Computer Simulations Show Nuclear “Pasta” May Be 10 Billion Times Tougher Than Steel

What is the toughest material in the universe? Most people would probably guess something like diamonds, titanium alloys, or carbon-treated steel. A reader up to date on their contemporary science knowledge might say graphene a substance so tough it would take the weight of an elephant to break a sheet the thickness of saran wrap. Well, it might be that all of those answers are wrong, as a new study conducted by a trio of researchers from institutes in the U.S, and Canada has potentially identified the strongest substance known to man.

Computer simulations run by the researchers indicate that elastic structures composed of nuclear material under the crust of neutron stars may qualify as the toughest material in the universe, far stronger than steel, titanium or even graphene.

An artist’s imagining of a neutron star. Source: Kevin Gill via Flickr

The material, dubbed “nuclear pasta” due to the pasta-like shapes it takes on, potentially exists about 1 km under the crust of a neutron star, the densest objects known to man. The researchers refer to this nuclear pasta as a “liquid crystal” and it has a potential shear modulus strength of up to 1023 J/cm3.  This high shear modulus means that in order to bend these nuclear pasta, once would have to apply a mechanical force equivalent to that released by one quadrillion tons of TNT (1,000,000,000,000,000 tons) per cubic centimeter of the material. These structures can only exist in the most extreme of environments, under the intense pressure and gravitation of a neutron star. The study is also notable for being the largest computer simulation of nuclear pasta to date.

Neutron Stars And Nuclear Pasta

When stars get old the explode into supernovas and then collapse back in on themselves. Some stars are big enough to collapse all the way into a black hole, while some stars are too small and instead collapse into faint white dwarfs. Others are just the right size so instead of turning into a black hole or a white dwarf, they will shed their protons and electrons and collapse into an extremely dense composed almost entirely of neutrons. Neutrons stars are extremely dense, so much so that a single teaspoon-sized bit would weigh 900 times more than the Great Pyramid of Giza. The average density of a neutron star is about 3.7–5.9×1017 kg/m3. Neutrons stars are the densest known kind of star, except for hypothetical stars made from quark-gluon plasma.

The interior structure of a neutron star. Neutrons are relatively small but extremely massive and dense.  Source: WikiCommons

The neutrons in neutron stars are as close to one another as the particles in an atomic nucleus of ordinary matter. at the surface, neutrons tend to arrange themselves in a relatively orderly and periodic lattice structure. However, deep inside the star, the pressure and gravity press the neutrons even closer together, causing chains of neutrons to arrange in complex shapes called nuclear pasta. Previous computer simulations have modeled the crust of the neutrons star as a more or less isotropic lattice of particles. The current study is an attempt to understand the dynamics of theses nuclear pasta, specifically how they shift under the surface and produce high energy tectonic behavior in neutron stars.

A more detailed graphic of the interior of a neutron stat showing the densities at the various levels. Nuclear pasta exist from the inner crust down. Source: WikiCommons

The researchers modeled the nuclear pasta using powerful software meant for modeling molecular dynamics. By defining simple rules that govern the behavior between two simulated nucleons, the software is able to extrapolate and simulate the behavior of thousands of particles interacting at the same time. The team ran a number of situations with the pasta shapes oriented in different directions to chart out the dynamics of how nuclear pasta reacts to force and movement. For example, in their first run of simulations, the researchers modeled nuclear pasta in its “lasagna” shape, flat plates of the material that extend under the crust and layer on top of each other. This particular run simulates the effects of transverse, longitudinal, and compressive stress on sheets of nuclear “lasagna” to see how the material rearranges itself. Specifically, the researchers were looking for the point where the lasagna sheets would break and change the stress in the interior of the neutron star.

In the second run of simulations, they modeled the dynamics of the “waffle” formation, a lattice of parallel plates with hexagonal holes. According to the authors, this simulation had been the largest of nuclear pasta yet. The waffle shape showed relatively little breaking compared to the lasagna shape as the hexagonal holes on the material gave suitable room for the material to deform instead of breaking.

So what are we to make of nuclear pasta? Well, for one, the simulations were idealized, so it is likely that actual nuclear pasta will not behave exactly as simulations predict. Notably, their simulations left out any possible effect from the superfluidity of neutrons or any superconductive behavior that may have an effect on the star’s magnetic field. Also, the elastic constants meant to represent the elasticity of the nuclear pasta were assumed rather than achieved through calculation.

However, the study provides valuable information about the geological properties of nuclear pasta. Specifically, nuclear pasta is extremely strong. Even when the shear modulus estimate of 1024 J/cm3 is cut down due to inadequacies in the simulation, the resulting number is still stronger than any known material and incredibly stronger than any material humans have been able to manufacture. In addition to the pasta shapes under the crust, the researchers’ model also predicts that neutron stars contain buried “mountains” in the crust. The mountains of neutrons could be a major source of gravitational waves.

Incidentally, this prediction indicates a relatively new turn in science; the reliance on computer simulations to test hypotheses and make predictions. The rise of computers makes it so scientists can run accurate simulations of extremely complex phenomena, sometimes to the effect of discovering something new about those phenomena. This lesson has been applied to studies on black holes, whose powerful gravitational fields make traditional methods of observation impossible.

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