Neutron stars as extreme laboratories for gravity tests

中子星:引力检验的极端实验室

Neutrons were discovered 90 years ago by James Chadwick.The concept of neutron stars was hypothesized around that time by Lev Landau,Walter Baade,and Fritz Zwicky,and it was further developed by Richard Tolman,Robert Oppenheimer,George Volkoff,and other physicists.Neutron stars are astrophysical compact objects formed after the death of massive stars(more massive than our own Sun).According to current understanding,the typical mass of neutron stars is comparable to that of the Sun,while the radius is only~10km.As its name implies,the main interior ingredients of such stars are neutrons,but they also consist of protons,electrons,muons,and presumably even more exotic particles,like hyperons,kaons,or quarks.The field of neutron stars was bolstered by the discovery of radio pulsars by Jocelyn Bell Burnell,Antony Hewish,and collaborators in 1967[1].A famous achievement brought forth by the first binary pulsar,the so-called Hulse-Taylor pulsar,is the first-ever empirical proof that gravitational waves exist in our Universe[2].In 2017,the first binary neutron star merger was observed directly via gravitational waves(ripples of spacetime)and accompanied by enormous electromagnetic follow-up observations[3].