PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 622 January 27, 2003  by Phillip F. Schewe, Ben Stein, and James
Riordon

BUTTERFLIES AND PHOTONIC CRYSTALS. 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,
jean-pol.vigneron{at}fundp.ac.be, 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 at www.aip.org/mgr/png ), 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)

SYNCHRONIZATION TOMOGRAPHY.  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, p.tass{at}fz-juelich.de,
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)

THE PHYSICS OF STONE THROWING.  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,
lyderic.bocquet{at}lpmcn.univ-lyon1.fr) 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://lpmcn.univ-lyon1.fr/~lbocquet )

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