This Week in Science (April 9-15 2019)

1.) Astronomers Take the First Direct Image of a Black Hole

This week, astronomers released the first ever picture of a black hole. The picture of a supermassive black hole at the center of the M87 galaxy shows a bright disk of orbiting gas circling around a black hole and achieved an angular resolution of 20 micro-arcseconds. The center of the picture is the black hole’s shadow. The image was taken by the Event Horizon Telescope (EVT), which is a network of eight ground-based radio telescopes across the globe. The network includes the Atacama Large Millimeter Array (ALMA) located in Chile, the Atacama Pathfinder Experiment (APEX) also in Chile, the Institut de Radioastronomie Millimétrique (IRAM) in Spain, the James Clerk Maxwell Telescope (JCMT) and Submillimeter Array (SMA) in Hawaii, the Large Millimeter Telescope Alfonso Serrano (LMT) in Mexico, the South Pole Telescope, and the Arizona Radio Observatory in the United States of America.

The picture represents the culmination of decades of work to construct the EHT, which required collaboration and funding from institutions and agencies across the globe. Sheperd S. Doeleman, the project director, stated

“We have achieved something presumed to be impossible just a generation ago. Breakthroughs in technology, connections between the world’s best radio observatories, and innovative algorithms all came together to open an entirely new window on black holes and the event horizon.”

2.) New State of Matter is Identified that Shows Properties of Both a Liquid and a Solid

Potassium in paraffin oil (Jurii/ CC3.0)

Computer modeling using a limited form of artificial intelligence has confirmed the existence of a new phase-state of matter. This new phase, chain-melting, exhibits the properties of both a solid and a liquid. For now, this new phase has only been confirmed for potassium atoms that are compressed under immense pressure.  The experiment, published in the Proceedings of the National Academy of Science, showed that potassium atoms under extreme pressure could arrange into an elaborate structure of sub-lattices. The study authors used a neural network to model the interactions of several thousand potassium atoms under 20,000-40,000 times the atmospheric pressure of Earth. Under these conditions, the atoms would enter a phase called a chain-melted state. This new phase joins the other exotic states of matter that include plasma, bose-einstein condensates, superconductors, superfluids, degenerate matter, and quark-gluon plasma.

A new type of matter interaction has been identified in a new study published in Nature. The study’s authors showed a new attractive interaction occurring between a pair of particles that were surrounded by a quantum gas. This interaction is a mediated interaction where the particles do not touch but do interact over when separated by distance. This was the first observation of a mediated interaction occurring in an ultracold mixture. When the authors

DeSalvo and colleagues used a mixture of bosonic caesium-133 atoms and fermionic lithium-6 atoms — bosons have integer spin angular momentum, whereas fermions have half-integer spin. The atoms were confined near the minimum of a harmonic trap, which is a type of potential-energy surface (often simply called a potential) that has a parabolic form. The authors cooled the mixture to extremely low temperatures, so that the caesium atoms formed an exotic state of matter known as a Bose–Einstein condensate (BEC), whereas the lithium atoms instead formed a quantum gas called a Fermi gas.

he direct interaction between the caesium atoms was weakly repulsive, whereas the direct interaction between the caesium and lithium atoms could be tuned to be either attractive or repulsive. There was no direct interaction between the lithium atoms because of a fundamental principle of quantum mechanics that states that two fermions cannot occupy the same space. This principle also meant that the lithium gas had a much larger volume than the BEC of caesium atoms, which was located at the centre of the gas. DeSalvo et al. demonstrated that the presence of the lithium gas had two effects on the caesium BEC.

First, when the BEC was set to oscillate back and forth in the harmonic trap, the oscillation frequency changed when the lithium gas was present. The reason for this change is that the energy of the caesium atoms was shifted by the direct interaction between the caesium and lithium atoms. This energy shift was spatially dependent because the density of the lithium gas decreased as a function of distance from the centre of the trap. As a result, the BEC was subjected to an additional trapping potential from the lithium gas that altered the oscillation frequency.

The second, and arguably more intriguing, effect was that the lithium gas made the BEC decrease in size. By carefully analysing their experimental data, DeSalvo and colleagues concluded that this effect was caused by an attractive interaction between the caesium atoms that was mediated by the lithium gas. The results were in fair agreement with theoretical predictions based on a type of interaction called a Ruderman–Kittel–Kasuya–Yosida (RKKY) interaction, which has been shown46 to exist as an interaction mediated by a Fermi gas precisely like the lithium gas.

RKKY interactions give rise to a great variety of magnetic phenomena in rare-earth elements — the collective name for 17 chemically similar metallic elements — in which electrons play the part of the Fermi gas. In addition, electronic devices, such as hard drives, that exploit a phenomenon known as the giant-magnetoresistance effect contain magnetic layers that are thought to be coupled by RKKY-like interactions7.

Although the observed mediated interaction between the caesium atoms was quite weak, DeSalvo et al. used a clever trick to show that it could still have spectacular effects. The authors tuned the direct interaction between the caesium atoms to be only very weakly repulsive so that the attractive mediated interaction was comparatively stronger. The combination of the direct and mediated interactions then gave rise to a net attraction between the caesium atoms. Because, unlike for fermions, nothing prevents bosons from occupying the same space, the BEC collapsed. The authors observed this collapse through the formation of small, soliton-like blobs of caesium atoms — solitons are spatially localized states that are characteristic of BECs.

Given the unrivalled versatility of atomic gases, DeSalvo and colleagues’ results open up the possibility of exploring mediated interactions in detail and probing interactions that have never been seen before. So far, only a weak mediated interaction has been observed, and it would be useful to study stronger interactions. Such interactions should greatly affect the energy spectrum of excitations of the BEC8 and give rise to a range of exotic phases of matter in mixtures of bosons and fermions9,10.

Future work should also explore the reciprocal case of an interaction between fermions that is mediated by a BEC. This mediated interaction is, in general, much stronger than the fermion-mediated interaction because of the large compressibility of a BEC compared with a Fermi gas, and it could also lead to previously unobserved phases of matter1113. It will be exciting to see what discoveries follow the breakthrough results of DeSalvo and colleagues.

4.) Z-pinch Breakthrough may Allow for Sustained Nuclear Fusion

st experimental results demonstrating sustained, quasi-steady-state neutron production from the fusion Z-pinch experiment, operated with a mixture of 20% deuterium/80% hydrogen by pressure. While the Z-pinch is not a new plasma confinement concept, it was largely abandoned as a path for fusion energy because the plasma was not stable, which limited how long it could be confined. To get around this issue, we exploited the fact that flows can stabilize plasma, and our flowing plasma was maintained five thousand times longer than a static plasma. We also observed the sustained release of telltale energetic neutrons signaling nuclear fusion. Because this approach provides a path to nuclear fusion without coils, it could be used in the future for long-duration fusion burns in a compact and low-cost device.

“Magnetic field coils drive fusion devices to larger size and larger costs,” he says. “The coils are also particularly sensitive to neutron damage, which requires more shielding, further driving size and costs.”

A more efficient way to achieve these streams of plasma may be what is known as a Z-pinch confinement system. Rather than intricate webs of expensive magnetic coils, these systems pin the plasma in place with an electromagnetic field generated within the plasma itself. Z-pinch systems have been referred to as the dark horse of the nuclear fusion race, as the upside is a far simpler plasma configuration. The downside, however, is that instabilities cause distortions in the plasma that quickly cause it to hit the walls of the container vessel and collapse.

“Compressing and confining a plasma with magnetic fields in a Z-pinch configuration is prone to instabilities since the plasma can escape between the parallel magnetic field lines,” Shumlak tells us. “The magnetic field forms circular loops around the plasma column which confines the plasma radially, but the plasma can form bulges, like an aneurysm, which locally weakens the magnetic field and allows the bulge to grow.”

These problems have plagued the Z-pinch approach since its inception in the 1950s and effectively drove the development of tokamaks and stellarators, but the UW researchers say it still has something to give.

The reason for this, is that they believe they have figured out a way to stop the distortions that occur in the plasma and cause it to collapse. Through making slight adjustments to the behavior of the plasma by inducing what is known in fluid dynamics as sheared axial flow, the researchers were able to break new ground in their 50-cm-long (20-in) Z-pinch plasma column.

“The primary innovation is using plasma flows, specifically sheared axial flows,” Shumlak tells us. “The sheared flow stabilizes the plasma by constantly smoothing the plasma surface and preventing the bulges from developing.”

While the potential of sheared axial flow in Z-pinch plasma streams has been explored or years, the researchers write that this is the first time that they have produced “evidence of fusion neutron generation from a sheared-flow stabilized Z-pinch.” More specifically, their flowing plasma was held in place 5,000 times longer than a static plasma, and they were able to observe energetic neutrons that are the telltale signs of nuclear fusion.

While enthused by the breakthrough, given the unstable history of Z-pinch confinement systems and uncertainty of nuclear fusion research as a whole, they are taking a cautiously optimistic outlook.

“As a scientist, I would state that we do not know for certain if this advance will lead to a new dawn,” says Shumlak. “However, the results are particularly encouraging. Sheared flow stabilization of the Z-pinch approach has been thoroughly investigated, and the predictions of scaling to current performance have been demonstrated. The evidence is described in our Physical Review Letters article. If our current scientific understanding continues to hold, then we should be able to reach even higher performance.”

5.) NASA Announces Funding for 18 Projects in their Innovative Advanced Concepts Program

This week, NASA announced that they had allocated funding for 18 new projects under their Innovative Advanced Concepts Program. The projects include space to ground power beaming, a self-healing spacesuit, electro-static microprobes, space debris collection, polar ice mining, light sails, beamed propulsion, and neutrino detection. The NASA Innovative Advanced Concepts (NIAC) Program  is designed to nurture visionary ideas that could transform future NASA missions with the creation of breakthroughs that are radically better or entirely new aerospace concepts  Jim Reuter, the acting associate administrator of NASA’s Space Technology Mission Directorate stated that “Our NIAC program nurtures visionary ideas that could transform future NASA missions by investing in revolutionary technologies. We look to America’s innovators to help us push the boundaries of space exploration with new technology.”



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