In a generic curved spacetime, massless particles such as photons and gravitons do not travel solely along null geodesics. Rather, they propagate both on and inside the light cone of their sources. This inside-the-null-cone propagation is called the tail effect. I will explain how the tail effect induces a self force for compact objects orbiting, and subsequently plunging into, super-massive black holes at the center of many galaxies. Because such Extreme-Mass-Ratio-Inspiral (EMRI) systems produce gravitational waves that could potentially be detected by future space-based observatories such as NGO/eLISA, it is important to understand the tail effect in black hole spacetimes.

I am interested in whether the causal structure of wave propagation in 4D Schwazschild spacetime can be elucidated in terms of waves traveling in 6D Minkowski, since the former can be embedded in the latter. I will provide two examples where such an embedding perspective is useful for understanding wave tails: odd dimensional Minkowski, as well as de Sitter spacetime (of any dimension).

I will also discuss two other examples of wave tails: photons traveling in a slightly inhomogeneous universe like ours; and the spontaneous quantum pair production of massless charged fermions in 2D, due to the presence of an external electric field.