Neutrons Become Cubes Inside Neutron Stars

Intense pressure can force neutrons into cubes rather than spheres, say physicists

Emerging Technology from the arXivarchive page

August 11, 2011

Inside atomic nuclei, protons and neutrons fill space with a packing density of 0.74, meaning that only 26 percent of the volume of the nucleus in is empty.

That’s pretty efficient packing. Neutrons achieve a similar density inside neutron stars, where the force holding neutrons together is the only thing that prevents gravity from crushing the star into a black hole.

Today, Felipe Llanes-Estrada at the Technical University of Munich in Germany and Gaspar Moreno Navarro at Complutense University in Madrid, Spain, say neutrons can do even better.

These guys have calculated that under intense pressure, neutrons can switch from a spherical symmetry to a cubic one. And when that happens, neutrons pack like cubes into crystals with a packing density that approaches 100%.

Anyone wondering where such a form of matter might exist would naturally think if the centre of neutron stars. But there’s a problem.

On the one hand, most neutron stars have a mass about 1.4 times that of the Sun, which is too small to generate the required pressures for cubic neutrons. On the other, stars much bigger than two solar masses collapse to form black holes.

That doesn’t leave much of a mass range in which cubic neutrons can form.

As luck would have it, however, last year astronomers discovered in the constellation of Scorpius the most massive neutron star ever seen. This object, called PSR J1614-2230, has a mass 1.97 times that of the Sun.

That’s about as large as theory allows (in fact its mere existence rules out various theories about the behaviour of mass at high densities). But PSR J1614-2230 is massive enough to allow the existence of cubic neutrons.

Astrophysicists will be rubbing their hands at the prospect. The change from spherical to cubic neutrons should have a big influence on the behaviour a neutron star. It would change the star’s density, it’s stiffness and its rate of rotation, among other things.

So astronomers will be getting their lens cloths out and polishing furiously in the hope of observing this entirely new form of matter in the distant reaches of the galaxy.

Ref: arxiv.org/abs/1108.1859: Cubic Neutrons