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The long battle to reach for the stars: the life and times of Cecilia Payne-Gaposchkin

“I have reached a height that I should never, in my wildest dreams, have predicted 50 years ago. It has been a case of survival, not of the fittest, but of the most doggedly persistent,” remarked Cecilia Payne-Gaposchkin, before her death in 1979. The trailblazing astronomer had many firsts to her name – she discovered that stars are mainly made up of hydrogen and helium.

In 1925 she became the first person to earn a PhD in astronomy from Radcliffe College at Harvard University; and she was also the first woman to become a professor at Harvard and then to head its astronomy department. But despite her many achievements, she was frequently overlooked, with her work not receiving the attention it warranted during her time; and her significant contributions often omitted from the annals of scientific history.

Which is why it was delightful to see the first full biography outlining her life, work and trials, in What Stars Are Made Of: the Life of Cecilia Payne-Gaposchkin by US author and journalist Donovan Moore. Extremely well-researched, detailed and engaging, with a perfect mix of history, anecdotes and scientific explanations, Moore brings to life the bright light that was Payne-Gaposchkin.

Born in Wendover, England, in 1900, she was a curious child who already bore the makings of a scientist. It’s interesting to note that Payne-Gaposchkin had a number of influential women in her early life who moulded her views and acted as the mentors and role models that would shape her career – from her mother, to her school teachers to her botany professor Agnes Arber at Cambridge. It was perhaps the support and encouragement that she received during these early years that helped her face the many hurdles in her later life, especially when it came to the extreme bias she faced as a woman.

Despite winning a scholarship to study botany, physics and chemistry at Newnham College, University of Cambridge, she was not awarded a degree, as she was a woman. Payne-Gaposchkin realized that her only option to make a career as an astronomer was to leave the UK for the US. She moved to Harvard College Observatory in 1923, to join a graduate programme in astronomy, where her 1925 thesis was famously described by astronomer Otto Struve as “the most brilliant PhD thesis ever written in astronomy”.

Her research, that looked into the abundance of chemical elements from stellar spectra, would begin a revolution in astrophysics – but she would have to fight hard for her work to be trusted (she was told by Henry Russell, director of the Princeton Observatory, that her findings were incorrect, only for Russell to later take credit for the very same research).

Despite ultimately achieving great success in her academic career, What Stars Are Made Of highlights the incredible struggles that Payne-Gaposchkin faced at each and every step – battles that she had to fight mostly single-handed. While her story is inspiring and encouraging, I can only hope that bright young women today do not have the same mountains to climb, as they reach for the stars.

  • 2020 Harvard University Press 320pp £26.95hb

Decommissioned Namibian telescope to be brought back to life

A defunct telescope at the High Energy Stereoscopic System (HESS) site in Namibia is to be refurbished and brought back online to discover and monitor variable and transient sources in the sky. The telescope once belonged to the Robotic Optical Transient Search Experiment (ROTSE) – a network of four now-decommissioned robotic telescopes around the world. The decision to refurbish the telescope was approved in October by the African Astronomical Society (AfAS) science committee.  

The ROTSE project was originally funded by NASA but when the funding ran out for operational costs, the telescopes were deactivated. They were designed to continuously observe the celestial sphere and to respond to alerts from spacecraft of transient events, particularly gamma-ray bursts, novae and supernovae.

As well as Namibia, the other locations include the Siding Springs Observatory near Coonabarabran, Australia, the Tübitak National Observatory in Antalya, Turkey, and the McDonald Observatory in Texas.  

David Buckley, chair of the AfAS science committee says that the refurbishment will be paid for by the AfAS science committee, adding that the South African Astronomical Observatory will provide an initial CCD camera and may also assist with technical assistance. “The major costs of operating the facility are electrical power and Internet connectivity, both of which will be supplied by the HESS consortium,” he adds. 

Once brought back online in the coming months, the refurbished telescope will conduct sky surveys and follow-up observations to allow AfAS astronomers to discover or monitor variable and transient sources.

“This includes the discovery and study of remote solar system bodies, such as trans-Neptunian objects, as well as search for extrasolar planet transits,” says Buckley, who adds that the telescope also provides African participation in global collaborations focusing on the “new and vibrant field” of multimessenger astronomy. 

Carl Akerlof, an astronomer at the University of Michigan in Ann Arbor, Michigan, who initiated and led ROTSE, told Physics World that he is “delighted” by the AfAs’s decision to bring the telescope back online.

“The main question will be selecting someone with the skills to keep it operable,” says Akerlof, who believes there are opportunities for such a small telescope to capture transient images such as gamma-ray bursts for which it was designed. He also thinks the telescope could be made available to university students by taking advantage of the telescope’s remote computer control system.   

New Oak Ridge centre to boost US isotope production

Construction has begun on a new facility in the US that will produce a wide range of isotopes including those that can be used to make radioisotopes for medical applications. When it fires up in 2026, the Stable Isotope Production and Research Center (SIPRC) at the Oak Ridge National Laboratory (ORNL) in Tennessee is expected to establish the US as a domestic and global supplier of isotopes to industry, government and research institutions.  

ORNL already produces, purifies and ships more than 300 isotopes for medical, research, industrial and space applications. “Providing isotopes that can’t be made anywhere else is central to our identity as a national laboratory,” says director Thomas Zacharia. “We are fortunate to have some of the most talented experts in the world.”  

The current production of isotopes at ORNL is based on three enrichment technologies. SIPRC will house two of those technologies for large-scale production. Yet officials have refused to identify which techniques the new facility will use. Instead, they note only that the new centre will be capable of “simultaneously enriching multiple stable isotopes” from across the periodic table. 

“The mission of the isotope programme is to reduce US dependence on overseas supplies,” says Balendra Sutharshan, ORNL’s associate director for isotope science and engineering. “Some important isotopes come from Russia – the only other country capable of producing radioisotopes in large quantities.”  

Sutharshan says that isotopes that are used for cancer treatment will be “in great demand”. Stable isotopes that SIPRC might produce are needed as target material for such radioisotopes. These include lutetium-177 and carbon-14 as well as technetium-99m. Sutharshan notes that, while other US laboratories can produce some critical isotopes, Oak Ridge currently produces the greatest volume.

“We’ve picked up steam,” he adds, because we recognized that if we close our borders [to overseas suppliers], we cannot perform some of the medical procedures and industrial uses that we use now.”

The Department of Energy (DOE) will provide $75m to support SIPRC, which will occupy about 6000 m2 on ORNL’s campus. This money will come from the $1.5bn Inflation Reduction Act, which is aimed at supporting “critical infrastructure upgrades and other projects at 13 national labs across the country”. 

Other grants awarded from the act are expected to focus strongly on climate-friendly technologies. “The fundamental science and technology development that’s really occurring at the national labs can unlock the clean-energy technologies that we need to tackle climate change,” says US energy secretary Jennifer Granholm.  

Correction 23/11: An earlier version of the article suggested that SIPRC will be able to produce radioisotopes for medical applications. It will instead produce stable isotopes that are then used elsewhere as target material to make radioisotopes.

Work begins on ground station in South Africa to support NASA’s lunar programme

Officials gathered on 8 November to break ground on a new facility in South Africa that will support human spaceflight missions to the Moon and possibly Mars. Matjiesfontein in the central Karoo region of the Western Cape will host the new Lunar Exploration Ground Sites (LEGS) antenna to help ensure near-continuous connectivity between astronauts on NASA’s Artemis spacecraft.  

Located about 240 km north-east of Cape Town, Matjiesfontein has a dry and clear climate. It will host one of three LEGS antennas to provide communications to those working on and around the lunar surface. The other LEGS facilities are in NASA’s White Sands Complex in Las Cruces, New Mexico, and a still-to-be-determined location in Australia. 

Badri Younes, deputy associate administrator for NASA’s pace communications and navigation (SCaN) programme, says that the radio antenna is designed to provide direct-to-Earth communication and navigation for missions operating up to two million kilometres away. Younes says they anticipate the LEGS antenna in South Africa will cost around $25–30m, with the South African National Space Agency supplying the funds to manage and cover operational costs. 

Bill Marinelli, a SCaN development director at NASA who helped to pick the site, says the choice was based on many factors, including how governance and bilateral agreements could boost co-operation and technical expertise. South Africa first started working with the US on space projects in the 1950s and 1960s when it helped gather data for NASA’s Apollo Moon landings.  

A letter of intent has also been signed by NASA and the South African Department of Science and Innovation (DSI) to formalize the space exploration partnership.

“We see this partnership as mutually beneficial,” says Phil Mjwara, director-general of South Africa’s department of science and innovation. “Matjiesfontein ground station will alleviate increased demand for NASA’s Deep Space Network, allowing Artemis to meet its goals and expand our scientific knowledge of key challenges to astronaut health and safety, such as space radiation, isolation and confinement, closed environments, and extreme and prolonged distance from Earth.” 

The construction of the new deep-space ground station is set to begin early next year and aims to be complete before the first crewed Artemis mission to the lunar surface in mid-2025. 

New SI prefixes go large and small, using physics to avoid sauce splatter

Say hello to the first new SI prefixes since 1991. At the humongous end of the scale, ronna and quecca now denote 1027 and 1030, respectively. Apparently, the mass of the Earth is six ronnagrams, or 6 Rg. At the miniscule end of things ronto and quecto denote 10–27 and 10–30, respectively.

The new prefixes were announced today at the General Conference on Weights and Measures, which is being held near Paris. As well as giving nice and simple numbers for the masses of planets, the large prefixes will probably come in handy for describing the vast and growing amount of data that is being created by the Internet. So, get ready for the ronnabyte. Indeed, some people have already been calling 1027 bytes a brontobyte or a hellabyte, much to the horror of metrologists – and this is rumoured to be one of the reasons behind the announcement.

As for the ronto and quecto, it has been suggested that they could be used to describe extremely weak phenomena such as the cosmic microwave background that permeates the universe

Physics of splatter

I do love mayonnaise on a sandwich, but I have learned the hard way to stand well back when squeezing the sauce out of the bottle – especially when the bottle is getting close to being empty. But I must admit that I have never thought about the physics behind sauce splatter – until now.

That’s because Callum Cuttle and Chris MacMinn at the University of Oxford have just published a paper about why a smooth flow of liquid can suddenly become an annoying splatter. The duo did experiments where bubbles of air were injected by a syringe into an oil-filled capillary tube.

“Our experimental system is simple, but it replicates all of the essential parameters of a more complicated system, such as a squeezy ketchup bottle,” explains Cuttle. Pressure was exerted on the oil and bubble mixture, causing it to flow through the tube. At low driving pressures, the mixture flowed smoothly thorough the tube – so no splatter when the bubbly oil emerged. However, at higher pressures, friction inside the tube resists the flow and the air bubbles become compressed – storing up energy, and trouble. When a compressed bubble exits the tube, it can expand rapidly causing splatter to occur.

“Our analysis reveals that the splattering of a ketchup bottle can come down to the finest of margins: squeezing even slightly too hard will produce a splatter rather than a steady stream of liquid,” Cuttle concludes.

The duo describe their findings in a preprint on arXiv.

High-resolution MRI enables direct imaging of neuronal activity

Medical television shows sometimes depict thoughts skipping across the brain as action potentials that ignite like exploding stars. While it looks dramatic and impressive, today’s brain-imaging technologies can’t visualize brain activity so sensitively. A new magnetic resonance imaging (MRI) technique called DIANA – direct imaging of neuronal activity – may get us closer, though.

An alternative to BOLD fMRI

A brain signal begins with an action potential caused by rapid changes in voltage across cellular membranes. Researchers involved in this proof-of-concept study, reported in Science, say that DIANA might measure this neuronal activity by capturing the intracellular voltage of a group of neurons.

The technique could fill a gap in brain imaging. Functional MRI (fMRI), for example, allows us to view neuronal activity in the brain non-invasively by measuring a surrogate – fluctuations in the blood oxygenation level-dependent (BOLD) signal that result from changes in neuronal activity. But the temporal specificity of fMRI is too slow (on the order of 100 ms) to follow neuron activation during cognitive processes. Alternatives such as electroencephalography (EEG) and magnetoencephalography (MEG) have higher temporal resolution, but spatial resolution that’s limited to the centimetre range.

DIANA can achieve both temporal and spatial resolutions on the order of cognitive processes (5 ms and 0.22 mm, respectively) by isolating groups of neuronal signals with a distinct data acquisition scheme, explains senior author Jang-Yeon Park.

Park, a professor of biomedical engineering at Sungkyunkwan University in Korea, was inspired by a study in Nature Methods that described how data can be split to acquire images in pieces for BOLD fMRI. After mulling over the theory, Park says he realized that the method could be adopted to ultrahigh-temporal resolution MRI.

Acquiring images in pieces

Data from MR signals are stored in a temporary image space called k-space. Information about image contrast is held at the centre of k-space, which contains low-spatial frequency information, while image details are held at the high-spatial frequency edges of k-space. Over the course of an MRI scan, k-space is filled. At the end of a scan, k-space is full, and data are reconstructed to produce an image.

The most common way to fill k-space is to acquire data line-by-line and collect a series of complete images to follow a signal through time and space. DIANA, on the other hand, collects a series of partial images using a 2D fast line-scan approach. Here, single lines of k-space are acquired repeatedly between the intervals of a repeated stimulus, and different k-space lines are acquired in different stimulus periods. This way, each stimulus period adds one line of k-space to all the time-series images within the period.

DIANA requires no contrast agents or new equipment. Neural activation can be imaged on an ultrahigh-field scanner using a conventional 2D gradient-echo imaging sequence with a short echo time and short repetition time in a line-scan acquisition scheme.

To produce repeatable neural action potentials, the researchers repeatedly flicked the whiskers of anesthetized mice, a technique common to brain imaging studies. They observed that in response to the stimulus, neurons in the mouse somatosensory cortex were activated after deep regions, such as the thalamus, were activated. Imaging this level of neuronal activity, the researchers say, could help us understand communication between areas of the brain in the future.

“If it works in the human body, it can be a game-changer for neuroscience, because if we detect neuroactivation directly at high temporal, high spatial resolution, then I think we can really start looking at the brain network as a neural network in time and space,” says Park. “Using BOLD fMRI…it is very difficult to look at the dynamic neural network and it is also difficult to explore the hierarchical functional connectivity in the neural network.”

T2 to the future

The biophysical source of the DIANA signal isn’t clear, but the researchers think they have a strong hypothesis supported by additional experiments and simulations – that changes in neuronal membrane potential are reflected in a positive correlation with the transverse relaxation time (T2) of the MRI signal, which determines how quickly MRI signal disappears.

Such neural activation signals haven’t been observed in BOLD fMRI experiments because they capture haemodynamic responses on the order of several seconds rather than milliseconds, Park explains.

He and his group still take inspiration from fMRI experiments, however, as they look to future work. BOLD fMRI measures signal in the absence of stimulus using resting state techniques, and sensitivity is increased by processing data using a neuronal response function. The team hope to develop analogous techniques for DIANA.

Park says that his research group is using DIANA to study visual neural networks in mice and how the neural network changes in animal models of neurodegenerative disease. They are also translating the technique to use in human studies, where motion artefacts and trial-by-trial variability may be prevalent and activation patterns may span multiple time scales.

“Neuroscientists and people working on neuroimaging studies, they want to understand the brain and how it works,” Park says. “I think if we understand the real dynamic neural network, we can really start understanding how the brain works, as well.”

I’ve found a major physical flaw in an iconic childhood game

When I was eight years old, my sister and I would play a game where we would jump across furniture, leap over floorspace, and basically do anything we could to avoid touching the ground. If any part of our body did hit the floor, we were out of the game. If you’ve played this game – or have kids who do – you’ll know it’s called “the floor is lava”. The aim is to avoid any contact with the ground because – obviously – it’s made of molten lava.

The trouble is that the game isn’t really consistent with the laws of thermodynamics. Of course, when I was eight, I didn’t realize all the inconsistencies, such as the fact that if we were actually that close to lava, the air temperature would reach dangerously high levels in a matter of seconds, causing severe burns. We also didn’t consider that the furniture itself would rapidly disintegrate.

Recently, however, I have started to question how the game would work if the floor actually were made of lava. I started by assuming that I live in a standard two-storey home, with the floor consisting of basaltic lava, roughly a third of a metre deep. The short answer is that the house would be totally destroyed in a matter of seconds. The long answer is much more interesting.

When in a molten state, basalt rocks reach temperatures of about 1100 °C, making them hot enough to glow bright orange. So if our floor did suddenly turn into basaltic lava, it would create a rapid and massive temperature spike in our house, spreading out within seconds and ultimately setting the property on fire. Our parents wouldn’t exactly be happy.

But can we model what would happen when the lava spreads? During a game of the floor is lava, furniture and other miscellaneous items are usually strewn haphazardly throughout the room, which means there’s ample floorspace for the lava to flow smoothly. We can also assume that all energy is conserved within the household, implying that all windows are shut, the walls don’t radiate heat to the outdoors, and that no players leave the house after reaching thermal equilibrium.

A house has lots of thermal conductors, into which heat can be transferred, including the walls, furniture, players and the air. When the lava floor and the air eventually reach thermal equilibrium, my calculations suggest that the final distributed temperature of the house will be given by [(CmT)lava + (CmT)air]/[(Cm)lava + (Cm)air], where C is heat capacity, m is mass and T is temperature.

Based on figures for the US, the average house has a floorspace of around 215 m2. If we assume the entire floor is transformed into molten lava 30 cm deep, then given basaltic lava’s density of 2800 kg/m3, the total mass of the lava would be around 180,000 kg. That would be enough to collapse your floor, but let’s ignore that slight problem and carry on with the calculation.

The lava and air would eventually reach a final equilibrium temperature of just under 1100 °C. Clearly, any children playing the floor is lava are ignoring this limitation

The heat capacity of basaltic lava is 840 J/kg°C, which means, with a heat influx of 840 J, one kilogram of basaltic lava would be heated by 1 ºC. The air in a house is harder to heat than the lava; its heat capacity is a shade over 1000 J/kg°C. Unfortunately, air has such a low density that all the air in a household may only weigh around 1100 kg. The lava will therefore have a significant effect on the final temperature, with not much heat being absorbed by the air.

Substituting all the numbers into our formula, we find that the lava and air would eventually reach a final equilibrium temperature of just under 1100 °C. Clearly, any children playing the floor is lava are ignoring this limitation. Air that hot would – obviously – be fatal to a human, which feels like the biggest inconsistency with the game. Given that the ambient air temperature is roughly 20 °C, the air would increase in temperature by about 1080 °C, while the lava would only cool by a few degrees. Better luck next time.

In practice, if the floor actually were covered in lava, the entire house would quickly combust, dramatically increasing in entropy, and reaching thermal equilibrium before slowly cooling. So while my findings do not represent an urgent physics emergency, it may be necessary to alert new parents who happen to have physics degrees. But I’d better dash, I’m sure I can feel the floor starting to get a bit toasty.

UK firm Universal Quantum wins major quantum-computing contract

A British quantum start-up firm has been awarded one of the largest government quantum-computing contracts ever given to a single company. Universal Quantum has won the €67m contract from the German Aerospace Center (DLR) to build fully scalable quantum computers based on trapped-ion technology.  

Founded in 2018 as a spin-off from the University of Sussex, Universal Quantum aims to build a fully scalable quantum computer based on trapped ions while making use of standard silicon-chip processing technology.

Trapped-ion technology is considered one of the most mature approaches to building quantum computers but will likely eventually require millions of quantum bits (qubits) to perform useful computations. 

Achieving this requires several challenges to be solved, such as building reliable connections between chips, as well as providing ultra-low cooling temperatures of several millikelvin.

Universal Quantum claims to be able to solve both of these issues by developing modular chips that can easily be connected to scale to high qubit numbers, as well as operating them at moderate temperatures of only 70 K.  

Previous versions of trapped-ion quantum computers required a pair of laser beams for every qubit, making this difficult to scale to machines with high qubit numbers.

Universal Quantum’s machines on the other hand execute quantum gates by applying voltages to a microchip. This approach makes use of electric field links between quantum-computing modules, which are much faster than photonic interconnects and operate with much smaller errors. Both these innovations enable scaling to much larger qubit numbers.  

The money from the DLR will be used to build a single-chip quantum computer, as well as a multi-chip quantum computer that demonstrates how the technology could scale to very large qubit numbers. The basis for both machines – to be built within four years at the DLR facilities in Hamburg – will be the most powerful chip ever developed for a quantum computer.

The two machines will allow researchers to test new concepts for software development as well as build real-world quantum-computer applications. Sussex physicist Winfried Hensinger, who co-founded Universal Quantum, says the DLR contract gives “tremendous validation” to the company’s technology. 

The contract is part of the German government’s Quantum Computing Initiative, which involves the German research and economics ministries providing the largest investment ever made by a European government for this purpose. The DLR is allocating one-third of this funding to establish a cluster of excellence for quantum computing at innovation centres in Hamburg and Ulm. 

Universal Quantum grew out of the UK’s National Quantum Technology Programme, which began in 2013. The €67m contract outshines others, such as €14m from the DLR to the photonic quantum-technology company QuiX and $2.9m from the US agency DARPA to US firm Rigetti Computing. “Our mission is to solve important industry problems and as such we aim to build quantum computers with millions of qubits – this is the next step along the way,” says Hensinger.   

Meteorites and magnetostrophic mathematics reveal unsung scientific heroes of the past

In this episode of the Physics World Weekly podcast the physicist Susanne Horn talks about the career of Donna Elbert, an American applied mathematician who worked on Nobel-prize-winning physics but did not get the credit she deserved. Based at Coventry University, Horn also talks about her recent research, which builds on Elbert’s pioneering work in magnetostrophic convection. You can read more in: “The Elbert range of magnetostrophic convection. I. Linear theory”.

Also in the podcast is the planetary geochemist Áine O’Brien of the University of Glasgow. She chats with Physics World’s Margaret Harris about her shocking discovery of a biological toxin in a Martian meteorite – and how that led her to investigate the history of Black students at Purdue University in the US.

O’Brien also talks about her recent research on the Winchcombe meteorite, which is described in: “The Winchcombe meteorite, a unique and pristine witness from the outer solar system”.

Black holes could reveal their quantum-superposition states, new calculations reveal

Quantum superposition is not just a property of subatomic particles but also of the most massive objects in the universe. That is the conclusion of four theoretical physicists in Australia and Canada who calculated the hypothetical response of a particle detector placed some distance from a black hole. The researchers say the detector would see novel signs of superimposed space–times, implying that the black hole may have two different masses simultaneously.

Black holes are formed when extremely massive objects like stars collapse to a singularity – a point of infinite density. The gravitational field of a black hole is so great that nothing can escape its clutches, not even light. This creates a spherical region of space around the singularity entirely cut off from the rest of the universe and bounded by what is known as an event horizon.

An active area of research into the physics of black holes seeks to develop a consistent theory of quantum gravity. This is an important goal of theoretical physics that would reconcile quantum mechanics and Einstein’s general theory of relativity. In particular, by considering black holes in quantum superposition, physicists hope to gain insights into the quantum nature of space–time.

Unruh–deWitt detector

In the latest work, reported in Physical Review Letters, Joshua Foo and Magdalena Zych of the University of Queensland together with Cemile Arabaci and Robert Mann at the University of Waterloo outline what they describe as a new operational framework for studying space–time superpositions. Rather than using a “top-down” approach to quantize general relativity they instead consider the effects of a black hole’s quantum state on the behaviour of a specific physical device called an Unruh–deWitt detector.

This is a hypothetical device that comprises a two-state system, such as a particle in a box, coupled to a quantum field. When in its low-energy state and exposed to electromagnetic radiation of just the right frequency, the system jumps to its higher state and registers a “click”.

This kind of detector can in theory be used to measure Unruh radiation, a heat bath of particles that is predicted to appear from the quantum vacuum to an observer that is accelerating through space. In the scenario laid out in the new research, it would instead capture Hawking radiation. This is radiation that is predicted to be created when virtual particle–antiparticle pairs within the quantum vacuum are ripped apart at a black hole’s event horizon – the antiparticle then disappearing into the void and the particle emitted into the surrounding space.

In their thought experiment, the quartet envisage an Unruh–deWitt detector located at a specific point outside a black hole’s event horizon, with the detector’s fixed position enabled by an acceleration away from the black hole that yields the Hawking radiation. The researchers consider the effect of a superposition of the black hole’s mass on the output of that detector.

Superposition of distances

As they explain, the two masses yield different solutions to the field equations of general relativity and thereby distinct space–times. The resulting superposition of space–times in turn leaves the detector in a superposition of distances from the event horizon, creating what is in effect an interferometer whose arms are each associated with one of the black hole masses. The probability that the detector clicks depends on which masses are present in the superposition.

Doing the calculations for a relatively simple black hole described in two spatial dimensions by the Banados–Teitelboim–Zanelli formulation, the physicists obtained a striking result. They plotted the probability of detecting a particle emitted by the black hole as a function of the square root of the superposition mass ratios and found sharp peaks when those values were equal to 1/n, with n being an integer.

The researchers attribute this behaviour to constructive interference between the radiation in the interferometer arms that correspond to the black hole masses predicted by the American-Israeli physicist Jacob Bekenstein in the 1970s. He showed that the surface area of a black hole’s event horizon – and therefore its mass – is an adiabatic invariant. This is a physical property that remains constant when acted upon slowly and which results in the mass being quantized.

“This result provides independent support for Bekenstein’s conjecture,” the researchers write in Physical Review Letters, “demonstrating how the detector’s excitation probability can reveal a genuinely quantum-gravitational property of a quantum black hole”.

The four physicists stress that the result emerged from their calculations without assuming that the black-hole mass had to fall within the discrete bands predicted by Bekenstein’s conjecture. They add that their technique could be extended to more complex descriptions of black holes in three spatial dimensions, which they say, would provide additional insights regarding the effects of quantum gravity in our universe.

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