At the end of a nondescript corridor at the University of Nottingham is a door labelled simply: Black Hole Laboratory. Within, an experiment is under way in a large, hi-tech bathtub that could offer a unique glimpse of the laws of physics that govern the real thing.
The lab is run by Prof Silke Weinfurtner, a pioneer in the field of analogue gravity, whose work has demonstrated uncanny parallels between the mathematics describing fluid systems on Earth and some of the most extreme and inaccessible environments in the universe.
“It is easy to get intimidated when thinking about black holes. All the effects predicted to occur around black holes seem so bizarre, so weird, so different,” she says. “Then it helps to remind yourself, ‘Wait a second, it happens in my bathtub. Maybe it’s not so strange after all.’”
Previously, Weinfurtner’s team has used the bathtub setup to investigate Hawking radiation, a process by which black holes are predicted to “evaporate” and eventually disappear. She and colleagues are now working on a more advanced simulator, which they believe will provide even more sophisticated insights into the behaviour of black holes.
“All these effects are tremendously beautiful and of fundamental importance,” she says. “For example, does a black hole evaporate or will it just stay there for eternity?”
The basic idea is that the flow of fluid down a plughole mimics, in a mathematical sense, the curving of space time itself by the extreme gravitational field of a black hole.
“Physics repeats itself in many places. It’s a set of mathematical models that are very universal. And if the maths is the same, the physics ought to be the same,” Weinfurtner says. “To me, the analogues are a gift from nature. There is a whole class of systems that possess the same physical processes.”
Weinfurtner believes the parallels between the two situations be exploited to explore what happens when gravitational fields and quantum fields interact. This has been arguably the central quest in physics for the past century. Gravitational and quantum theories work well individually – and this is often sufficient to describe the world around us because at large scales gravity tends to dominate, while at atomic scales quantum effects rule.
But in black holes, where a lot of mass is crammed into a very small region of space, these worlds collide and there is no theoretical framework that unifies the two.
“We have a great understanding of both individually, but it turns out extremely hard to combine these two theories,” says Weinfurtner. “The idea is that we want to understand how quantum physics behaves, on what we call a curved space time geometry.”
In the new setup, the black hole is represented by a tiny vortex inside a bell jar of superfluid helium, cooled to -271C. 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 swirl at certain fixed values. Ripples sent across the surface of the helium, tracked with nanometre precision by lasers and a high-resolution camera, represent radiation approaching a black hole.
Weinfurtner is planning to use the setup to investigate a phenomenon known as superradiance, a seemingly paradoxical prediction that radiation that comes into the vicinity of a black hole (without straying over the event horizon) can be deflected out with more energy that it had on the way in. Through this process, energy can be extracted from a black hole, causing its rotation to gradually slow down.
This phenomenon has been predicted theoretically, but never observed. And it is possible, Weinfurtner says, that a rotating black hole could display quantum effects something like those seen in superfluid helium.
The simulator could also be used to make predications 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.
Analogue gravity experiments were, until recently, considered a fringe element of the physics community, but are now growing in popularity, according to Weinfurtner. The helium black hole simulator was funded by a £5m grant, shared across teams at seven top UK institutions (including Weinfurtner’s). Collaborators at the University of Cambridge are simulating the first moments after the big bang.
The approach has critics, who question whether, despite remarkable mathematical parallels, fluid systems can really provide fundamentally new insights into cosmological processes. Weinfurtner is unfazed, noting that gravitational wave physics had detractors until the breakthrough detection was made and that her work also has value in the field of superfluids.
“Many things have been controversial in the past, which we now take for granted,” she says.
