Weird and wonderful stories from the world of physics
If you want a challenge for the new year, the Institute for Cosmic Ray Research (ICRR) in Tokyo, Japan, has just the thing. It’s created a 300-piece jigsaw of the Super-Kamiokande neutrino detector in Kamioka, Japan. The detector is a giant stainless-steel tank filled with 50,000 tonnes of ultra-pure water and lined with 13,000 photomultiplier tubes that detect the Cherenkov radiation released when a neutrino collides with a water molecule. In other words, it’s a jigsaw puzzle featuring water and lots and lots of identical tubes. Costing ¥1500 (£10) and with a finished size of 38 × 26 cm, a limited number of the jigsaws went on sale in late October. But its fiendish nature doesn’t seem to have put anyone off: the puzzle sold out within days. Jigsaw enthusiasts, however, will be pleased to know that, as Physics World went to press, the ICRR was planning to release more.
Once and once again
This magazine’s “once a physicist” column has for many years profiled people who have gone on to do something else, such as Elon Musk, who founded PayPal and Space X. But we recently spotted someone who’s a “once-and-now-again-a-physicist”. Sandra Miarecki did a bachelor’s degree in astronomy at the University of Illinois Urbana–Champaign, graduating in 1986 before becoming a US Air Force pilot for 20 years. In 2010 she then came back full circle and began a PhD in neutrino physics at Lawrence Berkeley National Laboratory, which she completed in 2016. “I debated whether my 42-year-old brain would be spongy enough to tackle a PhD programme,” Miarecki told Symmetry. She’s now an assistant professor at the Air Force Academy in Colorado Springs, teaching classical mechanics and electromagnetism. Maybe that actually makes her a “once-a-once-and-now-again-a-physicist”.
Anchovies in space
Italian astronaut Paolo Nespoli seems to have been having a fun time on the International Space Station (ISS). In November he became the first person to contribute to Wikipedia from space when he recorded and uploaded two audio messages, one in English and one in Italian. In the 44-second clip, Nespoli outlines the three times he travelled into space, the first being in 2007 when he was on the Discovery shuttle that helped to build the ISS. Shortly after his Wikipedia exploits, Nespoli and his five colleagues on the ISS then spent time painstakingly crafting their own pizzas using ingredients in a kit delivered on a cargo resupply mission that contained flatbread, tomato sauce, cheese, pepperoni, olives, olive oil, anchovy paste and pesto. After playing around with the pizzas like they were frisbees, the astronauts cooked them in foil. Nespoli, who has been in orbit since July, declared them “unexpectedly delicious”. It must be the furthest fast-food delivery in history.
The end of the holiday season can mean a lot of recycling. And as we all know, if you put a dent into the side of an aluminium can, it’s easier to crush from top to bottom. But predicting the exact force needed to crumple a dented can is notoriously difficult, requiring knowledge of the exact dimensions and position of the flaw. Thankfully, physicists at the École Polytechnique Federale de Lausanne and Harvard University have now worked out a relationship between the size of the dent and the force needed to buckle the can (Phy. Rev. Lett. 119 224101). They put an empty Coke can in between two metal plates to produce a vertical force. The researchers then poked the side of the can using a metal ball attached to the end of a rod, continuously pushing the ball further into the can until it buckled with a loud snap. By varying the size of the ball and the force of the plates, they determined that the cans generally buckle with a force greater than 700 N. The researchers say that the work could help with the design of rockets, aeroplanes and, of course, beer cans.
Quantum information has been transferred between a cold atomic gas and a solid crystal using photons. Carried out by Nicolas Maring and colleagues at the Institute of Photonic Sciences (ICFO) at the Barcelona Institute of Science and Technology in Spain, the work could lead to significant advances in quantum computing and even to the creation of a “quantum internet”.
A big challenge in building a quantum computer is transferring quantum bits (qubits) of information between the “quantum nodes” of a system. These nodes can consist of different types of matter, including cold atomic gases and solid crystals doped with impurities. If two nodes are the same, it is relatively straightforward to transfer qubits – in the form of single photons, for example. In this process, one node emits a qubit-encoded photon that is then absorbed by another node.
In practical quantum communication systems, however, it is often better to use different types of quantum nodes to perform different functions. This is because some nodes are better than others at doing certain tasks. Cold atomic gases can easily produce qubit-encoded photons, for example, while doped solids can store quantum information for long periods.
The snag is that different types of nodes usually emit and process photons at different wavelengths and bandwidths, making qubit transfer between nodes tricky. “It’s like having nodes speaking in two different languages,” says Maring. “For them to communicate, it is necessary to convert the single photon’s properties so it can efficiently transfer all the information between these different nodes.”
In the ICFO study, a “hybrid” quantum network link between two different quantum nodes in separate labs was established for the first time. The first node, a gas of laser-cooled rubidium atoms, produced a qubit-encoded 780 nm photon. The photon was then converted to 1552 nm and sent down an optical telecoms fibre into a next-door lab, where it was converted again to a wavelength of 606 nm. Its quantum information was then processed by a second quantum node – a crystal doped with praseodymium ions. It could store qubits for 2.5 μs while retaining most of the original quantum information.
The research could lead to the creation of quantum networks that take advantage of the different processing and storage capabilities of different quantum nodes. The ICFO scientists hope that larger scale, more complex hybrid networks will be built, made from many different nodes and links between them (Nature551 485).
The first convincing evidence that lightning strikes can synthesize radioactive isotopes in the atmosphere has been unveiled by physicists in Japan. It was obtained by Teruaki Enoto of Kyoto University and colleagues, who have shown that the isotopes are produced in reactions triggered by gamma rays from the lightning. The research, which was initially financed by crowdfunding, follows several previous, inconclusive observations.
Lightning strikes produce gamma rays when relativistic electrons, accelerated by strong electric fields, collide with air molecules. Dubbed gamma-ray flashes, these events are usually directed up towards outer space, but on rare occasions they shoot down at Earth. Enoto’s team has been operating detectors at Japan’s Kashiwazaki-Kariwa nuclear-power station since 2006 to detect gamma-ray emissions from heavy, low thunderclouds that are common to the region.
On 6 February last year they got lucky when their four detectors recorded a powerful gamma-ray flash, lasting under 1 ms, from two simultaneous lightning strikes less than 2 km away. These gamma rays reacted with stable nuclei, such as nitrogen-14, knocking out neutrons that were captured by other nuclei. These nuclei then decayed to produce further gamma rays – forming an “afterglow” that faded over the next 200 ms (Nature551 481).
The researchers then detected another signal in detectors downwind of the lightning strikes. Emerging slowly, this signal peaked about a minute after the initial flash at 0.51 MeV, which is almost exactly the energy of gamma rays produced when electrons and positrons annihilate. The team believe that the positrons are made through the decay of unstable radioactive isotopes, such as nitrogen-13, that are created when gamma rays from the lightning reacted with stable isotopes like nitrogen-14. Nitrogen-13, for example, decays to stable carbon-13 by emitting a positron with a half-life of 10 minutes.
This conclusion is supported by other research that shows unambiguous direct detection of neutrons from a downward gamma-ray flash created when lightning hit a wind turbine in Japan (Geophys. Res. Lett.44 10063).
Disturbances in the Earth’s gravitational field caused by the 2011 Tohoku earthquake have been spotted in data recorded at the time by a network of seismometers spread throughout East Asia. The signal was identified by Martin Vallée and collaborators at Sorbonne Paris Cité, the French atomic-energy commission (CEA) and the California Institute of Technology in the US. Their analysis provides a faster and more accurate way than conventional methods of estimating the magnitude of large earthquakes.
Usually, the first physical indication of a distant earthquake is received in the form of elastic P-waves, which travel from the rupture site to a seismometer along arc-shaped paths through the crust and upper mantle. These pressure waves typically propagate at 6–10 km/s, meaning that for seismic stations more than 1000 km from the epicentre, several precious minutes can elapse between the earthquake and the arrival of the first direct seismic signal.
However, large earthquakes can rearrange the Earth’s mass in such a way that they can be detected more immediately via perturbations to the gravitational field. What happens is that as P-waves spread out from the ruptured fault, the solid medium is alternately squeezed and stretched, causing transient changes in rock density. Far beyond the primary seismic wavefront, these gravitational effects, which move at the speed of light, can trigger secondary seismic waves that can be picked up by seismometers before the direct waves arrive.
The ground accelerations measured by the gravitationally induced seismic waves in Vallée and colleagues’ data were hard to spot, being barely 1–2 nm/s2 – about 100,000 times smaller than from the P-waves. Another problem was that immediately after the fault slipped, the direct and induced effects of the gravitational perturbation cancelled out, meaning an identifiable signal became apparent only about 60 seconds after the event. The gravitational effect was most easily observable in traces from stations 1000–1500 km from the epicentre, where the P-wave delay was long enough for the signal to emerge before being overwhelmed.
The researchers also simulated the effects of earthquakes of different sizes on the data, and found that the immediate gravitational signal recorded by stations about 1300 km from the epicentre set a lower limit of magnitude 9 for the Tohoku event. In 2011, however, when the earthquake struck, the difficulty of judging magnitude based on the instrumental peak amplitudes measured at nearby stations meant that geologists underestimated its size. Measuring earthquake sizes using the current approach would therefore have allowed Tohoku’s power to have been estimated in minutes, rather than hours (Science358 1164).
A new and rapid way to pack identical cubes in a dense configuration has been discovered by physicists in Spain and Mexico led by Diego Maza of the Universidad de Navarra. They poured 25,000 small plastic dice into a clear cylindrical barrel with a radius of 8.7 cm (left) before repeatedly twisting it back and forth. Slow twists tended to align the dice at the edges, while leaving those in the middle disordered. When the twist acceleration increased to 0.52g, however, the dice ended up in horizontal layers after about 10,000 twists, and were arranged in nearly perfect concentric rings within each layer (right). The twisting itself does not agitate the dice; they’re instead jolted by the change in direction, which induces shear. The team also ran experiments with more and fewer dice: the heavier load of more dice competed with the shear process to stop the dice from densely packing together, while fewer dice required a far smaller acceleration to become aligned. The work could be useful in industry as pharmaceutical powders, for example, are packed by repeatedly tapping them with ever-decreasing intensity, but this is slow and inefficient (Phys. Rev. Lett.119 228002).
The Physics World 2017 Breakthrough of the Year goes to an international team that ushered in a new era of multimessenger astronomy, as Hamish Johnston reports
The Physics World 2017 Breakthrough of the Year has been awarded to the international team of astronomers and astrophysicists that made the first ever multimessenger observation involving gravitational waves. On 17 August 2017 the LIGO–Virgo gravitational-wave detectors in the US and Italy, plus the Fermi Gamma-ray Space Telescope and the INTEGRAL gamma-ray space telescope detected nearly simultaneous signals. They came from the merger of two neutron stars – an event now called GW170817 (Phys. Rev. Lett.119 161101).
This was the first time that LIGO–Virgo scientists had seen a neutron-star merger, but five hours later they had already worked out the location of the source in the sky (see November 2017). Over the next hours and days, more than 70 telescopes were pointed at GW170817 and a wealth of observations were made in the gamma-ray, X-ray, visible, infrared and radio portions of the electromagnetic spectrum (ApJL848 L12). Astrophysicists also searched for neutrinos, but none were seen.
These co-ordinated observations have already provided a vast amount of information about what happens when neutron stars collide in what is called a “kilonova”. The observations have yielded important clues about how heavy elements, such as gold, are produced in the universe. The ability to measure both gravitational waves and visible light from neutron-star mergers has also given a new and independent way of measuring the expansion rate of the universe. In addition, the observation settles a long-standing debate about the origin of short, high-energy gamma-ray bursts.
The observation of GW170817 is a shining example of how our knowledge of the universe can move forwards when people from all over the world join together with a common scientific cause
While some awards, notably the Nobel prizes, are given to individuals and not groups, Physics World recognizes that science is a collaborative effort. Furthermore, the multimessenger observation of GW170817 epitomizes the collaborative nature of science and is a shining example of how our knowledge of the universe can move forwards when people from all over the world join together with a common scientific cause.
On a recent trip to LIGO Livingston, we spoke to LIGO scientist Amber Stuver.
Highly commended research
The top 10 breakthroughs were chosen by Physics World editors from a shortlist based on the fundamental importance of the research; representing a significant advance in knowledge; having a strong connection between theory and experiment; and being of general interest to all physicists. Physics World also picked nine other pieces of research that were highly commended, which follow in no particular order.
In quantum physics, Hatim Salih of the University of Bristol, UK, and colleagues and Jian-Wei Pan of the University of Science and Technology of China and colleagues carried out theoretical and experimental work into the realization of transmitting information using quantum physics without exchanging any particles. Salih and colleagues first proposed a new quantum-communication scheme that does not require the transmission of any physical particles four years ago (Phys. Rev. Lett. 110 170502). While some physicists were sceptical, a team, led by Pan, created such a system in the lab and used it to transfer a simple image while sending (almost) no photons in the process. Dubbed “counterfactual imaging”, the technique could prove handy in imaging delicate pieces of ancient art that cannot be exposed to direct light (Proc. Natl Acad. Sci.114 4920).
Another breakthrough in quantum physics came from Sascha Agne and Thomas Jennewein of the University of Waterloo, Canada, and colleagues, and Stefanie Barz, Steve Kolthammer and Ian Walmsley of the University of Oxford, UK, and colleagues for independently measuring quantum interference involving three photons. Seeing the effect is difficult because it requires the ability to deliver three indistinguishable photons to the same place at the same time and also to ensure that single-photon and two-photon interference effects are eliminated from the measurements. Three-photon interference could also be used in quantum cryptography and quantum simulators (Phys. Rev. Lett.118 153603 and Phys. Rev. Lett.118 153602).
Meanwhile, Boubacar Kanté and colleagues at the University of California, San Diego, US, created the first “topological laser”. The device involves light snaking around a cavity of any shape without scattering – much like the motion of electrons on the surface of a topological insulator (Science358 636). The laser works at telecom wavelengths and could lead to better photonic circuits or even protect quantum information from scattering.
In applied physics, Ronggui Yang and Xiaobo Yin of the University of Colorado, Boulder, US, and colleagues created a new metamaterial film that provides cooling without the need for a power source (Science355 1062). Made out of glass microspheres, polymer and silver, the material uses passive radiative cooling to dissipate heat from the object that it covers. It emits the energy as infrared radiation, so it can travel through the atmosphere and ultimately into space. The material also reflects sunlight, which means that it works both day and night. But perhaps most importantly, it can be produced cheaply at an industrial scale.
Work on cosmic rays resulted in two entries in the top 10. The Pierre Auger Observatory collaboration showed that ultrahigh-energy cosmic rays come from outside the Milky Way. For decades, astrophysicists have believed that the sources of cosmic rays with energies greater than about 1018 eV could be worked out from the arrival directions of these particles. This is unlike lower-energy cosmic rays, which appear to come from all directions after being deflected by the Milky Way’s magnetic fields. Now, Pierre Auger’s 1600 Cherenkov particle detectors in Argentina have revealed that the arrival rate of ultrahigh-energy cosmic rays is greater in one half of the sky (Science357 1266). Moreover, the excess lies away from the centre of the Milky Way – suggesting that the cosmic rays have extragalactic origins.
Meanwhile, the ScanPyramids collaboration used cosmic-ray muons to find evidence for a hitherto unknown large void in Khufu’s Pyramid at Giza, Egypt. By placing different types of muon detectors in and around the pyramid, the team measured how showers of muons were attenuated as they passed through the huge structure. Computer algorithms analysed the data and revealed an unexpected and very large void deep within the pyramid (Nature 10.1038/nature24647).
In imaging, Francisco Balzarotti, Yvan Eilers, Klaus Gwosch, Stefan Hell and colleagues at the Max Planck Institute for Biophysical Chemistry, Uppsala University, Sweden, and the University of Buenos Aires, Argentina, developed a new type of super-resolution microscope that can track biological molecules in living cells in real time (Science355 606). The new technique is called maximally informative luminescence excitation probing (MINFLUX) and it combines the merits of two Nobel-prize-winning techniques – one of which was developed by Hell. MINFLUX attains nanometre-scale resolution more quickly and with fewer emitted photons than previously possible.
In atomic physics, Christopher Monroe at the University of Maryland, US, and colleagues and Mikhail Lukin of Harvard University, US, and colleagues are credited for their independent creation of “time crystals”. Like conventional crystals, which spontaneously break translational symmetry, time crystals spontaneously break discrete time symmetry. Time crystals were first predicted five years ago and now two spin-based systems with properties resembling time crystals have been created. Lukin used spins in diamond defects (Nature543 221) while Monroe’s spins were trapped ions (Nature543 217).
Finally, Teruaki Enoto of Kyoto University, Japan, and colleagues provided the first detailed, convincing evidence that lightning strikes can lead to the synthesis of radioactive isotopes in the atmosphere (Nature551 481). Physicists already knew that lightning strikes can produce gamma rays and neutrons, and had suspected that interactions between this radiation and nitrogen nuclei in the air could create radioactive nuclei. Enoto and colleagues confirmed this by measuring a gamma-ray signal indicative of nuclear decay that peaked about 1 minute after a lightning strike. This, they say, is evidence for the production of radioactive nuclei such as nitrogen-13.
For more on the winning work, see “A new cosmic messenger” by Imre Bartos
The $9bn James Webb Space Telescope (JWST) has completed its final round of cryogenic testing at NASA’s Johnson Space Center in Houston, Texas. The tests began last July when the spacecraft’s optical telescope and integrated science instrument module were sealed in a chamber, which is a huge cylindrical vacuum chamber 27 m tall and 17 m in diameter. There, it was cooled to temperatures as low as 11 K using cold helium gas and then put through a three-month testing programme to ensure that the JWST functions in an environment similar to space. The instrument module and optics will now be shipped to Northrop Grumman Aerospace Systems in Los Angeles, where it will be integrated into the JWST spacecraft. Once this is complete, the spacecraft will be subject to a final round of “observatory-level testing” before being launched in the spring of 2019. The telescope will then journey to Lagrange point 2, a location 1.5 million kilometres from Earth.