Planetary waves

Phys. Rev. Lett. 98, 198501 (2007)

Credit: RETO STÖCKLI/ROBERT SIMMON/MODIS

As well as seasonal changes, the Earth's atmosphere exhibits variability on shorter timescales. Tropical rainfall, which fluctuates in 30–60-day cycles, is an example of intraseasonal oscillations. The enhanced or suppressed rainfall affects mainly the Indian and Pacific Oceans, but it does have an impact on weather further away; in North America, for instance, it can lead to heavy rain and flooding in winter. As for any complicated global phenomenon, there are many open questions. Elena Kartashova and Victor L'vov offer a wave approach to address some of them.

Their model is based on interacting planetary waves — in particular, the interaction of four resonant clusters, each composed of three modes. As the model is independent of the topography of the Earth, their solution explains why intraseasonal oscillations affect both the Northern and Southern hemispheres. Moreover, they can also explain why such oscillations are more observable in winter, as well as the origin of the different periodicity reported. It remains to be seen, however, whether they can predict the weather.

So far and yet so near

Science Express doi:10.1126/science.1140300 (2007)

Secret messages can be transmitted between two parties with — in theory — absolute security once quantum entanglement is brought into play. Several entanglement-based quantum communication schemes have been proposed, but setting up a practical quantum network is a delicate task. Chin-Wen Chou and colleagues now report progress towards providing the physical resources needed to implement one of the most promising proposals for long-distance quantum communication, known as the DLCZ scheme.

Photons are the prime candidates for carrying quantum information through quantum networks, but unavoidably imperfect optical fibres cause an exponential decrease in communication efficiency as the distance between sender and receiver grows. The DLCZ scheme describes how quantum communication can be extended over larger distances than dictated by the signal attenuation in the optical fibres. Key ingredients of the scheme are quantum nodes, where quantum states are mapped between photons and atomic ensembles. Chou et al. essentially demonstrate the operation of a functional segment of a DLCZ network, consisting of two nodes placed three metres apart, between which they successfully distribute entangled states.

Anisotropy builds strong bones

Nature Mater. 6, 454–462 (2007)

It is well known that many of the superlative properties of bone — such as its elasticity, strength and hardness — arise because it is not a single homogenous material but a composite of many different materials and structures. Yet precisely how the microscopic anisotropy of bone causes these properties is poorly understood.

Using an atomic force microscope, Kuangshin Tai and colleagues identify complex nanoscopic variations in the stiffness of bone (pictured). Surprisingly, none of these variations seems to correspond to any specific compositional or structural variations in the bone itself. Even so, the authors were able to use their results to develop a plausible mechanism for how these variations dissipate energy and confer a greater degree of ductility and strength than would be likely to arise in a more homogeneous substance.

Measure of a scientist

Europhys. Lett. 78, 30002 (2007)

Jorge Hirsch proposed an objective metric to gauge the research output of any given scientist; for example, someone having an 'h index' of 30 has published 30 papers that each have at least 30 citations. In a study of influential writers with h indices near 100, he showed that the number of self-citations — that is, citations to an author's own work — contributes little. Thus it seemed robust and fair. And since then, the concept has been applied to assess the impact of topics and even journals.

But it can be manipulated. Michael Schreiber corrected for self-citations for 16 (relatively) young researchers and found that their h indices fell by 10–46%. So citing your own work can boost your h index. Of course, some self-citations are perfectly legitimate, but Schreiber argues that they don't reflect the true impact of the paper and should not be counted. Until his correction techniques are taken into account, such metrics should be used with care, he says, particularly for younger scientists whose citation records are under the most scrutiny, as they apply for jobs and research grants.

Tight fit

Phys. Rev. Lett. 98, 181101 (2007)

Why is gravity so much weaker than the other forces of nature? It could be that its effect is diluted in our four-dimensional world because gravity seeps out into other dimensions. Such extra dimensions are part of the 'braneworld' model for quantum gravity. However, torsion-balance experiments — starting with that of Henry Cavendish in 1798 — have verified Newton's inverse-square law down to separations of tens of micrometres, and constrain the size of any extra dimensions to similar distances. Dimitrios Psaltis has arrived at a similar level of sensitivity, but by a very different route.

Psaltis uses a connection that can be drawn, in braneworld, between the curvature of anti-de Sitter space in an extra dimension and the lifetime of a black hole. From astrophysical measurements of the black hole XTE J1118+480, he calculates a lower limit on its kinematic age (11 million years, at 95% confidence) which in turn translates into an upper limit on the size of extra dimensions of 80 μm — a limit that could be tightened with improved data on the black hole. For an 'ideal', accurately measured black hole, says Psaltis, the limit could be made as tight as 6 μm.