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Why do beer bottles foam when struck on top?

A foamy mess in the making (Courtesy: Javier Rodríguez-Rodríguez) — A foamy mess in the making. (Courtesy: Javier Rodríguez-Rodríguez)

By Hamish Johnston

We’ve all had a friend who does it – you’re deep in conversation at a party, beer bottle in hand, when someone sneaks up and taps the top of your bottle with theirs, causing a foamy mess to erupt from your bottle. And to add insult to injury, their bottle doesn’t foam.

Now, physicists in Spain and France have studied this curious effect and gained a better understanding of how it occurs. While their work won’t prevent wet shoes and slippery floors at university social gatherings, the researchers believe their work could provide insights into geological features such as oil reservoirs, mud volcanoes and “exploding lakes”.

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Model explains why liquid suspensions suddenly turn solid

A new model combining fluid dynamics with granular dynamics can provide important insights into discontinuous shear thickening – a curious effect that causes some fluid-like materials to suddenly behave like a solid. Developed by physicists in the US, the model agrees with key experimental observations and could help researchers develop new technologies based on shear thickening, such as flexible body armour.

Suspensions of tiny solid particles dispersed in fluids can exhibit a range of bizarre properties that have proven difficult for physicists to fully understand using traditional fluid dynamics. Most notably, they do not have a constant viscosity: some suspensions become runnier when they are stirred faster or pushed harder, while others become thicker. Albert Einstein first evaluated the effect of suspended particles on viscosity in 1906, and there have been numerous theoretical models since then. None of these, however, has adequately explained discontinuous shear thickening: the abrupt solid-like stiffening that is easily observed in a suspension of corn starch in water.

In the new research, Ryohei Seto and colleagues at the City College of New York considered the essential nature of a suspension – a mixture of solid grains and liquid medium. This allowed them to combine ideas from fluid dynamics with those from granular physics. In fluid dynamics models, suspended particles are forbidden to touch, and all interactions are mediated by the fluid medium. In contrast, there is no medium in granular physics, or the medium is a gas that offers little resistance to particle contact. As a result the macroscopic dynamics of granular physics are the product of individual interactions between the granules.

Chains of force

At low shear, the viscosity is mainly caused by the difficulty the medium has in squeezing itself between the solid particles as the suspension is moved. In reality, there are direct contacts between the particles, and these do add to the viscosity. Forces applied to one particle can sometimes be passed directly to the next, creating a chain of particles moving together. However, these force chains are well dispersed and too short to have a significant impact on the macroscopic behaviour of the suspension.

Above a certain shear rate, however, the direct contacts between particles become more numerous as the particles are forced closer together more and the medium is unable to flow between them fast enough. This creates more chains of particles moving together, and allows the chains to get longer. At a certain point, the chains get tangled up in each other and lock together, and the suspension becomes an amorphous solid with liquid in the gaps between particles.

While this kind of jamming behaviour is well understood in granular physics, the New York researchers’ model is the first to start from the basic forces on a suspended particle and capture this liquid–solid crossover. “Starting from basic assumptions familiar to the hydrodynamics community,” says Seto, “we introduced some particle contact, and we borrowed a model from granular physics to describe contact. So our work is bridging two fields.”

The model does a good job of reproducing a range of experimental results obtained in several different labs. Indeed, Seto explains that the group’s model provides a unified explanation of results that had been interpreted in different ways in the past.

Heinrich Jaeger at the University of Chicago is impressed by the team’s work: “I’m very excited about this paper combining ideas from two perspectives – the granular perspective, which starts from solid on solid and then worries about what happens when you put liquid in, and the suspension rheology perspective that starts from a liquid and wonders what happens when you put particles in.”

Aside from scientific interest, both Seto and Jaeger say that the work has technological applications. “Discontinuous shear thickening is typically thought of as a problem in industries,” says Jaeger. “If you are trying to transport these dense suspensions through pipes and they suddenly lock up this would be a potential disaster. Therefore it is very important to have a good approach to controlling this behaviour.” He adds that there are also applications in which it is desirable, such as flexible stab vests or other protective clothing. “At slow agitation the material behaves rather liquid-like, while sudden impacts can activate this shear thickening, which immediately turns the material into a more solid-like form.”

The research is published in Physical Review Letters.

What are the big unanswered questions in nuclear physics?

In less than 100 seconds, Peter Butler explains that while nuclear models have been highly successful, there are still many phenomena in nuclear physics yet to be fully understood. This includes the reasons for the existence of exceptionally tightly bound nuclei with “magic numbers” of protons and neutrons. Butler is also excited by the fact that experimentalists are starting to recreate conditions similar to those that existed just an instant after the Big Bang, when matter was severely compressed.

Watch more from our 100 Second Science video series.

Postcard from Rio – checking out Brazilian physics

Beach scene on a gloomy day in Rio de Janeiro. — Not your typical summer’s day in Rio de Janeiro.

By Matin Durrani in Rio de Janeiro

Sun, sand, sea – that’s Rio de Janeiro surely?

Well, sadly, I’ve had to make do with just two out of the three as it’s been distinctly cloudy, rainy and cool since I flew in to the Brazilian metropolis yesterday at the start of a week in the world’s fifth largest country (by both size and population).

So what, you may ask, am I doing in Brazil?

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Safe graphene, Martian mollycoddling, mathematical tales and more

The “Telescope names” comic from xkcd. (Randall Munroe/Creative Commons)

By Tushna Commissariat

Just when we thought that it couldn’t possibly have any more practical applications, everybody’s favourite “wonder material” graphene is going to be used to develop “stronger, safer, and more desirable condoms”. Thanks to a Grand Challenges Explorations grant of £62,123 from the Bill and Melinda Gates Foundation, scientists at the University of Manchester will use graphene to develop new “composite nanomaterials for next-generation condoms, containing graphene”. Unsurprisingly, the story made all the national newspapers with the BBC, the Guardian, the Telegraph and the Independent all having their say. The Guardian also noted that industrial graphene-producer Applied Graphene Materials’ shares jumped by 40% during its stock-market debut, the day before the above story broke. You can read more about graphene’s many potential applications on page 50 of Physics World’s anniversary issue, a free PDF download of which is available here.

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Physicist and philanthropist Fred Kavli dies at 86

The Norwegian-born physicist and philanthropist Fred Kavli has passed away at the age of 86. Kavli, one of physics’ biggest benefactors, set up the Kavli Foundation in 2000 that has endowed research institutes at leading universities worldwide. Today, there are 17 such institutes around the world dedicated to astronomy, nanotechnology, neuroscience and theoretical physics. Kavli died at his home in Santa Barbara a year after contracting cholangiocarcinoma, a rare form of cancer.

Speaking to Physics World in 2007, Kavli said that physics was, for him, “the most interesting and fascinating subject because it deals with the most fundamental questions and forms a foundation for most science because it [gives] us an understanding of nature, the universe and the world in which we live”.

Cambridge University astrophysicist Martin Rees told physicsworld.com that he first got to know Kavli through the latter’s “generous” backing for the Kavli Institute for Cosmology at the University of Cambridge, as well as through his support for the Royal Society. “[Kavli] believed fervently in the value and potential of fundamental science,” says Rees. “His foundation – now a permanent memorial to his vision – succeeded because he chose excellent advisors and staff, and also because he had a genuinely global vision.”

From missiles to nanoscience

Kavli was born in 1927 on a small farm in Eresfjord, Norway. He chose to study physics at the Norwegian Institute of Technology (now known as the Norwegian University of Science and Technology) in Trondheim. After he graduated in 1955, Kavli applied for a visa to work in the US. A year later, with visa in hand, Kavli travelled to California, where he became chief engineer of a small firm that designed and manufactured sensors for the Atlas missile – the US’s first successful intercontinental ballistic missile.

Two years later, Kavli founded his own business, becoming head of the Kavlico Corporation, which grew into one of the world’s largest suppliers of sensors for aeronautic, automotive and other industrial applications. He remained chief executive and sole shareholder of the company until it was sold in 2000.

It was then that Kavli turned his focus to philanthropy. He established the Kavli Foundation, which aims to “advance science for the benefit of humanity, promoting public understanding of scientific research, and supporting scientists and their work”. The first such institute – the Kavli Institute for Theoretical Physics – was created at the University of California, Santa Barbara in 2002, and has now been joined by 10 other institutes in the US, one in Japan, two in China and one each in the Netherlands, Norway and the UK. “I realized that I wanted to use the fruits of a lifetime of hard work in an efficient way for the long-term benefit of humanity by supporting basic science,” Kavli told Physics World in 2007.

Hitoshi Murayama, director of the Kavli Institute for the Physics and Mathematics of the Universe at the University of Tokyo that was established in 2012, says that Kavli was “an amazing man of incredible vision”. “[Kavli] believed firmly in the importance of basic science and its long-term benefit to humanity,” adds Murayama. “His passion was contagious.”

Funding more

In 2008 Kavli enlarged the foundation’s activities, setting up the $1m Kavli Prize, which is awarded in three subjects: astronomy, neuroscience and nanotechnology. Each prize is worth $1m and is bestowed every two years.

George Efstathiou, director of the Kavli Institute for Cosmology at Cambridge Univeristy, told physicsworld.com that Kavli’s motivation for the prizes was to generate public interest – particularly in young people – in Kavli’s three favoured areas of science. “Kavli was a visionary figure and a generous and enthusiastic supporter of basic, curiosity-driven, research,” adds Efstathiou. “The Kavli Institutes are [his] way of saying that curiosity-driven research is important”.

Since 2008 Kavli has also established Kavli professorships, Kavli symposia and endowed the AAAS Kavli Science Journalism Awards, which is managed by the American Association for the Advancement of Science. The Kavli Foundation has even helped to fund a new “residential working retreat” for scientists at the 18th-century Chicheley Hall in Buckinghamshire, UK, known as the Kavli Royal Society International Centre. The hall was bought for £6.5m by the Royal Society, part-funded by Kavli himself.

“We will forever be grateful to Fred Kavli – someone who, with the foundation, invested his heart and soul into ensuring that science will make this a better world for future generations,” Robert Conn, president of the Kavli Foundation, said in a statement. “And we will carry forward this mission with the same commitment and dedication that he gave to science and his life.”

An interview with Fred Kavli appeared in the November 2007 issue of Physics World: “Once a physicist: Fred Kavli”

IceCube finds cosmic neutrinos at the South Pole

An enormous “telescope” buried deep under the ice of Antarctica has made the first observation of cosmic neutrinos. The international collaboration operating the IceCube laboratory says that the detection of these chargeless, almost massless and very high-energy particles marks the beginning of a new era in astronomy in which electromagnetic radiation is no longer the only means we have for probing the distant universe.

Detecting neutrinos from space is not new. For decades physicists have been able to observe the neutrinos generated by nuclear reactions inside the Sun, as well as those produced by cosmic rays interacting with nuclei in the Earth’s atmosphere. But neutrinos from further afield have until now remained elusive, their extremely high energies making them rarer and much harder to detect than those from closer to home.

At the same time, cosmic neutrinos are particularly prized as information carriers because their inertness allows them to pass through clouds of gas and dust that would otherwise keep distant astrophysical objects hidden from view. In particular, they might be able to reveal the origin of cosmic rays. Cosmic rays are charged particles and the paths they take to Earth are bent by galactic and intergalactic magnetic fields, which obscure their origins.

Photomultipliers watch the ice

The $275m US-led IceCube telescope, located at the Amundsen–Scott research centre at the South Pole, comprises 86 cables, each up to 2.5 km long, suspended inside vertical holes in the ice. Attached to each cable are dozens of photomultiplier tubes. The photomultipliers record the Cerenkov radiation given off by the secondary particles created when incoming neutrinos collide with hydrogen or oxygen nuclei inside the ice.

The tubes and cables are spaced so as to create a total detector volume of 1 km³. Neutrinos interact with other matter only extremely weakly, which means that the detectors have to be as large as possible if they are to register a significant number of neutrinos in a reasonable timeframe. To maximize its chance of detection, the IceCube collaboration had originally focused its efforts almost exclusively on muon neutrinos, since these generate muons that continue to travel in a forward direction for several kilometres after the neutrino has collided, so allowing interactions from beyond the photomultiplier tubes to be included in the dataset and thereby effectively increasing the detector volume.

However, a twin discovery made using data collected between May 2010 and May 2012 persuaded the team to take a different approach. The data contained two collisions – nicknamed Bert and Ernie and each involving a whopping 10¹⁵ eV of kinetic energy – that were located inside the bounds of the detector. As a result, the researchers started a new analysis using only high-energy events originating inside the instrumented cubic kilometre of ice.

Larger showers

This move limited the amount of data the researchers had to work with but made it easier to filter out collisions involving the far more abundant atmospheric neutrinos. This is because high-energy events of cosmic origin tend to create showers of secondary particles in the detector, whereas lower-energy atmospheric neutrinos tend to produce single muon tracks (see figure “A shower called Ernie”).

Shower of particles associated with a cosmic neutrino event called Ernie — **A shower called Ernie** This shower of particles was detected by IceCube on 3 January 2012 and was created by the highest energy neutrino ever observed. Called Ernie, its energy is estimated to be 1.14 × 10¹⁵ eV. The image shows the signals detected in various photomultiplier tubes strung along several cables in the ice. (Courtesy: IceCube Collaboration)

The change of strategy paid off and the researchers have since found another 26 events with energies of at least 3 ×10¹³ eV. Furthermore, the team calculates that only about 11 of the total of 28 events were likely to be caused by atmospheric neutrinos or muons. These results, the researchers concluded, provide what is known as 4σ evidence for the detection of cosmic neutrinos; in other words, their statistics suggest only a one in 15,000 chance that all of their events can be explained via purely atmospheric phenomena. “This is the first evidence we have of neutrinos that are not of atmospheric origin,” says IceCube principal investigator Francis Halzen of the University of Wisconsin-Madison. “It opens up a new wavelength in astronomy, thanks to a different kind of particle.”

Emilio Migneco of Italy’s National Institute of Nuclear Physics in Catania, who is not a member of the IceCube collaboration, points out that 5σ evidence (less than one in a million chance of statistical fluke) is the standard usually required to claim a new discovery. Nevertheless, he says that “the signal seems to clearly emerge from the background”, adding that the results are “extremely exciting and will open a new window on the observation of the universe”.

Low angular resolution

Migneco cautions, however, that the window has not been opened just yet, pointing out that astronomical observations will require the neutrinos that are detected by IceCube to be correlated with specific objects in the sky. The latest results provide a hint that at least some of the cosmic neutrinos detected were generated in the centre of the Milky Way, but the angular resolution of the measurements is not high enough to prove this, and indeed the IceCube collaboration makes no such claim in its paper. Migneco, who is former co-ordinator of the KM3NeT neutrino telescope that is currently under construction in the waters off Sicily, says that a facility in the northern hemisphere, such as KM3NeT, “may help in solving the puzzle since it will have a better view of this region of the sky”.

IceCube itself will also concentrate on trying to resolve this issue. “Now that we know what we are looking for, we will probably find lots of other events fairly easily,” says Halzen. “Statistics is the key. Especially those from muon neutrinos, the tracks of which help determine where they come from.”

What might the data tell us? “It would be disappointing if we didn’t manage to pinpoint the sources of cosmic rays this way,” says Halzen. “But there are likely to be surprises as well. One surprise would be identifying the source of cosmic rays and finding out it is nothing that theorists have thought of in the last 100 years.”

The findings are described in Science.

Oldest minerals from Mars found on Earth

A meteorite recently retrieved from the Sahara Desert bears the oldest known minerals ever seen from the planet Mars, say scientists in the US, Australia and France. These minerals are 4.4 billion years old and therefore formed just 150 million years after the red planet’s birth. Their age confirms earlier indications that the Martian crust formed quickly, as did the crusts of the Earth and Moon.

Today, Mars is a cold, dry, nearly airless desert, but ancient Mars was warmer and wetter, and may even have given rise to life. The planet had a thicker atmosphere then and dried-up riverbeds – signs of a milder climate – course through parts of the planet’s oldest terrain, highlands that blanket most of the southern hemisphere. Scientists know that the highlands are ancient because these regions bear the most craters.

Although NASA has tried to land eight spacecraft on Mars – and succeeded seven times – these landers were not able to measure the ages of rocks nor determine all the chemical elements that constitute them. Scientists therefore value Martian meteorites that they can scrutinize in the laboratory – rocks with isotopic compositions that reveal they came from Mars. Unfortunately, with only one previous exception, all the other Martian meteorites are so young that they originated long after the climate deteriorated.

First-ever rock from the highlands

“It is very exciting that we have the first-ever sample to come from the Martian highlands,” says Munir Humayun, a geochemist at Florida State University in Tallahassee. He and his colleagues studied a Martian meteorite named NWA 7533. It was one of five stones that landed together in northwest Africa, which is what NWA stands for.

The meteorite is a so-called breccia, a collage of different rock fragments. Breccias can form after an asteroid hits a planet and throws up chunks of rock that later conglomerate. These impact breccias litter the cratered highlands of the Moon and presumably Mars. But breccias can also arise after violent volcanic eruptions.

Which type of breccia is NWA 7533 – impact or volcanic? “When we got this meteorite, we looked inside the breccia for clues as to impact,” Humayun says. The search focused on siderophiles, which are “iron-loving” elements. These are rare in a planet’s crust because they sink with iron into the core but are common in asteroids. Sure enough, the meteorite abounded with siderophiles such as nickel and iridium, indicating it was an impact breccia – and therefore probably came from the heavily cratered highlands. “So we thought, ‘Wow! This looks like a highland breccia’,” Humayun says. If so, it had to be old.

Zircons remember

But science is rarely that simple, and this interpretation conflicted with a report earlier this year by other researchers who found that another Martian stone – which fell with NWA 7533 – was too young to have come from the ancient cratered highlands. Fortunately, NWA 7533 contains tough minerals called zircons. “From a geochronological point of view, zircons are like elephants: they remember,” says Humayun. “They are resistant to almost anything.” Even impacts do not destroy them. Moreover, zircon crystals trap uranium, which decays into lead, allowing scientists to date them. With one exception, every zircon in the meteorite had the same age – 4.4 billion years. Humayun’s reaction? “Like, eureka!”

“This is a really exciting find,” says Harry McSween of the University of Tennessee at Knoxville, a planetary scientist unaffiliated with the research team. “It is a much more representative sample of the ancient crust of Mars than we have seen before [among meteorites],” he says. “The zircon method is the best method there is for defining the age of crystallization of a magma, so I’m real confident in this 4.4-billion-year age.”

Only one other Martian meteorite is ancient. In 1996 scientists claimed that a meteorite named ALH 84001 harboured fossils of Martian life. The meteorite was then thought to be 4.5 billion years old, but like the fossils themselves, that figure has not held up to further scrutiny. Later work lowered the age to 4.1 billion years. Therefore, the 4.4-billion-year-old zircons in the new meteorite are the oldest known minerals from Mars.

Fast-forming crust

As the meteorite is a breccia, the zircons came from different rocks throughout the Martian crust, yet all but one have the same old age. That means the crust formed fast, says Humayun. Otherwise, the zircons would have a spread of ages. In addition, the zircons are as old as the earliest zircons on the Earth and Moon, so all three worlds created their crusts at the same time.

Although the new meteorite’s zircons are ancient, the rock itself may be younger. The scientists do not know when the various fragments cemented together to assemble the breccia.

The meteorite contains soil with a composition that nearly matches what NASA’s Spirit rover saw at Gusev Crater. Unlike modern Martian soil, however, the meteoritic soil lacks much sulphur or chlorine – probably because flowing water on a warmer, wetter world carried these elements away.

The scientists publish their work today in Nature.

Letting the sunshine in

Photo of a burst of sunlight and a golden stalk of wheat — **Sunny outlook** Solving the dual problems of food and fuel for a growing world population all comes down to harnessing the Sun, according to Tony Ryan and Steve McKevitt. (Courtesy: iStockphoto/mycola)

By 2050 the world’s population will have reached nine billion. How can the Earth provide the resources to sustain all these people? This topic is tackled in the book Project Sunshine, in which authors Tony Ryan and Steve McKevitt explain how world population growth has led to a “perfect storm” of intertwined water, food and energy shortages, with climate change as a backdrop.

The book’s preface and its first two chapters describe how the discovery of fossil fuels was followed by a population explosion, albeit one that is predicted to plateau at the afore-mentioned figure of nine billion. Here, the authors introduce their ideas (which are fleshed out later in the book) on how it is possible to provide sufficient food and energy for nine billion people in a “sustainable” way. The book then takes what appears at first to be a detour, as the authors go back to the Big Bang to explain the various sources of energy that exist. Subsequent chapters describe the evolution of humans and give a brief history of societal developments, showing how human activities before the discovery of fossil fuels were constrained by the time it took for the principal source of energy – wood – to replenish itself.

Descriptions of nuclear power, non-solar renewables, solar power and energy storage show how sufficient energy can be provided for the expanding world population. Food production is also examined, discussing contentious aspects such as genetically modified plants. The final chapter stresses the need to view solar power “as a must-have, not a nice-to-have”, reminding us that this means that the wheel has turned full circle: “Until 350 years ago, mankind was living like any other animal, beholden to the solar cycle.”

The authors – who are respectively a polymer chemist and pro-vice chancellor at the University of Sheffield, and an expert in communications and consumerism – could hardly have picked a more important or universally interesting topic. The latest report by the Intergovernmental Panel on Climate Change (released on 27 September this year) lends their book topicality, as global warming (although not their main focus) is mentioned in many places, while the gap between energy requirements and supply and how to address it are constant preoccupations for anyone who has the remotest interest in current affairs. Ryan and McKevitt address these questions, treading the fine line between being too technical or too woolly. While the authors do have a solar axe to grind, their book also complements Sustainable Energy – Without the Hot Air by the Cambridge physicist David MacKay (UIT, Cambridge 2009), in that both agree it is possible to act to solve energy problems – and, moreover, that it is imperative to do so.

I have quibbles. The authors state early on that “all of our energy comes from the Sun”, yet this is not quite true. Geothermal energy, which is mentioned in the book, results from the radioactive decay of elements inside the Earth; like nuclear power, it originates in elements that were created mainly in supernovae. And on the subject of nuclear power, it is also worth noting that the chapter devoted to it does not talk about waste disposal, even though difficulties with the permanent disposal of high-level waste argue against this form of electricity generation (see “Too hot to handle“). Another quibble is that while the book shares its name with a network of researchers at the University of Sheffield, and the two are related (the aim of the Sheffield “Project Sunshine” is to develop new ways to use the Sun’s energy more efficiently to increase food production and provide renewable energy), this link is not mentioned until page 197.

The authors of Project Sunshine also have little to say about sourcing materials for renewable-energy generation in a sustainable way. For example, a shortage of indium metal is currently forcing a rethink of how to make the transparent conducting films needed for electrodes in solar cells, as these films are currently made from indium tin oxide. And in the chapter on solar power, there is no discussion of thin-film solar cells made from inorganic semiconductors such as copper indium gallium selenide (CIGS) and their sustainable replacements, even though this type of cell offers many advantages. For example, they are typically 50 times thinner than cells made from multicrystalline silicon, and the efficiency of some CIGS cells made on flexible polymer substrates has reached 20% – not far off the 28% of single-crystalline silicon.

Finally, there is no mention of the crash in the price of solar photo-voltaic (PV) modules in the last few years. A consequence of excess production in China, the price crash has made solar cells much cheaper than could have been anticipated just a short while ago, but it has also led to the loss of 100 out of 350 companies that produce them and inhibited development of new PV technologies. As noted in Project Sunshine, the cheapness of the new technologies relative to crystalline silicon has been an important rationale driving their progress. In the light of the fast-changing nature of the field of energy supply, I suggest that the authors might wish to cover such topics in a blog, perhaps one linked to the Sheffield Project Sunshine.

No book can cover everything without sacrificing readability. Several books have been published recently that address the politics of energy, an example being The Energy of Nations by Jeremy Leggett (Routledge, London 2013). Project Sunshine complements these books by placing energy in the context of the history of the planet going back to the Big Bang, and as a result is an absorbing read with a clear story to tell.

2013 Icon Books £16.99hb 320pp

Web life: Empirical Zeal

What is the site about and who’s behind it?

Empirical Zeal is a blog that covers mind-blowing science from a wide range of disciplines, but especially physics and evolutionary biology. Its zealously empirical author is Aatish Bhatia, a PhD student in the physics department of Rutgers University in New Jersey, US. For a year or so in the late 2000s Bhatia wrote a conventional physics blog called High Energy Mayhem in which he described typical PhD-student activities such as going to conferences, attending seminars and attempting to get to grips with various mathematical tools. Since then, however, both Bhatia’s scientific interests and his blog have evolved considerably. His current research focuses on developing and applying new computational tools for genome analysis, and his new, general-public-friendly blog is an irresistible mix of everyday and esoteric science, backed up with some serious but accessible quantitative analysis.

Can you give me some examples?

Bhatia often uses blog posts or videos created by others as jumping-off points for his own work. For example, one post from July features a video in which the British science presenter and comic Steve Mould makes a long chain of beads “levitate” as it is poured out of a jar. The video is jaw-dropping, and Mould offers a good description of the physics behind it. However, Bhatia takes things several steps further, creating a mathematical model of the falling chain and using motion-tracking software to compare the model’s prediction to a slowed-down version of Mould’s video. Another recent post begins with a video of a “rolling swarm” of caterpillars, and goes on to explain that these swarms are essentially a squishier version of the moving walkways in airports: caterpillars in the top few layers can travel at speeds several times faster than an individual bug can crawl, thanks to the motion of the bottom layers. Calculating the speed of the Nth layer of caterpillars is, however, left as an exercise for the reader.

Anything else I should know?

In addition to physics and evolution, Bhatia also has a strong interest in education. Earlier this year, he helped make a video for TED-Ed (the educational wing of the online science/culture/ideas juggernaut) about the physics of fluids and the ways in which large and small swimming creatures have adapted to move through them. The video carries the arresting title “The physics of sperm vs. the physics of sperm whales”, and is worth watching for the animations alone. Another post takes an in-depth look at the “monkey and the hunter” problem found in many undergraduate physics textbooks, analysing not only the problem itself but also how its presentation has changed over the years.

Can you give me a sample quote?

From a post in June: “I just read an interesting new physics paper. It’s called ‘Statistical mechanics of the US Supreme Court’, and it attempts to understand how Supreme Court judges influence each other when voting, using techniques from the physics of magnetism… Imagine you had a magnet. If you zoom in to this magnet with the right kind of microscope, you’d see tiny little microscopic magnets – each of which can either align with or against each other. These micro-magnets (or spins, which is what physicists call them) can flip their directions, and they can influence each other – every micro-magnet tries to get the other ones to align with itself. Some micro-magnets are more influential than others, and they can convince many others to flip in their direction. Turns out, this magnet model maps nicely to the Supreme Court problem. Just as the micro-magnets influence each other’s orientation, and arrive at an emergent magnetization, the Supreme Court judges can influence each other’s votes, and from their deliberations emerges a final vote.”

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Why do beer bottles foam when struck on top?