Black holes are perhaps the most mysterious cosmic bodies in the universe. They’re elusive and packed with secrets. The physics that explains black holes isn’t any less esoteric either, such that only a few, if anyone, are aware of their inner workings. But sometimes, complex physics can be demonstrated with deceivingly simple, elegant experiments. Galileo Galilei proved in the 16th-century that all objects, no matter their size or weight, fall at the same speed by dropping balls weighing different amounts from the Leaning Tower of Pisa in Italy. Thomas Young proved light is a wave in 1803 by flashing light through two narrow slits on a board. And in 2017, at the University of Nottingham, UK, physicists proved the existence of a physical process around black holes using a specially designed bathtub.Bathtub physicsBlack holes are the most massive, most mysterious objects in the universe. A black hole does not have a surface, like a planet or star. Instead, it is a region of space where matter has collapsed in on itself. The gravitational pull of this region is so great that nothing can escape – not even light — which makes the black hole virtually invisible. But we definitely know these exist. We might even get the chance to image one — a black hole’s event horizon that is — after physicists made a radio observatory equivalent to the size of planet Earth for this purpose.