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Magnetic skyrmions could help make low-energy artificial ‘brains’

Simulations suggest that magnetic skyrmions could form the basis of ultra-low-power-consumption devices that mimic the memory and learning functions of neural synapses.

Despite advances in computer power, there are still tasks that are best done by biological brains. Efforts to emulate the way the brain is wired have led to work on “artificial synapses” as connections for use in “neuromorphic” computers that try to emulate the functionality of a biological brain. Researchers in China have now demonstrated that the skyrmion – a type of magnetic quasiparticle – could be used to create energy-efficient synaptic devices.

New challenges are not always best met with old tools, and as challenges go, emulating synaptic connections in a scalable system – the human brain contains hundreds of trillions of synapses – is no mean feat. Synapses do more than connect neurons, they weigh how well neurons are connected through signal spiking and modulation processes that are thought to be the basis of human learning and cognition. While some progress in the development of synaptic devices has been made using phase-change memories, Ag-Si memories and resistive memories, studies of magnetic skyrmions suggest they may be a promising alternative.

Collective excitations

Skyrmions are particle-like regions within a field where all of the field vectors point either towards or away from a single point in space. They were originally proposed in the 1950s by British physicist Tony Skyrme to explain aspects of particle physics. Researchers have since discovered that some collective excitations of electron spins in solids behave much like skyrmions, and the first observation of a magnetic skyrmion lattice was reported in 2009. These solid-state skyrmions could be potentially useful in next-generation electronics and spintronics.

“My supervisor Weisheng Zhao told me to investigate applications of skyrmions,” says Yangqi Huang, a researcher at Beihang University in China. He came upon the idea of using skyrmions in synaptic devices through discussions with members of his research group, which includes spintronics theorists – people who design devices and specialists in fabrication and circuit design – as well as people working in neuromorphic computing. “A skyrmion is a particle-like structure, so I thought it’s very similar to a neurotransmitter.”

Huang and his colleagues at Beihang University and the Chinese University of Hong Kong, Shenzhen, simulated their skyrmions as 2D discs 50–60 nm in diameter. The circumferential edge and centre of the discs are opposite magnetic poles separated by a chiral domain wall. The skyrmions are incorporated within a device comprising a ferromagnetic layer that has perpendicular magnetic anisotropy, modelled as cobalt, and a heavy-metal layer modelled as platinum. Together, the two components comprise a “racetrack” that magnetic skyrmions can move along.

Energy barrier

Skyrmion racetracks have been studied before as possible electronic memory components. However, by adding an energy barrier at the centre of the racetrack, the researchers simulated the presynaptic and postsynaptic regions where neurons connect to a synapse. Current flow through the heavy-metal layer from one end of the device to the other injects a vertical spin current into the ferromagnetic layer, which drives skyrmions between the pre and post-synaptic regions.

In a biological synapse, prior signal activity causes changes in the number of neurotransmitter receptors, leading to “depression” or “potentiation” – which is the weakening or strengthening of the synaptic connection. In the proposed skyrmion synaptic device, the change in magnetoresistive properties that occurs as skyrmions move either side of the energy barrier mimics this depression and potentiation. Huang and colleagues showed that their system has both short-term plasticity and long-term potentiation. These are synapse-like behaviours that are linked to long- and short-term memory.

The simulations suggest that the skyrmion synaptic devices operate with very low energy dissipation, explains Huang. In addition, the electrical current density needed to drive the skyrmions is very low, as has already been shown for skyrmions in previous theoretical and experimental studies. The result is a power consumption of just 1 pJ per synaptic event, making it a contender for making practical synaptic devices.

“Only a simulation”

“But it is only a simulation,” adds Huang, emphasising that most other synaptic devices have already been built and demonstrated. While the racetrack can be readily fabricated from metals with a capping layer to produce the energy barrier, an effective way of detecting skyrmions based on electrical signals is still a challenge. So far, other groups have used the Kerr effect – an optical phenomenon – to observe skyrmions. Huang has also begun experimental work on skyrmions in germanium thin films using Lorentz transmission electron microscopy, but this is limited to very thin films and work in this area is ongoing.

“Skyrmions have unusual topological properties,” says James Gimzewski, director of the UCLA CNSI Nano & Pico Characterization Core Facility, who was not involved with the current research. As one of the pioneers in artificial synapses based on nanostructures he adds: “It is interesting to see that they can now be used to mimic synaptic excitation and depression opening a new avenue for neuromorphic devices.”

The research is described in Nanotechnology.

Flash Physics: Artificial skin feels heat, Earth’s footprint on the Moon, baryon decay glimpses CP violation

Artificial skin has snake-like feelings

A new artificial skin can sense temperature changes like a pit viper senses its prey. Researchers from Caltech in the US and ETH Zürich in Switzerland have developed a flexible skin-like material out of pectin and water. The film generates an electrical response to temperature changes in a manner similar to the way pit vipers sense warm prey. The snakes’ pit organs contain ion channels in the cell membrane of its sensory nerve fibres. These expand with temperature increase, allowing the flow of calcium ions and therefore triggering electrical impulses. In comparison, the artificial skin releases calcium ions that are within the weakly bonded structure of pectin molecules. Chiara Daraio and colleagues suggest that the combination of increased ion concentration and increased ion mobility causes a decrease in electrical resistance. By testing over a range of 5–50 °C, the researchers found the skin could sense temperature changes of a mere 0.01 °C – almost 10 times more sensitive than existing electronic skins. The new skin can be as little as 20 μm thick and has many potential applications. It could be used on prosthetic limbs allowing amputees to sense temperature changes and, if included in first aid bandages, it could alert health professionals to temperature changes caused by wound infections. The team also plans to increase the functional temperature range so the skin can have industrial applications such as robotic skins and thermal sensors. However, this requires a new fabrication process because the water within the material bubbles and evaporates at high temperatures. The research will be published in Science Robotics on 1 February.

Earth’s footprint on the Moon

Oxygen from Earth reaches the Moon's surface when it protects the Moon from solar winds
Oxygen from Earth found on the Moon’s surface. (Courtesy: Osaka Univ./ NASA)

Oxygen from Earth’s atmosphere has been detected on the Moon’s surface. Scientists in Japan have analysed data from the lunar orbiter Kaguya taken when the spacecraft and the Moon were sheltered from solar winds by the Earth’s magnetosphere. For all but five days of the lunar orbit, the Moon is bombarded by solar wind. For those other five days, when the Earth lies between the Moon and Sun, the Earth’s magnetic field deflects the solar wind away from the Moon and ions from the Earth are able to reach the lunar surface. Previous studies analysing the Moon’s soils have shown the presence of terrestrial nitrogen and noble gases. Now, Kentaro Terada and colleagues have found evidence that oxygen from Earth’s biosphere also reaches the Moon. The orbiter Kaguya measured the mass and energy of oxygen ions reaching the Moon while it was sheltered from solar wind. It measured a significant number of oxygen ions when the Moon was within the Earth’s plasma sheet (the region of reduced magnetic field between the north and south lobes). The energy of the ions combined with the depth of oxygen in the lunar soil implies that the gas from Earth’s lower atmosphere has been depositing on the Moon since oxygen became abundant on Earth about 2.5 billion years ago. The findings, reported in Nature Astronomy, suggest that the lunar soil could provide a footprint of our planet’s ancient atmosphere, although Kentaro Terada and team stress that differentiating between solar and Earth winds will complicate the investigations.

Matter–antimatter asymmetry glimpsed in baryon decay

Photograph of physicists on the LHCb collaboration
Physicists on the LHCb collaboration have spied CP violation in lambda baryon decay. (Courtesy: CERN)

The first sighting of matter–antimatter asymmetry in the decay of a baryon has been reported by physicists working on the LHCb experiment at the Large Hadron Collider (LHC) at CERN. The finding is the first potential observation of the violation of charge-parity (CP) symmetry in a particle comprising three quarks and, if verified, could provide important clues about why there is much more matter than antimatter in the universe. The study looked at how bottom lambda baryons (comprising up, down and bottom quarks) and their antimatter counterparts decay after being created by collisions in the LHC. The team looked at about 6600 events in which the baryons (or antibaryons) decayed to create a proton and three pions (or corresponding antiparticles). It found that the spatial distribution of the matter and antimatter decay products was different at a statistical significance of 3.3σ. A similar measurement of about 1000 baryon (or antibaryon) decays that created a proton, pion and two kaons (or corresponding antiparticles) found no evidence for a matter–antimatter asymmetry. While 3.3σ is smaller than the 5σ required for a discovery in particle physics, this is the first measurement of these two decays and the statistics will improve as more data are collected – making it clearer with time if the asymmetry exists or not. CP violation was first seen in the decay of kaons in 1964 and more recently in B-meson decays – with both particles containing two quarks. In the Standard Model of particle physics, CP violation is explained by the Cabibbo–Kobayashi–Maskawa (CKM) mechanism. However, the CKM is not able to explain why there is more matter than antimatter, and therefore studies of CP violation in baryons could provide important clues towards solving this mystery. The study is described in Nature Physics.

 

  • You can find all our daily Flash Physics posts in the website’s news section, as well as on Twitter and Facebook using #FlashPhysics. Tune in to physicsworld.com later today to read today’s extensive news story on later today to read today’s extensive news story on how skyrmions can be used to create artificial synapses.

A fusion fly-over

 

By Michael Banks

To the critics, a working fusion power plant is always 30 years away.

But in the past decade, progress has been made at the construction site of the ITER fusion reactor in Saint-Paul-lez-Durance, France.

Ten years ago – on 29 January 2007 – preparation work began on ITER’s home in the large stretch of national forest. Within two years, more than three million cubic metres of rocks and soil had been removed to level the site ready for the behemoth.

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Analogue black hole could be made from plasma mirror

An analogue to the creation of Hawking radiation at the event horizon of a black hole could be made by firing an intense laser pulse at specially designed targets. That’s the conclusion of physicists in Taiwan and France, who say that the “plasma mirror” created in the proposed experiment could be used to study the relationship between quantum particles inside and outside a black hole. The researchers have calculated that the experiment could be done using existing technology and that it could shed important light on the black-hole information-loss paradox.

The idea of Hawking radiation has been around since the 1970s when Stephen Hawking considered what would happen to pairs of “virtual particles” created near the event horizon of a black hole – the region beyond which not even light can escape the tug of gravity. Quantum mechanics dictates that pairs of such particles can pop into and out of existence within a vacuum, and Hawking reasoned that one particle from each pair would be swallowed up by the black hole, while the other would be emitted to form “Hawking radiation”. This process would remove energy from the black hole, making it evaporate and eventually disappear in the absence of any other nearby sources of matter.

Because the emitted radiation is generated at the edge of a black hole, it tells us no more than an external observer can learn about the black hole – its mass, charge and angular momentum. All other information regarding individual objects that have been sucked into the black hole would be lost forever. The problem with this loss of information is that it violates a principle of quantum mechanics that says that the complete information about a physical system at one point in time will dictate its quantum state at any point of time in the future.

Thought experiments

Research into the information-loss paradox has been mostly theoretical as it is hard to make the appropriate measurements on real black holes. Physicists are therefore keen to create systems in the lab that are analogous to black holes, with the Hawking-like radiation associated with these analogues potentially providing important clues to resolving the information paradox.

Now, Pisin Chen of National Taiwan University and Gerard Mourou of Ecole Polytechnique in Paris have proposed a way of using a plasma mirror to create a black-hole-like system. Plasma mirrors are created when an intense pulse of radiation strikes a solid material, such as glass, and separates electrons from the atoms to make a plasma. When this occurs, the material changes from being transparent to being highly reflective.

To mimic Hawking radiation at the event horizon of a black hole, Chen and Mourou propose creating a plasma mirror that accelerates rapidly and then stops abruptly. This, they say, could be done by firing an intense laser pulse at a solid target to create an intense pulse of X-rays. This X-ray pulse would then be directed at a second solid target that has a density varying on the nanometre scale. A plasma would be created in this second target and the density gradient would make the plasma accelerate in the direction of the X-ray pulse.

Imperfect mirror

The plasma acts as an imperfect mirror, reflecting one half of a virtual photon pair created at its surface and allowing the other photon to pass through. These reflected photons are analogous to Hawking radiation. The un-reflected photons become trapped in the plasma and are analogous to photons within a black hole.

According to Chen and Mourou, the trapped photons should be released when the mirror stops as it reaches the end of the second target. The reflected and trapped photons would then be detected and physicists could look for correlations between the photons to determine if the photons are quantum-mechanically entangled. The virtual pairs are entangled when they are created, and a measurement of entanglement between the reflected and trapped photons could provide important information about the information-loss paradox.

Chen and Mourou say it should not only be possible using advanced laser and nanofabrication techniques to do the experiment but also measure correlations between the photons of interest, despite the presence of a large background of other photons created in the experiment.

The research is described in Physical Review Letters.

Physics in the US: no longer business as usual

New rules: citizens of seven countries have been barred from the US. (Courtesy: CBP)

By Matin Durrani

Over the last couple of years here at Physics World, we’ve been publishing special reports examining the state of physics in different nations around the world, including Brazil, China, Japan, India, Korea and Mexico.

When we decided in September last year to publish our next special report in 2017 on the US, it seemed reasonable to expect that Hillary Clinton was going to be elected president. For science, a Clinton presidency would pretty much have been “business as usual” and so, probably, would have been the tone of our special report.

But now that Donald Trump is in the White House, it looks as if we’re entering a period where the US is as far removed from “business as usual” as you could imagine.

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Flash Physics: Detecting life’s signature, UK to pull out of Euratom, nuclear-clock lifetime measured

Detecting the signature of life on other planets

A new chemical test could determine whether life has existed on other planets. Scientists at NASA’s Jet Propulsion Laboratory, California, have developed a quick and simple method for analysing amino acids using capillary electrophoresis (CE). Amino acids are the building blocks of life as we know it. Made predominantly of oxygen and carbon, the organic molecules exhibit chirality. This involves two molecules having the same composition, but being mirror images of each other – like human hands. The amino acids related to life forms on Earth are left-handed. If we assume this applies to other planets, their presence could be seen as a signature of life. However, amino acids are also present because of non-life sources such as meteorites. These sources have equal amounts of left- and right-handed molecules. Therefore a test to identify the different amino acids is needed to determine the source. Peter Willis and colleagues have developed a simple, automated “mix and analyse process” using CE. In CE, molecules are identified based on their movement under an electric field. Using a laser detection system, the molecules can be seen moving at different speeds. The method, described in Analytical Chemistry has been used to test samples from Mono Lake in California. The lake’s high salt content and high alkalinity make it an excellent substitute for the waters believed to be on Mars, Saturn’s moon Enceladus and Jupiter’s moon Europa. Tests have shown that the method is 10,000 times more sensitive than that used by the Mars Curiosity rover and can detect very low concentrations of amino acids. If deployed during explorations of other planets, the method could help in the search for extra-terrestrial life.

UK to pull out of EU nuclear agency

Photograph of the interior of JET
Funding gap: inside the JET at the Culham Centre for Fusion Energy. (Courtesy: EUROfusion)

The UK has confirmed that it intends to pull out of the European Atomic Energy Community (Euratom), the international organization that develops nuclear power in Europe. The intention was set out in explanatory notes that accompanied a bill the UK government published on Thursday to start the process for the country leaving the European Union (EU) by triggering the Article 50 exit clause. If the UK does go ahead and leave Euratom then it could threaten the UK’s participation in two major fusion facilities. Experiments on the Joint European Torus (JET), which is based at the Culham Centre for Fusion Energy in Oxfordshire, are funded by the EUROfusion consortium until 2018. Discussions are currently under way to extend this to 2020. JET receives funding of €69m, 87.5% of which comes from the European Commission and 12.5% from the UK. The EU is providing half the cost of ITER, which is currently being built in Cadarache, France, and is seen as a successor to JET. If the UK pulls out of Euratom then it could follow Switzerland’s lead by becoming an “associate” member. This will allow the UK to participate in ITER and it may be enough for Euratom to continue to support JET.

“Nuclear-clock” lifetime is measured

The lifetime of the first excited state of the thorium-229 nucleus has been measured for the first time. The measurement provides important information to physicists who hope to use an optical transition from this excited state to create a “nuclear clock” that could outperform existing atomic clocks. The work was done by Benedict Seiferle, Lars von der Wense and Peter Thirolf at Ludwig Maximilian University of Munich, who have measured the half-life of the state as it undergoes an internal conversion process, which involves the decay energy being transferred to an atomic electron. The team found the half-life to be about 7 μs, which confirms calculations that internal conversion dominates the decay of thorium-229 nuclei. This measurement also confirms that the emission of light only occurs in about one in a billion decays. Making a clock from thorium-229 requires a decay resulting in the emission of light, not internal conversion. It is likely, therefore, that a practical nuclear clock based on thorium-229 will have to employ a scheme to suppress internal conversion in favour of an optical transition. The measurement is described in Physical Review Letters. Last year, Seiferle, von der Wense, Thirolf and colleagues made the first direct detection of this transition.

 

  • You can find all our daily Flash Physics posts in the website’s news section, as well as on Twitter and Facebook using #FlashPhysics. Tune in to physicsworld.com later today to read today’s extensive news story on a new black-hole analogue.

Great wagers in physics, CERN’s pine marten gets stuffed, Doomsday Clock moves closer to midnight

Flat out: Wallace saw him coming (Courtesy: PI)
Flat out: Wallace saw him coming. (Courtesy: PI)

By Hamish Johnston

I bet you can’t resist clicking on “Great wagers in physics history” – which has been compiled by Colin Hunter at the Perimeter Institute for Theoretical Physics in Canada. A surprising number involve Stephen Hawking, whose record on winning is quite abysmal according to Hunter. Hawking’s fellow Cantabrigian Isaac Newton also enjoyed a flutter and accepted Christopher Wren’s offer of 40 shillings to anyone who could – in two months – derive a force law that explained Keplers laws of planetary motion. Newton succeeded, but ran overtime so he didn’t collect the cash. In the image above you can read about another wager involving a “flat-Earth theorist”.

(more…)

Light recorded mimicking a sonic boom

The optical equivalent of a sonic boom has been filmed for the first time. The feat involved two important breakthroughs, slowing the light to create the effect and developing an ultrafast imaging technique to record the phenomenon.

When a jet aircraft travels faster than the speed of sound (343 m/s), it produces an immensely loud sonic boom that can smash windows and set off car alarms. This phenomenon is related to pressure waves. As an object, such as an aircraft, pushes air out of the way, it creates pressure waves that move at the speed of sound. If the object also reaches the speed of sound, known as Mach 1.0, the waves build up and create a shock wave, or sonic boom. Continuing at or above Mach 1.0 means the sonic boom trails behind the object in a conical shape – called a Mach cone.

A Mach cone is created whenever a wave emitter travels faster than the waves it creates and therefore the event is not limited to sound. However, while the speed of sound is achievable by modern aircraft, bullets and even bullwhips, the same cannot be said for light. It is a fundamental law of physics that nothing travels faster than the speed of light in a vacuum (299,792,458 m/s). So how could an emitter travel faster? Lihong Wang and Jinyang Liang of Washington University in St Louis, the lead researchers on the current study, get around this problem by taking advantage of the fact that light will travel significantly slower when in a medium rather than a vacuum.

Appears faster than light

To create their Mach cone, the team made two display panels of silicone rubber doped with aluminium oxide powder. These flanked a thin channel containing air and dry-ice fog. A green laser pulse lasting 7 ps is fired down the channel. As the short laser pulse travels through the channel, the dry ice fog scatters some of the light into the panels. The speed of light in the display panels is slower than in the channel. Therefore the light is slowed as it travels through the panels above and below the channel, making it appear that the pulse is travelling faster than the scattered light. As the scattered wavelets of light superimpose in the panels, they create a wave front, analogous to the sonic boom shock wave, and a Mach cone of light is seen trailing behind.

Yet, even with this reduced speed, it is still difficult to record the propagation of the light in real time. “Capturing a photonic Mach cone’s movement in real time to produce an intuitive movie has been a long-standing challenge owing to the lack of single-shot light-speed 2D imaging,” say Wang and Liang. The feat of freezing light’s motion requires an imaging speed of a billion frames per second, but most cameras can only achieve 1000 frames per second. Furthermore, most ultrafast technologies are pump-probe devices. They take thousands of measurements that then need to be stitched together. These require the events to be accurately repeated, something that is not achievable for many physical events.

Ultrafast solution

To overcome these challenges, Wang, Liang and colleagues developed a single-shot ultrafast imaging technique to record the real-time propagation of a laser pulse travelling through a scattering medium.

For the imaging set-up, the group used lossless-encoding compressed ultrafast photography (LLE-CUP). The LLE-CUP system is a step on from past devices because it is ultrafast and takes only one snapshot. The set-up Liang and team used was a complex arrangement of optical devices that captured the event through three different cameras. The first camera recorded a direct image of the scene while the second two recorded temporal information. The combination allowed the scientists to reconstruct the scene frame by frame. The result is the first ever recording of a photonic Mach cone in real time.

The LLE-CUP system provides a new approach for recording complex, unique events in real time. It has particular potential in the field of biomedical imaging. “Our camera is fast enough to watch neurons fire and image the “live traffic” in the brain. We hope we can use our system to study neural networks to understand how the brain works,” say Wang and Liang.

The photonic light cone and LLE-CUP system are described in Science Advances.

Has metallic hydrogen finally been seen?

The first observation of the low-temperature transformation of solid hydrogen into a metal – first predicted over 80 years ago – has been claimed by researchers in the US. The material needs further investigation – it is not clear whether it is a solid or a liquid – but some theoreticians have predicted exotic, and potentially useful, properties for metallic hydrogen such as room-temperature superconductivity. At least one leading high-pressure physicist, however, remains unconvinced by the results.

Hydrogen is a colourless diatomic gas under standard conditions. However, in 1935, Eugene Wigner and Hillard Huntington predicted that, at a pressure of 25 GPa (250,000 times atmospheric pressure) or higher, it would form an atomic, solid metal. This pressure was later shown to be hugely underestimated, as hydrogen becomes less compressible as its density increases. Liquid metallic hydrogen comprises the majority of the planets Jupiter and Saturn and this liquid metal can be produced by heating hydrogen up at high pressure until it crosses the so-called plasma phase transition. It was first observed in static experiments by Isaac Silvera and colleagues at Harvard University in 2016.

However, the so-called Wigner-Huntington transition, in which solid metallic hydrogen forms without heating at even higher pressures, had not been definitively observed, despite several suggestions that the material might have interesting properties. In 1968, Neil Ashcroft of Cornell University in Ithaca, New York, suggested that it could be a high-temperature superconductor. Then in 2011, David Ceperley and Jeffrey McMahon of the University of Illinois predicted that, at 500 GPa, the transition temperature would be well above room temperature.

Fade to black

In 2016, Silvera’s team reported compressing hydrogen in a diamond anvil cell to 420 GPa – the highest static pressures then reported, in a paper on the arXiv preprint server. At 335 GPa, the sample turned from a transparent phase to a black one, but concluded that it was not metallic. Intriguingly, Mikhail Eremets and colleagues at Max Planck Institute for Chemistry in Mainz, Germany, published another arXiv paper in 2016 identifying a “possible metallic” phase in hydrogen at 360 GPa. Silvera and colleagues believe this is likely to be the same phase that they observed.

In the new research, Silvera and his colleague Ranga Dias modified their apparatus to increase the pressure even further. They found that at 495 GPa, the sample changed from black to highly reflective – which Silvera and Dias say is evidence that the hydrogen has become a metal. Numerous questions remain, however, such as the sample’s state: “It’s possible that at low temperatures, the ground state of hydrogen is a liquid,” says Silvera, “If it’s a liquid, then it’s all part of the same phase of liquid metallic hydrogen. If it’s a solid, which I think it is, then that’s interesting too.”

Silvera and Dias have maintained the sample stably at liquid nitrogen temperatures for about three months. They now intend to conduct a series of ever-more challenging tests such as Raman and X-ray scattering to determine its state and structure and resistance measurements to determine its electrical conductivity. Perhaps most tantalizingly, it wants to release the pressure to see whether it remains metallic: “It’s been predicted that metallic hydrogen is metastable,” explains Silvera. If it turns out to be a superconductor, this would be especially interesting, although what would happen to the transition temperature remains uncertain: “I would expect that, if it was a superconductor at very high pressure, and you released the pressure and it was metastable, the critical temperature would change somewhat, but probably not a great deal,” Silvera says.

There have been many false claims in the past, so I think everyone will look for confirmation and for more data about the new phase
David Ceperley, University of Illinois

Ceperley is cautiously enthusiastic: “The search for metallic hydrogen has been kind of a contentious field,” he says. “There have been many false claims in the past, so I think everyone will look for confirmation and for more data about the new phase.”

Eremets, however, is not convinced, saying “We observed much stronger evidence of metallicity but we did not claim that it was really metallic, just possibly metallic.” He criticizes the absence of repeated experiments and describes the techniques used to measure pressure as “ambiguous”, saying the true pressure could be anywhere between about 400-630 GPa. Finally, he criticizes the researchers’ reliance on reflectivity measurements as proof of metallicity without data on conductivity: “What they observe could be from a semiconductor,” he says, “Because narrow-gap semiconductors reflect very well.”

Silvera disputes this interpretation, saying that the reflectivity of a semiconductor should increase with temperature, whereas their material became more reflective as they cooled the material: “This is the expected behaviour for a metal,” he concludes.

The research is described in Science.

Flash Physics: Sound can halt tsunamis, scientists to march on Washington, H0LiCOW measures Hubble constant

Sound waves could halt tsunamis

The devastating effects of a tsunami could be mitigated by firing underwater sound waves at the giant wave. That is the claim of Usama Kadri, who is a mathematician at Cardiff University in the UK. He has calculated that when the outgoing acoustic gravity waves (AGUs) collide with a tsunami the height of the incoming wave is reduced – thereby lessening its impact when it reaches shore. AGUs occur naturally and are created by violent geological events such as earthquakes. Kadri admits that creating artificial AGUs with sufficient energy to dissipate a tsunami would be a huge technological challenge. However, he points out that the great expense of developing and deploying the technology would be offset by its ability to save lives and protect property. The research is described in Heliyon.

Scientists plan march on Washington

A group of scientists in the US is trying to organize a scientists’ march on Washington to protest science-related policies of President Donald Trump and his new administration – including the potential muzzling of scientists working for the federal government. “An American government that ignores science to pursue ideological agendas endangers the world,” said a statement on the group’s Scientists March on Washington website. Several Facebook accounts set up by the group and like-minded supporters have gathered hundreds of thousands of followers over the past few days. A Twitter account associated with the movement, @ScienceMarchDC, has also attracted more than 136,000 followers. The group says that it is “working to schedule a March for Science on DC and across the United States. We have not settled on a date yet but will do so as quickly as possible and announce it here”. According to a report in the Washington Post, organizers will meet this weekend and plan to announce the date of the march next week.

H0LiCOW’s Hubble constant not consistent with theory

Optical image of quasar RXJ1131-1231 taken by the Hubble Space Telescope
H0LiCOW!: Distortions in quasar light reveals Hubble constant. Quasar RXJ1131-1231 was studied in the current work. (Courtesy: NASA/STScI)

A new measurement of the Hubble constant – the rate at which the Universe is expanding – has strengthened the argument against the standard cosmological model. The H0LiCOW collaboration has independently measured the Hubble constant by studying how the light from quasars is distorted by gravitational lensing. Quasars are supermassive black holes located at the centre of galaxies. They emit huge amounts of electromagnetic energy that randomly varies. We see this as an apparent flickering in their intensity. However, each image detecting the flickering shows a different time-delay of the event. This is because the emitted energy takes different paths to reach us due to the enormous mass of foreground galaxies bending space-time. This distortion is called gravitational lensing. The international collaboration led by École Polytechnique Fédérale de Lausanne (EPFL) and the Max Planck Institute, measured the time-delays to determine the Hubble constant because the distance the quasar light travels is dependent on the universe’s expansion. The current measurement of the Hubble constant agrees with other recent independent studies of the local universe. However, they all disagree with measurements of the cosmic microwave background made using the Planck satellite in 2015 and the predictions of the standard cosmological model. The current result, presented in a series of papers in the Monthly Notices of the Royal Astronomical Society, has strengthened the idea that there is new physics beyond the standard cosmological model.

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