  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. The only assumption
in this analysis is that causality is maintained to leading order in gently
curved spacetime. Thus if echoes are actually detected, then we would face a
profound change in our understanding of physics.

General Relativity and Quantum Cosmology;High Energy Astrophysical Phenomena;High Energy Physics - Theory