Dublin, together with Bryan Reed of Integrated Dynamic Electron Solutions in the US and colleagues in the UK and Japan, inverted the traditional measurement protocol by measuring the time required to achieve a fixed number of scattered electrons from every pixel. Jones offers an analogy: “If you look at the weather forecast on TV you see the rainfall in millimetres per hour,” he says; “If you look at how that’s measured by weather forecasters they go and put a beaker outside in the rain and, one hour later, they see how much is in the beaker…If I ask you how hard it’s raining, you’re going to go outside, stick your hand out and see how long it takes for, say, three drops to hit your hand…After you’ve reached some fixed [number of drops], you don’t wait for the rest of the hour in the rain.”
Event response
The researchers implemented an event-responsive microscopy protocol in which the individual scattered electrons from each pixel is recorded, and this information is fed back to the electron microscope. After the set number of scattered electrons is recorded from each individual pixel, a “beam blanker” is switched on until the end of the normal pixel waiting time. “A powerful voltage is applied to skew the beam off into the sidewall,” explains Jones. “It has the same effect of opening and closing a shutter on a camera.” This allowed the researchers to measure the scattering rate from all the sample points without subjecting any of them to unnecessary electron flux. “It’s not a slow process,” says Jones; “The image is formed in front of the user in real-time.”
The researchers used their new protocol to produce images of biologically and chemically fragile samples with little to no radiation damage. They now hope it will prove possible to produce electron micrographs of samples such as some catalysts and drug molecules that are currently obliterated by electron beams before an image can be formed. They are also exploring the protocol’s use in other imaging techniques such as electron energy loss spectroscopy and X-ray microscopy. “It will probably take a number of years for us and other groups to fully unpick what such a fundamental shift in how measurements are made will mean for all the other kinds of techniques that people use microscopes for,” says Jones.
Electron microscopy expert Quentin Ramasse of the University of Leeds is enthusiastic about the work. “It’s inventive, it’s potentially changing the way we record data in a STEM and it’s doing so in a very simple fashion. It could provide an extra tool in our arsenal to not necessarily completely remove beam damage but certainly to minimize it,” he says. “It really is [the result of] clever electronics, clever hardware and a very clever take on how to drive the motion of the probe as a function of what the sample’s response has been up to that point.”
