It’s hard to make a black hole in the lab. You have to gather a bunch of mass, express until it collapses gravity on itself, work, work, work. It’s so hard to do that we’ve never done it before. However, we can make a simulated black hole with a tank of water, and it can tell us interesting things about how black holes work.
Water simulations of black holes are possible because the math that describes the behavior of water is similar to the math that describes the behavior of things like gravitational waves. Gravitational interactions take place in fluid-like ways, so you can use a fluid to study it. However, there are limitations to these water models, so you need to be careful when studying water simulations.
One problem with black hole water models is that you have to run the simulation to keep it going. Suppose you want to study how matter can be trapped through a black hole. You can simulate the black hole through a vortex of water, similar to the tornado-like vortex you sometimes see when you drain a bath. To keep the vortex running, you need to power your system so that the cartridge stays stable long enough for you to get good data.
As a result, it was widely thought that water models could not have an effect that can occur with real black holes, also known as backlash. Feedback occurs when there is an interaction where an object reacts with its environment. For example, catching a black hole increases the mass. This increase in mass changes how the black hole curves the space around it, thus slightly changing the surrounding space. Feedback is an important phenomenon, but it is subtle and difficult to study.
Recently, however, a team found that backlash can be seen in water simulation models. The research studied how a background of gravitational waves with a rotating black hole can vary. In their water model, they created a water vortex that simulates a black hole and then generated a ripple wave to the vortex. The reaction between the vortex and ripples caused the vortex to grow faster than it normally would. In this way, gravitational waves can accelerate the growth of a black hole through a back-reaction effect.
In the water simulation, the reaction was strong enough that the team would visibly drop the water level of their tank when it occurred, proving that the reaction can occur on short time scales.
Although this study is interesting in itself, the work also shows that feedback must be taken into account with many water simulations. It is generally accepted that water vortex simulations can assume a stationary background, which means that any backlash in the model can be ignored. This work shows how the assumption might not work if you study other black hole effects such as Hawking radiation.
It will take a while before real black holes can be made in the lab. Fortunately, water simulations like these still have a lot to teach us.
Reference: Goodhew, Harry, et al. “Feedback in an analog black hole experiment.” Physical overview letters 126.4 (2021): 041105