Light rings (LRs) play an important role in gravitational wave observations and black hole photographs. In this paper, we investigate general features of LRs in stationary, axisymmetric, asymptotically flat spacetimes with or without horizons. For a nonextremal black hole, we show explicitly that there always exist at least two LRs propagating in opposite directions, where the outermost one is radially unstable. For an extremal black hole, we show that there exists at least one retrograde LR. We find that there is at least one LR which is angularly stable. The stability analysis does not involve any energy condition. Our method also applies to horizonless spacetimes and we prove that LRs always appear in pairs. Only some natural and generic assumptions are used in our proof. The results are applicable to general relativity as well as most modified theories of gravity. In contrast to previous works on this issue, we obtain much stronger results with a much more straightforward approach.

BIO
Dr. Gao received his Ph.D. degree from the University of Chicago in 2002. He has been doing research on fundamental issues of general relativity and gravitation, including classical relativity, black hole thermodynamics. In the past years, Dr. Gao has been continuously supported by National Science Foundation of China (NSFC). He is an editorial board member of the international journal Classical and Quantum Gravity since 2020.