University of Chicago physicists have succeeded in creating a vortex knot—a feat akin to tying a smoke ring into a knot. Linked and knotted vortex loops have existed in theory for more than a century, but creating them in the laboratory had previously eluded scientists.
Vortex knots should, in principle, be persistent, stable phenomena. “The unexpected thing is that they’re not,” said Dustin Kleckner, a postdoctoral scientist at UChicago’s James Franck Institute. “They seem to break up in a particular way. They stretch themselves, which is a weird behavior.”
This behavior culminates in what the UChicago researchers call “reconnection events.” In these events, the loops elongate, begin to circulate in opposite directions, move toward each other and collide (the reconnection). Parts of the vortices then annihilate other parts, changing their configuration from linked or knotted into one that is unlinked or unknotted.
Kleckner and William Irvine, assistant professor in physics, report their findings on the creation and dynamics of vortex rings in Nature Physics, published online Sunday, March 3. Their work relates to deep questions in a variety of physics subfields, including turbulence, plasma physics, ordinary fluids and the more exotic superfluids. Knotted structures are thought to occur in all these phenomena but are difficult or impossible to observe.
“We look at plasma physics and turbulence every day in the sun,” Irvine said, yet such phenomena pose longstanding, unsolved scientific puzzles. But knots may offer a means of untangling the complicated behavior of the electrically charged gas in plasma flows, for example, and for understanding the energy transport of complex flows in regular fluids and superfluids.
Conservation of quantities like energy and momentum are among the most important principles in physics. In many systems, the degree of “knottedness” can be represented as a precise physical quantity that also is conjectured to be conserved. “If confirmed, this would deepen our understanding of the dynamics and connections between many disparate physical fields,” Irvine said. “We don’t know if it's true or not, but I think we can finally test this in experiment. There’s actually around 50 years of theory on this subject with no clean experiments.”
Colliding smoke rings
Irvine became interested in knot physics as a postdoctoral scientist at New York University after watching a smoke-ring demonstration in Washington Square Park. He wondered if he could get colliding smoke rings to become tangled. After unsuccessfully trying to make them himself he learned that others had tried before and failed.
“At some point the enthusiasm wanes and you worry about whether there’s a very good reason why nobody has ever done this,” Irvine said. “But sometimes going into a new field with a little naivete can be helpful.”