Build a white hole in your kitchen sink
A white hole is the opposite of a black hole in that matter can only come out of a white hole and never penetrate it’s event horizon from the outside. It turns out that you can make a simple hydrodynamic analogy in your sink with exactly the same mathematical description as a white hole. Experimenters have shown for the first time that this truly is a white hole in the sense that nothing can penetrate from the outside.
To make a white hole in your sink all you need to do is run water at a high enough speed that it carves out a circle around where it hits the sink. The circle is a shockwave where water from the outside is moving too slowly to be able to penetrate any further upstream.
In the photo, there is a needle stuck into the stream, showing what is called a Mach cone. Mach cones form behind anything moving at supersonic speeds, and it is the Mach cone hitting you that you hear as a sonic boom when a supersonic plane flies overhead.
One of the predictions about a white hole is that Mach cones should form around objects inside the hole with the angle of the Mach cone getting greater until you get to the very edge where it forms a right angle. Testing these angles is the process the experimenters used to show that the white hole analogy holds up exactly.
In this series of images, the needle is placed at different distances out from the source of water and you can see the angle getting larger as it moves toward the boundary and then outside the boundary there is no Mach cone, as predicted.
Knowing that the mathematics and properties of white holes really do stand up, it seems that these hydrodynamic systems could be more useful than previously thought in understanding the behavior of black holes, which are just white holes run in reverse.