It has been argued that ultracompact objects, which possess light rings but no horizons, may be unstable against gravitational perturbations. In this seminar, I will talk about our recent work on the light rings in quasi- black hole solutions which is a family of horizonless spacetimes whose limit is the extremal Reissner- Nordstr¨om black hole. We find a critical parameter at which the light rings just appear. By calculate the quasinormal modes of the quasi- black holes, we find that long- lived modes survive for the range where light rings exist, indicating that horizonless spacetimes with light rings are unstable. Our work provides a strong and explicit example that light rings could be direct observational evidence for black holes.
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