Waves on a string, ripples on a pond are transverse waves generated by mechanical energy and in the simplest form oscillations. Sound also is a form of energy and basically oscillations. What makes sound unique in the fact that it is longitudinal ?

There are a few coordinate systems to describe the inside of the BH: Schwarzshild, behaves oddly inside the BH Kruskal-Szekeres, space and time do not swap Gullstrand-Painlevé, t is the time measured on a clock held by an observer falling freely into the black hole Eddington-Finkelstein, incom...