Physics News Update No. 622

In recent years, scientists have discovered that the iridescence of various colorful creatures, from beetles to birds to butterflies, is often due to microscopic structures known as photonic crystals. Unlike pigments, which absorb or reflect certain frequencies of light as a result of their chemical composition, the way that photonic crystals reflect light is a function of their physical structure. That is, a material containing a periodic array of holes or bumps of a certain size may reflect blue light, for example, and absorb other colors even though the crystal material itself is entirely colorless. Because a crystal array looks slightly different from different angles (unlike pigments, which are the same from any angle), photonic crystals can lead to shifting shades of iridescent color that may help some animals attract mates or establish territories.

A collaboration of researchers from Hungary and Belgium (Jean-Pol Vigneron, Universitaires Notre-Dame de la Paix, Brussels, 011+32-81 724711) may have discovered why the males in certain populations of lycaenid butterflies carry the striking, photonic crystal coloration, and males in other lycaenid populations do not. The researchers examined butterfly scales through high-resolution scanning electron microscopes (see image), and confirmed that indeed the colorful butterflies' scales included arrays of submicron-sized holes that formed natural photonic crystals. Their closely related brethren from higher elevations did not have the hole arrays in their scales, and their wings were dull brown rather than iridescent blue. The difference, it seems, may be due to a question of survival. The researchers found that the plain brown butterfly wings warmed much more than the iridescent blue wings when each were exposed to identical illumination. The researchers believe that the butterflies at high elevations trade flashy iridescence for light-absorbing brown so that they can withstand colder temperatures, and survive long enough to mate.

If photonic crystals can have such a dramatic impact on butterfly thermal management, suggest the researchers, manmade photonic crystals may someday provide flexible thermal protection in extreme environments, possibly being incorporated into such things as space suits or desert garments. (L. P. Biro et al, Physical Review E, February 2003)

A new brain imaging method pioneered by a German research group from several institutions can now produce images that localize the areas of the brain involved when test subjects perform physical activities, and can show how portions of the brain interact with each other. The technique, dubbed synchronization tomography, involves mapping the fluctuating magnetic fields produced by tiny electrical currents in the brain, and determining which brain regions are synchronized with an activity - such as a test subject's tapping finger. The researchers (Peter Tass, Institute of Medicine, Research Center, Juelich, 011+49-2461-61-2087) asked test subjects to tap their finger in time to a rhythmic tone, and to continue tapping at the same rate after the tone was switched off. Meanwhile, their brain activity was mapped with a magnetoencephalography (MEG) machine. The maps showed that the same regions of the brain areas are active both as people tapped to a beat and as they paced the tapping themselves, but that the synchronization between the different brain areas changes dramatically.

Other brain imaging methods, including functional magnetic resonance imaging (fMRI) and positron emission tomography (PET), can also provide insight into which regions of the brain are involved during various activities, but they take too long to acquire images to disclose how the brain regions interact with each other, and therefore overlook important details of brain function which are clearly revealed with synchronization tomography. In addition, a related synchronization technique may help in the study of rapidly changing signals in the heart detected with magnetocardiography systems. (P. A. Tass et al., Physical Review Letters, upcoming article)

Prompted by his son's questions on the subject and the need to furnish his mechanics textbook with commonplace examples, physicist Lyderic Bocquet of the Universite Claude Bernard Lyon (France) has investigated the science behind stone skipping. The chief parameters that determine whether your stone goes right in or skims across the lake are as follows: the mass of the stone, its angle with respect to the horizon, its angle with respect to the water surface (lower is better), its spin rate (more is generally better, for stability), and its horizontal velocity. Armed with calculations on energy loss, Bocquet (33-472-43-2796) has worked out an expression for the maximum number of skips one can expect. According to Bocquet, the world's record for stone rebounds is 38. (American Journal of Physics, February 2003; see also http://dpm.univ-lyon1.fr/~lbocquet/ )

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