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Space hurricane observed in the Earth’s upper atmosphere

A space hurricane – complete with electron “rain” – has been detected in the Earth’s upper atmosphere for the first time, an international team of researchers has reported. With the requisite plasma and magnetic fields needed for such storms present in the atmospheres of planets across the universe, the researchers suggest that such phenomena should be commonplace.

The hurricanes with which we are more familiar form in the Earth’s lower atmosphere over warm bodies of water. As warm, moist air rises, it creates a pocket of low pressure near the ocean’s surface, which in turn sucks in the surrounding air, generating strong winds and creating clouds that lead eventually to heavy rainfall. As a result of the Coriolis effect, the inward rushing air is deflected on a circular path – forming the characteristic spiral shape of a tropical storm.

Hurricanes have also been spotted in the lower atmospheres of our neighbouring planets of Mars, Jupiter and Saturn, while similar phenomena – so-called “solar tornados” – have even been spotted churning the surface of the Sun. However, such swirling masses had never before been detected in the upper atmosphere of a planet.

The space hurricane in question was recorded above the North Pole, some several hundred kilometres up into the ionosphere, back in August 2014 by four satellites in the US Defense Meteorological Satellite Program. However, it was only revealed in the data by recent retrospective analysis led by researchers from China’s Shandong University.

Using three-dimensional magnetospheric modelling, the team was able to create an image of the phenomenon – a swirling, 1000-km-wide funnel composed not of air, but plasma. It rotated around in an anti-clockwise direction, sported multiple spiral arms, had a calm “eye” at its centre and lasted for a duration of around eight hours before gradually breaking down.

“Until now, it was uncertain that space plasma hurricanes even existed, so to prove this with such a striking observation is incredible,” says paper author and space scientist Mike Lockwood of the University of Reading. “Tropical storms are associated with huge amounts of energy, and these space hurricanes must be created by unusually large and rapid transfer of solar wind energy and charged particles into the Earth’s upper atmosphere.”

Based on their model, the team believe that the phenomena formed as the result of interactions between incoming solar wind and the Earth’s magnetic field. Notably, the hurricane appeared during a period of low solar and geomagnetic activity – with the interplanetary magnetic field pointing northward – suggesting that such hurricanes may be frequently occurring phenomena in the atmosphere of both Earth and other planets.

Schematic of the space hurricane
Schematic of the space hurricane in the northern polar ionosphere and its formation mechanism. (Courtesy: CC BY 4.0/Nat. Commun. 10.1038/s41467-021-21459-y)

“Vorticity is well-known to be associated with field-aligned current flows, but it is intriguing to see such an intense current during northward interplanetary magnetic field, when one would generally expect the currents flowing to be smaller,” comments John Coxon, a space physics researcher from the University of Southampton who was not involved in the present study.

As radar observations can directly measure the plasma flow speed from the ground, “it will be interesting to see whether radars see the large-scale vorticity that the authors report, and if not, why that might be,” Coxon adds.

“We have known for a while that interesting energetic interactions such as the ones described in the paper also exist during northward interplanetary magnetic field, but these are often overlooked as unimportant,” says Maria-Theresia Walach, a solar terrestrial physicist from Lancaster University, who was also not involved the study. But she questions the name chosen by Harwood’s team. “The phenomena that has been observed here is not new, so from a scientific perspective I find renaming it a ‘space hurricane’ not useful, despite being catchier than ‘high-latitude dayside auroral [HiLDA] spots’.”

Nevertheless, she says, “this study shows a very nice case-study of some of the interactions between the solar wind, the magnetosphere and the ionosphere at Earth.”

Lockwood, however, disagrees with this interpretation. “I have no doubt that the auroral spot at the centre of the event in our paper is what has been called HiLDA event, but this paper is only marginally about the auroral spot,” he tells Physics World. “What marks this particular event out is its longevity, the spiral arm structure that forms in the field aligned currents and aurora, the extremely large energy deposition at a time of minimal geomagnetic activity, and the lobe reconnection extending unusually far onto the nightside because of the unusual combination of interplanetary conditions.”

While the space hurricane would have had little tangible impact down on the Earth’s surface, the electron precipitation from such storms in the ionosphere does have the potential to disrupt communications, GPS satellites and radar operation, as well as potentially altering the orbital patterns of space debris at low orbital altitudes. This, the researchers concluded, highlights the importance of continued and improved monitoring of space weather.

The study is described in Nature Communications.

Is the answer to climate change lying beneath your feet?

About 15 years ago I bought a house in Cornwall. It wasn’t any ordinary house but a “future-technology” building that had been fitted with a ground-source heat pump and underfloor heating. It was a fantastic place to live in – well insulated and always cosy. As soon as my family moved in and we had unpacked, I simply had to find out how it all worked.

Built in 2001, the house had been fitted with a “GeoKitten” ground-source heat pump made by a local Truro-based firm called Kensa, which pioneered the adoption of the technology in the UK. The pump is a bit like a fridge that works in reverse, pulling heat in from liquid circulating through coils buried underneath my front garden. Coupled to the underfloor heating circuit, the pump made the house a fabulous place to live in, especially when winter storms were raging outside in Falmouth Bay.

Lovely warmth was sent by the pump around the house, with every room having an adjustable flow rate and precise temperature control

Lovely warmth was sent by the pump around the house, with every room having an adjustable flow rate and therefore precise temperature control. But as it was the only form of heating, I was expecting a massive electricity bill. Turns out heat pumps are incredibly efficient, with the coefficient of performance (COP) – the ratio of heat output to electrical-energy input – being about 3–4.5. My fears were unfounded.

In fact, Kensa has been a great UK success story. Founded in 1999 by two former marine engineers who’d spent years installing heat pumps on luxury yachts in the Mediterranean, the company’s products are now used in many domestic and commercial settings. Realizing they were local, I once called the owners, who kindly showed me round their factory. It was small back then, but the firm has grown and expanded. David Cameron even visited in 2009 before becoming prime minister.

Too hot to handle

Ground-source heat pumps are a great way of heating. Apart from being quiet and cheap to run, they need little maintenance and last for ages. Heat pumps will help the shift away from traditional gas- and oil-fired boilers – in fact, the UK’s recent 10-point plan for a “green industrial revolution” envisages 600,000 heat pumps being installed in Britain every year by 2028. That’ll save the equivalent of 71 million tonnes of carbon-dioxide emissions (16% of the country’s total).

Ground-source heat pumps are a great way of heating. Apart from being quiet and cheap to run, they need little maintenance and last for ages

First developed in the 1940s by an American inventor called Robert C Webber after a bizarre incident in which he burned his hand on the hot outlet pipe from his domestic deep freezer, there are now countless ground-source heat pumps around the world. Study after study has shown that such systems are both effective and efficient. The reason why they’re not the number-one choice for heating buildings is simple: the up-front costs are high.

A conventional domestic gas boiler uses about 20 kW of power and costs £50 per kilowatt, whereas a comparable ground-source of 17 kW costs more like £500/kW. Of course, if your house is well insulated you need less capacity. Costs will also fall over time as demand for such heating systems increases (regulation will be vital for that to happen). I suspect that  the eventual mature market price of ground-source heat pumps will be around the £100–£200/kW mark.

That’s roughly similar to the cost of air-source heat pumps, which transfer heat from the outside air to the inside of a building and are likely to be the main competitor to ground-source pumps if gas boilers are banned. Ground-source heat pumps are harder to install. Installing the coils (or “slinkies”) in the ground has to be carefully done. You also need to dig quite a few large holes, which is tricky for anything other than a new or refurbished property.

But if your house is well insulated, as it should be, then you can expect to save money compared to an LPG boiler within seven years. Unfortunately, compared to a mains-gas system, there’s no payback at all over that time with a ground-source heat pump. Gas, despite being a carbon-dioxide-producing fossil fuel, is currently so cheap.

And there’s the rub. People are generally poor at making long-term decisions based on a potential future payback. If it’s a choice between saving money now or in the future, most of us can only think of the here-and-now. And unless customers want to pay extra for environmentally friendly systems, house builders have no reason to invest in them. That’s why new regulations, such as those outlined in the UK’s 10-point plan, are so vital. They will force the construction industry towards this green technology.

Zeroing in 

The beauty of ground-source heat pumps is that they can be fitted even in built-up areas where space is at a premium. A great example is the new headquarters of the Institute of Physics (IOP) in King’s Cross, London. It won a CIBSE building-performance award in 2020 for its green technology, which includes photovoltaics, LED lights and an advanced ground-source heat pump consisting of 10 vertical boreholes extending 60 m below the building – the first time such a system has been used anywhere in the UK.

After its first full year of operation, the system had a COP rating of 3.4 – a figure that’s likely to improve still further as the system gets tweaked and optimized. It’s a success story for the IOP, proving that the physics community is leading by example. I believe green technology and growth go hand in hand, and that, with sufficient focus, the UK can meet its net-zero-carbon goal by 2050. Thanks in part to heat pumps, we have the means to tackle the threat of climate change, which is one of the most enduring dangers that we face.

Quantum mechanics gives new insights into the Gibbs paradox

Entropy has been a subject of debate among physicists ever since it was formulated in classical thermodynamics some 150 years ago. One such debate centres on the so-called Gibbs paradox, in which the entropy of a system seems to depend on how much an observer knows about it. Astounding and confounding the physics community when it was first put forward by the American physicist Josiah Willard Gibbs in 1875, the paradox has since found numerous resolutions, albeit mainly in the classical setting with ideal gases.

Researchers at the University of Oxford and the University of Nottingham, UK, have now shed light on what the Gibbs paradox may look like in the quantum realm. By leveraging quantum effects, they show that more work can be extracted from a system than would be possible classically. Their result lays the theoretical groundwork for an experimental demonstration in the future, and could have applications in the burgeoning effort to manipulate large quantum systems.

The classical Gibbs paradox

The classical Gibbs paradox takes the form of a thought experiment involving a box with a partition that separates two bodies of gas. When the partition is removed, the two gas bodies mix spontaneously. To an informed observer who can distinguish the two gas bodies, the system’s entropy increases. On the other hand, for an ignorant observer who cannot discern any differences between the two gas bodies, there is no visible mixing and the entropy remains unchanged.

This difference of opinion has a physical significance since work can be extracted through the mixing process when the entropy increases. That suggests that the system’s entropy should be an objective quantity – something that does not reconcile with the existence of the different outcomes for the two observers. Gibbs, however, noted that the extraction of work depends on the experimental apparatus of the observer. Hence, the informed observer can extract work, whereas the ignorant observer has to contend with their inability to do so. This makes each observer’s reality consistent with the entropy change they witness.

Quantum effects

In the new work, the Oxford-Nottingham team considered how quantum effects such as superposition would affect the thought experiment. As in the classical case, the informed observer witnesses an entropy increase. For the ignorant observer, however, there is a marked difference after transitioning to the quantum realm. Although they are still unable to distinguish the two gases, they, too, can now witness an entropy increase. At the macroscopic limit, this entropy increase can even become as large as that which the informed observer perceives, providing the maximal discrepancy to the classical case.

Though the result might seem surprising at first, the researchers behind it say that it is a stark reminder that the classical limit is not always the same as the macroscopic one. “The classical limit is not just about large particle numbers, but also about limited degrees of control,” Benjamin Yadin, Benjamin Morris and Gerardo Adesso explain in an e-mail to Physics World. By giving the ignorant observer complex control over microscopic quantum degrees of freedom, they add, that observer becomes able to derive quantum effects at the macroscopic scale.

While some consider quantum mechanics to be a resolution to the classical Gibbs paradox, Yadin, Morris and Adesso note that their result indicates otherwise. “Our work shows that quantum effects can add an additional layer of seemingly paradoxical behaviour,” they say. They emphasise that their result is impossible in classical physics, as it relies on the symmetry requirements of bosons and fermions – a property not found in classical mechanics.

The researchers are now working on a proposal for demonstrating this effect experimentally. They explain that doing so requires a degree of quantum control, which may be possible in optical lattices and Bose-Einstein condensates. In the long term, they believe it could be possible to use this theory to build an effective quantum heat engine, one that could operate in regimes where a classical heat engine would fail.

“The question of how quantum features of identical particles may be harnessed for thermodynamical advantages is currently gaining a lot of interest – and we would like to see our work inspire other novel ideas in this area,” Yadin, Morris and Adesso conclude.

The research is reported in Nature Communications.

Spoilt for choice: APS March meeting explores the wide range of careers available to physicists

Before I chose to study physics, I remember hearing more than once that ”you can do anything with a physics degree”. As encouraging as that statement sounds, it is also vague. Although I already knew that most people who study physics don’t become professional academics, the overriding picture I had of a physicist was of someone working in a lab at a university or research institute. Where did all those missing physicists end up?

I now have a renewed interest in this question, having recently taken on the role of reviews and careers editor at Physics World, where we aim to spread the word about the possible careers you can pursue with a physics background. Apart from regular careers articles online and in the print magazine, we also publish annual career guides for the Institute of Physics and the American Physical Society (APS). That’s why I was keen to attend the APS March meeting 2021, which demonstrated some of the many directions that a degree in physics can take you in.

One session I attended focused on early-career physicists, and kicked off with a talk from Maika Takita, who studied physics at Barnard College, New York, before doing a PhD in the department of electrical engineering at Princeton University. Now a quantum computing researcher at IBM Quantum in New York, she spoke about her work on superconducting qubits – a career she never imagined she would have – and described how her academic background led her into industry.

Kenneth Gotlieb, who studied physics at Harvard University and completed a PhD in applied physics at the University of California, Berkeley, also spoke about making the transition from academic to industry research. He is now a senior scientist at Triple Ring Technologies, which has branches in California and Massachusetts, and described some of the medical devices he works on, from X-ray machines mounted on robots for taking X-rays of horses to non-invasive oxygen monitoring systems for use in surgery.

The speakers compared industry research with academic research, mentioning the importance of data analysis and creative problem solving within a team in both environments. A key difference was the strong focus in industry on clients’ needs and a willingness to change your line of work depending on their wishes.

Other talks in the session showed how a background in physics can be used more indirectly. Yue Zhang, a machine learning engineer who works on recommendation algorithms at Facebook, spoke about how she uses mathematical tools that she became familiar with during her PhD in applied physics at Rice University. I was fascinated to learn that vectors, which are used in physics to describe a distance, can be used in machine learning to represent similarities between items in a numerical way, and thus be used in recommendation algorithms.

Similarly, Calvin Patel, a stock analyst at Morgan Stanley, emphasized the value of mathematical skills, which he developed during his PhD in physics at the University of California Irvine, for careers in finance. He described the function of finance as the evaluation of the profitability of ideas, and explained the importance of statistics and probability in the field.

Physics is also essential to tackling some of the most pressing problems we face today, which was highlighted by a session on ”Seeing the energy future”. Among several talks was one by Denise Gray, president of LG Energy Solution, Michigan. Having worked on electric vehicles for over a decade, she outlined the remarkably quick progress that has been made in the area, with the distance they can travel on a single charge soaring from a little over 100 km to nearly 500 km. She attributes this to the collaboration between researchers who saw the potential in the technology and to funders who have supported their work. She also spoke about the promising future of the technology and the challenges currently being addressed, such as increasing the speed at which the vehicle batteries charge.

These are just a handful of the many careers at the APS March meeting. Indeed, it’s hard, impossible even, to summarize what physicists end up doing, because there are so many branches and niches in which physics is useful. In a world where science and technology are so advanced and ubiquitous in daily life, the answer to where physicists end up is easy: they’re everywhere.

Physics World is always on the lookout for people to talk about their careers in our magazine and online. If you’re interested in spreading the word about your field, get in touch at pwld@ioppublishing.org

World’s smallest origami bird, why hummingbirds hum, physics meeting ‘ain’t got that swing’

 

Spring has arrived here in Bristol and the birds are going bonkers in our garden, especially the amorous wood pigeons. So this edition of the Red Folder is dedicated to our feathered friends.

Cornell University is famous for its ornithology lab, but now physicists at the US university have also gone to birds and created what they describe as the “world’s smallest origami bird”. Measuring about 60 microns across, the folding bird is actuated by an extremely thin layered material that bends when a voltage is applied to it. It was created by Qingkun Li, Itai Cohen, Paul McEuen and colleagues – who explain how and why they have created the tiny folding bird in the above video.

Hummingbirds are not quite as small as the Cornell origami bird, but their tiny size and habit of feeding on nectar from flowers means that they have a unique way of flying. Indeed, hummingbird refers to the humming sound that the birds’ wings make as they flap furiously to hover in front of flowers.

Now researchers at Eindhoven University of Technology and the spin-out company Sorama, both in the Netherlands, and Stanford University in the US have used a myriad array of sensing equipment to work out how the hum is generated. They found the hum is made by changes in the pressure differences between regions above and below the wings as the birds flap to create their upward hovering force. The team have developed a model to describe hum generation and say that it could be used to design quieter fans. You can read more in a paper in the journal eLife.

The saxophonist Charlie Parker was famously known as Bird and is much revered in music circles for his development of bebop in the 1940s. With a fast tempo and complex chord changes, this form of jazz continues to have an important influence on musicians today.

The riddle of swing

Bebop often incorporates highly syncopated rhythms, which I am sure delight the German physicist Theo Geisel who uses mathematics to study musical rhythms. At this week’s virtual March Meeting of the American Physical Society, Geisel gave a talk on “Psychophysics of musical rhythms and the riddle of swing”.

Geisel and colleagues at the Max Planck Institute for Dynamics and Self-Organization have shown that “certain systematic microtiming deviations between musicians do enhance and are relevant for the swing feel in jazz”.

Unfortunately, the recording of Geisel’s talk seems to have been expunged from the video of the meeting session, E14 Physics of Social Interactions III. However, you can read more about this fascinating topic in this open access paper: “Microtiming deviations and swing feel in jazz”.

Spacecraft in a ‘warp bubble’ could travel faster than light, claims physicist

Albert Einstein’s special theory of relativity famously dictates that no known object can travel faster than the speed of light in vacuum, which is 299,792 km/s. This speed limit makes it unlikely that humans will ever be able to send spacecraft to explore beyond our local area of the Milky Way.

However, new research by Erik Lentz at the University of Göttingen suggests a way beyond this limit. The catch is that his scheme requires vast amounts of energy and it may not be able to propel a spacecraft.

Lentz proposes that conventional energy sources could be capable of arranging the structure of space–time in the form of a soliton – a robust singular wave. This soliton would act like a “warp bubble’”, contracting space in front of it and expanding space behind. Unlike objects within space–time, space–time itself can bend, expand or warp at any speed. Therefore, a spacecraft contained in a hyperfast bubble could arrive at its destination faster than light would in normal space without breaking any physical laws, even Einstein’s cosmic speed limit.

Negative energy

The idea of creating warp bubbles is not new, it was first proposed in 1994 by the Mexican physicist Miguel Alcubierre who dubbed them “warp drives” in homage to the sci-fi series Star Trek. However, until Lentz’s research it was thought that the only way to produce a warp drive was by generating vast amounts of negative energy – perhaps by using some sort of undiscovered exotic matter or by the manipulation of dark energy. To get around this problem, Lentz constructed an unexplored geometric structure of space–time to derive a new family of solutions to Einstein’s general relativity equations called positive-energy solitons.

Though Lentz’s solitons appear to conform to Einstein’s general theory of relativity and remove the need to create negative energy, space agencies will not be building warp drives any time soon, if ever. Part of the reason is that Lentz’s positive-energy warp drive requires a huge amount of energy. A 100 m radius spacecraft would require the energy equivalent to “hundreds of times of the mass of the planet Jupiter”, according to Lentz. He adds that to be practical, this requirement would have to be reduced by about 30 orders of magnitude to be on par with the output of a modern nuclear fission reactor.  Lentz is currently exploring existing energy-saving schemes to see if the energy required can be reduced to a practical level.

Any warp drive would also need to overcome several other serious issues. Alcubierre, who regards Lentz’s work as a “significant development”, cites the “horizon problem” as one of the most pernicious. “A warp bubble travelling faster than light cannot be created from inside the bubble, as the leading edge of the bubble would be beyond the reach of a spaceship sitting at its centre,” he explains. “The problem is that you need energy to deform space all the way to the very edge of the bubble, and the ship simply can’t put it there.”

Spacecraft doubts

Lentz describes his calculations in Classical and Quantum Gravity, where other recent research on the topic is outlined in an accepted manuscript from Advanced Propulsion Laboratory researchers Alexey Bobrick and Gianni Martire. The duo describes a general model for a warp drive incorporating all existing positive-energy and negative-energy warp drive schemes, except Lentz’s which they say “likely forms a new class of warp drive space–times”.

However, they argue that a Lentz-type warp drive is like any other type of warp drive in the sense that, at its core, it is a shell of regular material and therefore subject to Einstein’s cosmic speed limit, concluding that “there is no known way of accelerating a warp drive beyond the speed of light”.

Though he recognizes these huge hurdles to building a warp drive, Lentz feels they are not insurmountable. “This work has moved the problem of faster-than-light travel one step away from theoretical research in fundamental physics and closer to engineering,” he says.

After addressing energy requirements, Lentz plans to “devise a means of creating and accelerating (and dissipating and decelerating) the positive-energy solitons from their constituent matter sources”, then confirm the existence of small and slow solitons in a laboratory, and finally address the horizon problem. “This will be important to passing the speed of light with a fully autonomous soliton,” he says.

Nanoparticle-based vaccine offers new approach to COVID-19 immunity

As the international effort to vaccinate the population against COVID-19 gathers pace, the demand for vaccine doses that can be used in all countries and climates is enormous. Researchers from Cleveland Clinic and Chungbuk National University have described a new vaccine candidate that triggers an immune response using antigens attached to nanoparticles, potentially bypassing the need for cold storage during delivery. They report their findings in mBio.

All vaccines approved to date cause an immune response against the same part of the SARS-CoV-2 virus: the receptor binding domain (RBD) of the spike protein. However, they employ different mechanisms to bring this about – from using inactivated viruses to cause RBD production to delivering genetic instructions directly into cells. This new candidate provides an alternative approach – using inert nanoparticles to carry the RBD and display it to the immune system.

Direct delivery

Delivering the RBD protein directly, rather than causing the cell to produce it, seems an appealing option for vaccination. However, the body’s immune defences won’t respond to such a small molecule. Attaching multiple RBD units to a larger nanoparticle overcomes this challenge and makes them visible to the immune system. The nanoparticles used in this study are built from ferritin – a naturally produced protein existing in most organisms that can self-assemble into a useful nanoparticle structure.

The researchers tested the vaccine candidate in ferrets – which are susceptible to the same respiratory infections as humans. They saw that after three injections with this vaccine, the vaccinated ferrets had high levels of antibodies against SARS-CoV-2 in their bloodstreams. Ferrets treated with the vaccine and then exposed to the virus did not experience symptoms and cleared the virus from their system far quicker than unvaccinated ferrets.

The researchers were even able to show how vaccinated ferrets avoided lung damage caused by the infection. They note that combining intramuscular injection with introducing the vaccine through the nose – where SARS-CoV-2 commonly enters the body – produced an even stronger protective effect.

A hot topic

One of the biggest challenges in vaccinating the world’s population is getting the vaccine efficiently to every place it is needed. The vaccines in current use all need consistent cold storage, and in some cases ultracold storage, to remain effective. By being built from a nanoparticle structure that is naturally very thermostable, this new candidate may not need such conditions.

“This protein is an attractive biomaterial for vaccine and drug delivery for many reasons, including that it does not require strict temperature control,” says study author Jae Jung.

“This would dramatically ease shipping and storage constraints, which are challenges we’re currently experiencing in national distribution efforts. It would also be beneficial for distribution to developing countries,” adds co-first author Dokyun Kim. The authors note that this stability needs to be more rigorously verified, but were it to remain true, this could be one tool to help reduce global inequities in vaccine availability.

Any vaccine candidate still has many stages to progress through before it is approved for widespread use in humans, but the data so far for this approach are promising. If the vaccine comes to fruition, it will add a different type of weapon to the already diverse arsenal available to combat the continuing COVID-19 pandemic.

Hydroplaning of tyres is imaged using tiny fluorescent particles

Detailed images showing how water drains through tyre grooves during hydroplaning have been obtained by Serge Simoëns and colleagues at France’s University of Lyon. Their technique could provide crucial guidance to engineers trying to design tyres that are better suited to driving in wet conditions.

When a tyre rolls over a wet or flooded road, a build-up of water pressure at the front of the tyre can generate a lifting force. Known as hydroplaning, the effect can cause tyres to lose all contact with the road if this lift becomes greater than the weight of the car. To minimize its influence, tyre treads must drain as much water as possible from front to the back, without significantly reducing road adhesion. Since the fluid dynamics involved in hydroplaning are highly complex, tread designs must be informed by detailed information about these flows.

Particle imaging velocimetry (PIV) is a widely used technique for measuring flow velocities in 2D. It involves seeding fluid with fluorescent tracer particles that must be small enough to accurately reflect the dynamics of the fluid surrounding them. Then, a 2D slice of the fluid is illuminated by a laser sheet, causing the particles to glow and create a direct image of the flow.

Fluorescent test track

In their study, Cabut’s team used PIV to image a thin film of water on a test track, as a car drove through it at several different speeds. Their images were captured from below, through a transparent window embedded in the road. To overcome the optical constraints of the setup, the researchers combined their fluorescence images with measurements of laser sheet refraction at the interface between the window and the flowing water.

Inside the grooves, Cabut and colleagues observed white elongated filaments, which hinted at a gaseous phase – possibly cavities or air bubbles – within the liquid water. In the largest grooves, these columns showed some local periodic distortions. The team suggests that the nature of this phase could be linked to properties of a tread including groove widths, spacings between adjacent grooves, and the locations of the transverse grooves connecting them. The team also observed swirling vortices in some of the grooves. These could have arisen from flows around the sharp edges of the tyre’s ribs, and their number may be related to the ratios between the heights and widths of the grooves.

For now, it is not yet possible for Cabut’s team to determine exactly how these vortices and bubble columns came about, and further studies will be needed to pin down their formation mechanisms. However, their innovative new setup will likely be a key first step in these efforts. With a greater knowledge of the flow velocities involved in hydroplaning, engineers could design tread patterns that are better suited to minimizing the effect, while maintaining overall tyre performance.

The research is described in Physics of Fluids.

Artist draws on her physics background for inspiration, improving proton therapy for better cancer care

This episode of the Physics World Weekly podcast features the artist Geraldine Cox, who draws on her background in physics to create pieces inspired by the patterns of nature. Cox talks about her ongoing collaboration with physicists at Imperial College London and also about her work with World of Atoms, a UK-based organization that uses art, experiment, poetry and dance to teach children about atoms.

Also featured this week is the University of Liverpool accelerator physicist Carsten Welsch. He talks about his role as coordinator of Optimization of Medical Accelerators, which is a European training network, and he explains how the network is working to improve cancer care by optimizing proton therapy technologies.

 

Ultracold atoms permit direct observation of quasiparticle dynamics

Theories of how quasiparticles form have been around for more than 80 years, but direct observations of the process have remained elusive due to experimental challenges. A team of researchers at the Center for Complex Quantum Systems, Aarhus University, Denmark has recently overcome these obstacles by studying quasiparticle formation and dynamics in ultracold atoms.

The Soviet physicist Lev Landau developed a theory of quasiparticles – emergent phenomena that arise from a complex interaction between many real particles – in the 1930s. This theory, which is still routinely used in practical applications ranging from superconductivity to transport processes in electronic devices, considers the motion of an electron through a solid and describes how the electron (the quantum impurity) triggers the formation of a quasiparticle within the solid.

An ultracold analogue

Due to the high densities and fast timescales of this system, however, experiments cannot directly probe such quasiparticle behaviour in solids. The Aarhus team instead studied an analogue system: a quasiparticle called a polaron in a Bose-Einstein condensate (BEC). This dilute gas of ultracold atoms offers a pristine, controlled environment in which to study the quantum dynamics of many-body phenomena.

Photo of optical elements used in the experiment, bathed in purple light from a laser
Simulation centre: The region of the Aarhus experiment where atoms are initially trapped and cooled. (Courtesy: Lars Kruse/AU foto)

“Quasiparticles are exceedingly interesting to study since they may be composed by numerous particles and their excitations,” explains lead author Magnus Skou, a PhD student at Aarhus. “The Bose polaron is an excellent example of such a challenging quasiparticle that nonetheless holds great potential for helping us to understand exotic technologies like organic semiconductors and superconductors. This inspired us to investigate the polaron in an ultracold cloud of atoms and, in particular, to see if we could observe its gradual formation.”

Witnessing the formation of a Bose polaron

The team created the impurity not with an electron, but by manipulating the quantum state of only a few atoms in the BEC. Through theoretical modelling of the system, the authors identified three different dynamical regimes to describe the state of the impurity. By tuning the interaction strength of the atoms in the condensate and evolving the experiment for different lengths of time, the group experimentally probed each of these regimes. Their experiments, which Skou and fellow co-author Kristian Nielsen describe in a video posted to Twitter, showed how the impurity gradually evolved to form the polaron.

Skou predicts that their experiments offer promising pathways to better understand the interactions between quasiparticles. “Now that we have a better understanding of polarons,” he says, “it will be interesting to study how they interact with each other. These intricate interactions were recently predicted to enable the formation of a completely new quasiparticle known as a bipolaron. This quasiparticle has not yet been observed in an ultracold atomic gas, but we now believe that our experiment may allow for it to finally be seen.”

The research is described in Nature Physics.

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