Simulated black holes in water reservoir reveal ‘backlash’ for the first time

It’s hard to make a black hole in the lab. You have to collect a bunch of mass, squeeze it until it gravitationally collapses on itself, work, work, work. It’s so hard to do that we’ve never done it. We can, however, make a simulated black hole using a water reservoir, and it can tell us some interesting things about how black holes work.

Black hole water simulations 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 happen in a fluid way, so you can use a fluid to study them. However, these water models have limitations, so you should be careful when studying the water simulations.

One problem with aquatic models of black holes is that you have to drive the simulation to keep it going. Suppose you want to study how matter can be captured by a black hole. You can simulate the black hole as a vortex of water, similar to the tornado-shaped whirlpool you sometimes see when draining a tub. To keep the vortex going, you need to power your system so that the model stays stable long enough for you to get good data.

For this reason, it was generally believed that water models could not exhibit an effect that should occur with real black holes, known as feedback. Feedback occurs when there is an interaction where an object reacts with its environment. For example, as a black hole captures matter, its mass increases. This increase in mass changes the way the black hole distorts the space around it, thus slightly altering the surrounding space. Feedback is an important phenomenon, but it is subtle and difficult to study.

Recently, however, a team discovered that the reverse reaction can be observed in water simulation models. The research investigated how a background of gravitational waves could interact with a rotating black hole. In their water model, they created a water vortex simulating a black hole, then generated a ripple of waves towards the vortex. The reaction between the vortex and the ripples made the vortex grow faster than it usually would. In this way, gravitational waves could accelerate the growth of a black hole through a feedback effect.

In the water simulation, the feedback was strong enough that the team could visibly see the water level in their reservoir drop when it did occur, proving that the reaction can occur on short scales of time.

If this study is interesting in itself, the work also shows that the feedback must be taken into account in many water simulations. Usually, it has been assumed that water vortex simulations can assume a stationary background, meaning that any feedback can be ignored in the model. This work shows how this hypothesis might not work when studying other effects of black holes such as Hawking radiation.

It will be some time before real black holes can be made in the lab. Fortunately, water simulations like this still have a lot to teach us.

Reference: Goodhew, Harry et al. “Feedback in an analog black hole experiment.” Physical examination letters 126.4 (2021): 041105