by Bart Verberck, Nature Physics 13, 623 (2017) doi:10.1038/nphys4207 (sci-hub)
Your Earthly carpet sweeper won’t do the job in the low-pressure, CO2
-dominant atmosphere on Mars. But Catalin Ticoş and colleagues
have now shown how to build a Mars-proof dirt broom, which can be used for removing sand and dust from equipment stationed on the Martian surface.
The authors’ experimental setup involved a coaxial plasma gun, capable of producing dense pulsed plasma jets, directed perpendicularly to the area to be cleaned. As a test surface, they used an array of photovoltaic cells, covered with a powder retrieved from volcanic ash, which mimicked Martian surface soil.
Ticoş et al.
measured the efficiency of their cleaning method in terms of how the voltage delivered by the cells increased during operation. An analysis of the plasma jets in a CO2
environment at the same pressure as that on Mars’s surface revealed an average plasma plume speed several orders larger than the planet’s typical wind speeds — implying that the plasma broom would indeed succeed in the Martian environment.
The inflaton has not been seen in the decay of B+ mesons at the LHCb. (Courtesy: LHCb Collaboration, CERN)
The hypothetical inflaton is almost certainly not the particle behind the universe’s rapid expansion soon after the Big Bang. This is according to an international collaboration of physicists working at the LHCb experiment on the Large Hadron Collider (LHC) at CERN, who have been looking for traces of the inflaton in the decay of B+ mesons. Back in 1981, Alan Guth proposed a new model of the early universe to explain why it looks the same in all directions today. He theorized that after the Big Bang the universe initially expanded slowly, allowing time for matter to interact and the temperature to level out. Then, there was a very short, extremely fast expansion of space–time, which happened so rapidly that the universe now appears uniform throughout. For such an expansion to take place, however, there must have been a force field behind it. “A new [force] field always means the existence of a particle that is the carrier of the effect,” explains team member Marcin Chrzaszcz from the Institute of Nuclear Physics of the Polish Acadamy of Sciences (IFJ PAN). For a while, it was thought that this particle was the Higgs boson – however, when it was observed in 2012, the boson was too heavy to be the correct candidate. So theoreticians proposed a new particle called the inflaton, which had the properties of the Higgs boson but a smaller mass. To prove its existence, physicists looked at the decay of B+ mesons, which sometimes decay into K+ mesons and Higgs bosons. According to quantum mechanics, the near-identical nature of the “brother” particles means that they transform and oscillate between each other, so the Higgs boson should then convert into the inflaton. Rather than directly measuring the inflaton or Higgs, the LHCb detects their decay into a muon and antimuon. “Depending on the parameter describing the frequency of the inflaton–Higgs oscillation, the course of B+ meson decay should be slightly different,” Chrzaszcz explains. “We found nothing. We can therefore say with great certainty that the light inflaton simply does not exist.” The work is presented in Physics Review D.
by Iulia Georgescu, Nature Physics 13, 529 (2017) doi:10.1038/nphys4169 (sci-hub)
Special relativity assumes that laws of physics are the same in all reference frames, a principle known as Lorentz invariance. This principle has been subject to numerous experimental tests, but no sign of Lorentz violation has yet been spotted: either a reassuring or disappointing revelation, depending on your stance. These results are now reinforced by a new test using a fibre network of optical clocks, which pushes the existing bound on Lorentz violation in experiments measuring time dilation.
Pacôme Delva and colleagues used strontium optical lattice clocks located at the LNE-SYRTE, Observatoire de Paris in France, the National Metrology Institute in Germany and the National Physical Laboratory in the UK and connected via state-of-the-art optical fibre links. Looking at the frequency difference between the clocks, they were able to test whether time dilation varies between the reference frames of the three geographically remote locations. This approach improves on previous tests — including other atomic clock comparison experiments — by two orders of magnitude. Moreover, it is only limited by technical noise sources, so further improvements are certainly possible.
by Rachel Won, Nature Photonics 11, 331 (2017) doi:10.1038/nphoton.2017.90 (sci-hub – paper)
Transparent ‘perfect’ mirrors — one-way mirrors that transmit or reflect light completely depending on the direction of view — are useful for security, privacy and camouflage purposes. However, current designs are not perfectly reflective. Now, Ali Jahromi and colleagues from the USA and Finland have demonstrated a new design based on a non-Hermitian configuration — an active optical cavity — that may overcome this limitation. At a critical value of prelasing gain that is termed Poynting’s threshold, all remnants of the cavity’s structural resonances disappear in the reflected signal. At this point, the reflection becomes spectrally flat and light incident on the cavity is 100%-reflected at all wavelengths continuously across the gain bandwidth independently of the reflectivities of the cavity mirrors. Thus, the device at Poynting’s threshold becomes indistinguishable from a perfect mirror. The researchers have confirmed these predictions in an integrated on-chip active semiconductor waveguide device and in an all-optical-fibre system. They note that Poynting’s threshold is, however, dependent on polarization and incidence angle, and that observing the reflection of coherent pulses may reveal the cavity structure via its decay time. Since the concept of Poynting’s threshold is a universal wave phenomenon, it can be exploited in many areas including microwaves, electronics, acoustics, phononics and electron beams.
Four Eyes of Tatooine by Stefan Lines (created whilst working on his PhD thesis in Dr Leinhardt’s Planet Formation Group at the University of Bristol). This computer generated image, based on data taken from actual super-computer simulations, shows the formation of a planet around a binary star — a so called ‘circumbinary planet’. Tiny unit vectors show the magnitude (colour) and direction (orientation) of the acceleration of millions of tiny rocky ‘planetesimals’ that eventually coalesce to form a planet.
Why do you think space inspires people so much?
I guess that it’s our human nature to explore and we see it, at least most of us can see it, at night and I think that it’s in our nature to ask why things look the way they do, or why a process happens. And since you can look up in the sky and see a bunch of lights, it’s natural to question what that is and want to be able to explain it and go there. So I think it’s just our natural instinct to want to explain what we don’t understand, especially if we can see it.
(Zoë Leinhardt – continue to read the interview)
Causality is a concept deeply rooted in our understanding of the world and lies at the basis of the very notion of time. It plays an essential role in our cognition — enabling us to make predictions, determine the causes of certain events, and choose the appropriate actions to achieve our goals. But even in quantum mechanics, for which countless measurements and preparations have been rethought, the assumption of pre-existing causal structure has never been challenged — until now.
Giulia Rubino and colleagues have designed an experiment to show that causal order can be genuinely indefinite. By creating wires between a pair of operating gates whose geometry is controlled by a quantum switch — the state of single photon — they realized a superposition of gate orders. From the output, they measured the so-called causal witness, which specifies whether a given process is causally ordered or not. The result brings a new set of questions to the fore — namely, where does causal order come from, and is it a necessary property of nature?
via Nature Physics (sci-hub)
Sketch showing the definition of phase angle
Muñoz, Antonio García, Panayotis Lavvas, and Robert A. West. “Titan brighter at twilight than in daylight.” Nature Astronomy 1, Article number: 0114 (2017) doi:10.1038/s41550-017-0114 (arXiv)
Investigating the overall brightness of planets (and moons) provides insights into their envelopes and energy budgets. Phase curves (a representation of the overall brightness versus the Sun–object–observer phase angle) for Titan have been published over a limited range of phase angles and spectral passbands. Such information has been key to the study of the stratification, microphysics and aggregate nature of Titan’s atmospheric haze and has complemented the spatially resolved observations showing that the haze scatters efficiently in the forward direction. Here, we present Cassini Imaging Science Subsystem whole-disk brightness measurements of Titan from ultraviolet to near-infrared wavelengths. The observations show that Titan’s twilight (loosely defined as the view at phase angles ≳150°) outshines its daylight at various wavelengths. From the match between measurements and models, we show that at even larger phase angles, the back-illuminated moon will appear much brighter than when fully illuminated. This behaviour is unique in our Solar System to Titan and is caused by its extended atmosphere and the efficient forward scattering of sunlight by its atmospheric haze. We infer a solar energy deposition rate (for a solar constant of 14.9 W m−2) of (2.84 ± 0.11) × 1014 W, consistent to within one to two standard deviations with Titan’s time-varying thermal emission from 2007 to 2013. We propose that a forward scattering signature may also occur at large phase angles in the brightness of exoplanets with extended hazy atmospheres and that this signature has a valuable diagnostic potential for atmospheric characterization.
Schematic illustration of columnar hexagonal arrangement of
discotic cores in 1-D semiconductor and charge migration.
Kumar, Manish, Ashwathanarayana Gowda, and Sandeep Kumar. “Discotic Liquid Crystals with Graphene: Supramolecular Self‐assembly to Applications.” Particle & Particle Systems Characterization (2017). doi:10.1002/ppsc.201700003 (sci-hub)
In past decades many breakthroughs have been witnessed in research on liquid crystals (LCs) and the application of LCs has spread. On another side graphene is considered as a rapidly rising star on the horizon of materials science, soft condensed matter physics and promising applications. Supramolecular chemistry of LCs and graphene together is described as “chemistry beyond the molecule”. A new class of 2D colloidal graphene oxide liquid crystalline material consisting discotic liquid crystallinity and their interactions with LCs present a platform for number of versatile properties and applications. This review focuses on discotic liquid crystalline (DLC) behavior of graphene oxide/reduced graphene oxide in various solvents, their characterization and application for energy storage, wet-spinning fibers, electro-optical devices, and displays etc. In the first part of this review, a brief introduction of discotic graphene oxide liquid crystals (GOLCs), their fundamental, synthesis process, supramolecular structures of graphene-DLC composites is highlighted. In the second part, some important physical studies and application of this largest polycyclic aromatic core of DLCs are discussed. Finally, an outlook on this emerging two dimensional material in liquid crystal field with relevant scientific application background is presented.
Working with creative mathematical tasks is important for pupils both to reflect on mathematics as well as for their subsequent test results. Being faced with creative tasks during exercise has evident effects on all pupils, both on weak and high performers. This according to studies at Umeå University in Sweden.
“The results of my dissertation show the importance for pupils to work with creative reasoning and not always get methods and rules presented in advance. This is something both publishers and teachers could take into account more often when designing mathematical tasks,” says Mathias Norqvist, doctoral student at the Department of Mathematics and Mathematical Statistics at Umeå University.
The studies show that pupils at upper secondary school who work with exercises designed to encourage creative mathematical reasoning more easily remember what they have learnt and, as a result, perform better.
“Contrary to common belief, it seems to be the low performing pupils who benefit most from practicing with creative tasks, in comparison to more imitative tasks where focus lies on how to use the given solutions,” says Mathias Norqvist.
There is a great risk that pupils who are presented one method, will use it without further reflection. Although, there are of course certain methods in mathematics that should be automated to relieve the pressure on the working memory, but it should not come at a cost to the understanding of the underlying mathematics. Since well-designed creative exercises can focus on central mathematical properties, they are important for all pupils since they force pupils to reflect on the mathematics and to base their reasoning on what they already know.
A total of about 300 upper secondary school pupils participated in the studies that formed the basis of the dissertation.