Jan 11, 2007
The luminouballs persisted for up to 8 seconds, with smoke trails that formed spiral shapes, suggesting the balls were spinning (Images: Gerson Paivas orbs the size of ping-pong
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 (http://www.newscientist.com/channel/fundamentals/mg19225831.700-blackholes-in-your-backyard.html).
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 Antônio Pavão 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 Pavão. 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, Pavão'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.
These are by far the longest-lived glowing balls ever made in the lab. Earlier experiments using microwaves created luminous balls (http://www.newscientist.com/channel/fundamentals/mg18925384.500-the-source-of-all-ball-lightning.html), but they disappeared milliseconds after the microwaves were switched off.
"The lifetimes of our fireballs are about a hundred or more times higher than that obtained by microwaves," says Pavão, whose findings will appear in Physical Review Letters. Abrahamson is thrilled. "It made my year when I heard about it," he says. "The balls, although still small, lasted long enough to come into the mainstream of observed natural ball lightning."
Pavão's team is currently working out the chemical reactions involved in the balls' formation, and experimenting with other materials that might work too, including pure metals, alloys and sulphur compounds.