It has been known for a century that electromagnetic fields can transport not
only energy and linear momentum but also angular momentum. However, it was not
until twenty years ago, with the discovery in laser optics of experimental
techniques for the generation, detection and manipulation of photons in
well-defined, pure orbital angular momentum (OAM) states, that twisted light
and its pertinent optical vorticity and phase singularities began to come into
widespread use in science and technology. We have now shown experimentally how
OAM and vorticity can be readily imparted onto radio beams. Our results extend
those of earlier experiments on angular momentum and vorticity in radio in that
we used a single antenna and reflector to directly generate twisted radio beams
and verified that their topological properties agree with theoretical
predictions. This opens the possibility to work with photon OAM at frequencies
low enough to allow the use of antennas and digital signal processing, thus
enabling software controlled experimentation also with first-order quantities,
and not only second (and higher) order quantities as in optics-type
experiments. Since the OAM state space is infinite, our findings provide new
tools for achieving high efficiency in radio communications and radar
technology.
pandomilla on 12 Apr 12I hate scorpions, but this could be a nice subject for a future Ariadna study! This north African desert scorpion, doesn't dig burrows to protect itself from the sand-laden wind (as the other scorpions do). When the sand whips by at speeds that would strip paint away from steel, the scorpion is able to scurry off without apparent damage.
LeopoldS on 13 Apr 12Nice research, though they have done almost all the work that we could do in an Ariadna, didnt they? "To check, they took further photographs. In particular, they used a laser scanning system to make a three-dimensional map of the armour and then plugged the result into a computer program that blasted the virtual armour with virtual sand grains at various angles of attack. This process revealed that the granules were disturbing the air flow near the skeleton's surface in ways that appeared to be reducing the erosion rate. Their model suggested that if scorpion exoskeletons were smooth, they would experience almost twice the erosion rate that they actually do. Having tried things out in a computer, the team then tried them for real. They placed samples of steel in a wind tunnel and fired grains of sand at them using compressed air. One piece of steel was smooth, but the others had grooves of different heights, widths and separations, inspired by scorpion exoskeleton, etched onto their surfaces. Each sample was exposed to the lab-generated sandstorm for five minutes and then weighed to find out how badly it had been eroded. The upshot was that the pattern most resembling scorpion armour-with grooves that were 2mm apart, 5mm wide and 4mm high-proved best able to withstand the assault. Though not as good as the computer model suggested real scorpion geometry is, such grooving nevertheless cut erosion by a fifth, compared with a smooth steel surface. The lesson for aircraft makers, Dr Han suggests, is that a little surface irregularity might help to prolong the active lives of planes and helicopters, as well as those of scorpions."
Ma Ru on 13 Apr 12What bugs me (pardon the pun) is that the dimensions of the pattern they used were scaled up by many orders of magnitude, while "grains of sand" with which the surface was bombarded apparently were not... Not being a specialist in the field, I would nevertheless expect that the size of the surface pattern *in relation to* to size of particles used for bombarding would be crucial.
