Turn on your kitchen tap and the steady stream of water will spread out into a thin circular disc when it hits the sink. This disc has an unusual property: it is surrounded by a circular “lip”, where the height of the water changes suddenly.
This so-called hydraulic jump has puzzled physicists for at least a hundred years (John Strutt, otherwise known as Lord Rayleigh, published the first mathematical description of the phenomenon in 1914). These kinds of hydrodynamic problems are notoriously difficult to tackle.
In recent years, the study of hydraulic jumps has intensified. That’s because various physicists have pointed out that hydraulic jumps are examples of much more exotic objects: white holes, the time-reversed equivalent of black holes. (A white hole is a region that can emit waves and particles but which waves and particles cannot enter.) While that’s an interesting conjecture, nobody has come up with experimental proof.