Physics news update 610

October 24, 2002

TRAPPING DNA THROUGH THERMOPHORESIS might have a bearing on the origins of life, as a new experiment shows. The DNA molecules in our bodies are protected behind a nuclear membrane and a cellular membrane, but on the early Earth nascent life forms might have consisted of DNA floating in a free aqueous environment. How would such fragile entities keep from diluting themselves to death? One answer might be thermophoresis, a process (known for almost 150 years) by which heat can repel polymers. Generally the longer the molecule the greater the thermal repulsion will be, just as molecules or particles will be separated in a centrifuge according to mass. An experiment conducted by Dieter Braun and Albert Libchaber at Rockefeller monitors fluorescent-tagged DNA molecules as they are harried by a laser-generated heat spot. As expected the DNA was repelled, carried along by a convective flow away from the heat. But surprisingly the DNA then came back; the convection, scrutinized more carefully, was seen to be a circular cell pattern. The DNA had become trapped in a small zone (20 microns across and with a DNA concentration enhanced by a factor of 1000) centered around the heat spot.

Braun (212-327-8160, says this is the first quantitative experimental evidence, on a microscopic level, that biological molecules (DNA was used rather than RNA because RNA can quickly degrade in the presence of proteins in the solution) can be trapped in this way. Demonstrating a mechanism for confining early metabolic and replicative life forms in a far-from-equilibrium environment such as localized heat sources (e.g., hydrothermal vents) immersed in a cold ocean, should be of interest to biologists who ponder the advent of life. (Physical Review Letters, 28 October 2002; see; independent thermophoresis expert: Werner Kohler in Bayreuth, Germany,

NONINVASIVE EEGs. Conventional electroencephalograms (EEGs) monitor electrical activity in the brain with electrodes placed either on the scalp (involving hair removal and skin abrasion) or inserted directly into the brain with needles. Now a noninvasive form of EEG has been devised by scientists at the University of Sussex. Instead of measuring charge flow through an electrode (with attendant distortions, in the case of scalp electrodes) the new system measures electric fields remotely, an advance made possible by new developments in sensor technology.

The device's sensitivity is demonstrated by watching electric activity change as the ambient relaxed brain signal (the so-called alpha wave, at a frequency of 8-14 Hz) gives way to the beta wave (14-35 Hz) as the subject opens his eyes (figure at The Sussex researchers (contact Terry Clark,, 44-127-678087) believe their new sensor will instigate major advances in the collection and display of electrical information from the brain, especially in the study of drowsiness and the human-machine interface. The same group of scientists has made remote-sensing ECG units as well. (Harland et al., Applied Physics Letters, 21 October 2002; text at; research website:

NAVAL NEUTRINOS, emitted by nuclear subs as a routine byproduct of the reactions producing propulsion, will have to be taken into account when studying neutrino oscillations, suggests a team of Stanford physicists. Oscillation experiments probe the fascinating process by which one type of neutrino turns into other types. The power generated by nuclear submarines (100-200 operating at any one time) is only a few percent of all nuclear-generated thermal power in the world, and the neutrino flux from a typical naval reactor is only about 200,000 per sq. cm per second at a distance of 40 km. This does not represent much of a background for the current generation of reactor-based neutrino-oscillation experiments. But for future reactor-based experiments, trying to perform higher precision measurements or those using a lower flux from a longer baseline (neutrino flux drops with the square of the distance), naval-reactor neutrinos will have to be factored in. Stanford physicist Giorgio Gratta (650-725-6509, says that, on the other hand, neutrinos from naval reactors may be used for a new breed of oscillation experiments in which the baseline for oscillations could be changed by simply "sailing the reactor" to a new position with respect to the (fixed) large detector. It is suggested that a nuclear ice-breaker could be chartered for this purpose. And, no, a sub's neutrino flux is not strong enough to give away its position. (Detwiler et al., Physical Review Letters, 4 November 2002; text at
Number 610 October 22, 2002 by Phillip F. Schewe, Ben Stein, and James Riordon

American Institute of Physics

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