Probably come from the cores of active galactic nuclei (AGN), where supermassive black holes are
thought to supply vast energy for flinging the rays across the
cosmos. This is the conclusion reached by scientists who operate
the Pierre Auger Observatory in Argentina. This gigantic array of
detectors spread across 3000 sq. km of terrain, looks for one thing:
cosmic ray showers.
These arise when extremely energetic particles strike our atmosphere, spawning a gush of secondary particles. Many
of the rays come from inside our own Milky Way, especially from our
sun, but many others come from far away. Of most interest are the
highest-energy showers, with energies above 10^19 electron volts,
far higher than any particle energy that can be produced in
terrestrial accelerators. The origin of such potent physical
artifacts offers physicists a tool for studying the most violent
events in the universe.
To arrive at Earth most cosmic rays will have crossed a great deal of intergalactic space, where magnetic
fields can deflect them from their starting trajectories. But for
the highest-energy rays, the magnetic fields can’t exert as much
influence, and consequently the starting point for the cosmic rays
can be traced with some confidence.
This allowed the Auger
scientists to assert that the premier cosmic rays were not coming
uniformly from all directions but rather preferentially from
galaxies with active cores, where the engine for particle
acceleration was probably black holes of enormous size. The very
largest of cosmic ray showers, those with an energy higher than 57
EeV (1EeV equals 10^18 eV), correlated pretty well with known
AGN’s. (Science, 9 November 2007)
Breathing Exercises for Enzymes
A new model of proteins seeks to
explain how enzymes extract energy form their vicinity and put it to
use in regulating cell chemistry. Enzymes are huge protein
molecules that play a crucial role in catalyzing chemical reactions
among other molecules or atoms by lowering the energy barrier that
would otherwise keep the reaction from happening. Enzymes can
therefore be considered as energy-processing
chemical-reaction-facilitating machines.
They are usually large,
typically containing thousands of heavy (non-hydrogen) atoms, but of
these only a few dozen atoms actually participate in the catalytic
process. Addressing this important issue, a team of scientists at
the Ecole Normale Superieure (Lyon, France) and the Ecole
Polytechnique Federale de Lausanne (Switzerland) have concentrated
on modeling the behavior of the stiff parts of the enzyme since they
believe that some of the energy used in carrying out the catalytic
task is stored not just as chemical energy (in the form of adenosine
triphosphate, or ATP, the all-purpose “food” of cells) but also as
mechanical energy in the form of a waggling or “breathing” motion in
the stiffer parts of the enzyme.
Extending this research to
proteins in general, Yves-Henri Sanejouand
(yves-henri.sanejouand@ens-lyon.fr, 33-04-72-72-8870) says that he
and his colleagues would like to scrutinize in more detail the
nonlinear process by which some proteins catch and store thermal
energy from their environment and also how chemical energy can be
turned into mechanical energy, such as in muscle contraction.
(Juanico et al., Physical Review Letters, upcoming article)
Digital Droplet Sorting.
A new microfluidic lab-on-a-chip setup
forms tiny droplets, passes them through a pair of electrodes which
can perform an identification of the droplets, passes them through a
second pair which gives them a charge, and then through a third pair
which sorts the drops according to their properties. Basically the
charge imparted to the
droplet is proportional to the droplet size, and the charge is
gauged by the effect it has when passing through the first set of
capacitor electrodes.
Scientists at the Hong Kong University of
Science and Technology form a supply of drops moving in a
microchannel by having the
fluid of interest (in one channel) merge with a running rivulet of
oil (silicon or sunflower oil) in a second channel (see the figure
at http://www.aip.org/png/2007/290.htm). By regulating the flow
rate of the fluid and the oil, droplets of many sizes and rates can
be formed. The Hong Kong Scientists currently can look at droplets
smaller than a pico-liter (10^-12 liter) in size with a capacitive
sensitivity of a pico-Farad (10^-12 F).
The detection rate right
now is about 10,000 drops per second, which is already pretty high.
According to one of the researchers, Weijia Wen (phwen@ust.hk), this
capacitance-based detection rate is better than that can be
accomplished with optical means (such as with a CCD camera), and the
capacitance method is intrinsically cheaper than the optical
equivalent.
In the Hong Kong approach the detection and the sorting
are both performed electrostatically: sorting happens when an
electric field sends the higher-charged drops into one channel, and
the lesser-charged drops into another channel. In this way nano- or
micro-particles can be sorted digitally.The goal is to furnish a
useful digitally-controlled bio-chemical chip for performing various
experiments with nano-liter volumes of reactants or biological
samples. (Niu et al., Biomicrofluidics, Oct-Dec 2007)
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