Despite the massless character of their associated particles, electromagnetic and gravitational radiation do not travel strictly on the null cone in curved spacetimes. This ‘tail’ phenomenon was first uncovered by the mathematician Jacques Hadamard in his study of partial differential equations; and introduced to physicists by Robert Brehme and Bryce DeWitt — they pointed out that there is a novel self-force on electromagnetically charged systems arising from this inside-the-light-cone signal. The gravitational counterpart of this self-force (derived by Mino, Misao Sasaki, Tanaka; and also by Quinn and Wald) is of relevance to understanding gravitational waves generated by compact bodies orbiting near supermassive black holes, such as the one residing at the center of our own galaxy.

 

I will discuss the basic physics behind this ‘tail’ effect and explore issues related to understanding the causal structure of massless signals in various spacetimes.