How can we understand environments that can't be replicated on Earth? That's a challenge astrophysicists face all the time. In some cases, it's largely a matter of figuring out how well-understood physics applies to extreme conditions and then comparing the output of those equations to observations. But a notable exception to that is a neutron star, where the relevant equations get completely intractable, and observations don't provide many details.
So, while we're pretty sure there's a layer of nearly pure neutrons near the surface of these bodies, we're very uncertain as to what might exist deeper in their interiors.
This week, Nature is publishing a study that tries to move us closer to an understanding. It doesn't give us an answer—there's still a lot of uncertainty. But it's a great opportunity to look at the process of how scientists can take data from a huge range of sources and start whittling away at those uncertainties.
What’s after neutrons?
The matter that forms neutron stars starts out as ionized atoms near the core of a massive star. Once the star's fusion reactions stop producing enough energy to counteract the draw of gravity, this matter contracts, experiencing ever-greater pressures. The crushing force is enough to eliminate the borders between atomic nuclei, creating a giant soup of protons and neutrons. Eventually, even the electrons in the region get forced into many of the protons, converting them to neutrons.
This finally provides a force to push back against the crushing power of gravity. Quantum mechanics prevent neutrons from occupying the same energy state in close proximity, and this prevents the neutrons from getting any closer and so blocks the collapse into a black hole. But it's possible that there's an intermediate state between a blob of neutrons and a black hole, one where the boundaries between neutrons start to break down, resulting in odd combinations of their constituent quarks.
These sorts of interactions are governed by the Strong Force, which binds quarks together into protons and neutrons and then binds those protons and neutrons into atomic nuclei. Unfortunately, calculations involving the strong force are extremely expensive, computationally. As a result, it's just not possible to get them to work at the sort of energies and densities present in a neutron star.
But this doesn't mean we're stuck. We have approximations of the strong force that can be computed at relevant energies. And, while those leave us with substantial uncertainties, it's possible to use a variety of empirical evidence to limit these uncertainties.
How to look at a neutron star
Neutron stars are notable for being incredibly compact for their mass, squeezing more than a Sun's worth of mass inside an object that's only about 20 km across. The closest one we know about is hundreds of light-years away, and most are much, much farther. So, it would seem it's impossible to do too much in the way of imaging these objects, right?
Not entirely. Many neutron stars are in systems with another object—in some cases a neutron star. The way these two objects influence each other's orbits can tell us a lot about the mass of a neutron star. NASA also has a dedicated neutron star observatory attached to the International Space Station. NICER (the Neutron star Interior Composition Explorer) uses an array of X-ray telescopes to get detailed images of neutron stars as they rotate. This has allowed it to do things like track the behavior of individual hot spots on the surface of the star.
More critically for this work, NICER can detect the distortion of space-time around large neutron stars and use that to generate a reasonably accurate estimate of its size. If that's combined with a solid estimate of the mass of the neutron star, then it's possible to figure out the density and compare that with the sort of density you'd expect from something that's pure neutrons.
But we're not limited only to photons when it comes to assessing the composition of neutron stars. In recent years, the mergers of neutron stars have been detected via gravitational waves, and the exact details of this signal depend on the properties of the stars doing the merging. So, these mergers can also help rule out some potential neutron star models.
Back on Earth
While we can't create an object out of neutron star-like material on Earth, it's possible to very briefly create some of the material. Particle accelerators like the Large Hadron Collider and the Relativistic Heavy Ion Collider reach energies that are well beyond those found in the interior of neutron stars. But the predecessor of the Relativistic Heavy Ion Collider (which is still in use as part of that accelerator complex) reached energies that are more relevant. And many of the researchers involved in the new paperwork at the Schwerionensynchrotron 18 accelerator in Germany, which is limited to energies in the right range.
Collectively, these measurements add an additional set of constraints on the properties of the matter at the center of neutron stars.
To put all these limits together, the researchers started with calculations that could approximate an equation of state for neutron matter, describing how things like the pressure and temperature relate to the total energy of the system. This approximation works at energies below that found in neutron stars but could be extrapolated into the relevant energy range.
The researchers then used the constraints provided by different observations—astronomical, gravitational, and collisional—and gradually eliminated potential solutions from the total area covered by the extrapolation. The combination of different limits was especially helpful at some energies, where astronomical observations rule out many of the extreme solutions to the extrapolation, and the collision data tends to prefer solutions where the neutron material is more rigid.
The team finished by calculating the probability that there's an additional phase transition in the interior of neutron stars that produces something other than a sea of neutrons. They estimate that this probability is about 0.419, which means it's slightly disfavored. But the uncertainties are still large enough that a phase transition is very much a possibility.
And slightly disfavored is where things have to be left for now—there's no clean answer and no sense of closure on the issues being addressed. But that's far more typical of the process of science than most of the results we cover. And in this case, the process involved everything from out-of-date particle accelerators to one of the most exciting developments in general relativity, the detection of gravitational waves.
Nature, 2022. DOI: 10.1038/s41586-022-04750-w (About DOIs).
