At the end of a nondescript corridor at the University of Nottingham, there is a door labelled simply: Black Hole Laboratory. Inside, a unique experiment is taking place in a large, hi-tech bathtub that could provide valuable insights into the laws of physics governing black holes. The lab is led by Prof Silke Weinfurtner, a pioneer in the field of analogue gravity, whose work has revealed striking similarities between fluid systems on Earth and the extreme environments found in the universe.
Weinfurtner emphasizes that while black holes may seem strange and bizarre, they can be understood by relating them to everyday phenomena. She states, “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. 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 used the bathtub setup to study Hawking radiation, the process through which black holes are expected to “evaporate” and eventually vanish. Now, they are working on a more advanced simulator that will offer even more sophisticated insights into the behavior of black holes.
The flow of fluid down a plughole is mathematically similar to the curvature of space-time caused by the powerful gravitational field of a black hole. Weinfurtner explains, “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. To me, the analogues are a gift from nature. There is a whole class of systems that possess the same physical processes.”
Weinfurtner believes that by exploring the interaction between gravitational and quantum fields, they can use these analogies to better understand what happens in black holes. Combining these two fundamental theories has been a major challenge in physics for the past century. While both theories individually work well to explain the world around us, they fail to be unified in the extreme conditions of black holes.
The new setup in the laboratory represents a black hole with a small vortex in a bell jar of superfluid helium, cooled to an extremely low temperature. At this temperature, helium exhibits quantum effects. The helium vortex can only swirl at specific fixed speeds, unlike water which can spin continuously. Ripples on the surface of the helium, tracked with precise laser and camera technology, simulate radiation approaching a black hole.
Weinfurtner plans to use this setup to investigate superradiance, a paradoxical phenomenon where radiation near a black hole can be deflected out with more energy than it had initially. This process can extract energy from the black hole and gradually slow down its rotation. The simulator could also make predictions about Hawking radiation and gravitational wave signals from merging black holes that can be detected by gravitational wave detectors like LIGO.
While analogue gravity experiments were once considered on the fringes of the physics community, they are now gaining popularity. Weinfurtner’s helium black hole simulator was funded by a £5m grant, shared among teams at several top UK institutions. Critics question whether fluid systems can truly provide new insights into cosmological processes, despite the mathematical parallels. However, Weinfurtner remains undeterred, noting that breakthroughs in physics have often faced controversy initially, and her work also has implications for understanding superfluids.