They say their theoretical studies could contribute to the debate on whether the electromagnetic fields of powerlines cause diseases such as cancer.

Many creatures, including some birds, amphibians and reptiles, navigate by sensing tiny changes in the Earth's magnetic field. Sea turtles, for instance, can sense changes as small as a tenth of a microtesla--less than 0.2 per cent of the typical geomagnetic field.

But nobody knows exactly how these biological compasses work. One theory is that the magnetite molecules found in some tissues act just like miniature compass needles.

Another is that animals sense changes in biochemical reaction rates caused by differing magnetic fields, which are known to alter the pace of a wide range of chemical reactions.

But no one had proved such a biochemical compass could work. As well as being influenced by magnetic fields, reaction rates fluctuate randomly and also change with temperature.

"All organisms, warm or cold-blooded, experience temperature variations," says James Weaver, a biophysicist at the Massachusetts Institute of Technology.

"The challenge is to understand how nature would be able to sense small magnetic field differences in the presence of all this competition."

Weaver and his colleagues decided to find out the mathematical way. They took equations describing a common reaction involving two reactive molecules called free radicals.

Then they looked at how much the reaction rate would be swayed by the "noise" of random fluctuations and temperature variations in the environment, as well as by magnetic variations like those sea turtles can sense.

Sure enough, it turned out that a small group of cells should be able to sense the magnetic field signal loud and clear above the noise. "An evolved animal sensory system like this should be possible, and it could have a rather fantastic performance," Weaver concludes.

He hopes that theoretical work like this could shed light on whether similar mechanisms might explain the possible, albeit contentious, health risk of electromagnetic fields from power lines and cellphones.

"That's a big hot potato," says Weaver. "But it would help if we can get simple models from physics and chemistry that give us some insight into what's possible and what's not."

"From the point of view of a chemical compass, this could be very interesting," says Nicholas Day, an epidemiologist at the University of Cambridge, whose studies have failed to find a link between childhood cancer and magnetic field exposure in Britain.

However, Day thinks that only epidemiology and carefully controlled lab experiments can lay the debate about the cancer link to rest.

Source: Nature (vol 405, p 707)

This article appeared in the June 10 issue of New Scientist New Scientist. Copyright 2000 - All rights reserved. The material on this page is provided by New Scientist and may not be published, broadcast, rewritten or redistributed without written authorization from New Scientist.

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