By manipulating a quantum fluid, researchers could form liquid knots that never unravel. These could help us shed light on odd quantum objects from the dawn of the universe.
When tiny whirlpools called vortices form in a fluid, they can make loops that can then be knotted like a loop of string. But while a string can form knots that won鈥檛 unravel without the help of scissors, knotted vortices in a fluid break free more easily. They can explode into a diffuse swarm of vortices, or the liquid 鈥渟trings鈥� can pass through each other and re-form on the other side. at the University of Chicago and his colleagues realised that this could be prevented by going quantum.
They proved that certain knots 鈥� a trefoil, a figure of eight and a knot called Solomon鈥檚 link 鈥� would be incredibly stable if created from vortices in a fluid that displays quantum effects, specifically one called a Bose-Einstein condensate (BEC). A BEC is made from atoms almost as cold as absolute zero, which stops them acting like particles and makes them behave more like a wave.
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Because fluids like BECs display this quantum waviness, they must be described with mathematical equations that are more complex than those that apply to ordinary fluids such as water. And these equations contain rules for what a vortex can, and cannot, do. The researchers determined that for atoms with a high value of a property called quantum spin, those rules could make vortex knots that are impossible to unravel.
鈥淲e previously found that [these knots] could mathematically be true. The thing that was left open was to find some [knots] that you could realise in a practical system, something like Bose-Einstein condensate, which you can create in laboratory,鈥� says Annala.
at Aalto University in Finland, who worked on the project, says that actually making one of these knots in an experiment will require lots of technical expertise and innovation, but the researchers have been involved in similar experiments before and are certain that there is a way forward.
鈥淭he mathematics is very, very generic [here], which means that if we can learn about it in the experimentally accessible BEC, that [knowledge] could then apply to systems that we can鈥檛 bring into the lab,鈥� says at the University of East Anglia in the UK. Implications could go as far as learning more about theoretical vortex-like objects in the early universe, he says.
Journal reference:
Physical Review Letters,
