In the mud at the bottom of a pond live bacteria that carry tiny magnetic compass needles inside them. These needles are chains of magnetite crystals that help the bacteria swim along Earth's magnetic field, like a compass needle, in search of better conditions. When the bacteria die, the chains remain in the sediment and can be preserved for millions of years.

Geologists find such chains in ancient rocks and use them to learn when life appeared on Earth. But how do you tell a chain assembled by a living bacterium from a random clump of magnetic particles? It turns out that biogenic chains have distinctive magnetic properties. Scientists measure how such particles magnetize and demagnetize, and from the shape of the curves they determine whether life was involved.

Until now, calculations were done for spherical particles, but in many bacteria the crystals are elongated, like grains of rice. In the new work, simple formulas were derived for the interaction of such elongated magnets, and the behavior of whole chains of them was calculated. This will allow a more accurate reading of Earth's magnetic record.