Physics Tip Sheet #47 - News from the American Physical Society
Mass Extinctions without Astrophysical Calamities
Phys. Rev. E 71, 052902 (2005), 20 May 2005
Mass extinctions seem to occur on Earth roughly every 26 million years, leading some scientists to propose that they may be caused by rare collisions with comets or asteroids. A researcher in Poland thinks it may be possible that extraordinary predators are at fault instead.
Adam Lipowski (Adam Mickiewicz University) constructed a numerical model of many species competing for both food and living space. The model also included a term that controls mutation rates, allowing new species to develop over time. The model shows that, much of time, the system is populated with "medium efficiency" predators whose numbers fluctuate only slightly as the prey population waxes and wanes. Inevitably, their stable community is disrupted when mutations lead to a super predator that quickly decimates the prey population, which in turn leads to its own demise. The few creatures that survive the predatory apocalypse gradually mutate to fill the existing ecological niches - and the cycle begins again.
The period of the cycle depends on mutation rates in the model. The lower the mutation rate, the longer the periods between super predators. For a sufficiently low mutation rate, the model can lead to cycles that correspond to our 26 million year mass extinctions.
Previous models that do not show these sorts of cycles could be faulty, according to Lipowski, because they failed to account for the effects of limited living spaces shared by a large number of different species.
High Resolution Nuclear Magnetic Resonance without a magnet
S. Appelt et al.
Phys. Rev. Lett. 94, 197602 (2005), 20 May 2005
A group of German researchers have shown that a powerful chemical analysis technique, Nuclear Magnetic Resonance (NMR) spectroscopy, could lead to a highly sensitive way to measure the magnetic fields around living creatures, sample the Earth's magnetic field, or test the composition of mineral oils in wells. The key to the advance is developing an NMR system that makes use of the relatively weak magnetic field of the Earth, rather than fields from manmade magnets.
NMR spectroscopy is an analytical technique that relies on the fact that the nuclei of atoms have magnetic fields similar to those of common bar magnets. Placing atoms in a strong magnetic field and applying the appropriate radio signal can cause the nuclei to flip over and create signals that reveal information about the atoms and their environments. This makes NMR spectroscopy an important method for analyzing chemical composition and molecular structure.
NMR is also the phenomenon at the heart of Magnetic Resonance Imaging (MRI), one of the most powerful diagnostic tools in modern medicine.
When they were first developed, NMR systems employed very strong fields, generated by large, power-hungry magnets. In recent years, researchers have developed NMR techniques in ever weaker fields. Now S. Appelt et al. report that they have developed high resolution NMR techniques that rely only on the Earth's natural, very weak magnetic field.
Previous researchers have shown that the Earth's field is adequate for performing NMR studies on large samples (such as a cubic meter of ice), but the new research extends the technique to samples sizes only centimeters across.
The advance may turn NMR into a valuable analysis tool in places where large magnets are impractical, such as the tight confines of well holes. It may also lead to precise measurements of very weak magnetic fields, including those surrounding living creatures and the Earth itself.
The Smallest Solenoid
T. Ono and K. Hirose
PRL 94, 206806 (2005), 27 May 2005
Nanoscopic wires grown in gold may be the world's smallest solenoids, according to new theoretical analysis of the structures. Solenoids are typically tubes made of coiled wire. They conduct electricity along the spiraled wires to generate magnetic fields that run the length of their interiors. Solenoids are often important components of circuits and switches in their macroscopic form.
The nanoscopic versions, known as Helical Gold Nanowires (HGN), consist of concentric layers of gold atoms. HGNs were first made in laboratories in 2000. In the simplest incarnation, a wire only one atom thick is surrounded by a tube of atoms.
The analysis of the HGNs, conducted by researchers at Osaka University in Japan, shows that electrical current may spiral around a wire as it travels along the wire length. Considering the size of the wires (0.6 nanometers in diamter, or a hundred million times thinner than a human hair), it is likely to be difficult to directly measure the magnetic fields they produce. Nevertheless, if confirmed experimentally, the solenoids add yet another tiny component to the list of nanoscopic electronic parts (resistors, transistors, capacitors) needed to construct highly miniaturized circuits and machines.
Source: Eurekalert & othersLast reviewed: By John M. Grohol, Psy.D. on 21 Feb 2009
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