This question is directed mostly at people giving lectures on black holes, but input by other physicists or students is very much appreciated.

Do you know a good (home)-experiment with a black hole analog (such as water in a bathtub) that allows to discuss most of the pertinent concepts and features of a black hole?

To clarify what I mean by 'good' let me provide what I consider as a good experiment with a white hole analog (see exercise 2.2 in this exercise sheet), namely a hydraulic jump.

This experiment allows you to discuss in very simple terms what a white hole is (the white hole region is visibly distinct from the exterior because of their different water depths). It is also quite easy to show experimentally that you cannot send information into the white hole (in the form of shallow water wave excitations aka 'ripplons'). Moreover, the white hole exhibits some interesting features in the near horizon region (see the concentric rings in the picture on the back of the exercise sheet). Most importantly, this experiment can be done basically by anyone - there is no need for access to lasers, waveguides, Laval nozzles, supersonic flow etc.

The purpose of the hydraulic jump experiment is to provide a pedagogic introduction to white holes. I would like to have a comparable (non-Gedanken-)experiment available to introduce black holes.

  • $\begingroup$ I don't know of a "home experiment" but I can direct you to the people who work on emergent spacetimes. You can search for their work on arXiv and find many examples of analog spacetimes in BECs and other manybody systems. These are Weinfurtner, Vachaspati, Volovik, Visser, Barcelo and Liberati. The last three have a Living Reviews paper ch. 4 of which is titled "a catalogoue of models". That might be the best place to start. $\endgroup$
    – user346
    Commented Feb 6, 2011 at 19:21
  • $\begingroup$ Thanks, I know these papers, but none of them provides an experiment that is simple enough for a home experiment and yet as illuminating as the white hole experiment I mentioned. Well, let me try for fun a bounty, maybe this inspires new suggestions. $\endgroup$ Commented Feb 11, 2011 at 0:30
  • $\begingroup$ Not a home experiment, but related: Liquid analog of the Black Hole (Part 2). $\endgroup$ Commented Oct 17, 2012 at 22:11
  • $\begingroup$ Related TP.SE question: physics.stackexchange.com/q/27175/2451 $\endgroup$
    – Qmechanic
    Commented Dec 9, 2012 at 14:11

2 Answers 2


This is a water flow analogy with a black hole. Take a fish tank filled with guppies and put a large drain or siphon in the tank. This opening has to be big enough to set up a decent water flow. When the water starts flowing the fish will generally swim against the current. Assume all the guppies are the same size and generally swim at the same speed.

model configuration

Then the rate they flow is a vector sum of the water flow and the swimming velocity in the absence of water flow. This will provide a visual model for how photons move in spacetime. The flow of water is a model for the flow of spacetime points. The guppies represent photons (modulo the fact they change direction at will), and the guppies within some approximate radius will tend to be taken up by siphon.

flow of spacetime point in a Schwarzschild geometry

  • $\begingroup$ Guppie analog spacetime. LOL. +1 Though one will have to do it carefully in order to not be cruel to the guppies. $\endgroup$
    – user346
    Commented Feb 6, 2011 at 20:40
  • 1
    $\begingroup$ Thanks for the suggestion. I think this experiment is a nice Gedanken-experiment (essentially building upon Bill Unruh's seminal suggestion in the 1980ies), but as a home-experiment I find it less suitable: the ingredients are not as widely accessible as for the white hole experiment I mentioned; moreover, how do you actually "see" the horizon in the experiment? The red circle in your picture will not show up ;-), and this is again a crucial difference to the white hole experiment, where the horizon is visible. $\endgroup$ Commented Feb 7, 2011 at 7:02

How deep do you want the analogy to go? If you just want some region of a fluid where information cannot escape, all you need to do is pump liquid out of the volume quickly enough that you get fluid flows speeds that exceed the speed of sound in the fluid. I don't know how easy it is to do this and not make the flow completely turbulent, and it would only be an analogy, as a "swimmer" with sufficient strength could overcome the flow, but the basic idea of water signals not being able to escape would be present.

  • $\begingroup$ I would like the black hole region to be visibly (or otherwise) distinct from the exterior - optimal would be a time-reversed version of st-andrews.ac.uk/~ulf/fib_ripplons.jpg , the white hole experiment. Of course, one of the points of the analogy is that nothing particular happens at the horizon (except for being a point of no return), so it is not straightforward to come up with a good experiment. In other words, what I am asking for is a simple black hole analog where you make visible the black hole region one way or another. $\endgroup$ Commented Feb 7, 2011 at 7:06

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