A common blue dye used in £5 Bank of England notes could hold the key to spintronic devices after physicists in the UK and Canada discovered that its electron spins have surprisingly long quantum decoherence times. Marc Warner and colleagues at University College London and the University of British Columbia have found that copper phthalocyanine (CuPc), which has been used as a dye since the 1930s, can have a decoherence time of as long as 3 μs. While this is still shorter than the times available using the spins associated with nitrogen vacancy (NV) impurities in diamond, the latter structures are difficult to work with.
Spintronics is all about exploiting the spin of an electron as well as its electrical charge, with, for example, spin-up corresponding to a binary “1” and spin-down to “0”. In principle, spintronic circuits could be made much smaller and more energy efficient than conventional electronics. The concept can also be extended to single electrons that could function as quantum bits of information – or qubits – which could form the basis of quantum computers. A big challenge, however, is to find materials in which the spin state lasts long enough to store and process information in practical devices.
In this latest study, Warner and colleagues used a molecular-deposition technique to create a film comprising about 1000 individual layers of CuPc and hydrogen phthalocyanine (H2Pc). They kept the ratio of CuPc to H2Pc molecules relatively low – between 0.1 and 10% in different samples – to ensure that the average distance between the copper atoms is large enough to prevent their spins from interacting with each other as this would shorten their relaxation times.
The team measured how long the spin states endure by using electron spin resonance (ESR), which involves placing the sample in a magnetic field and then using microwaves to cause transitions between spin states. The researchers were interested in two particular time parameters related to the spin states. One is the “population relaxation time” (T1), which indicates how long it takes for an ensemble of spins pointing in the same direction to reach the state where the spins are pointing in random directions. T1 is an important parameter for spintronics applications that do not involve quantum information. The other parameter is the “phase memory time” or decoherence time (T2), which says how long quantum information could be stored in the spin of copper atom.
When the team measured T1 in films, it found a rapid drop in the relaxation time as the amount of CuPc in the sample increased. In samples where the ratio of CuPc to H2Pc is about 0.1%, T1 was about 0.1 s. But when that ratio rose to about 10%, T1 fell to about 10 μs. These measurements were made at a chilly 5 K but when the temperature was increased to 80 K, the 0.1% T1 value also dropped to about 10 μs.
More potential for quantum computing
When it came to T2, the team found that spins in the 0.1% sample retained their coherence for about 3 μs at 5 K, dropping slightly to about 1 μs at 80 K. This performance is not quite as good as the spins associated with NV impurities in diamond, which some physicists believe could prove very useful for building quantum computers. However, CuPc is an easier material to work with than NV impurities and some other materials mooted for quantum computing.
“Our research shows that a common blue dye has more potential for quantum computing than many of the more exotic molecules that have been considered previously,” explains Warner, who is now at Harvard University in the US.
As well as having relatively long relaxation times, CuPc has several other properties that make it attractive for use in quantum computers. Unlike NV impurities in diamonds, CuPc interacts strongly with visible light: a property that could be used to create quantum devices that use both spins and light to process quantum information. The material is also easy to modify both physically and chemically, which means its properties can be changed to suit a range of applications.
The discovery of graphene is truly one of the “eureka moments” of our time. It is the story of how Andre Geim and Konstantin Novoselov realized that the discarded strips of Scotch Tape routinely used to produce clean surfaces on blocks of graphite were not useless – but might actually be covered with a type of carbon only previously spoken of in scientific fables. Having demonstrated the production of graphene for the first time, Geim and Novoselov worked with speed and purpose to show that this material possess all kinds of wonderful properties, including its unprecedented strength, electrical conductivity and complete impermeability. For their studies of this one-atom-thick form of carbon, the scientists were awarded the 2010 Nobel Prize for Physics just seven years after they had described their initial discovery.
Physics World invited Geim to Bristol to give a public talk about the discovery of graphene, as part of a special lecture series to celebrate 25 years of the magazine’s publication. At the sell-out event, attended by more than 400 people, the Nobel laureate talked about why he is still so passionate about fundamental research. He told the story of graphene and discussed some of the exciting applications of the material that are beginning to emerge. The audience was also treated to a description of Geim’s earlier work on levitating frogs, which led to him sharing the Ig Nobel prize in 2000.
Date: Wednesday 23 October 2013
Speaker: Andre Geim is a condensed-matter physicist at the University of Manchester in the UK. He shared the 2010 Nobel Prize with Konstantin Novoselov for “groundbreaking experiments regarding the two-dimensional material graphene”.
Waves can be particles, and particles can be waves. Scientists have struggled to come to terms with this strange feature of the quantum world ever since the French physicist Louis de Broglie first described wave–particle duality in 1926. Are entities waves and particles at the same time – “wavicles” maybe? Or do they switch somehow, depending on the situation? The nature of wave–particle duality seems impossible to understand, because no-one has ever observed something being both a particle and a wave in the everyday world.
At least, that’s what physicists thought. In 2005 Yves Couder and Emmanuel Fort at Paris Diderot University discovered an odd phenomenon. If they placed an oil bath on a vertical vibrator, any oil droplets released onto the surface would not coalesce with the rest of the fluid, as you would expect. Instead the droplets would bounce up and down – their impact cushioned by a pocket of air – while generating circular standing waves.
Adjusting the amplitude of the vibrations, Couder and Fort noticed something even stranger happening. The droplets began to fall onto the wave crests in such a way that they were propelled across the surface. They bounced off the sides of the bath and one another, but always at a distance and never coming into direct contact. It was as though the waves were guiding the droplets to perform an elegant dance – to flit past and spin around one another, but never to collide.
These wave–droplets – or “walkers”, as the researchers began to call them – appeared to be the first macroscopic example of wave–particle duality. The waves could not exist without the droplets, nor could the droplets move without the waves. When a droplet eventually sunk, its corresponding wave would vanish; similarly, if a wave was damped, the droplet would stop moving around the oil bath. Eight years on, Couder and Fort have discovered more and more ways in which their walkers can reproduce phenomena previously considered unique to quantum mechanics – from quantized orbits, to single-particle interference in a Young’s double-slit experiment. The similarities are so marked that many researchers have begun to question whether there could be more to it than mere coincidence. Could a simple, classical experiment reveal something about how the quantum world ticks, and even lead to a deeper theory? “I find these results really, really fascinating,” says Aephraim Steinberg, an experimental quantum physicist at the University of Toronto in Canada. “And I think many people should find them striking.”
Walk the walk
The physics of these wave–droplet interactions may seem complex, but the basic “walking” phenomenon is easy to grasp. If the amplitude of the bath vibrations is just right, the droplet will fall onto the side of a wave crest, bouncing off at an angle. The droplet takes a parabolic trajectory through the air and falls on the side of another wave crest, again bouncing off at an angle and repeating the cycle (see figure 1).
One of the first experiments Couder and Fort tried to perform was the double-slit experiment, which is famous in quantum mechanics for demonstrating the existence of wave–particle duality. Traditionally this experiment is performed with light: a laser beam is sent through two slits in a plate, and the resulting pattern is projected onto a screen behind. The pattern reveals light and dark patches, suggesting that the diffracted light from one slit is interfering, like a wave, with that from the other. However, the same wave-like interference pattern can be built up slowly even when only one photon is sent through at a time.
1 Guided by waves A droplet of oil released onto the surface of a vertically vibrating oil bath does not merge with the bath. Instead, the droplet, cushioned by a pocket of air, generates a circular standing wave and bounces straight back off the surface. The droplet then continues to bounce around, guided by the sloping sides of the ripples on which it lands. These snapshots show a droplet bouncing from left to right (a–d). (Courtesy: Yves Couder and Emmanuel Fort)
Couder and Fort’s version of this experiment, which they performed in 2006, uses the same basic arrangement, but with walkers instead of photons and with the plate with double slits partially submerged in the oil bath. The researchers launched walkers towards the plate: if a walker arrived at the plate itself, the walker’s wave component was damped, and the walker either stopped or was reflected. If, on the other hand, a walker arrived at one of the plate’s two slits, it passed through and the researchers measured its subsequent trajectory (see figure 2).
Once a walker had passed through a slit, its direction appeared to be random and had no correlation with its previous trajectory. Upon counting the final trajectories of more than 70 walkers, however, the researchers found that the walkers were in fact reproducing a characteristic interference pattern. Apparently each walker passing through a slit was interfering with itself, thanks to part of its wave component going through the other slit as well (Phys. Rev. Lett.97 154101).
2 Young’s double-slit experiment on a macro scale (a) A single droplet of oil passes through a double slit, in analogy with Young’s double-slit experiment using single particles. After going through, each droplet’s trajectory changes to an angle α to the normal of the plate. (b) When the number, N, of droplets at each angle was added up for 75 different droplets, the resulting histogram showed the characteristic Young’s double-slit interference pattern. Each slit is 7.6 mm wide and their centres are 14.3 mm apart. (Adapted from figures by Yves Couder and Emmanuel Fort)
“We were really amazed,” says Fort. “In any first-year undergraduate course on quantum mechanics you look at this experiment, and you try to think of a classical system that does the same, but there is none. So this was really a discovery.”
The success of their double-slit experiment inspired Fort and Couder to try some other experiments reminiscent of those in quantum mechanics. They placed walkers inside baths, and tried confining them in square enclosures with submerged barriers of different widths. Mostly, the walkers bounced around the smaller enclosures like billiard balls. In 2009, however, the researchers discovered that, with a certain small probability, the walkers could “tunnel” across the barriers. During this tunnelling process the droplets skimmed over a barrier, and once they were over, their corresponding wave reappeared and the walkers continued as normal.
The range of quantum effects that the bouncing-drop experiments reproduce is truly remarkable
In quantum mechanics, tunnelling arises from the Heisenberg uncertainty principle. For instance, an alpha particle may not have enough energy, classically, to escape an atomic nucleus, but there is still a chance it will escape thanks to the innate uncertainty in its position. Whereas the properties governing tunnelling in quantum systems are barrier thickness and particle energy, in the Paris group’s system they were barrier thickness and walker velocity (Phys. Rev. Lett.102 240401).
The following year, Couder and Fort discovered yet another analogy between their walkers and quantum mechanics. Placing the vibrating bath on a rotating pedestal, they found that after a while the walkers began to orbit the centre of the bath – and only at certain radii. It turned out that the walkers’ angular momenta were quantized, in the same way angular momentum is quantized for charged particles orbiting in a magnetic field in so-called Landau quantization. Simulations performed by the Paris group suggested that the quantization arose because of past waves emitted by the walkers, which over time built up into a wave field that guided the walkers into certain orbits (PNAS107 17515).
De Broglie’s mechanics returns
Many physicists agree that Couder and Fort’s walkers are the first classical example of De Broglie’s wave–particle duality. But some go further and argue that the walkers appear to implement De Broglie’s picture of quantum physics itself. De Broglie believed that particles really exist, and are guided by equally real “pilot waves” that force them to adhere to quantum statistics. The pilot waves in De Broglie’s picture make nature deterministic, although they operate in a nonlocal, faster-than-light manner (see November 2009, pp32–37).
The ease of performing walker experiments has led several groups over the world to try them out. Among them, a small minority believe the walkers are directly mirroring what is going on in the quantum world. “I strongly believe this is not a coincidence,” says mathematician Aslan Kasimov at the King Abdullah University of Science and Technology in Saudi Arabia. “The range of quantum effects that the bouncing-drop experiments reproduce is truly remarkable, and should serve as a strong indication that a similar mechanism exists in quantum mechanics.”
Couder and Fort do not share such unbridled optimism, pointing out that the walkers’ analogy with quantum mechanics fails in many respects. From an experimental point of view, one difference is obvious: their system is dissipative, so without an energy input in the form of bath vibrations the walkers simply disappear. Another difference is that the Planck constant – so important for describing the innate “granularity” of the quantum world – cannot so far be derived from walker observations.
Perhaps the most important difference, however, regards the form of the walker’s associated wave. In quantum mechanics, a pilot wave or a wavefunction exists in a mathematical space of 3N dimensions, where N is the number of particles in a system. Even if a system contains just two particles, that means six dimensions – and the dimensionality rises swiftly as the system gets more complicated. In a walker system, regardless of how many walkers are present, the number of dimensions occupied by the waves is always just two, given by the length and width of the oil surface.
This contrast in dimensionality is more than just a technical detail. In quantum mechanics, it is the dimensionality of the wavefunction that is responsible for the weirdest quantum phenomenon of all: entanglement. Without a highly dimensional wavefunction, many physicists argue, it is impossible to explain how entangled particles can affect one another over large distances, faster than the speed of light. Such nonlocality in quantum mechanics has been demonstrated since the experiments of the French physicist Alain Aspect and others in the early 1980s, based on earlier theoretical reasoning by the Northern Irish physicist John Bell.
The disanalogies between the walker system and the pilot-wave theory are just as important as the analogies
“When there is more than one particle, the pilot wave, or quantum wavefunction, is not a function of physical space, and so cannot be analogous to any water or oil wave,” says philosopher Tim Maudlin at New York University in the US. “This allows for the entanglement of a pair of particles, and all of the effects of entanglement. If one thinks of this behaviour as central to quantum theory, it cannot possibly be reproduced in the [walker] system.”
Indeed, Physics World contacted a number of physicists and philosophers with a background in quantum foundations, and found that most were sceptical that the walker systems could shed light on the mysteries of the quantum world. “The reproduction of two-slit interference is impressive,” says philosopher Peter Lewis from the University of Miami in Florida, US, “but I think the disanalogies between the [walker] system and the pilot-wave theory are just as important as the analogies, and perhaps more so.”
A lesson from gravity
Most were sceptical – but not all. One exception was theoretical physicist Antony Valentini at Clemson University in South Carolina, US, who points out that walker systems are not the first to present strong analogues of a hard-to-grasp theory: the last decade has seen numerous experimental attempts to produce analogues of gravity, using fluids and light. “People are now wondering if we should look at these [analogue gravity] models and use them for inspiration when we’re looking at how our current theories of gravity might break down at short distances,” he says.
Analogue gravity models took a long time to catch on since their original theoretical proposal in the early 1980s by the Canadian physicist William Unruh. In the same way, says Valentini, it might take a while for Couder and Fort’s walker systems to inspire those studying quantum foundations in searching for a deeper theory.
Valentini stresses that the walkers will not be able to exhibit true, faster-than-light nonlocality. But what if the Paris researchers discover that the dynamics of their oil bath is governed by a characteristic speed – a speed much less than, but analogous to, the speed of light? Then, suggests Valentini, it would not be too much to imagine that the walkers might affect one another faster than this characteristic speed, analogous to how entangled microscopic particles are able to affect one another faster than the speed of light.
3 Circular argument This figure shows a white circle, within which are squiggly lines with colours ranging from dark blue to red indicating droplet speed in mm/s. Dark blue is 0 mm/s, turquoise is 10 mm/s, green is 15 mm/s, yellow is 20 mm/s and dark red is 30 mm/s. What stands out in the jumble of squiggly lines are a set of four dark-blue concentric circles, as well as a dark-blue spot in the very centre. (Adapted from Phys. Rev. E88 011001)
“Such a [situation] might encourage people to think that the nonlocality we observe in the lab isn’t instantaneous, isn’t infinitely fast,” Valentini adds. “That it’s much faster than light, but still finite.” As such, he believes the walkers might give us clues as to where quantum mechanics might break down.
There is already good reason to think that the walkers might exhibit some sort of pseudo nonlocality. Fort and Couder find that the dynamics of walkers is governed largely by the “memory” of past waves, which gradually builds up over the oil bath into a wave field. In this way, the Paris researchers say, one walker can seem to nonlocally affect another walker on the other side of the bath, thanks to a wave – or combination of waves – it emitted previously. This “memory” effect was key to the observed quantization of walker orbits on a rotating oil bath. This year, the researchers demonstrated the memory effect in a more general sense: if a walker is left long enough, its trajectory becomes “entangled” with a wave field in the bath.
Curiouser and curiouser
Will Fort and Couder’s walkers inspire physicists to find a theory deeper than quantum mechanics? It may be too soon to tell, but one point does seem clear: every time they look, the researchers find more ways in which walkers exhibit supposedly quantum behaviour. In another paper published this year, for instance, they present a macroscopic analogue of a so-called quantum corral (Phys. Rev. E88 011001). In surface physics, a quantum corral is a ring of atoms that pens in, or “corrals”, the wavefunctions of electrons held within into certain modes. Fort and Couder find that, in a classical, circular enclosure, walkers tend to occupy positions that match these quantum modes (see figure 3).
Mathematician John Bush at the Massachusetts Institute of Technology in the US, who collaborated with Fort and Couder on the quantum-corral study, recalls when he first listened to a talk on walkers. “Some of my colleagues were very dismissive; they thought it was just a coincidence,” he says. “But once you’ve heard of De Broglie’s mechanics, and you realize that these experiments are basically a macroscopic realization of it, and that they give quantum-like behaviour – it seems like too much of a coincidence. And so when I heard about it, I basically went all in!”
Exhibit A: a 4 cm-wide meteorite created by the Chelyabinsk asteroid explosion with “shock veins” in it. (Courtesy: Science/AAAS)
By Matin Durrani
If there is one thing that will be remembered about Friday 15 February 2013, it’s that it was the day when a massive asteroid blew up above the city of Chelyabinsk in Russia – creating the largest explosion on the planet since the one that occurred over the Tunguska river in Siberia in 1908.
But whereas hardly anyone saw or recorded information about the Tunguska explosion, the Chelyabinsk asteroid blew up over a relatively densely populated region and – perhaps more importantly – its journey through the air was recorded by numerous cameras and webcams that nervous Russian drivers love to install on their cars. Video footage of the event was soon seen by people all over the world.
Now, based on data from those videos and visits to some 50 local villages, researchers from the Czech Republic and Canada have published a paper in the journal Science detailing the trajectory, structure and origin of what they call the “Chelyabinsk asteroidal impactor”. The paper goes live on Thursday 7 November.
To save you the trouble of reading the full article, I’ve picked out a couple of factoids that might intrigue and interest you.
A version of the quantum Hall effect (QHE) involving light rather than electrons has been created by physicists in the US. The team believes the demonstration could boost understanding of the QHE and perhaps lead to the development of better photonic circuits that use light to process information.
The QHE is a well-known phenomenon that occurs when a voltage is applied along a thin conducting sheet and a magnetic field is applied perpendicular to the sheet’s surface. Throughout most of the sheet, the magnetic field makes conduction electrons travel in circular orbits that are quantized. At the edge of the sheet, however, the electrons cannot travel in circles because they would have to leave the sheet and re-enter it. Instead, these electrons hop along the edge in repeated semicircles. Crucially, they will travel along the edge regardless of its shape, following any dents or bulges.
These “topologically protected” paths and other aspects of the QHE have proven to be a rich seam of physics research that has led to two Nobel prizes. However, certain key predictions of QHE theory, such as the presence of bound electron states called anyons, remain unproven. This is because QHE experiments require pure samples, cryogenic temperatures and an ultra-high magnetic field – making measurements difficult to do.
No magnetic field needed
In recent years, physicists have sought mathematically analogous topological edge states that are easier to work with. Systems based on photons rather than electrons have drawn particular interest. However, light is not affected by a magnetic field and therefore researchers need to find another way of bending the photons. The idea of photonic topological states was first proposed in 2008 and first observed in 2009. However, these experiments still required high magnetic fields. Then in 2011 Mohammad Hafezi and Jacob Taylor at the Joint Quantum Institute (JQI) of the University of Maryland and researchers at Harvard University proposed a true photonic topological system that would need no magnetic field. This would allow them, in principle, to be miniaturized for use in microelectronics.
This goal has been realized by Hafezi and colleagues and also by an independent group at the Technion-Israel Institute of Technology and the Friedrich Schiller University in Jena, Germany. The latter group used an array of coupled helical waveguides to make a photonic lattice with topologically protected edge states that could not be scattered by imperfections.
Hafezi, Taylor and colleagues at JQI took a different approach based on a lattice of ring-shaped silicon waveguides placed just nanometres apart, which allow photons to tunnel between them. To create robust topological edge states, the researchers needed something that would have the same effect on photons that a magnetic field has on electrons. This role is played by the phase change acquired by a photon as it travels around a ring-shaped silicon waveguide. “When an electron goes around a magnetic flux it acquires a phase called the Aharanov–Bohm phase,” explains Hafezi. “If I have another particle – even if it’s not charged – that goes around a closed loop and acquires a phase, it looks as though that particle is feeling some magnetic field.”
Neat detours
To confirm the existence of edge states, the researchers injected photons in one corner of the lattice and found that they propagated around the sides to a collection point at the corner. To check the robustness of these states, they removed a ring and watched as the photons made a neat detour around the defect before continuing along the edge – just like a QHE edge state.
Beyond studying the QHE, the researchers believe their work could allow the precise manipulation of photons in circuits, something that is necessary to make optical analogues of electronic components. “There is no specific proposal,” says Hafezi, “but now, with these skills, we have more control on the routing of photons within an array. So we hope that this will give us more knobs to tune and potentially to do logical computation with photons.”
One of the main challenges facing those designing such photonic circuits is that light can easily rebound off any imperfections in a circuit. In a standard waveguide, forward and backward-propagating photons can couple to each other to create a standing wave that prevents photon propagation. In a QHE system, however, the electrons always propagate in the same direction along an edge.
Different phase shifts
The JQI researchers recreated this effect by arranging the waveguide rings so that a photon travelling backwards would acquire a different phase shift than one moving forward. As a result, photons travelling in opposite directions could not couple.
Mordechai Segev, the head of the Technion-Israel team, says time will tell whether the Maryland group’s design or his own will be more useful. He says which one ultimately makes it into technology will depend on which is more easily miniaturized.
An analysis of more than 50 years’ worth of climate data has found scant evidence for a controversial theory that attempts to link cosmic rays and global warming. The theory suggests that solar variations can affect the number of cosmic rays reaching the Earth, which in turn influences climate by impacting on cloud formation. The latest study was done by Rasmus Benestad of the Norwegian Meteorological Institute and he concludes that changes to the Sun cannot explain global warming.
Benestad compared variations in the 1951–2006 annual mean galactic cosmic-ray-flux data with annual variations in temperature, mean sea-level barometric pressure and precipitation. The cosmic-ray data were obtained using a high-altitude neutron monitor located in Climax, Colorado.
He looked for meteorological responses to cosmic rays over timescales of more than a year, and for “fingerprint” patterns in both time and space. He also checked for responses to greenhouse-gas concentrations and the El Niño Southern Oscillation.
Little evidence
“The significance of the findings was that the results were negative – I found little evidence of the cosmic rays having a discernible affect on a range of common meteorological elements: temperature, the barometric pressure or precipitation,” says Benestad. “Not for the global mean at least. One possible exception may have been for parts of Europe, however.”
The galactic cosmic-ray flux was associated with lower temperatures in parts of Eastern Europe. Benestad is intrigued whether these results were a coincidence or do indeed show a connection between cosmic rays and both temperature and sea-level pressure. He plans to investigate further. “Why would a solar effect be seen only in a limited region?” he wonders. “This region is affected by the North Atlantic Oscillation, and this phenomenon is a bit special – a variation in the sea-level pressure over timescales of up to several years. The persistence in these variations may match the variations in the Sun by accident, but it could also be sensitive to variations in the Sun.” If there is a real connection between changes to the Sun and the North Atlantic Oscillation, Benestad believes that this knowledge could benefit decadal predictions.
On a larger scale, the analysis indicated that the weak global mean-temperature response associated with cosmic-ray flux could easily be down to chance. What is more, there has been no long-term trend in cosmic-ray flux. “Hence, there is little empirical evidence that links galactic cosmic-ray flux to recent global warming,” wrote Benestad in Environmental Research Letters.
Do we see a clear effect?
“To me, the question was whether galactic cosmic rays really matter when it comes to climate change – do we see a clear effect?” Benestad asks. “There have been some discussions about climate change over the last decade and whether these cosmic rays have played a role in the recent global warming – even to the extent to suggest the exclusion of other explanations such as greenhouse gases.”
Cosmic rays ionize the atmosphere and some scientists believe that water droplets can condense on the resulting ions and aerosol particles, assisting cloud formation. Experimental work in this area has not been conclusive. Sunspot activity, which ebbs and flows on an 11-year cycle, decreases the cosmic-ray flux by increasing the solar wind – charged particles emitted by the Sun. The solar wind’s greater magnetic field then deflects away some of the cosmic rays that would otherwise hit the Earth from elsewhere in the galaxy. So, if the theory linking cosmic rays and cloud formation is correct, then increased sunspot activity could potentially reduce cloud cover.
However, other researchers have found that the relationship between cosmic rays and changes in cloud cover, and hence the Earth’s surface temperature, is limited. And Benestad has found that there is no evidence of a link between cosmic rays and climate variables such as temperature, pressure and precipitation.
False dichotomy
“Some scholars have implied a false dichotomy between galactic cosmic rays and greenhouse gases, arguing that global warming caused by galactic cosmic-ray flux would be at the expense of an effect from rising concentrations of greenhouse gases,” Benestad explains. “Such propositions have resulted in public controversy.”
Benestad’s study was part of the European TOSCA project, which aims to understand the impact of solar variability on the Earth’s climate. The purpose of his research is to look for a clear, direct and lasting response to cosmic rays in general meteorological parameters affecting people, rather than to assess whether cosmic rays are important for physical processes in the Earth’s atmosphere.
“There has been a tremendous amount of effort [put] into collecting all the data – from the neutron counter, thermometers around the world, rain gauges and barometric readings – organizing the data, and making them available,” says Benestad. “The novel aspect was the strategy used for analysing the results – combining linear algebra with regression analysis. This approach made it possible to investigate a large number of measurements simultaneously.”
India launched its first mission to Mars today at 15:08 local time from the Satish Dhawan Space Centre on the country’s east coast in Sriharikota, Andhra Pradesh. Built by the Indian Space Research Organisation (ISRO), the mission, dubbed Mangalyaan (Mars craft), is expected to “focus on life, climate, geology, origin, evolution and sustainability of life on the planet”, according to the ISRO. In doing so, Mangalyaan will attempt to shed light on one big unanswered question about Mars: whether the planet has a biosphere or even an environment in which life could have evolved.
The probe, which was launched on a polar-satellite launch vehicle, is expected to arrive at the red planet in September 2014 after a 300-day trip. Costing $100m, the 1350 kg craft will then be placed in a highly elliptical orbit in the Martian atmosphere when it arrives, being 500 km from the red planet at its closest approach and 80,000 km away at its most distant. The mission will carry five scientific payloads. Among them are a multi-spectral camera and spectrometers, as well as a highly sensitive methane sensor to assess if the gas is of “biological or geological origin”.
Mangalyaan, which was approved for launch only a year ago, follows hot on the heels of India’s successful maiden mission to the Moon – Chandrayaan-1 – that found evidence of water on the lunar surface in 2009. “This is our modest beginning for our interplanetary mission,” says ISRO spokesperson Deviprasad Karnik.
Asian space race
Reaching Mars is not easy – about 50% of missions do not succeed. But if the satellite successfully reaches Mars, India will become the fourth power – after the US, Russia and Europe – to launch a probe to the red planet. Indeed, India will also become the first country from Asia to reach Mars after China’s maiden mission to the red planet – Yinghuo-1 – crashed shortly after take-off when it was launched together with the Russian satellite Phobos-Grunt in November 2011. Although China and Japan have beaten India in other aspects of space development, including manned and Moon missions, Mars is seen as an area where India could now take the lead in the Asian space race.
Are Earth-like planets very common? (Courtesy: Petigura/UC Berkeley, Howard/UH-Manoa, Marcy/UC Berkeley)
By Hamish Johnston
A decade or so ago it would have been reasonable to wonder if the universe contained any planets that resemble Earth. But now that astronomers have discovered more than 1000 planets orbiting stars other than the Sun, it seems perfectly sensible to think that the number of Earth-like planets could be huge. However, actually finding such planets remains a challenge.
How big must an ensemble of particles be before the exact number of individual particles becomes inconsequential and the entire system can be described using many-body theories? This is an important question in condensed-matter physics and one that has proved difficult to answer. Now, a group of researchers in Germany has observed the transition from “few” to “many” in an experiment using ultracold fermionic atoms. The results could help in modelling simple, few-body systems and to study mesoscopic quantum-mechanical systems.
Complex interactions
Studying systems with many particles can be challenging: while the microscopic behaviour of each particle by itself might be easy to determine, their combined macroscopic behaviour as they interact can become very complicated. Indeed, for systems with anything more than three particles it is often impossible to find analytical or numerical solutions. One of the ways of getting round this problem is to assume that the system is made up of an infinite number of particles. By doing so, the variables of the system go from being discrete to continuous and this makes the system much easier to study. But knowing exactly when this transition from discrete to continuous occurs can be tricky. In the past, experimental studies had been done with systems such as helium droplets but the results were inconclusive.
A drop in the Fermi sea
Now, Selim Jochim, Andre Wenz and colleagues at the University of Heidelberg and the Max Planck Institute for Nuclear Physics, both in Germany, have observed this crossover by studying a quasi-1D system of ultracold atoms using identical fermionic lithium atoms. The experiments look at how an increasing number of lithium atoms interact with a single “impurity” atom – a fermion that is in a different spin state from the rest. The impurity is used to probe the behaviour of the majority atoms. By measuring the energy of the system the team found that the atoms exhibited characteristics of a many-body system – which is known as a Fermi sea – when as few as four majority atoms were present.
In the experiment, the team uses a system made up of six lithium atoms – one of which is the impurity. The majority atoms themselves do not interact with each other because of the Pauli exclusion principle that dictates that no two identical fermions (particles with half-integer spin) can occupy the same quantum state simultaneously. However, as the impurity has a different spin, it is not affected by the exclusion principle and so can interact with all of the majority particles at the same time. Tuning the interactions between the majority atoms and the impurity – using a phenomenon known as “magnetic Feshbach resonances” – allows the team to explore the crossover from “few” to “many” at different interaction strengths. By studying samples with different particle numbers, the energy of the system is determined as a function of the number of majority atoms, for varying strengths of interparticle interaction. Initially, the system consists of the impurity (blue) and several majority atoms (green) without any interactions (see figure). Then, the researchers introduce an interaction between the blue and green atoms by applying a magnetic field. The energy of the system is then determined by changing the spin state of the impurity atom using a radio-frequency pulse. And it is this shift that acts as a probe – if the majority atoms were not already acting as a many-body system, the transition of the impurity atom with the pulse would occur at a particular frequency. In the presence of the majority atoms showing their collective behaviour, the frequency of this transition energy is shifted. “For a many-body system we know at which frequency this transition should occur and by comparing this with the experimental results we can find out whether the system has already reached the many-body limit,” says Thomas Lompe, another member of the team who is now at the Massachusetts Institute of Technology in the US.
Designer system
The researchers found that for weak and intermediate interactions, four atoms is sufficient to warrant the use of a many-body theory to describe the behaviour of the system. Wenz told physicsworld.com that the result was surprising and was not theoretically clear when the researchers began their studies. He also says that their “designer” ultracold system itself was an achievement – the system is “tunable” in the sense that the researchers can control its size on a single-particle level while maintaining full control over interparticle interactions. Wenz explains that such intermediate mesoscopic states – when a system has neither too few nor too many particles – are important and difficult to model. Studying them in a regime where one can control both the number of particles and their interactions was a key achievement. Also, theirs was the first system of ultracold atoms where only a single impurity interacted with few majority atoms – previous experiments in 2D and 3D systems with thousands of impurity atoms embedded in even larger clouds of majority atoms had been carried out. In terms of 2D and 3D few-particle systems, Wenz speculates that the minimum number of particles necessary for collective behaviour would probably be higher, but possibly 10 particles might suffice.
Currently, the team is doing condensed-matter lattice simulations using multiple traps filled with ultracold atoms, where each trap represents a fixed lattice point. In the long term, Wenz suggests that such few-particle systems are useful to study superfluidity, to better understand the behaviour of nucleons in atomic nuclei as well as to test the descriptions of finite physical systems, such as the dopants in the tiny transistors that are currently being used in computers.
This image of the Mona Lisa has been stabilized using technology developed by NASA to study solar flares. (Courtesy: Marblar)
By Hamish Johnston
The best thing about science fiction is that it is fiction, and nit-picking about scientific accuracy shouldn’t get in the way of telling a good story. That’s the theme of Roger Highfield’s review of the latest blockbuster Gravity. Writing in his old paper The Daily Telegraph, Highfield – who now works at London’s Science Museum – takes exception to a series of Tweets by the celebrity astrophysicist Neil deGrasse Tyson about the film. Among other things, the Tweets complain that Sandra Bullock’s hair should be wafting around in zero gravity, not hanging down as it would on Earth. Despite these and other “scientific holes big enough to fly a Saturn V rocket through” both Highfield and Tyson agree that Gravity is a film well worth seeing. The review is called “Gravity: how real is the science?“.
