In this ASMR video, black ink diffuses in water. When the video starts, the ink is so diffuse that it’s not apparent the video is playing backward. It’s only as specific structures — things like Rayleigh-Taylor instabilities, plumes, and jets — coalesce from the background that we recognize the time reversal. Though it’s probably unintentional, this makes for a neat, subtle commentary on the nature of isotropic turbulence. (Video and image credit: Wryfield Lab)
Waves contain lots of information. They are also time invariant, which means that they will behave the same regardless of whether time moves forward or backward. This isn’t a property we observe often in life since time just moves forward for us. But a new experiment has demonstrated a method of wave control that can, in a sense, roll back the clock.
To do this, the scientists created a instantaneous time mirror, or ITM. When they create a disturbance on the surface of a pool of water, it sends out capillary waves in the form of ripples. A short time later, they accelerate the pool sharply downward. This universal disturbance is their instantaneous time mirror, which generates backward-propagating ripples. Those new backward-propagating waves travel back toward the source and refocus into the shape of the initial disturbance. This works for both a simple point disturbance (top image) and for a more complicated geometry like a smiley face (bottom image). (Image credit: V. Bacot et al., source; submitted by @g_durey)
ETA: To be clear, this experiment does not refute causality. It’s more like saying that the information for the initial conditions is still carried on in the later state and that you can do something to extract that information.