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Solar wind and extreme heat creates ice on Mercury, say researchers

Some of the ice on Mercury is created by chemical reactions triggered by the planet’s extreme daytime heat according to Brant Jones and Thomas Orlando at the Georgia Institute of Technology and NASA’s Menelaos Sarantos.  The trio discovered the process by modelling the chemistry that unfolds as the solar wind impacts the planet’s surface. Their discovery could explain the presence of up to 10% of Mercury’s total water ice, and also provide new insights into how water could be created on the Moon.

Despite its daytime temperatures reaching as high as 400 °C, Mercury is known to host vast quantities of frozen water in the deep, permanently shadowed craters close to its poles. First discovered by Earth-based radar systems, this ice has now been precisely mapped by NASA’s MESSENGER spacecraft, which has been in orbit around Mercury since 2011. Astronomers believe that most of this water was delivered by impacting asteroids and comets. They also think the ice has remained trapped for millions of years in shady areas where temperatures remain permanently below -200 °C.

In this latest study, Jones, Orlando and Sarantos describe how protons in the solar wind may also be creating water on the planet. The trio developed a model that suggests that the protons can penetrate to depths of up to 15 nm into Mercury’s surface soil. There, the protons react with metal oxides to form hydroxyl (OH) groups. In Mercury’s extreme daytime heat, these groups can react with each other to form gaseous water, along with molecular hydrogen.

Exospheric journey

The trio then simulated how water behaves in Mercury’s exosphere – the planet retains no atmosphere but is surrounded by an exosphere of atoms and molecules kicked up from the surface. Water in the exosphere is transported across the planet through a variety of mechanisms. Some of these molecules rise far above the surface, while others are split into fragments.

However, the simulation revealed that some water does become trapped in the chilly polar craters, which occupy around 1% of Mercury’s total surface area. The trio predict that this process could account for around 10% of Mercury’s frozen water.

The result could shed new light on the differences between the ice-forming mechanisms found on Mercury, and those of other airless bodies like the Moon. The Moon has far cooler temperatures than Mercury, so the solar wind is far less likely to produce hydroxyl groups. This would explain why water ice does not appear to be nearly as abundant in the Moon’s craters as in Mercury’s. Instead, the trio hopes that their findings could lead to the development of new techniques for fabricating water on the Moon – which could be crucial for future space missions.

The research is described in The Astrophysical Journal Letters.

Diamond defects could boost the range of quantum cryptography

A new type of quantum memory that could extend the range of quantum encryption systems has been unveiled by physicists at Harvard University in the US. It offers a secure way of allowing an intermediate to assist in the transmission of quantum information and could lead to the more widespread use of quantum key distribution (QKD) cryptography.

Using QKD, two people (Alice and Bob) rely on quantum mechanics to exchange information secretly. Alice sends Bob a series of quantum bits (qubits) encoded into the polarization states of single photons (or weak coherent light pulses). By carrying out a series of measurements and communications over an insecure link, Alice and Bob generate an encryption key that they can use to send secret messages over an insecure link. Crucially, if an eavesdropper (Eve) intercepts and measures the quantum bits, Alice and Bob are alerted thanks to the laws of quantum mechanics.

Although some commercial QKD systems are in use, sending single-photon qubits over long distances in optical fibres is a significant technical challenge. The current record for QKD over a commercial telecom link (rather than a dedicated link) is 50 km.

“Photons get lost”

“At its core, the reason we don’t have a quantum internet right now is that photons get lost,” explains team member Bart Machielse: “Photons are scattered out of fibres, photons are absorbed, and as the links get longer the communication rate goes down.” Incorporating multiple photons into each pulse would remove the absolute security, as Eve could measure one photon without disturbing the others.

One possibility is to incorporate a third party (Charlie) between Alice and Bob to measure the states of the photons they exchange. However, if the security is to remain absolute, Charlie cannot simply measure the state of a photon from one party and compare it to the next photon he receives from the other, as he may not be trustworthy himself.

Here, too, quantum mechanics offers a solution: Charlie compares the polarizations without knowing their individual values.  “Charlie does measurements on both photons and says ‘These are the same’ or ‘These are different’,” explains Machielse, “Alice and Bob say ‘I know what photon I sent’ and ‘Charlie tells me our photons are the same or different’.” This preserves the security of the communication even over an insecure link.

Simultaneous measurements

One problem with current technologies is that to make a secure comparison, Charlie must receive the photons simultaneously from Alice and Bob – which happens rarely. Researchers have therefore tried to develop a quantum memory that allows Charlie to store the quantum state of a photon he receives without measuring it. “People have used memories ranging from trapped atoms and ions, quantum dots, different defects in diamonds, you name it,” says Machielse. None of these, however, have actually improved over what can be achieved by direct photon exchange.

In the new research, Machielse and colleagues created a memory using a silicon vacancy centre (Si-V) in a diamond. A Si-V is a defect formed when two carbon atoms in the diamond lattice are replaced by one silicon atom. This creates a quantum spin that is isolated from the environment and can be measured using laser light and microwave pulses.

The team placed their Si-V inside a nanophotonic cavity held at ultracold temperatures. The spin of the Si-V can be flipped by absorbing a 737 nm wavelength photon. If the spin state of the Si-V does not change after absorbing two photons, Charlie knows that the two photons had the same polarization as each other. If the state has been flipped, the polarizations must have been opposite. Crucially, however, Charlie does not know the polarization of either photon.

Another key feature of this implementation is that the photons from Alice and Bob do not have to arrive simultaneously at the Si-V. Instead, the Si-V stores the polarization of the first photon until the arrival of the second.

The Si-V-based quantum memory achieves both very strong and very reliable spin-photon interaction. “With a lot of the other memories, either not every photon that arrives is stored, or an error happens in the storage process and the information is essentially useless,” explains team member Ralf Riedinger, “We achieved low enough error rates that, even after correcting for the errors, we still achieved faster communication than anything possible with a direct communication link.”

The researchers describe their work in Nature. Sophia Economou of Virginia Tech in the US says “This is a very significant paper: I would call it a milestone in the field of quantum networks”. She believes the work opens up “several future directions” such as transferring information from the electronic spins of the Si-V centres to the more stable nuclear spins of the surrounding carbon-13 isotopes in the diamond: “Achieving this would allow storage of information for longer periods of time, boosting performance of the protocol and opening more opportunities for quantum networks,” she says.

Carbon nanotubes forecast when vegetables spoil and buds bloom

When one bad apple rots the bunch, it’s ethylene’s fault. Not only does this “universal plant hormone” trigger germination, flowering, ripening and rotting in seeds, flowers, fruit and vegetables, it’s also released during these processes, ensuring that a small problem quickly escalates. Ethylene is social media for plants, allowing them to communicate and synchronize – and it’s something the food and flowers industries would love to detect early, so they can forecast which products to sell first, and how to spot rot before it spreads.

The problem? “There really isn’t a good industrial sensor out there,” says Timothy Swager, a materials chemist at the Massachusetts Institute of Technology (MIT) in the US. Now, however, Swager, Darryl Fong and colleagues at MIT and the Nanotechnology National Laboratory for Agriculture in Brazil have combined the sensitivity of carbon nanotubes (CNTs) with a highly selective catalyst to produce a sensor that can detect ethylene at concentrations of as little as 15 parts per billion.

CNT sensing

Previous approaches to ethylene sensing have typically either been based on photoacoustic spectroscopy (which detects sounds produced in response to light) or on gas chromatography (which separates chemicals based on their different retention times in solvents). Neither technique is easy or simple enough for a grocer or florist to incorporate it into their work. Instead, Swager and colleagues looked at the reactions ethylene readily undergoes and focused on finding ways of detecting when such a reaction had taken place.

This is where the CNTs come in. Single-walled CNTs have several attributes that make them well-suited for sensing processes that involve the transfer of electrons – the basis of any chemical reaction. The CNTs Fong and Swager and their colleagues worked with are p-type semiconductors, so n-type dopants – anything that donates electrons to the CNT – will diminish their conductivity. The CNTs’ curved graphene surface also makes their electronic properties incredibly sensitive to dopants in their environment.

This may sound ideal for a sensor, but you can have too much of a good thing. “There’s a graveyard of CNT sensors with low specificity,” Swager tells Physics World. Because the CNTs are so sensitive to everything in the environment, he explains, all you detect is white noise. “You need a way of boosting the signal above the noise,” he adds.

Wacker reaction chemistry

Building on their expertise in analytic and synthetic chemistry as well as CNT technology, Swager and colleagues identified the so-called Wacker reaction as having both the specificity and sensitivity they needed. In this reaction, which was developed as a synthetic process in the 1950s, ethylene – a hydrocarbon containing two double-bonded carbon atoms – oxidizes into acetaldehyde. This chemical is perhaps best known as the primary cause of hangover-related headaches, although the researchers note that the test they developed does not produce it in high concentrations.

The Wacker reaction is not the only reaction ethylene undergoes, and it was not the first port of call for Swager and his group. First, they tried to mimic the plants themselves, which use copper ions to promote ethylene activity. However, trying to make a sensor out of copper proved almost as headache-inducing as acetaldehyde. Plants have a natural ability to keep copper in its Cu(I) oxidation state, which has one fewer electron than the material would have as a neutral atom. Ethylene readily binds to copper in this oxidation state, triggering a cascade of plant activity and ultimately the up-regulation of certain genes. However, in a synthetic device, copper tends to exist in the Cu(II) oxidation state, and preventing Cu(I) from oxidizing to Cu(II) proved too difficult for this approach to have commercial potential.

In the Wacker reaction, in contrast, ethylene oxidation is catalysed by palladium in the Pd(II) state. This is far more stable than Cu(I). Although palladium can catalyse other reactions as well, the palladium in the Wacker reaction is in an organic complex optimized to catalyse ethylene oxidation. The reaction also relies on a nitrite source, which contributes to the organic complex. The resulting ethylene reaction surrenders electrons that reduce Pd(II) to the n-dopant Pd(0), which the highly sensitive CNTs should announce with a dip in conductivity from the excess electrons.

Budding commercial potential

The researchers tested their proposed mechanism by replacing the CNTs with ZnO nanofibers, which are n-type semiconductors. This test showed that, as expected, the material’s conductivity increased in response to the Pd(0) produced in the presence of ethylene and the Wacker reaction Pd(II) catalyst. Next, they optimized their CNT-based sensor and used it to detect ethylene production from carnations (which, in their experiment, all burst into bloom on the same day) and purple lisianthus (which bloomed over the course of a week). They found that the ethylene they detected reflected these different blooming times.

The process is now licensed by a company that Swager co-founded, but some hurdles remain before the sensor is available commercially. “We could industrialize these sensors in a matter of months, but how long before it is commercialized is something the business world will control,” Swager says.

The researchers describe their work in ACS Central Science.

Physics in the pandemic: ‘Scientists don’t live in a vacuum’

The shutting down of my department was a surprisingly difficult day. The Imperial College chemistry department made the decision to shut on 16 March, when the first social distancing measures in the UK were announced, and I was not prepared for the emotional toll it would take on me. I’m in the last year of my PhD and was preparing to focus on a difficult year of work. Instead, I found myself putting away all my experiments and taping shut our freezers.

We’d all talking about it; we even had an informal bet on when the college would finally shut. Everyone had been on edge for a couple of weeks, and no-one could talk of anything else. I’m grateful that we shut down in the end though. Social distancing in London is very difficult and, as anyone who has been on the tube will know, sometimes it’s just impossible.

So now I’m at home writing my thesis. I have my desk set up by my window and a routine established. At least that’s what I’m supposed to be doing. My brain hasn’t quite worked out a way to stop worrying enough to work properly. I’m trying to be kind to myself; scientists don’t live in a vacuum and the state of the world will affect our work. Trying to adjust to the massive changes we’ve all experienced in the past month is no small task.

As an experimental scientist, all my work is lab based. Like many around the world, I’m faced with the uncertainty of not knowing when I’ll be able to get my next results. I have an ever-expanding list of experiments that I want to do when I can return to the lab. In the meantime, like many other scientists, I’m going to try to learn all these computational and programming techniques that have been passing me by for years. Prepare for some very mediocre modelling!

Whilst I was trying to settle into the new rhythm, one of my flatmates developed symptoms. Which means that now we’re self-isolating as well. This has added an extra layer of worry, as I’m constantly concerned about her health and waiting to see if I get it. Hearing her coughing from my room, I’m continuously reminded how serious this crisis is.

One of the positives I’ve managed to find though, is that my research group has banded together. Everyone has been checking in on each other and putting online coffee sessions and Friday drinks in the calendar. We had our first virtual group meeting and I was struck by how much it started to make me feel normal again. Through all of this I’ve been overwhelmed by the amount that people are reaching out and making connections. It’s these interactions that are becoming my inspiration, lifting my spirits enough to be able to put pen to paper.

Phase-contrast imaging could improve breast cancer diagnosis

Propagation-based phase-contrast CT (PB-CT) is an advanced X-ray imaging technology that can generate higher quality diagnostic breast images than absorption-based CT (AB-CT), at a glandular radiation dose comparable to, or lower than, conventional mammography and digital breast tomosynthesis (DBT). The technology is currently limited to use with synchrotron light sources, but the evolution of compact light sources may make clinical application feasible, improving the detection and diagnosis of breast cancer.

A multidisciplinary collaboration – including scientists from the University of Sydney, University of Melbourne, Monash Health, Maroondah Breastscreen and the Australian national science research agency CSIRO – is working on the clinical application of PB-CT using the Imaging and Medical Beamline at the Australian Synchrotron. The researchers have now optimized the technique using 12 mastectomy samples that included different tumour types or benign lesions. They showed that PB-CT achieved significantly higher image quality than AB-CT and demonstrated that substantially lower doses could be used with PB-CT (Acad. Radiol. 10.1016/j.acra.2020.01.009).

Phase-contrast imaging, which exploits both the refraction and the absorption of transmitted X-rays, offers potential to overcome the limitations of current breast imaging modalities. The 3D images produced by DBT reduce the tissue superimposition effects of 2D mammography, but have lower sensitivity in detection of calcifications. Breast MRI has higher sensitivity than mammography, but lower specificity. It is also a highly expensive examination. Breast CT, meanwhile, visualizes mass lesions better than mammography, but underperforms with respect to depiction of microcalcifications and has poorer spatial resolution. In addition, its radiation dose is the highest of the breast imaging modalities.

PB-CT, one technique for phase-contrast imaging, is based on free-space propagation and the use of phase-retrieval algorithms to fully use the refraction information. The PB-CT method measures the phase shift as intensity modulation at the detector by simply positioning the detector a few metres from the object. Unlike other phase-contrast imaging techniques, it does not require any special X-ray optical elements in order to render the X-ray refraction visible. The only key requirements for PB-CT are a long propagation distance between the object and the X-ray detector, and the use of highly spatially coherent incident X-rays, produced by synchrotrons or compact X-ray sources.

The group previously demonstrated that PB-CT could reconstruct images with high quality and high diagnostic value, with a dose comparable to that of 2D mammography. In this latest study, led by Patrick Brennan, the team employed 32 or 34 keV X-ray beams from the synchrotron to scan the mastectomy specimens using PB-CT and AB-CT techniques under varying conditions.

The researchers collected images at two sample-to-detector distances: 0.19 m to represent an AB-CT scan and 6 m for a PB-CT scan. All AB-CT images were collected at a “standard” mean glandular dose of 4 mGy, using 2400 projections with 0.075° angular steps. PB-CT images were collected at both 4 mGy (2400 projections with 0.075° angular steps) and 2 mGy (1200 projections with 0.15° angular steps). The team used an ionization chamber to measure the photon fluence rate and the corresponding rate of the surface absorbed dose to air at the ionization chamber plane.

AB-CT vs PB-CT
Absorption-based CT (left) and propagation-based phase-contrast CT (right) of a breast mastectomy sample, both using a dose of 4 mGy. (Courtesy: Seyedamir Tavakoli Taba)

After three radiologists selected the best quality AB-CT image set for each mastectomy specimen, 11 radiologists independently compared the overall image quality in PB-CT images, prepared in axial and sagittal planes, with the corresponding AB-CT images. They evaluated lesion sharpness, visibility of calcifications, image noise, perceptible contrast, visible artefacts and normal tissue interfaces.

The radiologists reported that PB-CT images acquired at both standard and low dose were of significantly higher image quality than the AB-CT images. The researchers also determined that PB-CT images obtained at 32 keV and reconstructed using half phase retrieval (rather than full phase retrieval) had the best overall image quality.

First author Seyedamir Tavakoli Taba tells Physics World that the team will soon start a receiver operating characteristic (ROC) study to compare the diagnostic efficacy of PB-CT with conventional breast imaging techniques, such as mammography and DBT. To date, the researchers have scanned over 75 fresh breast mastectomy samples and anticipate scanning another 50 before launching the first clinical trial, planned for early 2021.

“Our plan is to establish a world-first mammographic PB-CT clinic at the Australian Synchrotron in three to four years, to be used mainly for staging and treatment options,” says Taba. “The widespread clinical implementation of PB-CT can be delivered via commercially available compact X-ray sources in the future. This will allow PB-CT to be widely translated into specialist cancer care facilities across Australia and overseas.”

Condensed-matter physics pioneer Philip Anderson dies aged 96

The US condensed-matter physicist Philip Warren Anderson died yesterday aged 96. One of the most celebrated condensed-matter physicists of his generation, Anderson’s theoretical research into the electronic structure of magnetic and disordered systems led to an improved understanding of metals and insulators. For this work he was awarded the Nobel Prize for Physics in 1977, which he shared with the British physicist Sir Nevill Mott and the US physicist John Hasbrouck van Vleck.

Born on 12 December 1923 in Indianapolis, Indiana, Anderson was raised in Illinois, where his father taught plant pathology at the University of Illinois in Urbana. In 1940, Anderson went to study physics at Harvard University but during the Second World War was drafted to work at the US Naval Research Laboratory, spending the period from 1943 to 1945 researching antenna design. He then returned to Harvard working on a PhD under the supervision of  van Vleck, graduating in 1949.

Anderson then joined Bell Telephone Laboratories in New Jersey, which was part of the telecoms firm AT&T. It was there that he developed his theory of the electronic structure of solids.

Much of what we know about the electronic properties of metals and semiconductors is based on the idea that electrons with certain momenta can travel freely through a crystalline lattice, while others cannot. This is embodied in Felix Bloch’s 1928 quantum theory of conduction, which describes the lattice as a periodic electric potential through which some electrons (behaving as “matter waves”) diffract with ease. In the 1960s, Anderson worked out what would happen in such a system if the potential lost its periodicity. This could happen, for example, if the lattice remained periodic, but the potential has a different value at each lattice site.

Anderson found that electrons would be unable to move through such a “disordered” lattice, and instead become trapped by specific atoms. If the disorder is sufficiently strong, the electrons cannot form an electric current due to destructive interference between different scattering paths. Instead, they become localized and unable to propagate in space.

For this prediction of what became known as “Anderson localization” he was awarded the 1977 Nobel Prize for Physics, which he shared with van Vleck and Mott for their “fundamental theoretical investigations of the electronic structure of magnetic and disordered systems”. Anderson localization has since been seen in several systems including those based on light, microwaves and in atoms held in a Bose–Einstein condensate.

A ‘wonderful’ lab

The 1960s was a particularly productive time for Anderson. He also worked on the theory of superconductivity, in which the electrons in a material can flow without resistance, and explored the properties of helium-3. In 1967, Anderson spent eight years on a part-time basis at the University of Cambridge before returning to the US to work at Princeton in 1975, while still being affiliated to Bell Labs.

Anderson retired from Bell Labs in 1984 when the US government disbanded AT&T and began working full-time at Princeton where he continued his research on spin glasses – nonmagnetic metals embedded with randomly spaced magnetic elements – as well as high-temperature superconductors.

In an interview with Physics World in 2006, Anderson said that he mostly enjoyed his 35 years at Bell Labs. “For the first three decades it was the most wonderful laboratory in the world,” he said. “We had freedom, an enlightened management and a personnel department that never had any say in the direction of the research department. We had a very high opinion of ourselves, but it was justified. Those were the years when we invented modern technology.”

The ‘arrogance’ of particle physics

Anderson also made crucial contributions to other fields in physics. In particular, in 1962, he published a now-famous paper on how the photon acquires mass. It was cited two years later by Peter Higgs in his own paper on the discovery of a mechanism for understanding the origin of mass – a theory for which Higgs and François Englert won the 2013 Nobel Prize for Physics. The mechanism was later confirmed by the discovery of the Higgs boson at CERN’s Large Hadron Collider in 2012.

While Anderson had noted that the Higgs boson could have been called the “Anderson–Higgs boson” in recognition of his work, in 2013 he told Physics World that the Swedish Academy made “a perfectly reasonable decision” to award the prize to Higgs and Englert. “I also think the fuss over the theoretical part of the work a bit excessive relative to the gigantic experimental effort,” he added.

In the late 1980s, Anderson was a vocal critic of the $4.4bn Superconducting Super Collider (SSC), which the US was planning to build in Waxahachie, Texas, as the next big machine in particle physics. In 1987, Anderson famously gave testimony to the US Senate, in which he worried that the huge costs of the 87.1 km circumference circular collider would force cuts to other science budgets. He was far from the only physicist who had such concerns and, despite some $2bn eventually being spent on digging parts of the SSC’s underground tunnel and constructing various buildings, the collider was cancelled in 1993, by which time the project’s estimated final price tag had almost trebled to  $12bn.

Indeed, Anderson held a sceptical view of particle physics and the belief in the field by some that it deserved more funding that other areas. “There is a great arrogance and immodesty about that whole field, which gets on my nerves,” he told Physics World in 2006. “Particle theorists say [they’re] discovering ‘the mind of God’. It’s not the mind of God at all. In the first place, there’s no God, and in the second place, particle physics cannot explain things like superconductivity, life and consciousness. It makes no contribution to explaining how the world actually works.” He also held the view that particle theorists owe more than they realize to condensed-matter theorists like himself, particularly for having developed the concept of “broken symmetry” in the 1950s.

Emergent views

During his career, Anderson wrote several scientific books, including Concepts of Solids, Basic Notions of Condensed Matter Physics (1997) and More and Different (2012). He also contributed to the philosophy of science, writing a now famous article “More is Different” for Science in 1972. This set out the limitations of “reductionism”, according to which all of science can, in theory, be derived from just a few fundamental principles.

Anderson instead believed in “emergence”, which states that everything we observe at one level obeys the laws at a more primitive level, but that those observations cannot necessarily be deduced from that level. He even dubbed it the “God principle” but told Physics World that it did not reflect any religious beliefs. “I’m not quite as atheistic as [Oxford biologist] Richard Dawkins, but I’m very close,” he said.

Anderson received the National Medal of Science in 1982 and was involved with the formation of the interdisciplinary Santa Fe Institute, which explores the science of complexity. He joined as an emeritus professor in 1985 and in 1996 Anderson became an emeritus professor at Princeton.

Indeed, Anderson remained active as a physicist well into his 80s and 90s, even being named as the “world’s most creative physicist” by one statistical analysis in 2006. He continued to review books, including a review for Physics World in 2013 of a biography of his near-contemporary Freeman Dyson, who died in February. His last letter to Physics World was published in 2017.

Outside physics, Anderson was a keen hiker and gardener as well as an enthusiast of the Chinese board game Go where he was a certified “first-degree master”.

Physics in the pandemic: ‘I learned that my students miss the structure and support that the university provides’

It’s been two weeks since Trent University shut down. We all knew it was coming, but its abruptness came as a real shock. On Thursday night, I was making final preparations for Friday lectures (one of my heavy teaching days), and on Friday morning everything was closed. Now, the term is winding down and we’re all trying to figure out how to move exams online.

Despite the upheaval, it’s easy to feel removed from everything that’s going on in the wider world. Peterborough is a small city two hours north-east of Toronto, plunked in the middle of lakes, cottages and farmland. Birds have been streaming back into the area now that winter has ended, and the seasonal renewal contrasts strangely with the constant messaging around the virus. Social distancing is almost effortless here: I can easily wander the streets around my neighbourhood without crossing paths with another person, and a 10-minute drive takes me out onto quiet railway trails through the countryside.

In many ways, my life has become simpler since the shutdown. I still have lots of work to do, but the interruptions have disappeared. As I have recently learned, this is not necessarily true for my students. Indeed, the strangest aspect of the shutdown has been losing touch with them. Trent is a small university, with a lot of interaction between students and faculty, and I hadn’t realized how much student feedback informs my teaching.

Twelve time zones away

To fill the void, I posted a survey with one question (“How are you doing?”) and discovered that they were eager to share. Most have left town, and are back in their parents’ houses, including the international students who are now up to 12 timezones away. For some, the return home comes with emotional stress that makes it hard to focus on coursework; others are grateful for the extra support that their families provide; and more than a few confessed that they have been “on vacation” since the shutdown.

More seriously, a few students are anxious because they have family members with serious health problems that make them susceptible to the virus. Ultimately, I learned that my students miss the structure and support that the university provides. But they adapt: some continue to study together using Discord, a Skype-like app specifically designed for gamers, while others build new daily routines. I hope they are ready for the real challenge that comes next week: exams.

Black hole ‘subrings’ could be seen by putting a telescope on the Moon

Adding a space telescope to the earthbound Event Horizon Telescope (EHT) should reveal the delicate series of light rings surrounding a supermassive black hole – according to a team of astrophysicists in the US. As well as providing more precise values for the mass and spin of a black hole, observing these “subrings” could also be a benchmark test of long-baseline interferometry using telescopes on Earth and in space.

In April 2019, scientists working on the EHT observed a glowing ring of light surrounding the supermassive black hole that lies at the heart of the M 87 galaxy. This first observation allowed the EHT team to determine the mass of the black hole to 6.5 billion solar masses, give or take 10%. EHT scientists we also able to work-out the direction of rotation (spin) of the black hole.

This light comes from hot matter swirling around the black hole. The light is deflected by the black hole’s immense gravitational field, making it appear like a ring to a distant observer. However, what the EHT was not able to discern is a series of subrings within this ring that should provide important information about the black hole.

A glimpse of complexity

“With the current EHT image, we’ve caught just a glimpse of the full complexity that should emerge in the image of any black hole,” says Michael Johnson of Harvard University, who was involved in this latest research.

Each of these subrings corresponds to a specific set of trajectories taken by the deflected light. Most of the light in the ring is the result of small deflections, which create a diffuse halo-like subring that is denoted n=0. Light can also follow a parabola-like path, doing a half-orbit of the black hole before escaping. This light is focussed into a thinner ring within the halo denoted n=1 because the light has made one half-orbit of the black hole.

Some light will complete one orbit of the black hole before escaping to create the even thinner n=2 subring. Indeed, a series of increasingly thinner rings are created by light that completes increasingly higher numbers of half-orbits of the black hole. As the number of half-orbits increases, the subrings also shrink in diameter and become less bright.

Very, very large arrays

Now, Johnson and colleagues have calculated the structure of these subrings and concluded that it should be possible to observe them using telescopes that are separated by very large distances.

The EHT is a network of radio telescopes that span a hemisphere of the Earth. Using a technique called very-long-baseline interferometry, the EHT is effectively an Earth-sized radio dish, which gives it extremely high angular resolution. In this latest work, the researchers have calculated that even this huge telescope is not good enough to discriminate between the first few subrings.

One way of spotting the n=1 subring, they say, could be to use a ground-based array of telescopes that are sensitive to lower-wavelength signals than the EHT. Another, possibility would be to launch radio telescopes into low-earth orbit. Detecting the n=2 ring would require a telescope on the Moon and seeing n=3 would require a telescope at the L2 Lagrangian point beyond the Moon.

The team says that one future option would be to use the Russian Millimetron mission which is expected to launch to L2 in 2029.

The research is described in Science Advances.

How physics is helping in the war against COVID-19

The COVID-19 pandemic has led to a state of global emergency. In such an interconnected world, the virus is spreading fast and cares little about national borders. Fortunately, scientific knowledge and public health responses are more advanced now than ever before.

Medical doctors, nurses and other medical staff are on the frontline of dealing with the consequences of the pandemic. Behind them, scientists from a range of fields are working intensely to better understand the virus behind the pandemic: SARS-CoV-2.

How exactly does it infect humans then spread between us? Who is most at risk? Can we develop a vaccine and drugs to defend ourselves? Where should we focus our resources? These are just some of the many difficult questions faced right now by healthcare professionals, research scientists and politicians.

This short video looks at how physics and physics-based technologies can play a role in tackling these questions. To find out more about how physics is helping in the global response to COVID-19 see this article by science writer Jon Cartwright.

Physics in the pandemic: ‘I’m worried about paying for medical treatment’

Haley Harrison in front of her computer
Nanoscience PhD student Haley Harrison is catching up on writing, analysis and quality time with her five-year-old daughter – not necessarily in that order. (Courtesy: Haley Harrison)

I am a third-year doctoral candidate in nanoscience at the University of North Carolina, Greensboro, US, and my research focuses primarily on nanoscale surface modification. On most of my workdays, I’m in a typical wet chemistry lab or doing spectroscopy, but I also spend one week every month at NASA’s Glenn Research Center in Cleveland, Ohio.

Two weeks ago, when everything began shutting down and the US Center for Disease Control was increasingly urging people to travel home and stay there, I was doing work at Glenn Research Center and my five-year-old daughter was with my sister, who graciously watches her when I travel. When I got back, I briefly went to my university campus to finish up some last-minute experiments, although this was discouraged. Things happened fast, and my university has been diligent about shutting things down, but the labs are still open with some restrictions in place – for example, using gloves to open doors and only having one person in a lab at a time. They have also cancelled all instrument training, and technicians are operating most instruments for now.

Since then, my PhD adviser has pretty much ordered us not to work in the lab, so I won’t be back until May. I am currently working remotely and communicating with colleagues via e-mail and in shared Word documents. I know my project trajectory will be significantly altered if I can’t be in the lab this semester, which is hard to accept. I am sad that my research isn’t going to go as planned, but with the health of the world at stake it is a small price to pay.

Family concerns

On the personal side, my mom and I live together, and she just finished her cancer treatment. Normally, this is a momentous occasion, but due to the restrictions at hospitals she went to her last treatment alone and we didn’t get to see her ring the “I beat cancer” bell. She then drove directly to work. It is stressful knowing she is putting herself at risk, but her job isn’t guaranteed if she doesn’t go in. My daughter and I are taking extra precautions because Mom’s immune system is compromised from her cancer treatment, which means she is at a higher risk of complications from COVID-19. We send one family member to get groceries once a week and rely on various delivery services in the meantime.

My sister has been unofficially “furloughed”, which means she is on unpaid leave from her job. And she works for the state government! Don’t even get me started on how embarrassing it is that they would put her on unpaid leave. Although some employers are being proactive, others are clearly showing their complacency and leaving their employees to fend for themselves at this uncertain time.

As for me, I have a kindergartener at home and my priority will always be to make sure her needs are met. That means I’m at the mercy of the school system: as long as her school is closed, I’ll be at home, and caring for and educating a kindergartener doesn’t leave much time for doing anything that isn’t kindergarten.

Facing uncertainty

Apart from that, my biggest concern is the uncertainty. I am a graduate student, and our health insurance and pay is never guaranteed outside of our nine-month contracts. I am worried about paying for medical treatments if I do get sick, or if someone else in my family gets sick. I am worried about what will happen for us financially if social distancing goes into the late spring and early summer, as summer funding is always precarious even in a normal year and taking internships/childcare may not be an option.

There are some silver linings for me, though, at least temporarily. Because I have a bad habit of filling my lab notebooks with ideas and collecting lots of data all at once, I have lots of writing and analysis to catch up on. I find this one of the hardest parts of being a PhD student, so I could really benefit from taking this time to go through my work slowly and dig out mistakes so I can meticulously strengthen the skills I feel are weakest. Also, at my university, we have an open office setting, so it is nice to be able to think without having a colleague walking past every few minutes asking what I’m up to.

A final benefit is that I love spending unstructured time at home with my daughter. I’ve been a graduate student for most of her life, so I haven’t had this kind of time at home with her since she was very little. I suffer from “mom guilt” as a result – putting her into daycare was hard. Now I love sitting and listening to her tell me about all the things that go on in her little mind. We have been making the most of this time by playing, drawing, watching movies and catching up on endless craft videos on YouTube.

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