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.
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.
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.
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.
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.
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.
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.
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.
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”.
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.
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.
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.
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.
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.
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.
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.
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.
“The truth is in here” reads a line at the top of a new book about – you guessed it – aliens. In a series of 20 short, sharp essays by a mix of extraterrestrial scientists and experts, compiled and edited by physicist and TV presenter Jim Al-Khalili, Aliens attempts to succinctly answer some big questions beginning with “Do aliens exist?” You would be forgiven if at first glance, you think this book is more fiction than fact – the paperback cover with its tagline of “Science Asks: Is There Anyone Out There?” and its neon green backing may throw off the serious scientific reader. But do not judge this little book by its cover, for it does pack a punch. The book opens with an introduction from Al-Khalili, followed by an intriguing essay by cosmologist Martin Rees, in which he speculates about how future humans, travelling across the galaxy, may be the aliens that we seek today. The other 19 essays are divided into four categories: close encounters; where to look for life elsewhere; life as we know it; and alien hunting. What would motivate aliens to visit us; what are the necessary ingredients and conditions for life to form, evolve and flourish; what about some form of life elsewhere in the solar system; and what might aliens look like – all these themes and more are mentioned in the book by seasoned science writers, authors and scientists including Monica Grady, Lewis Dartnell, Louisa Preston and Paul Davies. Thanks to the discovery of thousands of exoplanets in the past decade, astrobiology and the search for life beyond our planet has become a common topic in the popular-science book market, and indeed may soon saturate it. What sets Aliens apart in some ways is the real expanse of topics covered. The two chapters you’ll guiltily enjoy the most deal not with science per se, but with the human aspect of alien existence. Science broadcaster Dallas Campbell’s entertaining chapter tells the tale of “five of the most notorious UFO stories that have taken the flying saucer from fringe subculture to mainstream modern folklore”. In a subsequent essay, psychologist and professor of paranormal belief Chris French looks into the psychology behind the many people world over who are convinced that they have had “close encounters” or been abducted by extraterrestrials. Aliens is an entertaining and educational if slightly basic read for anyone with a scientific interest in extraterrestrials.
Mention the Royal Institution and most people will know it best for its long-running and beloved “Christmas Lectures”. Indeed, the lectures have been run every year since 1825, only taking a hiatus between 1939 and 1942 as a result of the Second World War. The lectures were the brainchild of Michael Faraday, who wanted to bring science in an engaging manner to children and young adults. Although the lectures have been televised since 1936, each year’s lecture (which takes place over a few days) still has a live audience of school children. Over the years, the lectures have been on all aspects of science, but those mentioned in 13 Journeys Through Space and Time: Christmas Lectures from the Royal Institution have a bit more of a physics thread. Compiled by astronomer and writer Colin Stuart, the book features 13 chapters, each of which is a shortened version of an actual lecture based on the theme of space and time. Beginning with Sir Robert Stawell Ball’s 1881 lecture on the Sun, Moon and the planets and finishing with Kevin Fong’s 2015 lecture on how to survive in space, the book spans many decades and plots the huge advances that science has made in that time. Some of the more famous lecturers include James Hopwood Jeans and Carl Sagan. The book also boasts an introduction from British astronaut Tim Peake, who (virtually) participated in Fong’s 2015 lecture while he was on board the International Space Station. Grab a copy of the book to read about the whacky live demonstrations done by the lecturing scientists and for some Christmas nostalgia.
From a scientific point of view, the theories of relativity and quantum mechanics are often considered the 20th century’s most renowned and profound discoveries. But the past 100 years have also seen many other significant advances in science: from the discovery of penicillin to the structure of DNA, from continental drift to the Big Bang, and even that of information theory, which set the basis for today’s hi-tech society. However, there is an often forgotten but nevertheless crucial discovery in physics that, in my opinion, surpasses all the others. By that I mean the pioneering work done by physicist John Bell on “local hidden variables” of quantum mechanics, which ultimately led to his ideas of “non-locality” or Bell’s inequalities.
In John Stewart Bell and Twentieth-Century Physics: Vision and Integrity, fellow physicist Andrew Whitaker tells the story of Bell’s life and his revolutionary discovery that not everything in physics can be explained using only local variables. Back in 1935 Albert Einstein, Boris Podolsky and Nathan Rosen realized that two quantum particles can be in a state such that a measurement on one particle instantaneously affects the other – no matter how far apart they may be. This effect, more commonly referred to today as entanglement, upset the trio because such “spooky action at a distance” would require information to travel faster than the speed of light. We now know than entanglement emerges thanks to correlations between measurements made on the two particles, and that entangled particles have much stronger correlations than are allowed in classical physics. But it was Bell’s breakthrough in 1964 that laid the groundwork for this phenomenon, when the Northern Irish physicist calculated an upper limit on how strong these correlations could be, if they were caused by local physics alone. Bell reasoned that correlations stronger than this limit would occur only if the particles were entangled and this is Bell’s inequality.
Whitaker, a physics professor at Queen’s University, Belfast, tells the story of Bell’s main discovery, but the book also goes beyond that. Bell was no one-discovery-wonder and, peculiarly, pursued quantum mechanics as a “hobby” in his spare time. Indeed, he was a very successful high-energy theoretical physicist, spending most of his career at the CERN particle-physics laboratory in Geneva. The book sets the stage with Bell as a student at Queen’s, and then follows his dual career – from the early 1950s to his “decade of great success” in the 1960s – including the publication of his seminal paper in 1964, which he wrote while in the US on sabbatical from CERN.
Through the book, one reads a lot about Bell’s character and the many people with whom he interacted including Alain Aspect, Abner Shimony, Reinhold Bertlmann and even myself. Interestingly, despite the fact that Bell seemed to discuss his ideas with a number of fellow scientists, he had very few joint publications on his work on quantum foundations. It is also remarkable how few papers Bell published in refereed journals. It seems he didn’t quite like the referee reports he must have received about his fundamental work, which was initially ignored and did not truly gain favour until the 1970s.
Bell died unexpectedly at the relatively young age of 62, from a cerebral haemorrhage, which Whitaker describes as the “final tragedy”. The book continues with the far-reaching implications of Bell’s discovery, including brief descriptions of many of today’s active researchers. Bell’s inequalities are now experimentally testable and his concept of non-locality is gaining momentum. Violating a so-called Bell inequality shows that an experiment is truly quantum in nature and there are no “local hidden variables” at play.
Today, Bell’s non-locality is also being exploited for futuristic applications in a new field that would never exist without Bell’s seminal discovery – namely, “device-independent quantum information processing”. The idea is that a quantum protocol would be completely independent of the internal workings of the devices being used, which would therefore eliminate the risk of a quantum cryptographic system being hacked. That is because the protocol looks merely at the statistics of any measurement made, without the need to understand in any detail how the data were collected; it suffices to know that they were produced at separate locations that couldn’t communicate. The National Institute for Standards and Technology in the US has already tapped into this idea and has created a free, public random number generator that you can access online. Large sets of truly random numbers are difficult to produce, but they are used in a variety of applications today, including in unpredictable sampling and secure authentication methods.
I truly enjoyed reading this very informative book. Moreover, it is nicely illustrated with many pictures of John, his wife Mary and others such as Michael Horne, Daniel Greenberger and Artur Ekert. This is not a book to learn about physics, but to get to know a bit about the man who made one of the most profound, if not the most profound, discoveries of the 20th century.
