Probing Extreme (Astro)Physics with Neutron Stars

中子星是人类已知密度最大的天体，其质量是太阳的1.4倍，而球体直径只有20公里。顾名思义，它们主要含有中子，而中子是原子核的两种成分之一。事实上，人们可以把它们想象成巨大的原子核，尽管它们的平均密度大约是原子核的三倍，而且核心密度更高。我们还不知道物质在这些密度下是如何表现的，因为我们无法在实验室里达到这样的密度，而且我们还不够聪明，无法从理论上计算物质的行为。我项目的一部分旨在通过测量中子星的特性来找到答案。例如，可能在核心中，中子被挤得太紧，以至于它们溶解在它们的组成夸克中。如果发生这种情况，物质就会变得更容易压缩，在给定质量的情况下，中子星就会变小。我的总体目标是通过测量中子星的质量和半径，或者两者的组合，比如转动惯量，来检验这些假设。一个更具体的目标是找到最重的中子星。这测试了物质在高密度下的行为，因为随着中子星质量的增加，会有一个极限，超过这个极限，引力就会变得太强，物体就会坍缩，变成黑洞。这个极限取决于物质的可压缩性：可压缩性越强，最大质量越低。目前我帮助确定的最佳极限是2.0个太阳质量。我还发现了一颗可能更大的中子星，质量是太阳的2.4倍，我的目标之一就是证实或反驳这一点。***在未来一段时间里，我们一直在开发一种新技术，我们称之为“闪烁测量法”，这让我特别乐观。在这里，我们利用星际介质作为一个巨大的干涉仪，以极高的角分辨率测量射电脉冲星——这依赖于星际介质轻微偏转射电发射的事实，因此射电发射通过不同的路径到达我们。有了这项技术，我们应该能够测量脉冲星在天空中的轨道运动，使我们能够推断轨道的方向——这是测量质量所必需的——以及，原则上，精确的距离，这将有助于从引力波中确定合并的超大质量黑洞。

Neutron stars are the densest objects known to mankind, with a mass 1.4 times that of the Sun packed in a sphere with only 20 km diameter. They contain, as their name suggests, mostly neutrons, one of the two constituents of atomic nuclei. Indeed, one could envisage them as giant nuclei, although with a mean density about thrice that of atomic nuclei, and a core density that is higher still.***We do not yet know how matter behaves at these densities, being unable to reach such densities in laboratories and not yet smart enough to calculate the behaviour theoretically. Part of my programme aims at finding out, by measuring properties of neutron stars. For instance, it may be that in the core the neutrons are packed so closely together that they dissolve, in their constituent quarks. If this were to happen, it would make matter more compressible, and a neutron star would be smaller for a given mass. My general aim is to test hypotheses such as these by measuring neutron star masses and radii, or combinations of the two, such as a the moment of inertia.***A more specific aim is to find the heaviest neutron star. This tests how matter behaves at high densities because as one increases the mass of neutron star, there will be a limit beyond which gravity becomes too strong and the object collapses and becomes a black hole. This limit depends on the compressibility of matter: the more compressible, the lower the maximum mass. The current best limit, which I helped determine, is 2.0 solar masses. I also found a possibly more massive neutron star, with 2.4 solar masses, and one of my goals is to either confirm or refute that.***What makes me particularly optimistic about measure accurate properties in the coming period, is a new technique we have been developing, which we dubbed “scintillometry.” Here, we make measurements of radio pulsars at extremely high angular resolution by using the interstellar medium as a giant interferometer - relying on the fact that the interstellar medium slightly deflects radio emission, which thus reaches us through different paths. With this technique, we should be able to measure the orbital motion of the pulsars on the sky, allowing us to infer the orientation of the orbits – which is needed to measure the mass – as well as, in princple, precise distances, which will help pinpoint merging super-massive black holes from their gravitational waves.