I remember sitting in a meeting here at IOP Publishing about a decade ago when the idea of an ebooks programme was first mooted by one of our directors. With so much in the publishing world switching from print to online, it struck me as a timely and sensible concept. After much hard work, the IOP Publishing ebooks programme was launched in 2013.
In the video above, you can see Azahara Perez, IOP Publishing’s senior marketing executive for ebooks, talking to Duarte about his book. Often viewed as one of the strangest and most mysterious parts of physics, quantum entanglement refers to the fact that particles can be linked even if they are physically a long way apart.
Albert Einstein famously didn’t like the idea that the quantum state of one entangled particle in a pair can change instantly when a measurement is made on the other particle, dismissing it as “spooky” action at a distance. However, these days quantum entanglement has real-world applications in everything from fibre communications (both from Earth and via space) to quantum computing.
And its emergence as a real-world technology is where Duarte and Taylor’s ebook comes in, by providing scientists and engineers with practical mathematical tools to handle entanglement and to design functioning optical systems.
Researchers in France have pinned down the conditions under which “glacier tables” – large rocks perched atop thin columns of ice – form within retreating glaciers. Their results, which highlight the importance of the rocks’ surface area and heat conductance, could give scientists an alternative way of estimating the rate at which glaciers melt.
Glacier tables can appear strange, even unnatural, when seen for the first time. They form when a rock lying on top of a glacier shields the ice directly underneath it, decreasing the rate at which the ice melts. The un-melted ice then forms a column that can grow up to two metres tall as the rest of the glacier melts around it. When the column can no longer support the rock’s weight (usually after a few months), the rock topples over.
Miniature glaciers
Glacier tables are mainly found on low-altitude glaciers, where summer temperatures are high enough to melt ice. Only large rocks can create them since smaller ones invariably sink into the ice as it melts. Beyond these rough rules, however, little was known about the details of how they form. To better understand this phenomenon, physicists led by Nicolas Taberlet from the University of Lyon created miniature glaciers consisting of slabs of clear ice inclined at various angles. They left these slabs on a bench in their lab and measured the rate at which the slabs melted by monitoring how their thickness decreased over time. Under these circumstances, the main drivers of ice melting were infrared radiation coming from the walls of the lab and natural air convection.
Taberlet and colleagues then repeated the experiment using fresh slabs of ice, but this time they placed cylinders measuring between 4 and 14 cm in diameter and between 0.5 and 7 cm in height on top of them. These “rocks” were made of materials with different thermal conductivities and included polystyrene and granite.
The researchers observed that while some of the cylinders formed tables, others did not. For example, ice columns formed readily under cylinders made of polystyrene, but never under those made of granite. This is because polystyrene is a much worse heat conductor than granite, so it acts as an insulator, shielding the ice from the warmer environment.
To replicate glacier table formation, researchers placed cylindrical “rocks” made of polystyrene, PVC and granite on a plate made of ice. As the ice melted, the plastics (polystyrene and PVC) formed tables because they are relatively poor conductors of heat, whereas the granite – a stronger heat conductor – sank into the ice. (Courtesy: M. Hénot et al.)
Taberlet and colleagues also observed that thinner cylinders formed tables more easily than thicker ones. This is because a thicker structure has a greater surface area in contact with its environment, allowing it to absorb more heat, which causes the ice underneath to melt at a faster rate than the ice under a thinner cylinder.
One-dimensional conduction model
By inputting these observations into a one-dimensional conduction model that determines how quickly ice under a rock will melt relative to uncovered ice, the team estimated that the minimum size for a table-forming rock is between 10 and 20 cm. This is on par with the size of glacier tables observed in nature.
The model shows that ice-melting rates under different types of cylinders are controlled by a competition between two effects: the size and shape of the cylinder and a reduction in heat flux due to the higher temperature of the cylinder relative to the ice. The model also takes into account the transition between the two regimes and identifies a dimensional number, heffR=λ (where heff is the effective heat transfer coefficient between the cylinder and the ice, R is the radius of the cylinder and λ is its thermal conductivity) that controls the onset of glacier table formation.
This model might make it possible to develop new “benchmarks” for glaciology, Taberlet tells Physics World. For instance, glaciologists might be able to visit a site once a year, at the same time each year, and estimate glacier melting rates by measuring the height of the glacier tables, rather than by making repeated trips in a single season. He and his colleagues now plan to extend their study to the entire lifetimes of glacier tables and compare the results of their model with new field measurements.
Being very organized, decisive, co-operative, creative and mindful are important in my daily life. For over a decade, my husband and I have enjoyed the flexibility of working together from our home in Devon.
As expected, my work–life balance became more focused on family after our daughter was born four years ago. Among other things, I have become more skilled at multi-tasking: moving from role-playing, reading or painting with our daughter, to liaising with our content creators, and discussing website design and marketing strategies with my husband and our PR team. We often work late, so developing some mindfulness skills has helped to prevent burnout.
What do you like best and least about your job?
I have been feeling some eco-anxiety in recent years, as the world burns, floods and heats up. So I like that our latest project, SearchScene.com, has the potential to donate millions of pounds to environmental and humanitarian charities that are working hard to address the causes and effects of climate change.
SearchScene.com is a charitable search engine that we have spent the past few years building, with a small team of amazing people. Like Google, SearchScene makes money from search ads but, unlike Google, it donates 95% of its profits to charities that fight the causes and effects of climate change. Users can choose which of our nominated charities they prefer to support when they search the web. SearchScene also has great privacy features, and beautiful scenery on its homepage that changes regularly, reminding us of the precious things we can still protect if we make some changes, both individually and collectively. I love being part of a project that is making such a positive difference.
What I don’t like is that my job now includes dealing with climate-change deniers online.
What do you know today, that you wish you knew when you were starting out in your career?
I wish I had known that I was making a wise choice when I left an academic path that I had followed for many years and dearly loved, to become an Internet entrepreneur with my husband, and then a work-from-home mother. If people are willing to accept uncertainty, and take a leap into the unknown, there are often some wonderful new paths to explore.
Left: contact map highlighting beads that are close in space, comparing model simulations with experimental data for the modelled fragment. Right: DNA modelled as beads (connected by “spring” bonds, not shown). (Courtesy: CC BY 4.0/Nat. Commun. 10.1038/s41467-021-25875-y)
The regulatory patterns that underpin gene expression may originate from the spatial organization of the genome, according to a new study reported in Nature Communications. A collaborative research team, from the universities of Edinburgh, Göttingen and Oxford, has used simulations of a simple DNA model to provide insight into the mechanisms of this genetic control.
At the interface of cellular information storage and protein synthesis lies transcription: the process of copying DNA for subsequent translation into proteins. With more than 20,000 genes present in human cells, however, there is a need for tight regulation of this process. One mechanism for regulation involves transcription factors, which bind to promoters and initiate transcription by coupling to distant enhancers, forming a structural loop.
Experimentally, researchers employ genome-wide association studies to establish correlations between thousands of genes. Computer simulations can help link this whole-genome perspective to observations from classic experiments, for example by rationalizing how the addition of just a few proteins can drastically alter cell physiology.
“This is where the experience with this [DNA] model in the past helped a lot,” says corresponding author Davide Marenduzzo, referring to the representation of DNA as beads and springs. In this model, 3000 base pairs are each grouped into spheres, some of which act as promoter-like transcription units (TUs), which represent genes to be transcribed, or as transcription factor complexes (TFs).
University of Edinburgh researchers: Senior author Davide Marenduzzo (left) and first author Chris Brackley (right). (Courtesy: Davide Marenduzzo)
“We spent some time to develop the force field in the past, starting from the work where we uncovered the bridging-induced attraction,” Marenduzzo tells Physics World, referencing a core component of the model. The multivalent interaction between TUs and TFs leads to their clustering and strongly affects the patterns of observed transcription. “The link comes from a surprisingly simple assumption…as transcriptional activity is measured computationally as the fraction of time a [transcription] factor spends close to a transcription unit,” he explains.
First, the researchers looked at the behaviour of fragments comprising 1000 beads with randomly distributed TUs, interspersed by TFs. TUs that lie close together will cluster, which increases transcription locally but also leads to negative correlations of TUs that are further apart. In the case of a saturating number of TFs, the correlations remain local because there are enough TFs to bind all TUs without engaging in multivalent interactions. If, however, there are fewer TFs than TUs – as is the case in biological systems – the competition for TFs produces a well-linked, “small-world” network.
Within this framework, the researchers qualitatively recovered several characteristics of gene expression, such as short transcription bursts or a high variability between cells. Furthermore, they observed a rewiring of the network following the mutation of TUs or the introduction of fixed DNA loops.
Going beyond this so-called “toy system”, the authors succeeded in modelling entire human chromosomes based on experimental locations of the TUs. Marenduzzo reports that he was “very pleased we could get a significant agreement with transcriptional activity in human chromosome 14: while the correlation is not that large, what is good there is that there is no fitting [of the model parameters to match experimental data] in the model.”
Encouraged by these results, the team intends to “take these ideas forward by using much more sophisticated models…to build a compendium of 3D structures of all human genes and predict their transcriptional activities”.
The research groups of Marenduzzo and co-author Nick Gilbert have been awarded a Wellcome Trust grant to continue this collaborative work.
Asteroid quest: The $1bn Lucy craft will carry out several fly-bys of the Tojan asteroids over a 12-year period. (Courtesy: NASA)
NASA has launched a $1bn mission to study Jupiter’s Trojan asteroids – two large clusters of rocks that are believed to be remnants of primordial material that formed the solar system’s outer planets. The probe, dubbed Lucy, lifted off from Cape Canaveral Air Force Station in Florida at 5:34 a.m. local time on Saturday aboard an Atlas V rocket.
The 12-year mission will consist of several close-up fly-bys of Trojan asteroids to yield clues about these so-called “fossils” of the solar system. Given this link, the mission is named after the 3.2-million-year-old “Lucy” hominid fossil that was discovered in 1974 in Ethiopia and which is thought to be one of our earliest known ancestors.
The Trojans orbit the Sun “in front of” and “behind” Jupiter’s orbit. Both groups are located roughly at the same distance from Jupiter as the distance from Jupiter to the Sun. While Jupiter is large enough that it would scatter away nearby asteroids, due to the combined gravitational influences of the Sun and Jupiter, the Trojan asteroids have been trapped on stable orbits for billions of years.
The 14 m craft contains four payloads that include a suite of cameras, imagers and spectrometers. Lucy’s first target in 2025 en route to the Trojans will be a small body in the main asteroid belt. Lucy’s first Trojan asteroid encounter will happen two years later in the Trojan swarm that is in front of Jupiter. Lucy will then carry out remote sensing on four bodies – ranging from 20 to 60 km in diameter – that will include mapping their surface geology such as shape and crustal features as well as investigating their composition and searching for rings or satellites accompanying the bodies.
After four fly-bys in the front swarm, Lucy will then travel back to Earth to undergo further gravitational manoeuvres to travel to the Trojan swarm behind Jupiter, arriving in 2033. It will then carry out a fly-by of a Trojan binary asteroid pair that are both around 100 km in diameter.
Time capsule
Lucy was chosen as a NASA Discovery mission for launch in 2017. As well as the main payloads, Lucy also carries a plaque as a time capsule, which includes quotes from Albert Einstein and Carl Sagan as well as a diagram showing the positions of the planets and the date of Lucy’s launch. Indeed, it is expected that once the mission is complete, the craft will remain in a stable orbit for hundreds of thousands of years.
“Lucy embodies NASA’s enduring quest to push out into the cosmos for the sake of exploration and science, to better understand the universe and our place within it,” says NASA administrator Bill Nelson. “I can’t wait to see what mysteries the mission uncovers.”
Bill Gates begins How to Avoid a Climate Disaster: the Solutions We Have and the Breakthroughs We Need by acknowledging that he is an “imperfect messenger”. The Microsoft co-founder knows he is a billionaire technophile with a legacy carbon footprint the size of a small planet. Gates admits he flew into the 2015 Paris climate conference by private jet and wolfs down his share of Seattle beef burgers. For some readers, these facts may be too much to stomach.
But realistically, Gates’ influence and investment power are hugely significant. The book offers a pragmatic look at renewable energy and climate mitigation options from someone who profoundly understands how to innovate for mass markets. An alternative strapline could have read: “I’m ploughing my cash into these technologies because some might work at a meaningful scale.”
A key theme is that the green transition should not reduce living standards, especially in developing countries where the right to economic growth is sacrosanct. Gates’ priority is reducing the “green premiums” that customers pay for alternatives to carbon – because they will only help if average earners can afford them.
He professes his love for Sustainable Energy – Without the Hot Air, the 2008 book by the late University of Cambridge physicist David MacKay. Indeed, Gates shares MacKay’s knack for cutting through the hype to estimate scales, be they power outputs, costs or the amounts of land required. He insists that nuclear should remain in the mix because of renewables’ intermittency and the absence of a Moore’s law for batteries, wryly noting how much he has lost on battery start-ups.
Recent reports suggest Gates might not be the affable nerd he was once marketed as. But anyone who believes we should tackle the climate crisis while leaving the lights on would be naive to ignore this tech magnate’s opinion.
Environmental writers walk a tightrope. Offer relentless despair and readers may drift away; be too upbeat and you’re almost certainly downplaying the issues. Fail to offer an opinion and you can leave readers cold, but come across as preaching and critics will shout “hypocrisy!” In Under a White Sky: the Nature of the Future, Pulitzer-prize-winning author and journalist Elizabeth Kolbert masterfully traverses this tightrope, combining curiosity with an acerbic wit to explore humanity’s obsession with controlling nature.
The book’s title imagines the skies if they were sprayed with vast quantities of particles – a potential climate solution for which people including Harvard physicist David Keith (who is interviewed in the book) want more investment in feasibility studies. In theory this could seed clouds that reflect sunlight back into space – counteracting the greenhouse effect. Through a series of trips to projects trying to rectify problems created by humanity (a novel type of disaster tourism) Kolbert shows that this far-fetched suggestion is just one idea in a long history of environmental techno-fixes.
As we discover, even the best-laid plans frequently return to kick us in the teeth. The book’s recurring theme is that nature is always more fine-tuned and complex than we think, though we never seem to learn. As the author writes, “If control is the problem, then, by the logic of the Anthropocene, still more control must be the solution.”
Blending scientific reportage with literary flair, Under a White Sky presents a series of case studies written partly as a travelogue, as Kolbert meets scientists and engineers reshaping the natural world. Their projects range from diverting the Mississippi river to protect New Orleans from flooding, to breeding “super coral” resistant to bleaching in warming oceans. In one story we learn about the increasingly elaborate efforts to save the few remaining pupfish at the Devils Hole geothermal pool in the Nevadan desert. Humans have driven these metallic-blue creatures to the point of extinction – first with nuclear bombs in the 1950s at the nearby Nevada test site, then later by property developers draining a nearby aquifer.
Conservationists have recently created a multi-million-dollar replica of Devils Hole a mile from the real one. It quickly became infested by larvae-eating beetles that thrive on the artificial environment. “I was struck, and not for the first time, by how much easier it is to ruin an ecosystem than to run one,” remarks Kolbert. As a funny aside we hear how one Devils Hole scientist was brooding because a local newspaper had recently described him as “potbellied and stern”. When he asks Kolbert for her opinion, she suggests he might be better described as having a “paunch”. These moments of human banality help to keep you sane as the narratives of environmental destruction unfurl.
On a separate trip Kolbert visits Australia to hear about the giant cane toad imported from the Caribbean in the 1930s to deal with beetle grubs in sugar plantations. Unfortunately, the toads are tasty but toxic, so they’ve become the last supper for vast numbers of Australia’s native animals. To cut toad numbers, people “bash them with golf clubs, purposefully run them over with their cars, stick them in the freezer until they solidify”, but it’s done little to stem the toad tide. The latest proposal: genetically engineer “detox toads” that will make marsupials ill but not kill them, training them to develop a distaste for the toads. What could possibly go wrong this time?
Tales of toads and fish serve as an appetizer for the closing part of the book, when we finally get our teeth into engineering the atmosphere. Kolbert runs through the smorgasbord of negative-emissions technologies designed to sequester carbon dioxide. She visits a “direct air capture” facility in Switzerland where CO2 filtered from an industrial incinerator is piped into a neighbouring greenhouse to boost the growth of fruits and vegetables. The tomatoes were “perfect, in that greenhouse tomato-y way,” reports Kolbert. She also travels to Iceland to see a project that pumps waste gases including CO2 from a geothermal plant into volcanic rocks, forcing it to rapidly mineralize (see our recent feature that looks at this carbon-capture project).
To understand the mindset of geoengineers, Kolbert meets physicist Klaus Lackner, founder of the Center for Negative Carbon Emissions at Arizona State University. In Lackner’s view we need to move away from moral debates that equate carbon usage with blame and virtue. Carbon emissions, in his opinion, should be regarded in the same way as sewage. “Rewarding people for going to the bathroom less would be nonsensical,” is one comment that certainly evokes the senses.
One thing the book lacks is important detail on the economics of the interventions. As things stand, there is little agreement over who should pay for carbon-capture projects. Unless that changes – for instance with the establishment of a global carbon market – these technologies are never likely to scale up. Throughout the book Kolbert quotes eye-watering sums involved in human interventions in nature. But with little context or comparison, the figures are meaningless. What is the cost of doing nothing?
Kolbert’s skill is in presenting compelling stories from the Anthropocene and letting us judge for ourselves
Kolbert’s skill is in presenting compelling stories from the Anthropocene and letting us judge for ourselves. She’s unafraid to question our track record of interfering with natural systems, but never lectures her audience about what is right or wrong. Kolbert clearly empathizes with the motivations of the scientists and engineers seeking technical solutions, and there is an underlying sense of resignation that perhaps we’re already in far too deep to row back. We’ve reshaped the planet to such an extent that it might now be inevitable that we have to keep doing so. It’s a bleak message, beautifully told.
Three years ago, an international team of researchers observed something unexpected in a sample of chromium triiodide (CrI3): quasiparticles known as magnons appeared to be travelling along its edges, rather than moving through the sample’s bulk. This observation suggested that this two-dimensional layered magnetic material acts as a so-called topological magnon insulator – an unusual property with important applications in the field of dissipationless spintronics. But though the result sparked a flurry of interest in CrI3, the question of why the material behaves in this way remained unresolved.
Thanks to detailed neutron scattering measurements and extensive analyses, the same team has now found that the curious properties of CrI3 stem from the way its layers are stacked. Although single-layer CrI3 and the bulk material are ferromagnetic, two stacked layers are antiferromagnetic, meaning that the material’s magnetic moments point in opposite directions – a superficially simple difference with far-reaching consequences.
2D materials and topological insulators
Two-dimensional (2D) materials like CrI3 are made up of atomically thin layers stacked on top of each other. These layers are held together by weak van der Waals forces and the electrons in them behave very differently from those in bulk materials. For example, electrons in graphene, one of the best-known 2D materials, can move at almost relativistic speeds and behave as if they are massless.
Some 2D materials are also topological insulators – that is, materials in which electrons flow freely along the edges of a 2D sheet but cannot flow along the surface. This effect is related to the spin of the electrons, making these materials promising for spintronic devices, which store and process information using the electrons’ spin states.
Magnons for spintronics
Certain 2D magnetic materials are also predicted to be magnetic and topological insulators. In these materials, which are very rare, quasiparticles known as magnons travel along the edges of a sheet, much like electrons in a conventional 2D topological insulator. Magnons are collective oscillations of the spin magnetic moments of a material and they are expected to be massless (Dirac) particles too, meaning that they can travel over long distances without dissipating. This property would make them interesting for spintronics applications as well.
In the earlier experiments, Lebing Chen, a condensed-matter physicist at Rice University in the US, together with colleagues in the UK and South Korea, found such Dirac magnons in CrI3. The team came to this conclusion by studying the material using inelastic neutron scattering at the Spallation Neutron Source at Oak Ridge National Laboratory. In this technique, neutrons, which have magnetic moments, create magnons when they scatter from a sheet of CrI3, which has a honeycomb lattice structure much like graphene. By measuring the energy lost by the neutrons during the scattering process, Chen and colleagues were able to calculate the properties of these magnons. They found that they exhibit properties consistent with them being topological and having a dissipation-less edge mode.
Spin-orbit coupling
In the new work, described in Physical Review X, the researchers performed further neutron-scattering measurements with much greater precision and resolution. These results showed that the magnons’ topological properties arise thanks to spin-orbit coupling – a relativistic interaction between an electron’s spin and its motion. This coupling induces asymmetric interactions between spin of electrons in the materials, explains Elton Santos of the University of Edinburgh’s Higgs Centre, Jae-Ho Chung of Korea University’s Department of Physics and Rice’s Pengcheng Dai, who led the new study. These interactions make the spin “feel” the magnetic field differently, affecting their topological excitations. “Surprisingly the spins have some chirality, like in a mirror where left and right can work differently,” Santos says. “We observed that without such chiral interactions, or, in more complicated terms, Dzyaloshinskii-Moriya exchange, we can’t describe the data.”
Chung adds that the result has an accompanying magnetic phenomenon too: a stacking-dependent magnetic order in which a single layer of CrI3 is ferromagnetic but two stacked layers are antiferromagnetic. “The reason for this behaviour is that the interaction between stacked layers in CrI3 is a combination of ferromagnetic and antiferromagnetic exchanges – despite apparent ferromagnetic stacking.”
“Our new work also confirms the previously observed topological nature of the spin excitation based on the Dzyaloshinkii-Moriya exchange, and rules out the competing interpretation based on the Kitaev exchange,” Dai tells Physics World. “The latter is known to be an important spin-spin interaction in more complex materials, like spin liquids, but apparently not for CrI3. This came as a surprise to us.”
According to book of Genesis in the Bible, the city of Sodom was destroyed by God because of the wickedness of its people. While there are several historical sites that could have been Sodom – and some scientists have suggested that the city could have been destroyed by a natural event such as a meteor strike – the story is widely regarded as mythical.
Now, the geologist Sid Mitra at East Carolina University in the US and colleagues have come up with an explanation of where Sodom was and what happened to the city. They have focused their attention on a Middle Bronze Age city called Tall el-Hammam, which is in the Jordan valley. In 2005 archaeologists discovered a 1.5 m thick layer of debris and destruction in the city that comprises materials that have been subjected to intense heat.
“They found all this evidence of high-temperature burning throughout the entire site,” says Mitra. “And the technology didn’t exist at that time, in the Middle Bronze Age, for people to be able to generate fires of that kind of temperature.”
Ten nuclear bombs
The archaeologists hypothesized that the city was destroyed by an airburst meteor strike like the 1908 Tunguska event, which flattened forests across a swathe of Russia with the energy of 10 Hiroshima-sized nuclear bombs.
To test this idea, scientists in different disciplines have joined forces to study materials found at the site. Mitra is an expert in the analysis of soot and discovered that a large fraction of the organic carbon at Tall el-Hammam is in the form of soot. This, he says, points to a very high temperature fire at the site – something that could have been caused by a meteor.
Evidence studied by other scientists include diamond-like carbon, melted pottery and other materials affected by high temperatures and a pressure shock – which would have been delivered by an airburst meteor.
Mitra and colleagues describe their findings in Scientific Reports and speculate that this catastrophic event 3600 years ago may have been recounted in local oral tradition and then made its way into the Bible. The event could also explain why the region around Tall el-Hammam was abandoned for many years. It could even be the origin of another biblical story: the destruction of the walls of Jericho, which is about 22 km from Tall el-Hammam.
Collective motion
The 1950s science-fiction horror film The Blob features a growing, corrosive, alien entity that envelops everything in its path. Thankfully, researchers haven’t quite managed to recreate that but it is known that blackworms (Lumbriculus variegatus) can aggregate into “blobs” that are capable of collective movement.
Measuring up to 10 cm long, blackworms live in ponds or marshes in Europe and North America. To protect themselves from drought, they merge as entangled, shape-shifting blobs that contain about 100 individuals. The blob then moves to seek out cooler climes.
Researchers have now discovered that the collective movement of the blob can only emerge when there is a fine balance between their individual movement and how well they cling to each other.
The team says the results could be applied to the design of individual soft and flexible robots that entangle and move as a unit, which sounds like the basis for another science-fiction horror film.
And finally, scientists have carried out computer simulations showing that a nuclear bomb could be used against an Earth-threatening asteroid.
Such a scenario – as made famous by the films Armageddon and Deep Impact – could be averted thanks to a 1 megatonne nuclear bomb that if ignited near the surface of a 100 m-long asteroid two months before potential impact would result in up to 99% of the total mass of the asteroid missing Earth.
The LIGO–Virgo observatories are three kilometre-scale interferometers that detect tiny spatial displacements (10−18 m) that occur when gravitational waves – ripples in space–time – pass through Earth. The detectors are famous for detecting short, intense pulses of gravitational waves that are emitted in the final moments before pairs of black holes or neutron stars merge. But astrophysicists also want to use these huge facilities to observe much fainter continuous signals from objects such as rotating neutron stars. Meg Millhouse and Lucy Strang at the University of Melbourne and Karl Wette at the Australian National University are members of the Australian Research Council’s Center of Excellence for Gravitational Wave Discovery, or OzGrav, and they talk about their search for continuous gravitational waves.
Who belongs to OzGrav and what are its primary goals?
Karl Wette: OzGrav was funded by the Australian government in 2017 for seven years, with a mission to join with international collaborators in leading the exciting new field of gravitational astronomy, and to inspire the next generation of Australian scientists. It’s made a big difference to the field in Australia. When I did my PhD at Australian National University, well before OzGrav was funded, the number of people working on gravitational waves was a lot smaller, and I was one of the few people working on data analysis. We now have well over 100 students and postdocs working across instrumentation, data analysis, interpretation and astrophysics. We’ve recently submitted a proposal for a new centre to succeed OzGrav, and hopefully that gets funded so that we can continue to grow this new field Down Under.
Why do you expect neutron stars to emit continuous gravitational wave signals?
Meg Millhouse: For a neutron star to emit continuous gravitational waves it must have a time-varying quadrupole moment in its mass. In simple terms the star must be lumpy – not a perfect sphere – and it must be rotating. We believe that neutron stars have rigid crusts and previous research suggests that the crust could be deformed to create “mountains” on the surface. This is actually an overstatement because the deformities are thought to be on the order of millimetres – but that could be enough to create gravitational waves that we can detect.
We also know that many neutron stars rotate, because we can observe the pulses of radiation that they emit. It is these pulsars that we are studying.
In your latest study, you’ve targeted 15 neutron stars that have recently formed in supernovae. What’s special about these objects?
Lucy Strang: We expect young neutron stars to be lumpier and therefore emit more intense gravitational waves than older stars. Neutron stars are created in supernovae, so we target young supernovae remnants because these are relatively easy to find.
Big detector Aerial view of the Virgo interferometer in Italy, which has 3 km long arms. (Courtesy: LIGO–Virgo)
The LIGO–Virgo detectors are kilometre-scale interferometers. How do you point them at neutron stars?
LS: We can’t point the LIGO–Virgo detectors at specific parts of the sky – they observe the entire sky. Instead, we have a series of mathematical transformations on LIGO–Virgo data to look for signals that could correspond to the neutron stars we are interested in. So in a sense we are pointing a telescope using mathematics.
Are the signals that you are looking for different from the gravitational-wave signals that have already been seen from merging black holes and neutron stars?
KW: Mergers of black holes and neutron stars are violent events that produce short-lived and relatively intense pulses of gravitational waves. The continuous signals that we expect from slightly deformed and rotating neutron stars are much weaker – more like a very low background hum. While the low intensity makes the signals difficult to find, their continuous nature gives us an advantage. The signals are always humming in the background, so if we keep observing and analysing data over a long period of time, we may be able to extract the weak signals.
Ideally the search would require vast amounts of computing power – but this just isn’t available to us. So, as well as being important from an astrophysical perspective, our research is also a big data challenge – which is very exciting to work on.
Is LIGO–Virgo very sensitive to the gravitational-wave frequencies that you expect from rotating neutron stars?
KW: Most of the pulsars that we know of spin at about 1 Hz or slower. Unfortunately, LIGO–Virgo is not very sensitive to signals at these low frequencies, where seismic interference from things like human activity are a problem. Instead, we focus on neutron stars spinning at hundreds of hertz, where the detectors are much more sensitive. There are some neutron stars that fit the bill and we hope that we can see continuous waves from them
Have you spotted any continuous wave signals so far?
LS: Sadly, no – but that is not surprising because we know that there is currently a low probability of making a detection. However, we have established an upper limit on the strength of signals from neutron stars and that has allowed us to put constraints on some properties of neutron stars.
What sort of constraints?
KW: One thing we are interested in are r-modes. These are like giant ocean waves on the surface of a rotating neutron star, which could broadcast relatively intense gravitational waves. X-ray observations of the pulsar J0537-6910, for example, provide strong evidence that the neutron star is radiating gravitational waves via r-modes.
However, we haven’t seen such waves and that allows us to exclude certain models of r-mode emissions. This translates directly into a better understanding of the neutron star equation of state, which relates the radii of neutron stars to their masses. Even though we have not made a detection, we can already say something important about neutron-star physics.
Even though we have not made a detection, we can already say something important about neutron-star physics
LS: Another thing we can constrain is the shape of the neutron star. We talked earlier about how we expect lumpy (or, to put it another way, non-spherical) neutron stars to produce continuous waves. The less spherical and more lumpy the neutron star is, the louder we expect the signal to be. By constraining the size of the signal, we’re also constraining how non-spherical the neutron star must be.
As with the r-modes Karl just discussed, this restriction can be translated into constraints on the neutron star equation of state. The equation of state contains the fundamental physics governing the neutron star and determines its mass, radius and so on. The conditions inside a neutron star are impossible to replicate in a laboratory, so astrophysical observations are our only window into physics in these extreme conditions. With each limit we set, we get a step closer to understanding the underlying physics.
You are targeting neutron stars created in supernovae, but what if you are lucky enough to observe a nearby supernova?
MM: There are several different gravitational-wave signals that could come from a supernova. The explosion itself would create a short burst of gravitational waves, which would be a very exciting thing to detect. The astronomy community could do multimessenger observations of the event, capturing electromagnetic radiation, and possibly neutrinos, along with gravitational waves. This would give us a wealth of information about supernovae.
If there is a neutron star remnant present after the explosion, it should emit continuous gravitational waves. These could be difficult to detect because we expect that the rotational speed would be decreasing very quickly. If we don’t know what frequency we are looking for or how fast the star is spinning down, it can be a tricky observation.
LIGO–Virgo scans the entire cosmos. Are you also looking for neutron stars that we don’t already know about?
KW: Yes, that’s another strategy that we are pursuing – eyes wide open surveys that scan the entire sky over a range of signal frequencies. Estimates suggest there are approximately one billion neutron stars in the Milky Way, but we only observe a few thousand of them as pulsars. We hope that maybe a few of the billion are radiating gravitational waves that we can detect, and that would provide new insights into neutron stars.
We’re still looking at some of the data from the latest LIGO–Virgo observations and we are hoping that we will soon have more results to report. Next year will bring further upgrades to the LIGO–Virgo detectors, which will make them more sensitive to continuous waves. So, this is an ongoing story.
