Duck in the Bathtub

(© Terence Keller -

NOTTINGHAM, United Kingdom — For kids, bath time can be an entertaining experience — especially when they watch that cool vortex take all the water down the drain. For scientists, bath time may also unlock the secrets of powerful black holes out in space! Researchers from the University of Nottingham say experiments draining tanks of water, just like the typical bathtub, have successfully simulated how black holes all over the universe evolve.

Black holes are areas of space where gravity is so strong nothing, not even light, can escape their pull. Using a draining vortex, just like what happens when someone pulls the plug in a bathtub, scientists recreated a phenomenon they call backreaction. It’s the first study showing that black holes created by the fields surrounding them can be simulated in a lab.

What do black holes and drain water have in common?

Researchers say when waves of water swirl into the vortex, it’s mimicking the effect of a black hole’s gravitational pull. Water tank simulators continue to gain popularity with scientists, who use them to test gravitational phenomena in a lab setting.

Systems like these have even helped researchers to observe Hawking radiation in a stimulated black hole, using quantum optics. This radiation is named for legendary physicist and black hole expert Professor Stephen Hawking.

Using the water tank method, study authors demonstrated for the first time that when waves enter the black hole, the properties of that singularity actually change drastically. The British team explain this backreaction in very simple terms. When waves of water near the drain, they basically push more water down the drain hole. This causes the total amount of water in the tank to decrease. When this happens, it changes the water height, mimicking the change in properties occurring in black holes sucking in matter.

“For a long time, it was unclear whether the backreaction would lead to any measurable changes in analogue systems where the fluid flow is driven, for example, using a water pump. We have demonstrated that analogue black holes, like their gravitational counterparts, are intrinsically backreacting systems. 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 analogue black hole,” explains lead author and post-doctoral researcher Dr. Sam Patrick in a university release.

“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! 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.”

Money going ‘down the drain’

Thanks to a funding boost, the team at Nottingham is now working on a three-year project which aims to explore the physics of the early universe and creation of black holes.

Researchers will have the aid of quantum simulators which can mimic the conditions during this time in space. They’ll also be continuing these “bathtub experiments” with a hybrid superfluid optomechanical system, which simulates black holes.

The study appears in the journal Physical Review Letters.

About Chris Melore

Chris Melore has been a writer, researcher, editor, and producer in the New York-area since 2006. He won a local Emmy award for his work in sports television in 2011.

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