Download 2008 Term 1 No 5

a)
Gravity Aftermath of Deadly Earthquake
Scientists at the University of Texas have used a pair of satellites to measure the seismic deformations
produced in the Earth during and after the huge Sumatra-Andaman earthquake of December 2004, the one
whose associated tsunami killed hundreds of thousands around the Indian Ocean coastline.
The Gravity Recovery and Climate Change Experiment (GRACE) consists of two Earth-orbiting satellites.
The satellite's relative spacing, monitored continuously, can be altered by the shifting gravitational subtleties
triggered by the movement of massive objects beneath. This can mean big changes in land water and lakes,
sea level changes, polar ice sheet melting, or sea floor changes caused by earthquakes.
Essentially, GRACE maps the gravity field of the Earth's surface before and after an earthquake. In the case
of the Sumatra/Andaman quake numerous detectors were at work, but only GRACE could accurately measure
and map the offshore rupture over its entire 1800-km extent from space (see GRACE map at
http://www.aip.org/png/2007/288.htm).
They can make this map around the time of the quake and then afterwards, as the Earth slows its seismic
ringing. One of the researchers, Jianli Chen ([email protected]) says GRACE's most successful mission so
far has been to monitor terrestrial water storage change and polar ice melting, and so it is one of the most
important sentinels of potential climate change.
b)
Controlling Cardiac Chaos - a gentler approach
Physics can save lives: a new type of defibrillation aims to reduce the voltage needed to shock out-of-control
hearts back to a normal beating pattern. Ordinarily the beating heart is an orderly process (called systole) in
which the heart muscle cells contract cooperatively to insure that blood is pumped about once every second.
If, however, some portions of cardiac tissue are electrically triggered in a non-coordinated way, the overall
activity of the heart can become chaotic. An irregular systole (fibrillation) in the atrial chambers of the heart
can be tolerated for some time, but fibrillation of the ventricles can kill a person within a few minutes. The
most extreme remedy for ventricular fibrillation (VF) is the application of a huge electrical shock
(administered by paddles applied to the chest).
Conventional defibrillators applied to the outside of the body can deliver a voltage difference of up to 5000
volts and a current of 20 amps. The shock delivered by implanted defibrillators is much less, but can still
result in trauma. The goal of the shock is to overwhelm the electrical environment of the entire heart--disrupting electrical waves even in the parts of the heart beating normally---hoping a global coordinated
rhythm will resume. (One could compare this to brute-force method of chemotherapy, in which toxic
chemical meant to kill cancer cells will also kill many healthy cells, resulting in unpleasant side effects.)
To see how the general assault on fibrillation can be modified, consider that the threatening arrhythmias take
the form of rotating waves (spirals) of electrical excitation passing across the volume of the heart. These
spirals are enhanced (and dangerously pinned in position) by the presence of scars (dead tissue) on the heart
caused at the scene of previous attacks and even by other “heterogeneities” present in healthy hearts such as
blood vessels, connective tissues, and oriented bundles of cardiac muscle fibers.
Alain Pumir and Valentin Krinsky and their colleagues at the University of Nice, France and at the Centre
National de la Recherche Scientifique (CNRS) Nonlinear Institute try to undo threatening vortices not by
jolting the whole heart but by aiming their countermeasures at the vortices exclusively. This permits a much
smaller voltage to be used, and hence less trauma to the patient and less damage to the heart itself. One of
their earlier efforts in this direction (Physical Review Letters, 30 July 2004) allowed a rotating vortex in the
heart to be removed using an input electrical energy lower by a factor of 20.
Later the approach was confirmed to be effective using rabbit hearts. Now Pumir and Krinsky (33-6-68441415, [email protected] , [email protected]) have designed an even better scheme, one that
would counteract a chaotic cardiac crisis consisting of many vortices. In addition, this approach permits the
energy to be reduced by a factor of a hundred or a thousand from present levels.
A sophisticated implant device, programmed to mitigate potential fibrillation with the new shock method,
would be almost unnoticeable to the patient. Teams led by R. Gilmour (Cornell) and E.Bodenschatz (Max
Planck Institute, Goettingen [email protected] ) are currently testing the method. An
estimated 250,000 people have implanted defibrillators, so the scope for medical benefits are enormous.
c)
The Highest- Energy Cosmic Rays.
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.