Listening to the Horizon: Probing Near-Horizon Reflectivity with GW250114
The study used a prompt signal component of GW250114 called the Direct Wave (DW) to test if a compact object's surface reflects gravitational radiation. If the object is exotic, inward radiation can reflect off its surface and leak back, creating a delayed echo whose strength depends on how reflective the surface is. No such echo was found, setting an upper limit on reflectivity below 0.4% for this event.