Ball lightning in the lab

Seen infrequently and never scientifically studied in nature, the meandering globes of light known as ball lightning have nevertheless been reported thousands of times over the past few centuries, usually in the warm summer months when thunderstorms abound. Many dozens of theories have arisen, but few can explain the most puzzling properties of the atmospheric phenomenon—the balls, which can range in size from 1 cm to 1 m, last up to 10 seconds and move unpredictably through the air. They have even entered houses through chimneys and squeezed through small openings. One recent theory says that ball lightning arises from silicon nanoparticles that form in the soil when silicon oxides react with carbon. When a lightning strike vaporizes the oxides into metallic silicon, the vapor subsequently condenses in the air, electrostatically bound and glowing with the heat of oxidation. A team of physicists and chemists in Brazil has now given credence to that so-called Abrahamson–Dinniss theory. They mounted a silicon wafer on a steel plate and completed an electric circuit by touching the wafer with a movable tungsten or graphite electrode. When the electrode was slowly removed, an electrical arc formed with hot tiny silicon fragments flying everywhere. But at a separation of about 1–2 mm, luminous spheres formed and also flew off. The resulting balls were 1–4 cm across, had lifetimes up to 8 seconds, moved at speeds of 5–30 cm/s, and decayed with no trace. The figures show one such ball, at 80-ms intervals, passing through a small gap under an electrical conductor. The experiments were done in conditions not very different from those found in nature: room temperature, normal atmospheric pressure, and 70% relative humidity. (G. S. Paiva et al. , Phys. Rev. Lett. , in press.)