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STRETCHY materials, such as rubber or melted cheese, get thinner when you
pull them, right? Well, not always. A few substances do the opposite—they
get wider when stretched—and a group of scientists now believes that the
superdense shells of neutron stars have this odd property. This could shed light
on starquakes, in which giant cracks tear through neutron star crusts, they say.
It could also help produce exotic materials here on Earth.

One measure of the way a material responds to stretching is called the
Poisson’s ratio. Everyday materials have a positive Poisson’s ratio. But some
foams and polymers have a negative Poisson’s ratio, and they become wider when
stretched out.

Ray Baughman, a materials scientist at Honeywell International in Morristown,
New Jersey, wondered if the same might be true of the cores of the superdense
stars called white dwarfs and the solid crusts of neutron stars. These are
thought to consist of ion crystals in a sea of electrons at enormous pressures.
Calculations backed up this hunch.

To confirm it, they made their own ion crystals, by trapping beryllium
ions using electric and magnetic fields. By changing the force fields they were
able to stretch their crystals, and sure enough, they became wider in the
perpendicular direction.

“This should be important in understanding starquakes,” says Baughman. Cracks
ripping through the shells of neutron stars might explain some bright flashes of
gamma rays that astronomers see, although nobody has a full picture of exactly
how the quakes happen.

Baughman’s work may come in useful back here on Earth, too. Understanding and
engineering materials with very high negative or positive Poisson’s ratios could
be the key to very sensitive pressure sensors, he says.