Scientists Demonstrate Black Hole Evolution With “Bathtub” Plug Hole Simulation

The water black hole, which is basically a large bathtub. University of Nottingham

Black holes are mysterious, and there's a lot we still don't know about them. If there's one thing we do know, it's that nothing can escape them, including light, which makes them hard to study. Over the last decade, researchers have been able to recreate “black holes” in the lab. These are not the gravitational juggernauts found in space but analogous versions that behave like them.

In 2017, a team at the University of Nottingham in the UK first created a "water black hole". Now they've announced they've made it even more similar to its cosmic cousins. Using the upgraded setup, the team has demonstrated the phenomenon of backreaction in an analogous black hole. This is the first study to demonstrate that the evolution of black holes from the fields around them can be simulated in the lab.

The concept of backreaction comes from Einstein's theory of general relativity. It underpins the interaction of a black hole with the fields around it as its mass and spin changes – not something that can be easily translated into simulations. After all, a water black hole is essentially just a water tank with a plughole. Could they really show this complex property? As reported in Physical Review Letters, the answer is yes, and the effect is significant.

Water black hole simulations are relatively straightforward. It's a tank of water with a draining vortex, like when you pull the plug in the sink. When unplugged it creates a “black hole” at its center. Waves are created in the tank simulating the interaction between the black hole and the universe around it, and any waves that encounter the "black hole" get sucked in, sending more water down the drain. This changes the property of the black hole (it's now “bigger") and these changes don't vanish. 

When the waves get close to the drain they push more water down the plughole, causing the total amount of water in the system to decrease. This results in a lasting change of the properties of the "black hole", which the researchers say shows that when waves are sent into a "black hole", the property of the object itself changes.

“For a long time, it was unclear whether the backreaction would lead to any measurable changes in analog systems where the fluid flow is driven, for example, using a water pump. We have demonstrated that analog black holes, like their gravitational counterparts, are intrinsically backreacting systems,” lead author Dr Sam Patrick from the University of Nottingham said in a statement.

“We showed that waves moving in a draining bathtub push water down the plug hole, modifying significantly the drain speed and consequently changing the effective gravitational pull of the analog black hole.”

In fact, the effect was even more evident than expected.

“What was really striking for us is that the backreaction is large enough that it causes the water height across the entire system to drop so much that you can see it by eye!" Dr Patrick added. "This was really unexpected. Our study paves the way to experimentally probing interactions between waves and the spacetimes they move through. For example, this type of interaction will be crucial for investigating black hole evaporation in the laboratory.”

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