A variation on positron emission tomography (PET) offers a new way to diagnose hypoxia in tumours. Researchers at the University of Tokyo and Japan’s National Institute of Radiological Sciences demonstrated that positronium, which forms in tissues due to positron emission from a radiopharmaceutical agent, decays differently depending upon its chemical environment, and is especially sensitive to local oxygen saturation. This means that signs of tumour hypoxia could be spotted among the gamma rays collected routinely during PET imaging, providing clinicians with an additional source of information to guide treatment decisions.
Most positrons created during a PET scan are annihilated almost as soon as they are emitted: they lose energy through interactions with nearby molecules, then collide with electrons in those molecules to produce pairs of 511 keV photons. Some positrons hang around a little longer, however, and instead of annihilating the electrons that they encounter, they capture them, forming metastable positronium atoms.
When this happens, the positronium is created in one of two distinct configurations. The least stable is para-positronium (p-Ps), in which the spins of the electron and positron point in opposite directions. p-Ps atoms have a mean lifetime of only 125 ps, after which they decay into a pair of 511 keV photons. This process therefore adds to the near-immediate gamma signal produced by annihilation of those positrons that never form a positronium atom.
The other configuration is ortho-positronium (o-Ps), in which the spins of the electron and positron are parallel. Left alone, o-Ps would decay into three photons (with energies ranging from 0 to 511 keV) after a mean lifetime of 142 ns. This much longer period means that o-Ps atoms have more time to interact with their surroundings before they decay.
One of the routes open to o-Ps atoms is an interaction called spin exchange. In this process, the positronium’s electron switches with an electron of opposite spin in a nearby molecule. This converts the positronium atom into the less stable p-Ps form, hastening its decay.
The likelihood of spin exchange occurring depends on the availability of unpaired electrons in the vicinity of the o-Ps atoms. In tissues, such unpaired electrons are present primarily in oxygen molecules. This means that in oxygen-poor environments such as hypoxic tumours, more o-Ps atoms survive long enough to decay via the three-photon route. Measuring the timing and spectrum of the gamma rays emitted during the PET procedure should, therefore, yield information about the oxygen saturation.
As they report in Communications Physics, Kengo Shibuya and colleagues tested this principle by preparing samples of water saturated with either air, nitrogen or oxygen. Each sample also contained the unstable sodium isotope 22Na.
When 22Na undergoes beta decay, it simultaneously emits a high-energy gamma ray at 1.27 MeV. The researchers used this signal as the starter pistol for each measurement. In instances where the positron emission resulted in a positronium atom, the end of the measurement was marked by the detection of sub-511-keV photons announcing the positronium’s final decay.
By comparing the timing and energy of the photons emitted during millions of measurement intervals for the three samples, the team derived a linear relationship between oxygen saturation and positronium decay rate. They calculated that, in a clinical PET scanner, an acquisition time of around 30 min would yield enough measurements to distinguish a hypoxic tumour from normally oxygenated healthy tissue.
In terms of detector hardware, current PET devices are already suited to this task, although the researchers say that they will need new timer systems and software.
“The performance required by the new timers is comparable to those already used in conventional PET,” says Shibuya. “Therefore, I think it will not be difficult for medical device manufacturers to install them.”
Finding the right radiopharmaceutical agent might be more challenging, however. Whereas PET imaging techniques employ pure positron emitters, positronium imaging can only work if, like 22Na, the radioisotope emits a gamma photon and a positron simultaneously. Unfortunately, the 2.6-year half-life of 22Na makes it unsuitable for the clinic, for which sources with half-lives on the order of hours or days are required.
The formation of nanometre-sized crystals can – even in low concentrations – temporarily change the viscosity of magma and lead to violent volcanic eruptions. That is the conclusion of geophysicists in the UK, Germany and France, who say that their research helps to explain how otherwise calm and predictable volcanoes can turn unexpectedly explosive.
There are essentially two types of volcanic eruption: explosive and effusive. In the former, viscous molten rock with a high silica content tends to trap volcanic gases more readily. This increases the pressure on the magma column such that it can ultimately lead to an explosion. Basaltic magmas, meanwhile, have a low silica content and tend to be runnier. This usually allow gas to escape gently, leading to effusive eruptions that produce lava domes and flows of the kind seen on Hawaii.
This distinction is not clear cut, however, and basaltic magmas can sometimes unexpectedly produce explosive events. This happened at New Zealand’s Mount Tarawera in June 1886, which destroyed ten Māori settlements and killed around 120 people.
Explosive fragmentation
A common explanation for this unexpected explosivity is the growth in molten rock of microlites, which are small crystals ranging in size from 1–100 micron. In sufficient volumes, these can lock up magma flow, leading to explosive fragmentation. This occurs when magma flow switches from being a continuous liquid body containing gas and crystals to a turbulent flow dominated by gases that contain fragments of molten rock.
There is one problem with this theory, however, in that many such unexpected explosions (including Indonesia’s Mount Tambora in 1815 – the most powerful eruption in human history) occurred even though their microlite concentrations were seemingly not at a sufficiently high level of around 30% total volume.
In a new study, volcanologist Danilo Di Genova of the University of Bristol and colleagues considered the effects of nanolites, which are smaller crystals that are the precursors of microlites. Using scanning electron transmission microscopy and Raman spectroscopy, the team revealed the presence of previously-unidentified nanolites (20–50 nm in size) in ash samples from three low-viscosity eruptions: Italy’s Mount Etna (122 BC); the Colli Albani volcano (about 37,000 years ago); and Mount Tambora.
Rapid cooling
The team then created their own nanolites in the lab by melting and then cooling volcanic rock samples. Synchrotron-based X-ray spectroscopy revealed how nanolites grow under rapid cooling regimes of around 10–20 degrees per second. This cooling is typical of that experienced by magma that is ascending quickly at metres-per-second speeds. Further experiments with a synthetic magma, alongside modelling, suggest that the after-effect of so-called undercooling (where a magma, unable to solidify thanks to its dissolved water content, rapidly crystallizes once it degasses) provides the ideal scenario for nanolite growth.
Furthermore, the team found that even in low nanolite concentrations of 5% by volume, the spacing between each nanolite can be very small. “It is already accepted that even a low viscosity melt struggles to flow if it becomes locked by a network of crystals, but this usually requires a fraction of well over 30%,” Di Genova explained. “Nanolites also have a propensity to agglomerate together and form new and larger solid objects. During this process, volcanic liquid remains trapped inside these agglomerates to effectively increase the fraction of solids in the magma.”
Together, he explained, these processes account for how nanolites can increase magma viscosity for a short time as they form – leading to the previously inexplicable “sudden switch in behaviour,” that can cause calm volcanoes “to occasionally present us with a deadly surprise”.
“Nanoparticles suspended in a very low viscosity fluid appear to have a great effect increasing the bulk viscosity of the suspension even at low concentrations – a factor which could, if confirmed for magma viscosity, enhance explosivity during eruptions,” comments Francisco Cáceres, an experimental volcanologist from the Ludwig-Maximilians-University of Munich. He adds, “This work opens a window to new research that needs to be performed on natural magma, in order to test this claim”.
University of Cambridge volcanologist Marie Edmonds adds, “This work will have very important implications for our understanding of basaltic explosive eruptions but also more broadly for magma rheology, nucleation and growth of crystals and bubbles; and the strength of magma under shear. It will be important in the future to understand whether wet magmas – those containing lots of water, such as in subduction zones – behave in the same way as those in the experiments, which were dry.”
With their initial study complete, the researchers are moving to model the explosive impact of nanolites on real-life low-silica volcanic settings. When travel restrictions lift after the pandemic, they will be hunting for more examples of nanolites in volcanic rock from across the globe.
Our everyday experience shows that the macroscopic world is different from the quantum one. Unlike quantum particles, the objects in our daily existence do not, for example, exist in a superposition of different states. Traditionally, physicists explain the transition between the two worlds by saying that the quantum superposition principle, which is the building block of quantum theory, breaks down when measurements are performed. The wavefunction of this system is then said to “collapse” with the measurement.
Why such a collapse should happen is still unclear, but one model – developed by the mathematical physicist Roger Penrose, and drawing on earlier work by Lajos Diósi – suggests that gravity might play a role. Researchers in Italy, Germany and Hungary have now set important constraints on this so-called Diósi–Penrose model, in a work that could shed fresh light on a long-standing puzzle in quantum theory: why don’t the inherent properties of microscopic systems carry over to macroscopic ones?
Gravity-related wavefunction collapse
In 1996, Penrose suggested that the collapse of quantum superpositions might be caused by the curvature of space–time – that is, by gravity. The effects of gravitation, he reasoned, are negligible at the level of atoms, but increase dramatically at the level of macroscopic objects. Penrose also provided a formula to compute the decay of the superposition, using methods that were similar to Diósi’s earlier work.
In this Diósi–Penrose (DP) model, the gravity-related wavefunction collapse, which depends on the effective size of the mass density of particles in the superposition, induces a random motion of the particles. When the particles are charged (like protons and electrons), this “jitter” produces a characteristic and very faint emission of electromagnetic radiation.
A team of researchers led by Angelo Bassi of the University of Trieste has now computed the rate at which this radiation is emitted by solving the main equation of the DP model. They did this by calculating the number of photons emitted per unit time and unit frequency integrated over all spatial directions in the wavelength range λ∈ of 10−5 to 10−1 nm, which corresponds to energies E∈ of 10–105 keV. In a further development, a team of experimentalists led by Catalina Curceanu and Matthias Laubenstein from the INFN and Kristian Piscicchia from the Enrico Fermi Research Center, both in Italy, went on to measure this calculated radiation rate in an experiment at the INFN-LNGS Gran Sasso underground laboratory in Assergi.
Dedicated underground experiment
According to study lead author Sandro Donadi of the Frankfurt Institute for Advanced Studies, the experiment was designed to be sensitive to the faint X- and gamma-ray radiation that the DP model predicts. To this end, the researchers used a high-purity germanium detector to measure the radiation spectrum at the point at which theory predicts it should be enhanced. They also constructed their whole setup using materials with very low radioactivity and enveloped it in a complex system of shielding. Finally, they carried out their experiments in an underground facility specially built to have low background radioactivity. “All this effort was intended to minimize background noise sources that can mimic the collapse-related radiation that we are looking for,” Donadi says.
In addition to these measures, the researchers carefully characterized the background spectrum produced by known natural contamination sources that cannot be eliminated. By combining these experimental precautions with refined theoretical and statistical analyses of their data, they were able to set a lower bound on the effective size of the nuclei’s mass density. This lower bound is equivalent to about 1 Å, or approximately three orders of magnitude larger than previously reported bounds.
Not ruled out yet
“Our result suggests that more work needs to be done to relate gravity to wavefunction collapse, since it excludes the most natural (‘parameter-free’) version of the DP model,” Donadi tells Physics World. “However, it would be premature to dismiss gravity’s role at this stage.”
Donadi adds that Diósi and Penrose put forward good reasons for believing that there is a tension between the quantum superposition principle and general relativity. “We now intend to investigate possible solutions to this by developing refined wavefunction collapse models based on our recent findings,” he says.
In the short term, the researchers plan to apply a similar type of analysis to other collapse theories, such as the GRW model put forward by Giancarlo Ghirardi, Alberto Rimini and Tullio Weber, and another model known as Continuous Spontaneous Localization (CSL). These other models are more difficult to falsify, Donadi explains, because of the different mathematical relationships between the models’ parameters and the expected radiation emission rate.
“These studies will ultimately push us in the direction of realizing new, more sensitive experimental setups based on new vanguard radiation detectors, data acquisition systems and analyses methods,” Curceanu adds.
The upper limit on the speed of sound in solids and liquids depends on just two dimensionless quantities – the fine structure constant and the proton-to-electron mass ratio. That is the surprising conclusion of physicists in the UK and Russia, who calculate that the speed limit is twice that of the highest speed of sound measured to date.
Sound propagates as a series of compressions and rarefactions in an elastic medium, with its speed varying significantly from one material to another. Typically, sound is slowest in gases, higher in liquids and higher still in solids. In air at ambient conditions sound travels at about 340 m/s, while in water it reaches about 1500 m/s and in iron more than 5000 m/s.
These differences are due to the way that a passing wave disturbs atoms and molecules. Thought of as hard spheres linked to one another with springs, the particles are knocked forward by their neighbours in the direction of sound propagation and in turn go on to nudge other neighbouring particles ahead of them. But this transmission is delayed by inertia, meaning that waves move faster when the particles are less massive.
Stiffer links mean less delay
However, stiffer links also mean less delay – each particle having to move less before it triggers the movement of its neighbour. This is why sound travels faster through iron than it does through water, for example.
Expressed mathematically, sound’s longitudinal speed in a material is equal to the square root of that material’s elastic modulus – which quantifies its resistance to compression – divided by its density.
In the latest research, Kostya Trachenko of Queen Mary University of London and colleagues at the University of Cambridge and the Russian Academy of Sciences’ Institute for High Pressure Physics set out to recast this formula in terms of fundamental constants. Their first step was to link a material’s bulk modulus with the energy that binds its atoms together, given that greater stiffness implies a higher binding energy. They then assumed that the latter term could be equated to the Rydberg energy, which is the characteristic binding energy in condensed matter.
Eye-catching formula
The resulting formula proved eye-catching, particularly when expressed in terms of the fine-structure constant – which sets the strength of fundamental electromagnetic interactions – and then written specifically as an upper limit on the speed of sound. The upper limit comes about when the mass of the atoms in question is the lowest mass possible – that of the hydrogen atom. In that case, the velocity of sound can simply be expressed in terms of the proton-to-electron mass ratio (inverted, halved and square-rooted), the fine-structure constant and the speed of light in a vacuum.
Inserting the relevant numbers into their formula, the researchers worked out that the highest possible speed of sound in solids and liquids (exposed to moderate pressures) comes in at a little over 36,000 m/s. That is still nearly 10,000 times slower than light’s upper limit but about twice as high as the fastest ever recorded sound wave – which stands at around 18,000 m/s and obtained in (very stiff) diamond.
To establish whether their equation is broadly in line with measured velocities, the researchers compared its predictions against the experimentally obtained speed of sound in 36 different elemental solids. To do so they used a log-log plot to show how these speeds vary with the solids’ atomic mass and on the same graph drew the straight sloping line generated by their equation – which terminates at the high end with hydrogen. They conclude that the experimental data points more or less follow their sloping line, even if a coefficient they dropped to simplify their equation means that not all are that close.
Metallic hydrogen
As an additional check on their work, Trachenko’s team used density functional theory to calculate the speed of sound through metallic hydrogen from first principles. When exposed to very high pressures, hydrogen becomes a molecular solid, and at pressures above about 400 GPa it is predicted then to become an atomic metal. It is this metallic state that should hold the speed record. Modelling hydrogen in these conditions, they found that sound should propagate at up to 35,000 m/s – faster than in any other material, but still below their upper limit.
This work follows similar research that Trachenko and fellow group member Vadim Brazhkin published earlier this year, which revealed a universal lower limit on viscosity expressed in terms of the proton-to-electron mass ratio and Planck’s constant. By incorporating the fine-structure constant, they ended up with an expression containing two fundamental constants whose fine tuning yields a stable proton and also allows stars to ignite and produce heavier elements, enabling, as they put it, “the existence of solids and liquids where sound can propagate”.
Kamran Behnia of PSL University in Paris describes the work as “simultaneously simple and deep”. He says he had not expected to be able to work out the rough speed of sound in a particular material “by hand-waving arguments” – particularly, he adds, when the resulting formula involves only fundamental constants. As he points out, quantum mechanics is not needed to explain the propagation of sound – even if quantum mechanics is what makes the solid state possible. “This is why the main message of this paper comes as a surprise,” he says.
I grew up in Canada, which has two things in abundance: electric clothes dryers and snow. So, I was immediately drawn to a paper in PLOS ONE by Kirsten Kapp and Rachael Miller that quantifies the number of microfibres emitted by tumble dryers by extracting fibres from snow nearby to dryer exhaust vents. Their experiments were done in two snowy locations – Idaho and Vermont – using two different models of domestic dryer.
Kapp and Miller began their experiment with two pink polyester fleece blankets that were soaked and then placed in the dryers. The blankets were not washed to avoid introducing extra variables to the experiment. After the blankets were dried, the surface layer of snow was gathered from a dozen or so different locations near the dryer vents. The snow was then melted, and the fibres counted.
The duo found that the greatest concentration of microfibres were within about 1.5 m of the vents. However, several locations about 9 m from the vents also had significant numbers of microfibres.
There is growing concern about microfibre pollution – and it is already well known that wastewater from clothes washing machines is a major source of microfibres in the environment. “The goal of this study was to break the story about dryers and microfibre emissions, rather than answer all the questions,” says Miller. “Our intent is that these data now inspire researchers to investigate exactly what factors concerning dryer design, installation and settings increase or reduce shedding, inspire the white goods industry to include dryers in their discussions about microfibre pollution, and educate consumers about the potential effects that dryer use has on our environment.”
“I find it hard to think of anything that is more relevant to understanding how the world works than differential equations.” So says the physicist Sabine Hossenfelder in the introduction to her latest video “What are differential equations and how do they work?” which you can watch above.
Although Isaac Newton developed calculus – which underpins differential equations – our modern form of calculus is largely absent from his ground-breaking Mathematical Principles of Natural Philosophy. A rare first edition of the book printed in 1729 has been sold at auction for £22,000. Incredibly, the two-volume edition was discovered during a lockdown clear-out at a house in South Wales.
“It was on their shelves and they were looking for things to sell while they were in lockdown,” says Chris Albury of Dominic Winter Auctioneers in Cirencester, who according to the BBC “almost fell off” his chair when he realized it was “the greatest work of science in the English language”.
Autonomous vehicles (AVs) are supposed to be the future. So far, though, much of the work involved in building that future has focused on places like California and Arizona, where rain is rare and roads are wide and spacious. How, then, will AVs cope in the harsher conditions of northern Europe?
For Patrick Sachon, the question is more than hypothetical. As a scientist working in business development at the Met Office, Sachon is trying to understand whether the sensors that AVs use to navigate can do their job adequately in the UK’s famously unsettled weather. If the answer is “no” – or, more likely, a provisional “yes” that depends on weather conditions remaining within a certain safety envelope – then he and his colleagues aim to suggest improvements and explain to regulators what safe use looks like.
Speaking as part of a webinar series organized by the National Physical Laboratory, Sachon noted that the Met Office (the UK’s national weather service) has collaborated with partners in the road transport sector for years. “We’re always working on understanding what roads will be like to drive on,” he explains. While the Met Office’s publicly-funded forecasts are (usually) good enough for citizens to decide whether they need to pack an umbrella, the Highways Agency and local authorities need far more detailed information to help them deploy assets such as gritters, snowploughs and maintenance crews in an optimal way.
The Met Office’s experience of providing such information is useful for AVs, Sachon says, because such vehicles may require similar micro-forecasts to operate safely. As he puts, it, the “structure of precipitation” varies a lot during thunderstorms or heavy showers. While some areas experience high rain rates, others nearby remain relatively dry. And if AV sensors work fine in ordinary rain, but not when it’s chucking it down, then the basic, publicly-available weather forecasts won’t be enough to keep motorists safe.
As Sachon sees it, the key will be to develop vehicle sensors that work alongside weather sensors. The joint information from both types of sensors will then enable regulators and manufacturers to determine exactly how well – and how safely – an AV’s sensors will handle a given set of conditions. Such information will be helpful to the Met Office, too, Sachon adds, because vehicle-based weather sensors are a bit like miniature weather stations. For example, if Met Office scientists see that an AV’s windscreen wipers are on, they’ll know that moisture in the atmosphere is making it down to ground level – something that weather satellites don’t necessarily show. The scientists can then modify their forecasts accordingly, to more accurately reflect local driving conditions.
When asked which weather conditions are most challenging to AV sensors, Sachon began by reminding listeners that human-operated vehicles have drawbacks of their own. “[Artificial intelligence] won’t do the stupid things we do when we drive,” he said. “You can eliminate quite a lot of risks.” With that caveat, though, he acknowledged that AV sensors struggle when precipitation changes phase. While rain and snow are not so difficult on their own, he said, “the transition between phases can be quite a problem”.
Since the UK’s weather often hovers between fog, mist, drizzle, rain and snow, would-be AV purchasers in these islands may be in for a long wait. Another challenge is that hyper-local weather forecasts are not cheap, and it is far from clear who within the AV sector would pay for them. The equivalent service in aviation, Sachon notes, is paid for via a levy on all airlines that fly through UK airspace, with a small fraction of that levy going to the Met Office. In ground transport, however, the focus has traditionally been around keeping roads clear of ice and snow, and the burden of payment has fallen on local authorities rather than individual motorists. Whether any government – local or national – will take on those costs for the sake of an (initially) small number of AV users is, he suggests, an open question.
Simulated CT images of a COVID-19 XCAT phantom at three different radiation dose levels, representing (top left) 50, (top right) 25 and (lower left) 5 mAs settings. The lower right image shows a chest radiographic image of the same model. (Courtesy: American Roentgen Ray Society, American Journal of Roentgenology)
Medical imaging devices and methods are constantly evolving to meet rapidly changing clinical needs. CT and chest radiography, for example, could provide a means to detect lung abnormalities related to COVID-19, but the imaging parameters required to differentiate COVID-19 still need optimization. Clinical imaging trials, however, are often expensive, time consuming and can lack ground truth. Virtual, or in silico, imaging trials offer an effective alternative by simulating patients and imaging systems.
A group of researchers from Duke University took the first steps towards applying this approach to the current pandemic by developing the first computational models of patients with COVID-19. In addition, they showed how these virtual models can be adapted and used together with imaging simulators for COVID-19 imaging studies. Their proof-of-principle result, published in the American Journal of Roentgenology, paves the way towards effective and inexpensive ways of assessing and optimizing imaging protocols for rapid COVID-19 diagnosis.
Virtual COVID-19 patients…
Virtual imaging trials require accurate models of target subjects, also known as phantoms. For this study, the group modelled three distinctive features of the anatomy of a COVID-19 patient: the body; the morphological features of the abnormality; and the texture and material of the abnormality. For the first part, the researchers used the 4D extended cardiac-torso (XCAT) model developed at Duke University. These phantoms are constructed from patient data and include thousands of body structures, as well as anatomically informed mathematical models of texture and tissue-material properties.
To model the specific abnormalities found in SARS-COV-2 patients, the group manually delineated the unique morphological features characteristic of COVID-19 in CT imaging data from 20 patients with confirmed infections. They then incorporated these features, known as ground-glass opacities, into the XCAT models. To make their phantom as realistic as possible, the researchers also modified the texture of each segmented abnormality to that of a material filled with fluid to match the observed pathologies.
… virtually imaged
The group produced three COVID-19 computational models with abnormalities of different shapes and locations within the lungs. The researchers then used these models together with a validated radiographic simulator (DukeSim) to obtain clinically realistic virtual CT and chest-radiography images. “Subjectively,” the authors explain, “the simulated abnormalities were realistic in terms of shape and texture.”
First author Ehsan Abadi (third row, centre) together with the research group (Courtesy: Ehsan Abadi, Department of Radiology, Duke University)
“We have developed strategies to adapt and use virtual imaging trials for imaging studies of COVID-19,” first author Ehsan Abadi concludes. “This will provide the foundation for the effective assessment and optimization of CT and radiography acquisitions and analysis tools to manage the COVID-19 pandemic.” Future work will focus on modelling different stages and manifestations of the disease, with the aim of optimizing screening and follow-up of patients.
Physicists in Germany say they have made the world’s most precise measurement of the deuteron mass by comparing it to the mass of the carbon-12 nucleus. The new work, which was carried out by confining deuterons (which are nuclei of deuterium, or “heavy” hydrogen) and carbon-12 nuclei with strong magnetic and electric fields, provides a crucial independent cross-check with previous measurements that yielded inconsistent values.
Knowing the precise masses of simple atomic nuclei such as hydrogen, its isotopes deuterium and tritium, and the molecular hydrogen ions H2+ and HD+ (a proton and a deuteron, bound by an electron) is crucial for testing fundamental physics theories such as quantum electrodynamics. The mass of the deuteron can also be used to derive the mass of the neutron, which has implications for metrology as well as for atomic, molecular and neutrino physics.
To make these precise measurements, physicists often turn to Penning traps, which use extremely strong magnetic and electric fields to trap charged particles such as single deuterons and other simple ions. Once trapped, the particles oscillate at a particular (cyclotron) frequency that depends on their mass, with heavier particles oscillating more slowly than lighter ones. Hence, if the oscillation frequencies of two different ions are measured in the same trap, one after the other, the ratio of their masses can be calculated to a high precision (around one part in 8.5 x 10-12).
A cryogenic Penning trap mass spectrometer
In the new work, a team from the Max Planck Institute for Nuclear Physics, the Johannes Gutenberg University, the GSI Helmholtz Centre for Heavy Ion Research and the Helmholtz Institute in Mainz used a special cryogenic mass spectrometer that is dedicated to light-ion mass measurements. The setup of this spectrometer, which the researchers dubbed LIONTRAP, consists of a stack of Penning traps. This stack includes a highly optimized seven-electrode precision trap and two adjacent storage traps that sit within the homogeneous magnetic field of a superconducting 3.8 Tesla magnet. The entire set-up is also kept in a near-perfect vacuum (better than 10-17 mbar) at a temperature of about 4K.
The researchers placed a deuteron in the storage trap before transferring it to the precision trap. There, they determined its oscillation frequency with high precision by measuring the tiny alternating currents (known as image currents) induced at the inner surfaces of the trap electrodes by the charge of the moving ion. Finally, they compared this frequency measurement with a similar one made on a carbon-12 ion (12C6+) in the same apparatus.
Adjustable superconducting electromagnetic coil
Study lead author Sascha Rau explains that the team chose 12C6+ because it serves as the mass standard for atoms – meaning that its mass, by definition, is equal to 12 atomic mass units. Hence, measuring the ratio of the deuteron and 12C6+ ion oscillation frequencies gives the mass of the deuteron directly, in atomic mass units.
The precision of previous measurements made using this method was limited by deviations of the trap’s magnetic field from its ideal form. The German team overcame this problem by using an adjustable superconducting electromagnetic coil that is directly wound around the trap chamber, so that the trap can operate without disturbing the magnets in the other coils too much. This set-up allowed them to measure the magnetic field deviations and supress them by a factor of 100. In this way, they determined that the mass of the deuteron is 2.013553212535(17) atomic units, where the number in brackets indicates the statistical uncertainty of the last digits. The mass of the hydrogen molecular ion HD+, determined by the same method, is 3.021378241561(61) atomic units.
The new value for the mass of the deuteron is significantly smaller than the tabulated reference value. To validate their result, Rau and colleagues therefore also calculated the mass of HD+ using masses of the proton and the electron they previously measured. The new result for the deuteron is in excellent agreement with these values, they say, which suggests that the reference value for the deuteron needs to be corrected. The result, which is published in Nature, also agrees with a recent and precise measurement of the deuteron-to- proton mass ratio made by another group.
Nonlinear narrative Rosemary Teague left her PhD part-way through to investigate other careers in physics. (Courtesy: IOP/Lucy Kinghan)
Rosemary Teague is a trainee teacher on the physics PGCE programme at University College London
“What will you do with a physics degree?” asked my aunts and uncles when I excitedly told them I was going to study the subject at Imperial College in London, as an undergraduate. Well, I thought, I’m going to be a physicist. Back then, my understanding of what a career in physics entailed was not far beyond that of my family – I knew it as an academic pursuit and pictured myself, 30 years on, at the forefront of discoveries in renewable energy. Really, though, I wasn’t thinking that far ahead. I was thinking about the first four years: leaving home, making friends, and learning about the laws of the universe.
And that’s what I did. My undergrad years were filled with lectures, labs and dancing. As a self-proclaimed “country bumpkin” who grew up in Gloucestershire, I took to London life relatively well, making life-long friends and taking every opportunity the city presented. I spent my summer holidays in a similar fashion, either travelling or earning money to travel by teaching at summer camps. I spent my third year in Valencia, Spain, as part of the Erasmus programme, and surprised myself by choosing a computational research project.
At this point I noticed my friends and peers were already considering life after our degrees. We were on a four-year programme, so the upcoming summer was our last chance to get some work experience before applying for graduate jobs. I didn’t spend much time thinking about this myself as I was confident that I wanted to stay in academia, and follow that linear path. So, while my friends were off earning big bucks in the City, I crashed with a school friend for a couple of months and worked in a practical lab at the University of Bristol. While the city and research group were lovely, it confirmed that I’m more suited to a computer than optical mounts.
Always one to be prepared, I started looking into PhD programmes at the beginning of my final year. I soon discovered centres for doctoral training (CDTs), which tend to have a more interdisciplinary focus, and incorporate a Master’s qualification into a full PhD programme. CDTs also complement study with opportunities to develop outreach and other career skills. Among standard PhD applications, I applied to several CDTs including one on the theory and simulation of materials at Imperial. When I accepted this offer, I was excited about both the course and staying in London to take the next step in my personal life by moving in with my partner – he would be starting a more traditional PhD in ultrafast lasers, also at Imperial.
I struggled with impostor syndrome and would work long hours to prove that I deserved the place
Not long after starting the MSc, reality hit and my life started to get a bit less rosy. I struggled with impostor syndrome, as many postgraduate students do, and would work long hours to prove that I deserved the place. I also, crucially, wasn’t giving myself the time or space to grieve a close and unexpected loss in my partner’s family. I started to have panic attacks and increasingly depressive thoughts. Luckily, I have a strong network of family and friends who encouraged me to take a break, despite warnings from the university of the workload that would be waiting on my return. I escaped to my parents’ home, was prescribed antidepressants and arranged to start cognitive behavioural therapy (CBT) sessions back in London. These had an incredible impact and gave me a new perspective on my work–life balance.
I realized how much I hated the pressure and competition around working late and, instead of dismissing advice, I heeded it, making the decision to leave university before starting the PhD part of the course. This was an incredibly tough choice – while easier than if I was on a traditional PhD path, without my MSc graduation as an obtainable “end-point”, it felt like I was giving up, quitting on academia and failing to meet my ambitions.
So what then? I was still living in London and paying rent, so I desperately needed an income. I didn’t have the luxury of time to consider what sort of job I wanted and instead applied for anything and everything I could. A recruiter got in touch via LinkedIn and arranged an interview at Ocado Technology, the online supermarket. Two and a half months after handing in my MSc dissertation, I started working for Ocado as a cloud infrastructure engineer. It was a good job that paid well, but I wasn’t passionate about it and the hour-and-a-quarter commute quickly took its toll.
Exploring her options To help her decide what career pathway to follow, Rosemary Teague put together some data on her current expertise, as well as the proficiencies she would like to develop. She then matched these to three possible careers, with teaching emerging as the best path. (Click to enlarge)
After six months, in a move that I’m still not sure my parents understood, I sacrificed 30% of my salary for a job in the outreach and engagement department at the Institute of Physics [which publishes Physics World]. This was in May 2019, and it was the right decision for me – I hadn’t been this happy for a long time. More recently I have been considering what it is that I want out of a career. I know that collaborating and working with others is important to me, and whatever I do must be physics-focused.
By assessing my own skills, and thinking critically about what I truly enjoy, I narrowed down my options to a future career in one of three areas: public outreach and engagement; teaching; or academic publishing. Once I considered the skills these roles need and the opportunities they could offer (see graphic above) I realized that for me right now, a career in teaching is the most rewarding path, helping others to see the joys of physics while also developing myself. With this in mind, I applied for teacher training courses, and am incredibly excited as I began my PGCE last month.
By sharing my story, I hope it will show that there are many ways to start a career and that it’s okay to take time for yourself. My path recently has been far from linear and strongly supported by people I love. I am looking forward to seeing how it twists and turns in the years to come.
On the right track Amber Yallop needed to find a university set-up that worked for her circumstances. (Courtesy: IOP)
Amber Yallop is a new trainee on a graduate scheme with MBDA, after completing her BSc in physics at the University of East Anglia, UK
My degree pathway was not the most traditional or straightforward. It began in September 2016 when I left my hometown of Norwich, UK, to follow my dream of a degree in astrophysics, which has always been my specific area of interest. Although I loved my course, I struggled in the first few months, due to some personal circumstances that were further exacerbated by living away from home. By December 2016 I made the very difficult decision to drop out of my course and return home.
At this point I was unsure if I would ever return to higher education. I took a job at a local high school, where I supported students with special educational needs. The role was challenging, but also incredibly rewarding, and I thoroughly enjoyed my seven months at the school. During my time there, however, I heard that a local university – the University of East Anglia (UEA) – was going to launch a new physics degree, starting in September 2017.
I had previously considered teaching as a career, but in my time working at the school I realized that I much preferred working on a personal, one-to-one basis with the students and I wouldn’t get the same level of involvement and intervention as a class teacher. While I loved the job, I still had high aspirations and the physics course at UEA came along at the right time and in the right place. I chose to take the opportunity, and applied to the course. I was offered a place and decided to give university a second attempt, but this time living at home. This worked much better for me and despite further setbacks during my three years, including being diagnosed with epilepsy, I managed to stay on track and obtain a first-class degree.
I decided to give university a second attempt, but this time living at home. This worked much better for me
During my BSc, I volunteered at Institute of Physics outreach events, including a day at the Royal Norfolk Show, demonstrating and explaining hands-on physics experiments to the public, and also took part in the Norwich Science Festival. In my second year at university, I successfully applied to be the IOP campus ambassador for East Anglia. This role massively improved my confidence and organizational skills, including organizing events like a public lecture with BBC Stargazing Live’s Mark Thompson. This was a very popular event with both students and members of the public.
I was also lucky enough to represent East Anglia on the IOP’s Student Community Panel for the final two years of my degree. This involved travelling across the UK and Ireland to meet other regional representatives. We addressed issues facing physics students across the country, and supported the IOP with student events. My proudest achievement in this role was raising over £1000 in sponsorship for PLANCKS 2020, an annual physics competition for students around the world, hosted by the International Association of Physics Students. Each country holds its own preliminary round, with the winning teams progressing to the final competition, which takes place in a different country each year. The 2020 final was scheduled to be held in May in London, but was postponed as a result of COVID-19.
I have recently relocated to Stevenage to begin a “Hardware in the Loop” graduate scheme with the European defence company MBDA. This is a two-year programme with a permanent position on completion. It includes training, development, outreach and extracurricular opportunities. I am looking forward to applying the knowledge and skills learnt through my degree to the workplace, and I’m excited to see where this next chapter of my life will take me.
How the Institute of Physics can help you
Become a part of a vibrant student community and participate in a host of activities to develop your skills: iop.org/student-community
Network with physicists through our special interest groups – including our Early Career Members’ Group – and our local nations and branches network: iop.org/groups and iop.org/branches
Gain an advantage in the job market by attending our career-themed webinars iop.org/events and make use of the IOP Career Development Hub, which will support you in writing your CV, practising for interviews, delivering presentations and effective time management, and providing access to many other useful resources to support your future career choices: iop.org/member-services
Following graduation, join the Member grade and use the designatory letters MInstP after your name, to demonstrate your commitment and professionalism
Einstein’s general theory of relativity has triumphed once again after being tested in the strongest gravitational field to date. Dimitrios Psaltis at the University of Arizona and members of the Event Horizon Telescope (EHT) collaboration did their analysis using recent images of M87*, which is a supermassive black hole at the centre of a nearby galaxy. Their results set the stage for even more stringent tests of general relativity in the near future.
For over a century, general relativity has had an excellent track record in explaining observations of the universe. All the same, the theory leaves some big questions unanswered: including how to unify gravity with quantum mechanics and the surprise discovery in 1998 of the universe’s accelerating expansion. As a result, physicists are looking for subtle flaws in general relativity that could lead to the development of a more complete theory.
One way to study the theory’s limitations is to search for discrepancies in how it describes distortions of spacetime by the gravitational fields of massive objects. Initially, these tests used objects in the solar system – famously the motion of Mercury. More recently, gravitational waves created by merging black holes and observed by the LIGO–Virgo collaboration have enabled tests in the gravitational fields of objects as heavy as 150 solar masses. Yet despite the increasingly rigorous constraints imposed by these results, cracks have yet to show in Einstein’s theory.
Billions of Suns
M87* has a mass of about 3.5–6.6 billion Suns and its gravitational field is the largest ever used to test general relativity. In 2019, the EHT released its celebrated image of the shadow of M87* – a dark silhouette, surrounded by bright emission from hot plasma. General relativity provides a precise prediction of the size of the shadow and in the case of M87*, the observed size is within 17% of general relativity’s prediction.
It is possible, therefore that a modified version of general relativity could do a better job at predicting the size of the M87* shadow. To test this, Psaltis and colleagues considered alternative models of gravity that modify the general theory of relativity. They focussed on parameters of these alternative models that affected the models’ predictions of the size of the shadow.
By comparing these predictions to the observed shadow, they we able to constrain modifications to Einstein’s theory by a factor of almost 500 compared with earlier solar system tests. The new constraints are similar to those derived from gravitational wave observations. The EHT collaboration now hopes to impose even stricter limits by imaging the shadow of Sagittarius A* – the supermassive black hole at the centre of our own galaxy, whose mass is far more precisely defined than M87*.
