I thought Al Gore invented fire during one of his memory lapses.
Great balls of fire!
Mar 27th 2008
From The Economist print edition
Recreating the floating fireballs sometimes seen during thunderstorms
ON A stormy summer's day in the middle of the 18th century a physicist called Georg Richmann had an unfortunate encounter with a rare force of nature. He had set up a rod in his home in St Petersburg to study lightning strikes, but he got more than he bargained for when a pale blue ball of fire emerged from the rod and struck him in the head. Not only did the fireball kill him, but it also blew his shoes apart, knocked out his assistant and tore a nearby door off its hinges.
That fireball was what is now known as ball-lightning, a phenomenon as perplexing as it is spectacular. Thousands of people claim to have seen ball-lightning—glowing, roughly spherical balls of light, usually produced around the time of a thunderstorm—but the details of these sightings vary enormously. The size of the fireballs range from tennis balls to basketballs. They can be red, blue, yellow, white or even green. They may meander along the ground or drop out of the sky. They can pass through windows and emerge from fireplaces. They have even been seen to travel along aeroplane aisles in mid-flight. Philosophers and scientists, from Lucius Seneca to Niels Bohr, have studied the phenomenon and though few have met with the fate of poor Richmann, none has been able to produce a theory that can account for the full range of the lightning balls' observed characteristics.
Eli Jerby and Vladimir Dikhtyar, of Tel Aviv University, have not come up with a full explanation, either. But they may have a partial one. And, coincidentally, it involves those most fashionable of modern-day scientific objects, nanoparticles.
The story began in 2006, when Dr Jerby and Dr Dikhtyar managed to create ball-lightning-like fireballs in their laboratory. They did this by putting pieces of silicon or one of a number of other solid materials inside a shoe-box-sized cavity and zapping the material with microwaves from a metal tip. Once the material had melted, the researchers pulled the metal tip away, dragging material from the molten hotspot. That created a column of fire which then detached itself to form a floating, quivering fireball.
Now, Dr Jerby and Dr Dikhtyar have teamed up with Brian Mitchell of the University of Rennes and his colleagues at the European Synchrotron Radiation Facility (ESRF), a giant X-ray machine in France, to analyse the structure of their fireballs. By making fireballs in the path of the ESRF's X-rays, and observing how the rays are scattered, the researchers have concluded that the balls are full of particles with a diameter of about 50 billionths of a metre. In other words, 50 nanometres. In other words, nanoparticles.
This research, published recently in Physical Review Letters, supports a theory put forward several years ago by John Abrahamson, a chemical engineer from the University of Canterbury, in New Zealand. Dr Abrahamson believes that ball-lightning forms when a conventional lightning strike vaporises carbon and silicon oxides found in soil. The carbon strips the silicon of its oxygen, allowing the silicon atoms to group together into nanoparticles as they cool. These nanoparticles then oxidise back into silicon oxides, drawing oxygen from the surrounding air. As they do so, they emit heat and light.
The new work by Messrs Jerby, Dikhtyar, Mitchell and their colleagues is not proof that natural ball-lightning consists of silicon nanoparticles. For one thing, the fireballs produced last for only around 30 milliseconds after the microwave source has been turned off. Natural ball-lightning can last for many seconds. For another, the artificial fireballs seem to be glowing because of the excitation of the atoms within the vaporised material rather than because of oxidation. Moreover, Dr Abrahamson's theory fails to explain how ball-lightning can pass through windows and other solid objects, as some claim.
Excited states
If those observers are correct, some other type of explanation may be needed. One such is that a laser-like phenomenon is involved. Excited water molecules in the air emit microwaves that then stimulate the emission of further microwaves by their neighbours (this is how a laser works). According to Peter Handel of the University of Missouri that can, in the right circumstances, create a standing wave of electromagnetic energy which makes the surrounding air glow. Other, even more esoteric explanations involve antimatter and even tiny black holes left over from the Big Bang.
It is therefore, as Dr Jerby admits, too early to claim victory for the nanoparticle hypothesis. However, he is confident that, eventually, someone will recreate real, long-lived ball-lightning in the laboratory. And when they do he expects this to reveal its true nature without, it is to be hoped, killing anybody first.
economist.com |
