White holes, which are basically reversed black holes, may not even exist in our universe, but you can create a perfect substitute for them in your kitchen. Water flowing from a tap obeys exactly the same laws as white holes.


What are white holes? There are any number of jokes to be made here. All of them are inappropriate. Suffice it to say… Read more

White holes are theoretical structures that are essentially time-reversed black holes. While a black hole has its singularity in the future, so matter can only go into it, a white hole has its singularity in the past, meaning matter can only come out of it. We've never observed one, and the scientific consensus is we probably never will. That doesn't mean scientists can't figure out the physics of a white hole, and so we have a pretty exact idea of how they would have to act if they did exist in our universe.


That's where your kitchen sink comes into the picture. When the tap water hits the surface at the bottom of the sink, it spreads out in all direction, forming a stationary wave around the stream that is a little higher than the rest of the water. This momentary rise in the water level is known as a hydraulic jump, and if the waves inside the disc move faster than those outside the jump, it serves as the water equivalent of an event horizon.

Specifically, it's like the event horizon of a white hole, because water from the outside can't enter the region bounded by the jump, while water from the inside is obviously still able to get out. This isn't just a good way to think about the physics of a white hole - it's exactly the same laws of physics at work. Physicists Gil Jannes and Germain Rousseaux explain:

"The jump would therefore constitute a one-directional membrane or white hole. Surface waves outside the jump cannot penetrate in the inner region; they are trapped outside in precisely the same sense as light is trapped inside a black hole."


All of that assumes that the waves inside the jump really are moving faster than those outside. This is surprisingly tough to measure experimentally, as the water levels in a kitchen sink are too thin to use standard imaging methods. Instead, Jannes and Rousseaux decided to borrow from another area of physics and create a Mach cone.

As you might be able to guess, Mach cones are usually associated with breaking the sound barrier. When an object goes faster than the speed of sound, it emits sound waves in a cone shape. The same basic principle works here - place an object at the edge of the hydraulic jump to break the barrier, and the water creates the exact same type of cone. If the cone opens at an angle of precisely 90 degrees, then the waves on both sides of the jump are traveling at the same speed, which is just what would happen at the event horizon of either a white or black hole.


Jannes and Rousseaux used some fairly complex equipment to make sure their measurements were accurate - you can read more about their precise setup over at Wired, but they did indeed find that the cone was 90 degrees at the ridge of the hydraulic jump. That represents the first experimental proof that a kitchen sink is a match for a white hole.


They point out that this means "the concept of [event] horizons is not limited to relativity", and their colleague Ulf Leonhardt does them one better, pointing out this means anyone can create an event horizon in their own home:

"Germain Rousseaux and his team used sophisticated equipment and did very careful measurements, but at its heart, the experiment is based on a simple idea everyone can understand and try at home." [arXiv via Wired]