  [abridged abstract] No, within a broad class of scenarios. With the advent of
gravitational-wave (GW) astronomy, environmental effects on the GW signal will
eventually have to be quantified. Here we present a wide survey of the
corrections due to these effects in two situations of great interest for GW
astronomy: the black hole (BH) ringdown emission and the inspiral of two
compact objects. We take into account various effects such as: electric
charges, magnetic fields, cosmological evolution, possible deviations from
General Relativity, firewalls, and various forms of matter such as accretion
disks and dark matter halos. Our analysis predicts the existence of resonances
dictated by the external mass distribution, which dominate the very late-time
behavior of merger/ringdown waveforms. The mode structure can drastically
differ from the vacuum case, yet the BH response to external perturbations is
unchanged at the time scales relevant for detectors. This is because although
the vacuum Schwarzschild resonances are no longer quasinormal modes of the
system, they still dominate the response at intermediate times. Our results
strongly suggest that both parametrized and ringdown searches should use at
least two-mode templates. Our analysis of compact binaries shows that
environmental effects are typically negligible for most eLISA sources, with the
exception of very few special extreme mass ratio inspirals. We show in
particular that accretion and hydrodynamic drag generically dominate over
self-force effects for geometrically thin disks, whereas they can be safely
neglected for geometrically thick disk environments, which are the most
relevant for eLISA. Finally, we discuss how our ignorance of the matter
surrounding compact objects implies intrinsic limits on the ability to
constrain strong-field deviations from General Relativity.
