Ball lightning could soon lose its status as a mystery, now that a team in Brazil has cooked up a simple recipe for making similar eerie orbs of light in the lab, even getting them to bounce around for several seconds.
Thousands of people have reported seeing ball lightning, a luminous sphere that sometimes appears during thunderstorms. It is typically the size of a grapefruit and lasts for a few seconds or minutes, sometimes hovering, even bouncing along the ground.
One eyewitness saw a glowing ball burn through the screen door of a house in Oregon, navigate down to the basement and wreck an old mangle, while in another report, a similar orb bounced on a Russian teacher's head more than 20 times before vanishing.
One theory suggests that ball lightning is a highly ionised blob of plasma held together by its own magnetic fields, while an exotic explanation claims the cause is mini black holes created in the big bang (New Scientist, 23/30 December 2006, p 48).
A more down-to-earth theory, proposed by John Abrahamson and James Dinniss at the University of Canterbury in Christchurch, New Zealand, is that ball lightning forms when lightning strikes soil, turning any silica in the soil into pure silicon vapour. As the vapour cools, the silicon condenses into a floating aerosol bound into a ball by charges that gather on its surface, and it glows with the heat of silicon recombining with oxygen.
To test this idea, a team led by Ant?nio Pav?o and Gerson Paiva from the Federal University of Pernambuco in Brazil took wafers of silicon just 350 micrometres thick, placed them between two electrodes and zapped them with currents of up to 140 amps. Then over a couple of seconds, they moved the electrodes slightly apart, creating an electrical arc that vaporised the silicon.
The arc spat out glowing fragments of silicon but also, sometimes, luminous orbs the size of ping-pong balls that persisted for up to 8 seconds. "The luminous balls seem to be alive," says Pav?o. He says their fuzzy surfaces emitted little jets that seemed to jerk them forward or sideways, as well as smoke trails that formed spiral shapes, suggesting the balls were spinning. From their blue-white or orange-white colour, Pav?o's team estimates that they have a temperature of roughly 2000 kelvin. The balls were able to melt plastic, and one even burned a hole in Paiva's jeans.
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