At the end of a nondescript corridor at the University of Nottingham is a door labeled simply: Black Hole Laboratory. Inside, an experiment is taking place in a large, high-tech bathtub that could offer unique insight into the laws of physics that govern reality.

The lab is led by Professor Silke Weinfurtner, a pioneer in the field of analog gravity, whose work has shown uncanny parallels between the mathematics describing fluid systems on Earth and some of the most extreme and inaccessible environments in the universe.

“It’s easy to be intimidated when thinking about black holes. All the predicted effects around black holes seem so strange, so strange, so different,” he says. “So it helps to remember, ‘Wait a minute. Second, it’s happening in my bathtub. Maybe it’s not so weird after all.

Previously, Weinfurtner’s team used the bathtub setup to study Hawking radiation, a process by which black holes are thought to “evaporate” and eventually disappear. She and her colleagues are currently working on a more advanced simulator, which they believe will provide even more sophisticated information about the behavior of black holes.

The flow of fluid in a plug mimics, in a mathematical sense, the curvature of space-time by the extreme gravitational field of a black hole. Photography: Fabio de Paola/The Guardian

“All of these effects are extremely beautiful and fundamentally important,” she says. “For example, will a black hole evaporate or stay there for eternity?

The basic idea is that the flow of fluid in a well mimics, in a mathematical sense, the curvature of space-time itself by the extreme gravitational field of a black hole.

“Physics is repeated in many places. It is a set of mathematical models that are very universal. And if the math is the same, the physics should be the same,” says Weinfurtner. “For me, analogues are a gift of nature. There is a whole class of systems that have the same physical processes.

Weinfurtner believes that the parallels between the two situations should be exploited to explore what happens when gravitational and quantum fields interact. Arguably, this has been the central quest of physics for the last century. Gravitational and quantum theories work well individually, and that is often enough to describe the world around us because on a large scale gravity tends to dominate, while on an atomic scale quantum effects dominate.

But in black holes, where a lot of mass is concentrated in a very small region of space, these worlds collide and there is no theoretical framework that unifies them.

“We have a great understanding of both individually, but it’s proving extremely difficult to combine these two theories,” says Weinfurtner. “The idea is that we want to understand how quantum physics behaves, in what we call curved space-time geometry.”

In the new configuration, the black hole is represented by a small vortex inside a bell of superfluid helium, cooled to -271°C. At this temperature, helium begins to demonstrate quantum effects. Unlike water, which can spin at a continuous range of speeds, the helium vortex can only spin at certain fixed values. The waves sent to the surface of the helium, tracked with nanometer precision by lasers and a high-resolution camera, represent radiation approaching a black hole.

Weinfurtner plans to use the setup to study a phenomenon known as superradiance, a seemingly paradoxical prediction that radiation that is close to a black hole (without traveling far from the event horizon) can be deflected with more energy than it had at the time. Through this process, energy can be drawn from a black hole, gradually slowing its rotation.

This phenomenon has been theoretically predicted, but never observed. And it’s possible, Weinfurtner says, that a spinning black hole could display quantum effects similar to those seen in superfluid helium.

The simulator could also be used to make predictions about Hawking radiation and gravitational wave signals sent across the universe from merging black holes that can be detected by the LIGO gravitational wave detector.

Analog gravity experiments were considered, until recently, a fringe part of the physics community, but are now gaining popularity, according to Weinfurtner. The helium black hole simulator was funded by a £5 million grant, shared between teams from seven major UK institutions (including Weinfurtner’s). Collaborators from the University of Cambridge simulate the first moments after the big bang.

source: the University of Nottingham

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