Are there echoes of gravitational waves?

Understand

In several approaches to evading the information paradox, the semiclassical black hole is replaced by an Exotic Compact Object (ECO).

• It has been conjectured that gravitational waves emitted by the merger of ECOs can reflect off the ECOs and produce a detectable `echo'.
• We argue that while a part of the wave can indeed reflect off the surface of an ECO, this reflected wave will get trapped by a new closed trapped surface produced by its own backreaction.
• Thus no detectable signal of the echo will emerge to infinity.