A few weeks ago Deborah Jin was in London to accept the 2014 Isaac Newton Medal and Prize from the Institute of Physics. As is the custom, Jin also delivered the Institute’s Newton Lecture for 2014, which was called “Ultracold gases”. This is an apt title because Jin is an undisputed master in the control and study of gases that have been cooled to temperatures within a whisker of absolute zero.
As recent events have demonstrated dramatically, one intercontinental aeroplane flight can turn a regional virus outbreak into a global health event. Now, two physicists have used a computer model to show that the precise frequency of such long-distance jumps has a strong influence on the rate at which infections spread. The researchers’ simulations can also describe the spread of invasive species, genetic mutations within a population, and even rumours.
Throughout most of history, diseases, genetic mutations and species have usually spread relatively slowly, because individuals normally move only short distances in their lifetimes. This kind of spreading creates what University of California, Berkeley, physicist Oskar Hallatschek calls “wave-like” or “pancake-like” growth, whereby a population radiates outwards from a central core in a roughly circular fashion. The Black Death of the 1300s, for instance, spread this way, moving across Europe at between 300–600 km per year.
Today, however, a pathogen or potentially invasive plant can easily hitchhike across a city, a continent or even an ocean, in just a day or two. The individual can then seed a new population, which can itself launch additional long-range jumps to new territories. Seeking to predict mathematically how epidemics and invasions spread, scientists have developed computer simulations in which such long-range dispersal events happen continuously but at very low rates.
But Hallatschek and his colleague, physicist Daniel Fisher of Stanford University, realized that for real organisms, these hitchhiking events do not occur continuously but rather in discrete steps, like a aeroplane flight or boat trip. The researchers wanted to know how these rare, random events affect the overall rate of spread of a disease or mutation. “We were shocked that this was not understood,” Hallatschek says. “This was our motivation.”
To discover more, the researchers created a model of the world as a 2D grid. A simulation begins with one “infected” individual at a single point on the grid. During one time step, an infected individual has a certain probability of moving and thereby infecting another individual at another point on the grid. In general, an infected individual is more likely to move to a nearby point than to a faraway one. But in different runs of their model, the physicists varied the exact probability distribution of jumps of different distances.
Our model was purposely made so that it is as simple as possible but still sort of nontrivial, so that we can obtain some relevant insights
Oskar Hallatschek, University of California, Berkeley
The above video shows the time evolution of one such simulation. The epidemic begins in the centre of the grid and spreads out in space. Long-range jumps that mimic events such as air travel are shown to cause new outbreaks of the disease.
Hallatschek and Fisher found that the spreading rate depended sensitively on the probability of a long-distance jump. If this probability was low enough, the spread was slow and pancake-like. At higher probabilities, however, enough individuals seeded new growth far away from the original population that the overall rate of spread increased dramatically, resembling that of a metastasizing cancer.
Hallatschek notes that his team’s model cannot predict the spread of a real disease like Ebola or swine flu. These diseases spread along human travel networks, which make jumps between certain points – from Monrovia to Dallas, for example – far more likely to occur than an equidistant jump from one isolated rural area to another. The researchers’ model also does not contain specific details such as infection and recovery rates that would allow the researchers to predict the spread of a particular disease.
But Hallatschek notes that more complicated models containing such details are no better at forecasting the course of epidemics. Disease spread is idiosyncratic and strongly influenced by random events and initial conditions, which makes it very hard to predict. Opting for a very simple model allowed Hallatschek and Fisher to avoid these problems. “Our model was purposely made so that it is as simple as possible but still sort of non-trivial, so that we can obtain some relevant insights,” Hallatschek says.
“I think it’s an excellent paper,” says Dirk Brockmann, a physicist at Humboldt University Berlin. He applauds Hallatschek and Fisher for combining theory with numerical results to provide broad insight into how diseases, organisms and mutations spread. “The next step would be to see natural systems where you may observe this,” Brockmann says. “It would be great to see if there’s empirical evidence that this sort of thing is going on.”
The research appears in the Proceedings of the National Academy of Sciences.
The Italian particle physicist Fabiola Gianotti is to become the 16th director-general of the CERN particle-physics laboratory. Gianotti, 52, was selected today at a meeting of the CERN Council, making her the first woman – and fourth Italian – to hold the position. Gianotti, who will take up the position on 1 January 2016 for a five-year period, says that it will be a “great honour and responsibility” to lead CERN.
Three candidates were shortlisted for the job after being put forward by a search committee, but according to CERN Council president Agnieszka Zalewska it was Gianotti’s “vision for CERN’s future, coupled with her in-depth knowledge of both CERN and the field of experimental particle physics” that led them to pick the Italian. “Fabiola Gianotti is an excellent choice to be my successor, and I am confident that CERN will be in very good hands,” says current CERN boss Rolf-Dieter Heuer, who will step down on 31 December 2015.
Gianotti’s appointment has been welcomed by other physicists too. “I think that Fabiola is an excellent choice,” particle theorist John Ellis from King’s College London and CERN told physicsworld.com. “She is an outstanding scientist and communicator, who has demonstrated her leadership qualities as spokesperson of the ATLAS experiment.” Former CERN director-general Chris Llewellyn Smith, who was head of the lab from 1994 to 1998, says that he is “delighted” by the announcement. “She will do a great job,” he adds.
Gianotti received a PhD in experimental particle physics from the University of Milan in 1989 and then joined CERN, where she became a research physicist in 1994. From 2009 to 2013, she was spokesperson for the ATLAS experiment, during which time she played a key role in the discovery of the Higgs boson. Gianotti was also the public face of the discovery, presenting ATLAS’s results at CERN on 4 July 2012, in what went on to become a historic seminar. The finding led to François Englert and Peter Higgs being awarded the 2013 Nobel Prize for Physics.
As well as being on numerous international advisory boards, Gianotti is also a member of the recently established Scientific Advisory Board of the UN Secretary General. In 2012 she was awarded the honour of the “Grande Ufficiale dell’ordine al merito della Repubblica” by the Italian president Giorgio Napolitano, as well as being a co-recipient of the 2012 Special Fundamental Physics Prize. Last year, Gianotti bagged the 2013 Enrico Fermi Prize of the Italian Physical Society and the 2013 Medal of Honour of the Niels Bohr Institute of Copenhagen.
Indeed, Gianotti’s reputation has gone beyond the confines of particle physics. She was ranked fifth in Time magazine’s 2012 Personality of the Year and was included among the “Top 100 most influential women” by Forbes magazine in 2013. One of her first challenges will be to oversee the restart of the LHC, which is due to come back online next year when it will begin to ramp up to an energy of 13 TeV.
The magnetic properties of cobalt atoms lying on the surface of graphene can be controlled by the choice of substrate under the graphene sheet. This unexpected discovery has been made by physicists in Switzerland and could be exploited someday to create extremely dense magnetic memories or even quantum bits (qubits) for quantum-information processing and storage.
Graphene is a sheet of carbon just one atom thick. It has a number of unique electronic and mechanical properties that could be used to create new types of electronic technologies. These include spintronics, which aims to make use of the spin magnetic moment of the electron in circuits that are smaller, faster and more energy efficient than conventional electronics.
Now, Harald Brune and colleagues at the Ecole Polytechnique Fédérale de Lausanne (EPFL) and the Swiss Light Source (SLS) in Villigen have discovered an effect that could be exploited to create spintronics devices based on graphene. “The magnetic properties of transition-metal atoms on graphene were, so far, thought to depend only on the transition metals themselves,” explains Brune. “However, in almost all experiments, we need a substrate on which to grow graphene, and in our new work we show that this substrate greatly influences the magnetic properties of the transition metals that find themselves on top of it.”
In previous work, Brune and colleagues placed atoms of the transition metal cobalt on a graphene surface that had been grown on a platinum substrate. They found that the cobalt atoms have a magnetization that is in-plane – that is, pointing parallel to the surface of the graphene. However, in this latest work they discovered that when the graphene is grown on a ruthenium substrate, the magnetic moment of cobalt points out-of-plane. They also tried an iridium substrate and found that like platinum, the cobalt moment lies in-plane.
“The substrate thus plays a much more important role than previously thought and calculations, which until now considered graphene as freestanding, need to take this into account,” he says. “Our result also shows that we can actually tailor the magnetic properties of the transition-metal atoms, depending on the substrate they lie on.”
The graphene films were grown on ruthenium and iridium substrates using chemical vapour deposition. The graphene was then given a sparse coating of cobalt atoms using electron-beam evaporation.
The researchers made their measurements on samples at 3.5 K using X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) techniques. XMCD measures the magnetic properties of an atom using circularly polarized X-rays. “If the X-rays are polarized, we can infer whether the magnetic moments of the cobalt atoms lie along the direction of the incoming X-rays or in a direction perpendicular to them, and we can also calculate the size of this magnetic moment,” says Brune. “Applying an external magnetic field allows us to determine how much field is needed to align the magnetic moments of the individual cobalt atoms being probed.”
The team found that the magnetic properties of the cobalt are influenced by the strength of the bond between the carbon atoms in graphene and the substrate atoms. There are strong chemical bonds between the carbon atoms and ruthenium, for example, whereas there are much weaker weak van-der-Waals interactions with iridium and platinum substrates. As a result, the graphene is pulled much closer to the ruthenium substrate than it is to the platinum or iridium. The distance between the graphene and the substrate affects the graphene, which in turn affects the cobalt atoms.
“Put simply, we can imagine that the underlying metal surface transfers part of its electrons to the graphene, or the other way around, and this influences the electronic properties of graphene. In turn, this influences the magnetic properties of the cobalt atoms.”
If the magnetic states of transition-metal atoms on graphene are found to endure for long times, they could be used to create extremely dense information-storage devices. They could even be used as qubits, although Brune points out that they would have to be operated at extremely low temperatures.
The team says that it is now focusing its attention on identifying single atoms or molecules that have sufficiently long-lasting magnetic states, so that such applications might indeed be possible one day. “Ultimately, we might be able to store one bit of information in the magnetic state of a single transition metal atom,” says Brune. “Currently, magnetic hard disks use 107 atoms per bit.”
The research is described in Physical Review Letters.
The belief that scientific disciplines exist on a spectrum of “purity”, with physics and maths at one end and psychology and philosophy at the other, is fairly widespread – especially, it must be said, among physicists and mathematicians. But there is also an alternative view, one that treats science not as a linear spectrum, but as a loop in which the deepest problems in physics turn out to have philosophical roots, and vice versa. This second view is elegantly illustrated in At the Edge of Uncertainty: 11 Discoveries Taking Science by Surprise, which begins with a chapter on philosophy’s most physical problem – human consciousness – and concludes with one on the most philosophical question in physics: the nature of time. In the intervening chapters, author Michael Brooks writes about surprising discoveries in genetics, cosmology and many other fields with the same clarity and verve that made his earlier book in the “unsolved questions” genre, 13 Things That Don’t Make Sense, a bestseller back in 2009. Answers to some of the questions in At the Edge of Uncertainty may be closer than others. In the chapter on human consciousness, Brooks writes that “psychologists and neurologists are, in many ways, like Darwin aboard the Beagle, still gathering specimens and making observations”. On the other hand, he suggests that the Big Bang model of cosmology – or, as he disparagingly calls it, “Big Bang plus inflation plus dark matter plus dark energy” – could collapse under the weight of successive “fixes” in less than 10 years. It’s worth noting that Brooks’ paradigm-shift-o-meter has been faulty before: one of the 13 “nonsensical” things in his earlier book, the so-called “Pioneer anomaly”, has since been explained without the need for a new theory of gravitation. But when you write about cutting-edge science, a few incorrect predictions are probably inevitable – and when you do it as well as Brooks does, they’re forgivable, too.
When writing about quantum physics for a general audience, most authors still focus on the events and ideas of the 1920s and 1930s, when the likes of Niels Bohr and Werner Heisenberg brought forth a new physics base of ideas about uncertainty and wave-particle duality. By comparison, popular-level explanations of the “second quantum revolution”, which began in the 1960s with John Bell’s work on nonlocality, are often somewhat cursory. Not so for Quantum Chance: Nonlocality, Teleportation and Other Quantum Marvels. Written by Nicolas Gisin, a physicist at the University of Geneva, Switzerland, and a prominent figure in this ongoing second revolution, the book contains few equations. At just over 100 pages, it is also rather short. But brevity and mathematical simplicity can be deceiving, and this is definitely not a book for absolute neophytes. Indeed, even experienced physicists may find that certain passages (such as Gisin’s initial description of why, in the absence of nonlocal interactions, Alice and Bob cannot win a Bell-inspired “game” more than a certain fraction of the time) do not sink in on a first reading, or even a second. However, those who want to deepen their prior understanding of quantum entanglement, cryptography or teleportation should stick with it. For topics as complex and important as these, the rewards of even a limited increase in understanding are significant.
It’s not often that classical physics and Post-Impressionist painters collide, but when they do the results can be enchanting and intriguing. In one of the latest TEDEd videos, Natalya St Clair has created a short lesson that looks at “The unexpected math behind Van Gogh’s Starry Night.” The video above looks at the enduring mystery that is the turbulence we see in any kind of flows in the natural world and how the human brain can recognize and actually make some kind of sense of the chaotic random patterns turbulence describes.
As pointed out in the video, famous physicists such as Richard Feynman and Werner Heisenberg have noted the complexity of turbulence, with Feynman describing it as “the most important unsolved problem of classical physics” and Heisenberg saying that “when I meet God, I am going to ask him two questions: why relativity? And why turbulence? I really believe he will have an answer for the first”. But is it possible that the undoubted genius and troubled painter that was Van Gogh perceived something more about turbulence in nature and is this most clearly represented in his most famous masterpiece – the evocative painting known as Starry Night? Watch the video to find out.
Astronomers using the ALMA array of radio telescopes in Chile are the first to see a streamer of gas and dust flowing towards a single star in a binary system. This material is crucial for sustaining the formation of planets, and the observation could explain how planets can form around single stars in a binary system – something that had puzzled astronomers. This has important implications for astronomers studying planets outside of the solar system, because almost half of the Sun-like stars that we know of were formed in binary systems.
Astronomers have so far discovered more than 1800 planets orbiting stars other than the Sun. One striking thing about these exosolar planets – or exoplanets – is that they exist in a variety of systems, many of which are very different to our own solar system and include binary star systems. Sometimes planets in binary systems orbit both stars, and follow large “circumbinary” orbits. In other cases, planets will orbit tightly around one of the stars in a binary system.
It is this latter case that has puzzled astronomers, because it is not clear how such planets could form. The mystery is illustrated nicely by the subject of this latest study: a young system called GG Tau A that is 450 light-years away, and actually comprises three stars. Two of the stars orbit each other tightly, and a third star is some distance away. As a result, the third star (called GG Tau Aa) can essentially be thought of as one half of a binary system.
The system is surrounded by a large outer disc of dust and gas that orbits all three of the stars. One of its stars – GG Tau Aa – is also surrounded by its own compact inner disc of dust and gas, with a total mass on par with that of Jupiter. There is very little dust and gas in the large gap between the inner disc and outer disc, because the competing gravitational forces of the stars prevent matter from accumulating in this intermediate region.
The inner disc of GG Tau Aa has enough material to form planets, but its existence has puzzled astronomers. The problem is that matter is falling into the star at a very high rate, and therefore the disc should have vanished long ago. One possibility is that gas and dust is streaming in from the outer disc something that has been predicted by computer simulation, but not yet seen. But now, Anne Dutrey and colleagues at the University of Bordeaux, along with astronomers in Mexico, the US, France and Taiwan, have used ALMA to discover such a stream of gas and dust flowing from the outer disc, and into the inner disc around GG Tau Aa.
“These observations demonstrate that material from the outer disc can sustain the inner disc for a long time,” explains Dutrey. “This has major consequences for potential planet formation.” Indeed, she points out that an Earth-like planet could form within the habitable zone of GG Tau Aa – the region in which life could develop on such a planet. However, Dutrey cautions that the study does not allow the team to conclude that a planet will form around GG Tau Aa, but rather that there is enough material for this to happen.
If this streaming process occurs in other star systems, it could mean that planets are a common sight around single stars in binary systems. Team member Emmanuel Di Folco of the University of Bordeaux explains why this is exciting: “Almost half the Sun-like stars were born in binary systems. This means that we have found a mechanism to sustain planet formation that applies to a significant number of stars in the Milky Way.”
The discovery is described in Nature.
Some physicists can get a bit grumpy if talk turns to the supposedly dirty business of commercialization. They go into physics out of curiosity alone and have an innate dislike of ever having to justify their resarch in terms of potential spin-off benefits. But they can be thankful for the overall health and vitality of physics that some brave souls do risk their money and careers by setting up businesses to commercialize their findings.
The November 2014 issue of Physics World magazine gives a taste of some of the challenges in commercializing physics, as I describe with my colleague Margaret Harris in the video above. We kick off with one common problem for hi-tech start-ups, which is how to bridge the “valley of death” – in other words, what to do when your research funding has dried up but you’re not yet making any money from your product. Jesko von Windheim then examines why physics-based firms have a harder job than ordinary businesses, where succeeding is simply about finding a market and meeting its need, before we look back at some promising technologies tackled in Physics World’s Innovation column to see how they’ve fared. There are also some real-life lessons from Floor van de Pavert — a physicist who’s been at the business coal face — and we see how crowdfunding websites can help researchers get their ideas off the ground.
We’ve also been doing some innovation of your own and if you’re a member of the Institute of Physics (IOP), you can now enjoy immediate access to the new issue with the digital edition of the magazine. If you’re not yet in the IOP, you can join now to get full access to Physics World as well as many other member benefits.
For the record, here’s a run-down of other highlights of the November issue.
• Blue LED research wins Nobel prize – Catch up with the 2014 prize to Isamu Akasaki, Hiroshi Amano and Shuji Nakamura for developing efficient blue light-emitting diodes.
• Lost in translation – Asking physicists to tailor their research to deliver specific “impacts” has the potential to distort the scientific method, warns Philip Moriarty.
• From hype to hope – Graphene promises a whole host of commercial applications, but Norman Apsley says that business and physics will have to work together to make it happen.
• Bell’s theorem still tolls – The famous proof of entanglement by John Bell is half
a century old. Robert P Crease recalls the strange story of its origin and history.
• Navigating the valley of death – Taking an innovation from the lab to the market is hard in any discipline, but physics start-ups face some unique challenges crossing the so-called “valley of death”. James Dacey speaks to scientists and business professionals in the Boston area of the US who have dared to take on this journey.
• More push than pull – Meeting the demands of the market is usually vital to any new business. But as Jesko von Windheim explains, tech-based firms have it much harder as there might not yet be a market pull for the technology they are trying to push.
• Whatever happened to..? – Each month, Physics World covers commercially relevant breakthroughs in its Innovation column. Tushna Commissariat and Louise Mayor checked in with the physicists behind some of the most interesting and promising innovations we have featured over the years, to find out how they fared.
• The rocky road to success – Doing science involves complex experiments with uncertain outcomes, and starting a company based on innovations from the lab is no different. Floor van de Pavert shares some of the lessons she learned from co-founding her first spin-off company.
• A little help from the crowd – Whether they need start-up capital to fund an innovation or some novel research, physicists the world over are turning to crowdfunding websites to support their next projects, as Jon Cartwright reports.
• A lucky, lonely planet – Duncan Forgan reviews Lucky Planet: Why Earth is Exceptional – and What That Means for Life in the Universe by David Waltham.
• 3000 years of questions – John Singleton reviews Faith and Wisdom in Science by Tom McLeish.
• The view from the VC side – With a PhD in theoretical physics and more than a decade of experience in the investment world, venture capitalist Alexei Andreev has seen his share of innovation successes and failures.
• Once a physicist – Meet Jennifer (Jenny) Rollo, a systems biologist who studies Alzheimer’s disease at the University of Sydney, Australia.
• E-mail economics – In this month’s Lateral Thoughts column, Michael Hipkins wonders how to stem the deluge of e-mails we all receive each day.
A new way of creating a voltage by shining light on a solid has been developed by researchers in the US and Europe. Unlike most photovoltaic devices, the new system does not rely on semiconductors but rather on surface plasmons in tiny metal nanostructures. The team is now working to create new types of devices that convert light into electrical energy.
Surface plasmons are collective excitations of electrons at the surface of a metal that interact very strongly with light. As a result, plasmons are of great technological interest as an interface between photonics and electronics. This interaction is strongest at the plasmon-resonance frequency, which is defined by the size and shape of an object and its charge density. In 2009 Paul Mulvaney and colleagues at the University of Melbourne in Australia applied an electrical potential to gold nanoparticles, and found that they could tune the plasmon-resonance frequency by injecting or removing electrons.
In the new work, applied physicist Harry Atwater and colleagues at California Institute of Technology, together with researchers in the Netherlands, show that the reverse can also occur: a surface potential can be induced by using light to modify the charge density of a nanoparticle. The team made its plasmonic material by attaching gold nanorods with a plasmon-resonance wavelength of 550 nm to an indium-tin-oxide substrate. Then the researchers fired a tuneable laser at the structure, and swept the laser wavelength from 480 nm to 650 nm. During illumination, the electric potential on the surface of the material was monitored using the conductive tip of an atomic force microscope.
When the laser was on resonance with the surface plasmon, no voltage was induced. Irradiation either side of the resonant frequency, however, did produce a voltage. When the wavelength was below 550 nm a negative potential was measured on the gold nanorods, while longer-wavelength light created a positive potential. The team found that the magnitude of the potential related to the rate at which the light absorbance changed with respect to the frequency of the light. The largest potential (which was negative) was produced by illumination at 500 nm. Atwater offers a thermodynamic explanation for this observation: “If you shine light on the structure, free-energy minimization will cause the structure to try to adjust its charge density to bring itself into resonance with the exciting light.” The researchers have dubbed this phenomenon the plasmoelectric effect.
The team then used this model to predict the frequency at which the maximum potentials should be generated in its set-up, and found broad agreement with its experimental results. The researchers also checked that the model could be applied generally, by testing it in a different type of plasmonic material: a thin gold sheet studded with a periodic pattern of 10 μm holes mounted on a glass substrate. This too showed a plasmoelectric effect, with the peak negative and positive potentials as predicted by the model.
While the devices reported by the team simply produce a potential difference when illuminated, the team is now working on a device that will deliver usable electrical energy and thereby function as a solar cell. Atwater believes that such a device could complement traditional semiconductor photovoltaic cells: “Any given single-material solar cell can only convert power from photons that have energy greater than the band-gap energy,” he says, “[Our device] could potentially be used behind a conventional photovoltaic cell to harvest the infrared part of the spectrum, because I can design a plasmonic structure to have a resonance at pretty much any frequency.”
Nano-optics specialist Thomas Ebbesen of the University of Strasbourg, says: “I find it to be very impressive work. If something like this could become efficient as an energy conversion process that would of course be technologically important. But independent of that, I find the underlying physics very interesting just from a thermodynamic point of view.”
Ortwin Hess of Imperial College London is also impressed, and wants to know more: “Thermodynamics seems to be supporting their experimental and simulation work, and I’m really happy about that,” he says. “Nevertheless, from the microscopic perspective, plasmons are made up of electrons, and in the end I would like to see how that works.” The researchers are working on this question, and Atwater says there will be “a forthcoming theory paper in the near future”.
The research is published in Science.
Every once in a while we come across a physics story that seems very interesting – but we just don’t know what to make of it. The latest comes in the form of a press release from Brown University in the US and concerns “electron bubbles” in liquid helium.
These bubbles are about 4 nm in diameter and are formed when a free electron moves through liquid helium and repels surrounding atoms. Physicists have been studying these bubbles for decades and in the 1960s they discovered something very strange when firing electrons across a tank of liquid helium and measuring the time it takes the bubbles to reach a detector on the other side.
