Ball lightning is an odd and obscure phenomenon; reports describe glowing globes the size of footballs, which float along at walking speed, sometimes entering buildings, and whose existence sometimes comes to an end with a small explosion. Observations are generally associated with thunderstorms. I’ve never seen ball lightning myself, though when I was a physics undergraduate at Cambridge in 1982 there was a famous sighting in the Cavendish Laboratory itself. This rather elusive phenomenon has generated a huge range of potential explanations, ranging from the exotic (anti-matter meteorites, tiny black holes) to the frankly occult. But there seems to be growing evidence that ball lightning may in fact be the manifestation of slowly combusting, loose aggregates of nanoparticles formed by the contact of lightning bolts with the ground.
The idea that ball lightning consists of very low density aggregates of finely divided material originates with a group of Russian scientists. A pair of scientists from New Zealand, Abrahamson and Dinnis, showed some fairly convincing electron micrographs of chains of nanoparticles produced by the contact of electrical discharges with the soil, as reported in this 2000 Nature paper (subscription required for full paper). Abrahamson’s theory is also described in this news report from 2002, while a whole special issue of the Royal Society’s journal Philosophical Transactions from that year puts the Abrahamson theory in context with the earlier Russian work and the observational record. The story is brought up to date with some very suggestive looking experimental results reported a couple of weeks ago in the journal Physical Review Letters, in a letter entitled Production of Ball-Lightning-Like Luminous Balls by Electrical Discharges in Silicon (subscription required for full article), by a group from the Universidade Federal de Pernambuco in Brazil. In their very simple experiment, an electric arc was made with a silicon wafer, in ambient conditions. This produced luminous balls, from 1- 4 cm in diameter, which moved erratically along the ground, sometimes squeezing through gaps, and disappeared after 2 – 5 seconds leaving no apparent trace. Their explanation is that the discharge created silicon nanoparticles which aggregated to form a very open, low density aggregate, and subsequently oxidised to produce the heat that made the balls glow.
The properties of nanoparticles which make this explanation at least plausible are fairly familiar. They have a very high surface area, and so are substantially more reactive than their parent bulk materials. They can aggregate into very loose, fractal, structures whose effective density can be very low (not much greater, it seems in this case, than air itself). And they can be made a variety of physical processes, some of which are to be found in nature.