April 14, 2009

Neutron star crust is ten billion times stronger than Steel

Computer simulations show that the breaking strength of neutron star crust is about 10 billion times more than for terrestrial engineering materials such as metal-alloys where the strength is measured in fractions of a GPa. The largest contributor to this tremendous difference is of course the enormous pressure and thus density of the crust.

The screened Coulomb interaction is purely repulsive (in a neutron star) and has no explicit length scale, i.e. the system at twice the density behaves just like the system at the original density only at a lower temperature (Eq.2). This causes the material to fail abruptly in a collective manner at a large strain, rather than yielding continuously at low strain as observed in metals, because of the formation of dislocations. For example, the breaking strain of steel is around 0.005, some twenty times smaller than what we find for the neutron star crust. We speculate that the collective plastic behavior found here could help to improve design strategies that suppress the weakening effects of dislocations and other more localized defects in conventional materials. Note that small Coulomb solids have been studied in the laboratory using cold trapped ions.

Materials like rock and steel break because their crystals have gaps and other defects that link up to create cracks. But the enormous pressures in neutron stars squeeze out many of the imperfections. That produces extraordinarily clean crystals that are harder to break. A cube of neutron star crust can be deformed by 20 times more than a cube of stainless steel before breaking.

So if metals and other material can be made with perfect crystals then they would have 20 times the strength of regular materials.

Now, "all else being equal, the maximum height of a 'mountain' on a neutron star is now 10 times what we thought," Owen told New Scientist.

That would produce gravitational waves with 100 times the energy as those previously calculated, which could boost the likelihood that ground-based experiments like the US Laser Interferometer Gravitational-Wave Observatory (LIGO) could spot the signals, he added.

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