Scientists have looked inside neutron stars for the first time, making a breakthrough in their simulation on supercomputers

There are extreme physical processes going on inside neutron stars that will likely never be studied directly. Moreover, these objects are so compact that they are invisible to telescopes. All that science has is indirect data about neutron stars and the ability to roughly simulate their properties on computers. However, with some effort, the accuracy of such models can be increased to the highest level.

A combined image of the Crab Nebula with a neutron star in visible, infrared and X-ray light. Image source: NASA

The nearest neutron star is about 400 light years from Earth. We do not have, and for thousands of years will not have, the technology to send a research station there. At this distance, no telescope will be able to see a neutron star with a diameter of only 20 km. In addition, under terrestrial conditions it is impossible to reproduce the physical parameters inside a neutron star, where the density of matter is several times higher than the density of atomic nuclei.

A breakthrough in neutron star modeling will likely become possible with the advent of powerful quantum simulators. However, today we have supercomputers and developed quantum mathematics, which may be sufficient for an in-depth analysis of the physics of neutron stars. At least this was recently stated by scientists from the University of Colorado Boulder and the Massachusetts Institute of Technology.

The internal properties of a neutron star, such as pressure and density, are determined by the equations of quantum chromodynamics (QCD), which describe the strong interactions between protons, neutrons and their constituent quarks. However, these equations cannot be solved for the entire neutron star. By simplifying a number of variables, scientists can solve equations for the outer layer of the star and its core, but the intermediate layer has so far been described only by approximation. There was no direct solution.

To get around this limitation, the researchers used a different approach: lattice quantum chromodynamics. But even here there was a trick. Lattice QCD also does not allow one to directly solve equations for the entire volume of a neutron star. The equations become solvable if we take into account isospin, a characteristic that distinguishes protons from neutrons by the sign of their charge states.

Using the proposed model for describing neutron stars, scientists have established limits on the size of these objects and obtained new strict restrictions on the properties of their interior. One of the conclusions of this work was the assumption that the masses of neutron stars can exceed two solar masses, which was previously considered the theoretical limit for such objects. Supercomputer calculations provided a lot of interesting data. However, without the next step—confirming the calculated properties of neutron stars using astrophysical observations—these results remain a promising hypothesis and a tool for finding new ways to study them. And this is already no small achievement.