From one to many

Angew. Chem. Int. Ed. 47, 2427–2430 (2008)

Knowledge of the molecular structure of small organic molecules does not mean that the structure in which the molecules crystallize is determined. But a numerical approach developed by Marcus Neumann et al. takes us closer to that goal.

Neumann and his co-workers emerged as the winners of the fourth “blind test of crystal structure prediction”, hosted last year by the Cambridge Crystallographic Data Centre. The test involved calculating the crystal structure of four compounds within six months, after being given their molecular diagrams. Three predictions for each molecule could be submitted; Neumann and colleagues correctly calculated — investing some 280,000 CPU hours — all four structures.

The feat is all the more remarkable as in the previous run of the blind test, held in 2004, the results were sobering: of all submitted predictions, only one was successful for any of the 'blind' molecules. The pharmaceutical industry in particular is expected to benefit from reliable methods of crystal-structure prediction, given that many molecules of medical interest can crystallize to form more than one structure.

In black and white

Science 319, 1367–1370 (2008)

Once a photon has gone beyond the 'event horizon' of a black hole, it can no longer escape. However, particle–antiparticle pairs can spring from a vacuum, according to quantum mechanics, and if only one of the pair happens to lie outside the event horizon, we should be able to detect it as Hawking radiation. Unfortunately, any such radiation would be weak compared with the cosmic microwave background.

Thomas Philbin and colleagues circumvent this difficulty by creating event horizons in the laboratory. First they send laser pulses along an optical fibre. Each pulse increases the refractive index n of the fibre, and the 'extra' refractive index δn moves with the pulse. A probe pulse follows; initially it has a higher group velocity but cannot overtake the original pulse because δn slows the probe down to the speed of the pulse. As the probe pulse cannot 'enter' the trailing end of the pulse, the situation is equivalent to a 'white hole' — an object that nothing can enter. At the front end of the pulse, there is a black-hole horizon for probe light slower than the pulse.

Fly like a bird

Proc. Natl Acad. Sci. USA doi:10.1073/pnas.0707711105 (2008)

Credit: GETTY

“Do more with less” was the motto of the American aeronaut Paul MacCready, a pioneer of human- and solar-powered aeroplanes who died last year. He is also remembered for his theoretical work on optimizing soaring flight. Humans piloting paragliders, hang-gliders and sailplanes actively apply the MacCready formula to calculate a flight path that makes optimal use of the lifting potential of thermals — columns of rising warm air that provide an energy-saving means of gaining height. Birds, however, seem to have an innate ability to reproduce the MacCready formula, as Zsuzsa Ákos and colleagues report.

In their study comparing bird and human soaring strategies, Ákos et al. looked in detail at flight data collected with lightweight GPS devices attached to tame birds — peregrine falcons (pictured) and white storks — and to human-made gliders. They found that both bird and human, irrespective of their different sizes and approaches to manoeuvring in air, follow similar flight patterns and a soaring strategy that is close to the optimum predicted by MacCready's theory.

Nothing better

Phys. Rev. Lett. (in the press); preprint at arxiv.org/abs/0801.1874v2 (2008).

Fundamental constants are supposed to be constant. But according to certain cosmological models, some fundamental constants had different values in the early Universe compared with the present day, and could still be evolving. Moreover, these constants might be coupled to gravity.

Optical lattice clocks could, in principle, measure any drifts and gravitational coupling of fundamental constants, such as the fine-structure constant or the electron–proton mass ratio. Because the Earth's orbit is elliptical, such a clock would experience a varying solar gravitational potential over time. In fact, three strontium clocks — located in Boulder, Paris and Tokyo — have been measuring the clock transition of neutral 87Sr atoms confined in an optical lattice for the past three years. Sebastian Blatt et al. have analysed these data together with those from other clock species. Their verdict? We now know, within current accuracy, that there is no gravitational coupling and that the constants are fundamentally constant.

Sticking with convention

Phys. Rev. D 77, 052001 (2008)

That the three types of neutrino have different masses has been established by the observation of neutrino oscillations — one flavour of neutrino can change into another. But could the neutrino masses themselves vary?

The mass-varying-neutrino, or MaVaN, model posits that the mass of a neutrino changes depending on the density of matter in its path. If so, then MaVaNs could be a source for the 'dark energy' that seems to be driving the accelerating expansion of the Universe.

K. Abe et al. have analysed data on atmospheric neutrinos collected using the Super-Kamiokande detector between 1996 and 2001 for evidence of MaVaNs. Super-Kamiokande is a Čerenkov detector, holding 50 kilotons of water, buried in a Japanese mountain. Considering that MaVaNs would be sensitive to the varying electron density in the Earth's surface, crust, mantle and core (neutrinos can reach the detector by passing right through the Earth), Abe et al. conclude that, for oscillations of a muon neutrino into a tau neutrino, there is no sign of the mass effect.

The analysis does not rule out all variations of the MaVaN model, but as yet there is no evidence for anything other than 'conventional' neutrino oscillations.