Knotted structures called skyrmions seem to mimic ball lightning

A new type of skyrmion simulates linked magnetic fields that may hold glowing orbs together, too

skyrmion illustration

GREAT BALL OF FIRE  Scientists created a knotted structure called a skyrmion (illustrated) that mimics the magnetic fields described in a proposed theory of ball lightning, electrical orbs sometimes observed during thunderstorms.

Heikka Valja

The physics behind a weird electrical phenomenon — glowing orbs of lightning — may be mimicked by something even stranger. A magnetic structure proposed for the natural oddity known as ball lightning makes an appearance in a newfound variety of a knotlike entity called a skyrmion, a team of scientists reports.

Typically observed during thunderstorms, ball lightning is poorly understood. Anecdotal reports describe eerily glowing spheres that float through the air for several seconds before fading (SN: 2/9/02, p. 87). That’s much longer than standard lightning strikes, which last tens of microseconds, and researchers are still struggling to explain how the fireballs persist.

One theory, proposed in the 1990s, suggests that ball lightning is a plasma held together by magnetic fields arranged in rings that link together into a knot. “Because it’s linked up in this tight way, it can’t really fall apart,” says physicist David Hall of Amherst College in Massachusetts. “That could provide a reason why ball lightning survives as long as it does.”

LINKED UP Simulated magnetic fields produced by a 3-D skyrmion are arranged into linked rings (illustrated). The arrangement matches that of the magnetic fields proposed to explain ball lightning. D. Hall
Now, Hall and colleagues have created an analog of such linked magnetic fields in a seemingly unrelated type of knotted structure, a skyrmion. Found in a variety of substances — from thin films of magnetic materials to liquid crystals — skyrmions are a kind of disturbance within matter ( SN: 2/17/18, p. 18 ). The objects can move like independent particles , shifting from place to place within a material while maintaining their knotted configuration ( SN: 10/18/14, p. 22 ). And like a tight knot in a thread, skyrmions are difficult to undo, making them relatively stable structures.

Hall and colleagues created their skyrmion in a state of matter called a Bose-Einstein condensate, composed of atoms cooled to a temperature so low that they all take on the same quantum state and begin acting as if they are one unified entity (SN: 10/13/01, p. 230). The atoms that make up the Bose-Einstein condensate each have a quantum property called spin, which makes them behave like tiny magnets.

When the scientists switched on a specially designed magnetic field, the spins arranged into a twisting structure of loops, knotting up into a configuration known as a Shankar skyrmion. That arrangement was predicted theoretically about 40 years ago, but not seen in the real world until now. While skyrmions found in thin magnetic materials are two-dimensional whirls, the new skyrmion is a 3-D beast, the researchers report March 2 in Science Advances.

Within the condensate, the spins produced something analogous to a magnetic field: The condensate behaved as if it were a charged particle being pushed around by a magnetic field when in reality no such magnetic field existed. Like the skyrmion itself, the scientists realized, the imitation magnetic field was knotted, and it matched the interlinked rings of magnetic fields proposed for ball lightning.

Eventually, studying 3-D knotted magnetic fields like those potentially present in ball lightning might help scientists devise better ways to control plasmas within future fusion reactors for generating power, the researchers suggest.

The creation of knotted structures in Bose-Einstein condensates is in its infancy, and such efforts are “very welcomed” says physicist Egor Babaev of KTH Royal Institute of Technology in Stockholm, who was not involved with the research. “People are just starting to scratch the surface of these objects.”

Physics writer Emily Conover has a Ph.D. in physics from the University of Chicago. She is a two-time winner of the D.C. Science Writers’ Association Newsbrief award.

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