Jennifer Oullette in Gizmodo:
Theoretical physicists have been predicting that it should be possible for knots to form in quantum fields for decades, but nobody could figure out how to accomplish this feat experimentally. Now an international team has managed to do just that, tying knots in a superfluid for the very first time by manipulating magnetic fields.
Led by David Hall, a physicist at Amherst College, and Mikko Möttönen of Aalto University in Finland, the group describes their groundbreaking achievement in a new paper in Nature Physics. It’s tough to visualize these exotic objects, but they are essentially particle-like rings or loops in a quantum field connected to each other exactly once. A mathematician might not consider these structures to be true knots; typically a knot is defined as a knotted circle, like a pretzel, while a rubber band would be considered an “un-knot.” Hall and Möttönen prefer to think of their structures as knotty solitons.
And what’s a soliton, you may ask? There is a certain type of traveling wave that keeps rolling forward at a constant speed without losing its shape. That is a soliton, and such objects also show up in quantum field theory. As I wrote in a 2014 article for Quanta, “Poke a quantum field and you will create an oscillation [wave] that usually dissipates outward, but configure things in just the right way and that oscillation will maintain its shape” — just like a traveling wave.