Well, going back in time is of course not possible, but the group around our speaker Emmanuel Fort showed that time reversal transformations can be applied to surface waves (see video below). This means that propagating waves stemming from a point source, as you create when you throw a small stone into a lake, will come back (“refocus”) to the source. These time-reversed waves are generated by shaking the water bath, and thereby changing the effective gravity of the system. Mathematically, this is an inherent property due to the time-invariance of the wave equation.
In the video below you see the stages Emission (generation of waves) – ITM (shaking of the bath) – Refocusing (waves returned to source)! And all of this is done with a source in the complex shape of the Eiffel Tower. Very French 🙂
See the full paper on Nature Physics: