By Hamish Johnston
The physicist and best-selling author Frank Close has joined forces with Michael Marten – founder of the Science Photo Library (SPL) – and CERN Courier editor Christine Sutton to create a new app about particle physics. Called The Particles, the app is billed as an introduction to the Standard Model and is aimed at a wide audience that includes professional physicists, students and even amateur enthusiasts.
By Margaret Harris at the APS March Meeting in Baltimore
“Science remains institutionally sexist. Despite some progress, women scientists are still paid less, promoted less, win fewer grants and are more likely to leave research than similarly qualified men.” The opening lines of Nature’s recent special issue make an arresting – if depressing – summary, so it’s not surprising that Roxanne Hughes chose them to kick off yesterday’s press conference on women in science, technology, engineering and mathematics (STEM) at the APS March Meeting.
Hughes, an education expert at the US National High Magnetic Field Laboratory, spoke about a study she’d done of 26 women undergraduates. All of them entered university with the intention of studying a STEM subject, and 12 had enrolled in a “living and learning community” that offered specialized mentoring opportunities and the chance to live with other female science students. Such programmes have often been touted as a way of helping women persist in science, but on Hughes’ evidence, this particular one made not a whit of difference, at least in numerical terms. The 12 students in the study who switched to non-STEM fields were evenly split between those who participated and those who didn’t.
Scientists in the US have used data from sensors on aircraft to show that current strategies for reducing passenger exposure to ozone may not be effective.
Although ozone high up in the stratosphere provides protection from ultraviolet radiation, it can be harmful when circumstances such as air travel or ground-level pollution expose humans to high levels of the gas. Ozone can cause breathing difficulties, headache or even premature death. As a result, US Federal Aviation Regulations limit ozone levels to less than 0.25 parts per million, and to 0.1 parts per million as a three-hour average. Now Seema Bhangar and Bill Nazaroff of the University of California Berkeley are arguing that the route-planning measures used to limit ozone exposure may not be up to the job.
“[Our research] raises questions about the claim – which is written into current federal regulations – that aircraft can prevent high in-cabin ozone levels by modifying flight paths to avoid air that is expected to contain high concentrations of ozone based on statistical summary tables of the variation of ozone with altitude, latitude and season,” explains Bhangar. “The summary tables do not capture the rich spatial variation of atmospheric ozone concentrations.”
The team found that the mean height of the tropopause – the boundary between the troposphere and stratosphere – decreased towards the poles, as expected. This should mean that ozone concentrations at aircraft cruising altitudes are generally larger at high latitudes. But the pair did not find a systematic or monotonic increase with latitude in the ozone concentrations encountered by transatlantic flights.
“Our research findings suggest that the highest ozone levels encountered by aircraft may be tied to local, episodic disturbances in the tropopause height,” says Bhangar. “These high levels cannot be avoided through flight-path planning that is based only on mean annual variations in ozone with altitude, latitude and season.”
Many aircraft use ozone control devices, or “converters”, in their ventilation systems to remove ozone entering the plane. The team’s results indicate that as these devices age and approach the 60% efficiency limit at which they receive maintenance, cabin ozone levels between February and June could exceed the 0.1 ppm limit on transatlantic flights; the tropopause tends to be lowest in the spring and at its maximum height in the autumn.
“Though levels of ozone outside the cabin do not translate directly to exposures inside the cabin, outside ozone concentration is a key indicator of in-cabin exposures to both ozone and ozone-reaction byproducts,” says Bhangar. The presence of an ozone control device is also important.
Currently, only long-haul flights are required to have ozone control devices. But the team found that ozone levels greater than 0.1 ppm were routinely encountered outside the aircraft even in US domestic airspace.
In earlier studies Bhangar, Nazaroff and colleagues took a suitcase full of ozone measurement kit on board planes. Arranging this is complicated, expensive and time consuming, so when the researchers heard about the vast amount of data collected by the MOZAIC project, they were intrigued. MOZAIC has been running since 1993, using sensors installed on the shells of commercial aircraft.
The researchers looked at MOZAIC data from Airbus A340 planes travelling from Germany, to either Los Angeles, Chicago or New York between 2000 and 2005 – a total of 865 flights.
“This approach – making opportunistic use of an existing dataset – started as an engaging diversion from our primary air-sampling-based investigation, but proved to be a valuable complement to that campaign, which provided us with independent, useful insights,” says Bhangar. “The present work provides a way to build on the in-cabin work via a much larger sample size and lower investment of sampling effort per monitored segment.”
Peak one-hour ozone concentrations measured in the study ranged from 90–900 ppb, while the flight-average ozone level was 50–500 ppb. Flights to Chicago and New York typically saw higher average ozone concentrations than those to Los Angeles, as flights to the US West Coast tend to take a more northerly route, avoiding much of the “high ozone” region centred in the western North Atlantic, say the researchers.
The amount of ozone inside the cabin depends not only on the atmosphere outside, but also on the ventilation system, materials making up the cabin surface, density of occupants, and surface area to volume ratio. The team estimated that more than 95% of the flights between February and June analysed in the study would have exceeded the 100 ppb (0.1 ppm) mark for flight-average ozone levels inside the cabin if ozone converters were absent or ineffective.
“Our results improve the scientific basis for understanding conditions under which air travellers and flight crew are likely to be exposed to elevated ozone,” says Bhangar. “The findings can help regulators and the airline industry to be more efficient, effective and strategic in reducing human exposures to ozone and ozone byproducts in the air-cabin environment.”
The ideal next step, Bhangar believes, would be to add an indoor monitoring component to the MOZAIC endeavour. “If even one of the MOZAIC aircraft currently measuring outdoor parameters on all routine flights were equipped with an indoor ozone sensor, the resulting data – in conjunction with information on the ventilation system, ozone converter and occupant load – could give us an unprecedented look not only at real-time in-cabin and atmospheric ozone levels on a large number of flights, but also at what mediates the relationship between these two variables.”
In the meantime, the scientists are investigating levels of dynamic air pollutants in other occupied indoor environments, with a focus on bioaerosols. They reported their aircraft results in Environmental Research Letters (ERL).
Ozone is not the only health issue facing air travellers. Levels of neutron radiation on board aircraft are also monitored and some physicists are concerned about cosmic rays.
By Tushna Commissariat
It’s not often that we come across a mention of an astronomical event measured in Earth years, let alone months or hours. So suffice to say I was pretty surprised by a recent XMM-Newton finding that talked about a star orbiting a black hole at the furious rate of once every 2.4 hours! Further investigation revealed that this has only broken the previous record by an hour, but these extremely short orbits still have me rather amazed. Certain short orbital period binary stars or pulsars do have even shorter periods of less than an hour, but this star orbits a stellar-mass black hole (it’s about three times more massive than the Sun) that is roughly a million kilometres away from it. The video below, courtesy of the European Space Agency (ESA), is an animation showing one complete orbit of the star.
Physicists in the US say they are the first to store – and then retrieve – quantum information in a mechanical oscillator. Their device consists of an extremely thin disc of aluminium that is connected to a microwave circuit. Quantum information encoded in a microwave signal is transferred to the disc, which vibrates much like a drumhead. This information can then be retrieved by converting the mechanical oscillations back into microwaves.
Built by Konrad Lehnert and colleagues at the National Institute of Standards and Technology in Boulder and the neighbouring University of Colorado, the team says the device can store quantum information for long enough to make it a potential candidate for a future quantum computer. Such machines could, in principle, outperform conventional computers at certain tasks, but require quantum bits (or qubits) that can store and transfer quantum information without it being destroyed by interacting with the outside world.
Like many attempts to make “quantum memory”, the qubits built by Lehnert and colleagues are “mesoscopic” objects – small enough to behave as quantum systems, yet large enough to fabricate on a chip and connect to other qubits. Weighing just 48 pg, their drumhead is a circular sheet of aluminium 15 μm in diameter and 100 nm thick. The entire device was chilled to 25 mK and the drum itself is put into its lowest-energy vibrational state (its ground state) using a microwave cooling technique.
Quantum information is first encoded in the amplitude and phase of a microwave pulse. This pulse is then sent along a waveguide that sits next to a spiral-like resonant circuit that includes the drum (see figure; click to enlarge). The circuit is designed such that the pulse is completely absorbed by the circuit and the microwave energy is converted to vibrational energy stored in the drum. “At the end of the process, the mechanical oscillator quivers with a particular amplitude and phase, which is the amplitude and phase that the microwave signal had before it was absorbed,” explains Lehnert.
This absorption process is controlled using a second microwave signal, with the presence or absence of this “transfer field” determining whether the pulse can move from the waveguide to the resonant circuit and vice versa. To read out the quantum information, the transfer field is adjusted so that the vibration of the drum is converted back to a microwave pulse, which can then jump into the waveguide and be measured by Lehnert and colleagues.
In a typical experiment the team was able to store quantum information for about 25 μs without it degrading significantly. The team was also able to successfully store and retrieve the pulses in about 65% of attempts. According to Lehnert, this inefficiency arises because the microwave circuit is not perfect and some of the microwave signal is lost. But he is confident that smaller circuits will improve the performance. “We believe the prospects for improving this number are quite good, as other researchers have recently demonstrated microwave circuits with loss about 100 times smaller than we achieved,” says Lehnert.
Lehnert told physicsworld.com that his team’s drums could be used in conjunction with superconducting qubits, which also operate at low temperatures and microwave frequencies. While mechanical oscillators cannot easily be connected with each other to create logic devices, they could be used to store quantum information that is then processed in superconducting devices. In particular, Lehnert points out that the linear nature of the device means that it could be used to store more than one qubit at the same time.
Another possible application in quantum information is using mechanical oscillators as quantum transducers, which convert quantum information from one form to another. “We are working on using mechanical oscillators to convert quantum information encoded in microwave electrical signals into information encoded as an optical field [light],” Lehnert says.
Converting qubits from microwave to optical signals could play an important role in large-scale quantum information systems of the future. Although microwaves are very well suited for transferring quantum information in small ultracold devices, they cannot carry information over large distances. Optical signals, on the other hand, can travel tens or even hundreds of kilometres without losing their quantum nature.
The research is described in Nature.
By Hamish Johnston
“Beyond any reasonable doubt, it is a Higgs boson, and here we examine the extent to which its couplings resemble those of the single Higgs boson of the Standard Model.”
That’s taken from the abstract of a new paper by John Ellis and Tevong You of King’s College London. Ellis, of course, has been associated with CERN for decades and if he says it’s a Higgs that’s good enough for me!
Researchers in the US and Germany have gained important insights into how molecular coatings affect how nanometre-sized particles organize themselves into regular “superlattices”. By combining new X-ray techniques with thermodynamic analysis, the team has worked out why the same nanoparticles can assemble into several distinct structures. The scientists hope that their insights could lead to new designer materials.
Over the past decade or so, man-made metamaterials have been created to have desirable properties not found in natural materials. These include nanoparticle superlattices – regular arrays of nanometre-sized structures – that show promise for a range of technology applications, such as photovoltaics, light-emitting diodes and thermoelectrics. However, before these properties can be exploited commercially, researchers must first gain a better understanding of how the tiny nanoparticles come together via complex nucleation and other processes.
In the early stages of nucleation, nanoparticles aggregate randomly and form an amorphous structure with limited short-range ordering. The particles then minimize the total free energy in the structure by crystallizing into a long-range ordered superlattice. The nanoparticles appear to behave like hard spheres, packing together to form a fairly simple superlattice phase. However, they also appear to go through various quite intricate nucleation and growth steps that involve multiple nanoparticle interactions with surrounding solvents and surface-capping ligands, which are molecules that attach to the surface of the nanoparticles.
To better understand these mechanisms, the US–Germany team decided to look at how oleic-acid-capped lead-sulphide nanoparticles just 3.5 nm in size assemble in solution under a variety of experimental conditions and different temperatures and pressures. The changes to atomic structure occurring at the nanoparticle cores and at the capped ligands can be followed from the same volume of sample using both high-resolution synchrotron small-angle X-ray scattering (SAXS) and wide-angle X-ray scattering (WAXS) – new techniques developed by the researchers themselves.
The results show that the nanoparticles nucleate in three main superlattice structural phases or polymorphs. These are amorphous, body-centred-cubic (bcc) and face-centred-cubic (fcc) phases. The type of superlattice formed can be controlled by changing the distance between the nanoparticles without changing their size. For example, when the inter-nanoparticle distance is increased, the particles pack together in a slightly looser but more ordered way while the capping molecules rearrange and change structure, modifying the overall surface energy.
“Our analyses have shown that the nanoparticles appear to either maximize entropy or minimize the enthalpy in the structure to reduce the total free energy through multiple interactions with the surface molecules,” explains team leader Zhongwu Wang. “These findings will hopefully inspire both experimentalists and theorists to consider the importance of surface-capping molecules when modelling these superlattice structures. From an applications point of view, our phase diagrams could offer a guide to material fabrication and help make ‘designer’ solids with specific structures, inter-nanoparticle distances and sizes.”
The researchers now plan to look at binary nanoparticles and other more complex nanostructures. “Using techniques only available in our labs at the Cornell High Energy Synchrotron Source (CHESS), we aim to build a series of structure–property relation diagrams of confined nanoparticles with different sizes and shapes,” Wang told physicsworld.com.
The current work is presented in Nano Letters.
By Margaret Harris at the APS March Meeting in Baltimore
In industry, the gap between making a scientific discovery and turning it into a practical product is often termed the “valley of death”. Many an idea that seemed promising in the laboratory has failed to become a real application for want of funding, industrial know-how or, usually, some combination of the two.
The Industrial Physics thread of this year’s APS March Meeting – which my colleague Louise Mayor and I are attending this week on behalf of Physics World – includes a number of talks about the “valley of death” problem, and the one that kicked off yesterday’s session really brought home the importance of addressing it. The speaker, Robert Colwell, directs the Microsystems Technology Office at the Defense Advanced Research Projects Agency. Better known by its acronym – DARPA – the agency is part of the US Department of Defense, and one of the products that physicists in Colwell’s office have developed is a “blast gauge” for soldiers deployed in Iraq and Afghanistan.
New simulations of head-on collisions of particles travelling at nearly the speed of light show that black-hole formation can occur at lower collision energies than expected, according to a team of researchers in the US. The researchers attribute this to a “gravitational focusing effect” whereby the two colliding particles act like gravitational lenses, focusing the energy of the collision into two distinct light-trapping regions that eventually collapse into a single black hole. Although the work shows that black holes can form at lower collision energies than expected, the team says that the result has no impact on real particle collisions taking place at the Large Hadron Collider (LHC) at CERN.
From 2008 onwards, when the LHC was first scheduled to be switched on, there were rumours about what the experiment might create – extra dimensions, sparticles and strangelets, vacuum bubbles and, of course, planet-destroying black holes. Although the experiment ran seamlessly from November 2009 for more than two years and scientists found no evidence whatsoever for the formation of micro black holes, the fascination with black-hole formation and evaporation continues – among researchers and the media.
Frans Pretorius and William East of Princeton University in the US want to better understand the dynamics of particle collisions at the super-Planck scale.
Planck units are a system of units comprised of the simplest algebraic combinations of the fundamental constants of nature – the speed of light c, Newton’s constant G, Planck’s constant h and so on. For example, a combination of the constants to form a unit of Planck energy (Ep) is about 2 × 109 J. Pretorius explains that a super-Planck-scale collision is a collision between two fundamental particles where the total energy (rest energy (Er) plus the kinetic energy) exceeds Ep. At the Planck scale, quantum-gravity effects are expected to start playing a role in the interaction. However, at energies greater than Ep (and no-one knows exactly how much greater), classical gravity dominates the interaction.
So the researchers wanted a completely classical calculation, and this, explains Pretorius, is the “crucial ingredient in the argument that super-Planck-scale collisions form black holes, regardless of any non-gravitational interactions between the particles”. He goes on to explain that this is important, as currently we do not know exactly what quantum-gravity interactions occur at the Planck scale. According to Pretorius, the new results suggest that for energies sufficiently above the Planck scale it does not matter – a black hole will form around the interaction, hiding all quantum effects, at least temporarily.
While considering the super-Planck-scale regime, the researchers look at specific “gamma” (γ) values in the collisions. Pretorius explains to physicsworld.com that γ is a measure of the kinetic energy of the interaction; that is, if the rest energy of one of the colliding particles of mass m is Er = mc2, then the total energy of the particle in motion is Et = γ × Er. So, when two particles of mass m collide, each moving with velocity v towards each other in the reference frame of the lab, the total energy of the collision is 2 × γ × Er.
According to the researchers, the critical γ depends on the particular model of a particle, and in their simulations, the particles of choice are “fluid stars” – a hypothetical and perfect “star” or particle that Pretorius describes as a “classical model of a fermionic star”. They used the fluid star because its γ value is high enough that the total collision energy would allow for a black hole to form. “Since our calculation is purely classical, so no h, this serves as the proxy of the Planck energy,” says Pretorius.
A previous estimate for black-hole formation – known as the “Hoop Conjecture” and developed by Kip Thorne in 1994 – says that an object compressed in a highly spherical manner will “form a black hole around itself when and only when its circumference in all directions becomes less than the critical circumference”. This “critical circumference” is directly related to the Schwarzschild radius (rs) of the object. But in the case of their collisions with super-Planck-scale fluid stars, Pretorius and East found that black-hole formation “actually begins at a fraction of about one-third of this [Hoop Conjecture estimate] energy”.
In terms of the actual simulations, the researchers begin with the two model particles – the fluid stars – moving towards each other at high velocity. When they first collide, they are in a sense at such high velocities that the particles just smash through each other. “However, the gravitational field of each is so strong because of the extra energy coming from the large gammas, that the gravitational force, which is attractive, strongly focuses each particle towards the collision axis,” says Pretorius. The “fluid” material compresses because of this focusing, and eventually by so much that each particle collapses to form a black hole. “We measure this in the simulation by the apparent horizons, which baring technical details are essentially the event horizons of the black holes. These initially two separate horizons then merge to form a larger black hole,” he says.
You can see the simulations here – the first video is for γ = 8, which is strong enough to produce some lensing but not enough to form any black holes. The second is for γ = 10 and the black holes are formed.
Pretorius and East are clear that their simulation results have no real bearing on safety issues at any high-energy experiments. They say that even if such black holes are formed, they would still be completely benign given what we know about them. According to the team, the results might give researchers a better idea of the energies at which black holes might start to form. “Though the key question is what the true Planck scale is,” muses Pretorius. The units he mentioned above suggest that the Planck energy is 1016 TeV in LHC-like terms, so about 15 orders of magnitude above LHC energies. If this is the case, it is not even remotely possible for the LHC to form black holes, even with the factor of one-third decrease in the threshold energy. “However, if there are extra, small dimensions, as string theory predicts, then the true Planck scale could be lower. There are no firm predictions on how much lower, so this is a highly speculative scenario. If black holes were formed, it would strongly indicate that there are extra dimensions, which would be a very profound discovery,” says Pretorius.
The research is published in Physical Review Letters.
By Hamish Johnston
The BBC and the Open University have teamed up to produce two television programmes about Richard Feynman – the Nobel-prize-winning scientist who died 25 years ago.
Outside of the physics community, Feynman is probably best known for his diligent and outspoken role in the investigation into the causes of the Challenger disaster.
Like many of my generation, I can remember exactly where I was when the Space Shuttle Challenger exploded shortly after take-off on 28 January 1986: it was a snowy Tuesday in Guelph, Ontario, and for some reason I was still at home at 11.39 a.m. when the accident occurred.
