History of Physics: Long Before LIGO

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BIOMECHANICS Joseph Weber built the first detectors, now treatment results in a material that will deform Take the plunge known as Weber bars, in the 1960s. These were when heated. The composite’s shape-memory Proc. Natl Acad. Sci. USA gravitational wave antennas made of massive behaviour depends on the concentration and http://doi.org/brwm (2016) aluminium cylinders acting as resonators. distribution of the fillers — enabling control of Weber’s observations, claimed as evidence its thermoelastic response. Seabirds are notorious for plunge diving for detection, could not be confirmed by The authors further showed that at seemingly break-neck speeds. But does experiments performed during the following macroscopic bilayers of these elastomeric hunting like this really pose a threat to their two decades in laboratories worldwide. But composites with different deformational slender necks? And might humans be able despite the disappointment, work continued directions can result in any of five basic to withstand similarly treacherous descents? with cryogenic detectors. Going colder and thermomechanical deformation modes. The Brian Chang and colleagues sought to find out heavier did improve sensitivity and several future looks bendy. BV by developing a mechanical understanding of experiments ran in the 1990s and early 2000s. neck stability during plunge diving. However, evidence remained scarce. AGGREGATION PHENOMENA The team focused on two seabird species Resonant bar detectors never quite Collective diversity that both endure strong axial forces when they managed to detect gravitational waves, but New J. Phys. 18, 103005 (2016) dive, despite having distinctly different styles. the lessons learned during their construction A slender body subject to axial forces should and operation helped the development of buckle, but seabirds only injure themselves large-scale laser interferometers like LIGO and diving when they collide with other birds. To VIRGO. It seems now that 40 years of work make sense of this, Chang et al. conducted was worth all these efforts. IG experiments on a model seabird made of a long, thin elastic beam attached to a rigid ELASTOMERS cone, and performed a linear stability analysis Morph on demand to predict the buckling transition. Nat. Commun. 7, 13140 (2016) Their analysis suggested that the birds’ neck muscles and morphology render them Wouldn’t it be nice to have an elastomer ideally suited to plunge diving at high speeds. that could change into any given shape? A similar consideration of humans diving Most definitely — if only for applications feet first found that the compressive forces for like microfluidic valves, Braille readers or comparable speeds would be far too high for artificial muscles. / CORBIS © ANDRZEJ WOJCICKI / REINHARD DIRSCHERL DOCUMENTARY our bodies to endure. AK Andraž Rešetič and colleagues have made The concerted movements of birds and fish an important step towards the manufacture are captivating examples of collective motion. HISTORY OF PHYSICS of such functional rubbers. They took low- A number of models have been developed Long before LIGO viscosity polydimethylsiloxane, a conventional to describe such behaviour. However, which Euro. Phys. J. H http://doi.org/brwn (2016) elastomer, and added elastomeric liquid- pattern a given ensemble of self-propelled crystal microparticles exhibiting shape particles — be they starlings, bacteria or When the first successful detection of memory — the ability to return from a robots — will eventually assume, and the gravitational waves occurred early this year, deformed state into the undeformed state stability of that pattern, remains difficult to LIGO became a household name. But the through heating. Under the application predict. Now, Zhao Cheng and colleagues history of gravitational wave detection goes of an external magnetic field, the particles report that various ‘pattern phase transitions’ back to the early 1960s. And now, Guido collectively align, and the shape-memory can be captured in a model that factors in the Pizzella has given us a personal account of effect is carried over to the macroscopic vision range of the particles and their tendency these experiments — some already forgotten. structure. Subsequent hardening via thermal to avoid obstacles. The authors found that for short vision TOPOLOGICAL MATTER ranges particles move, gas-like, in a widely uncorrelated fashion. Longer-sighted particles A fine line Phys. Rev. Lett. 117, 136401 (2016) get locked into ‘crystalline’ patterns with identical inter-particle distances and, for Dirac semimetals are materials whose electronic band structures are three-dimensional even longer-ranging interactions, into liquid analogues of graphene, featuring electron bands with linear dispersion close to a Dirac point, patterns with varying, but short distances forming Dirac cones. Bulk cadmium arsenide was thought to fall into this class of material, between constituents. The liquid phase containing a single pair of Dirac cones that are protected by crystal symmetry, but experiments co-exists with various milling phases — by Ana Akrap and colleagues suggest that the situation may be a little more complex. stationary circulating patterns (as observed Previous experiments classified the charge carriers in bulk cadmium arsenide as massless in nature; pictured) that change depending symmetry-protected Dirac fermions, which is what you’d expect for a Dirac semimetal. And on the particles’ obstacle-avoidance tendency, although magneto-optical experiments by Akrap et al. support the idea that the carriers are which defines the short-range interactions. massless, for certain energies, they suggest that they are actually massless Kane fermions. Cheng et al. expect that these insights should Kane fermions are a type of carrier that can appear in some narrow-gap semiconductors enable, for example, changing the collective that have conical electronic band dispersions. And although they exhibit several behaviours motion of vehicles or robots by simply similar to Dirac fermions, they have quite a different origin, and are not protected by crystal tweaking a few dynamical parameters. AHT symmetry. Could this material contain both massless Dirac and Kane fermions over different energy scales? The authors think so. LF Written by Luke Fleet, Iulia Georgescu, Abigail Klopper, Andreas H. Trabesinger and Bart Verberck.