Dancing Ball Lightnings in the Lab

Ball lightnings are mysterious things: Small, bright balls of fire suddenly appear during a thunderstorm, swirl around, make sometimes funny noises, and leave behind a smell of ozone. My grandmother has often told me how she once has witnessed one when she was a little girl.


The sudden appearance of a ball lightning can be quite frightening. (Hergé: The Seven Crystal Balls)

In the Tintin adventure The Seven Crystal Balls, a ball lightning destroys the mummy of an ancient Inca King which was brought to Europe by seven explorers, all of whom succumb to a mysterious spell and fall into a deep sleep interrupted only by cruel nightmares.

Since ball lightnings are not only spooky, but also very elusive, there has been a lot of speculation how to understand and explain them in a scientific way: People have suggested that it may be some ionised balls of plasma held together by their own magnetic fields, or even such exotic things as mini black holes leftover from the big bang...

A more "down to Earth" explanation was proposed in a 1999 Letter to Nature: John Abrahamson and James Dinniss conjectured that a ball lightning may be caused by the burning of small particles created when a normal lightning strikes on soil. The stroke of the lightning would vaporise and reduce silica in the soil into pure silicon vapour. As this vapour cools, they suggested, the silicon condenses into a floating aerosol bound by electric charges on its surface and forming some kind of bubble. Like the magnesium in the flashlight of an old-fashioned photographer, the pure silicon immediately burns in the oxygen of the surrounding air. This then creates the bright light and the smell of the ball lightning phenomenon, which then would be not an electric discharge, but a chemical reaction.

The explanation proposed by Abrahamson and Dinniss has now been tested by by a group of chemists and physicists of the Universidade Federal de Pernambuco in Recife, Brazil, as reported in the New Scientist. The paper Production of ball-lightning-like luminous balls by electrical discharges in silicon by Gerson Silva Paiva, Antonio Carlos Pavao, Elder Alpes de Vasconcelos, Jr. Mendes Odim, and Jr. Felisberto da Silver Eronides has been accepted for publication in PRL in January 2007. In their experiment, the Brazilian scientists vaporised parts of silicon wafers by the application of high currents of up to 140 Ampere. In the electrical arc of this discharge, similar to a lightning hitting soil, small glowing fragments of silicon could be created, and, sometimes, Ping-Pong ball sized blobs persisting for several seconds!


Small balls of burning silicon vapour dancing on the floor (Espacociencia, Olinda, Brazil, mpg file)

There is a small movie available which shows little balls of light, burning silicon vapour dancing on the floor of the laboratory (unfortunately, the movie does not seem to work with Mac...). These small little things probably wouldn't frighten good old Captain Haddock, but they are quite impressive, nevertheless.

Even if it's not completely clear yet if the riddle of the ball lightning is solved, as claims a headline in the German news magazine Spiegel - this Brazilian experiment may bring us closer to 'home made' ball lightnings as a fancy show effect for the next cocktail party!

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TAGS: PHYSICS, BALL LIGHTNING