Neutron stars are the classic example of a synthesis between quantum mechanics and gravity. Formed after the collapse of the remnants of a supernova, a neutron star is generally smaller than the earth but extremely massive. Simple calculations show that the density of the star must be higher than that found in the nucleus of an atom, meaning that the star interior can only be described by quantum mechanics. On the other hand, the immense forces required to hold the star together come from gravity, a purely classical theory—making the neutron star the shining light of all of modern physics. Some recent research on the precession—the rotation of the axis of rotation—of neutron stars throws a spanner in the works. To understand why, we first need to look at the interior of a neutron star and why precession is such a problem.
The immense pressure in the interior of a neutron star causes all the protons to be expelled to the stellar crust, while the interior is made up solely from neutrons. The remaining electrons form a cloud around the stellar crust. In the interior, the neutrons pair up, so that they can act like bosons, which allows them to all fall to their lowest energy state, turning the interior into a superfluid. Likewise, the protons pair up and fall to their lowest energy state—unlike the neutrons, however, the proton soup is superconducting and couples strongly to the electron cloud. Since the neutron star is spinning, the superconducting shell generates huge currents and strong magnetic fields.
Focusing on the interior, which is a superfluid, we know that it cannot support a rotation. When we attempt to set superfluid helium rotating, the fluid sets up a regular grid of counter-rotating vorticies. However, if the motion is more complicated, e.g., precession is introduced, then the vorticies must continually rearrange themselves to keep countering the changing axis of rotation. This has been observed in model systems, where it causes the vorticies to become unstable and tangle up. Since precession has been observed in a single neutron star, and is a commonly observed feature of most stars and (solar) planets, the influence of precession on the stellar interior should be informative.
In research published in Physical Review Letters, scientists analyzed the neutron star as a two-component fluid—superfluid neutrons and the proton fluid, which experiences braking from electromagnetic interactions. The behavior of the system depends on the strength of the interaction between the proton component and the neutron component and how big the mountains on the stellar surface are, but the researchers were able to reach a number of conclusions about neutron star precession. If the interactions are strong enough, stable vorticies can never form, which implies that the interior must be turbulent. If the interactions are weak, then slow precession is possible, which happens to agree with the single real data point that the researchers had.
So what does all this mean? One commonly observed feature of neutron stars is their very regular rotational periods—a timing stability that is comparable with atomic clocks has been observed. Other neutron stars are observed to have timing jitter, which could be attributable to precession, although the observational data are inconclusive. Given that precession is likely to lead to a turbulent interior, it may be very very difficult to distinguish precession from noise. I would say that the strongest conclusion that can be drawn from this paper is that it sets very strict limits on the observation of easily detectable precession in neutron stars.
Physical Review Letters, 2008, DOI: 10.1103/PhysRevLett.100.081101