Seen something pale and round floating in the midst of a thunderstorm? If it lasted for a few seconds or less, it might be all in your head. Fluctuating magnetic fields, created by a nearby lightning bolt, could trick the brain into 鈥渟eeing鈥� round glowing objects, explaining at least some observations of mysterious 鈥榖all lightning鈥�.
The phenomenon is often described as a hovering orb of light 鈥� smaller than a beach ball 鈥� that lasts for a few seconds or minutes. It is thought to occur when lightning strikes the ground, but its exact cause is not clear.
Now it seems the glowing blobs may be a hallucination. Moving charges, in lightning strikes or in wire coiled around a patient鈥檚 head, generate magnetic fields. A fluctuating magnetic field induces an electric field that, if powerful enough, can make neurons fire in the visual cortex. Pale ovals, bubbles, lines, or patches are sometimes observed by patients who undergo transcranial magnetic stimulation (TMS).
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During a lightning storm, the initial stroke towards Earth is so short that we wouldn鈥檛 have time to react to it 鈥� two to three millionths of a second. After it hits the ground, a return stroke, which carries some of the charge back into the sky with a thunder-causing shockwave, lasts around 0.2 milliseconds 鈥� very near the shortest duration of the clinical stimulation.
A single flash generates an average of two to five return strokes. But some strikes can create more than 20 鈥� a protracted stream of events that could produce hallucinations lasting for multiple seconds, according to calculations by Josef Peer and , both of Innsbruck University in Austria.
Drifting through walls
In order to feel the effects of the magnetic field 鈥� but not be injured 鈥� a person would need to be 20 to 200 metres from the strike location, the researchers say. They estimate that in one per cent of close-lightning experiences, the fluctuating magnetic field could produce a hallucination in an observer.
鈥淭o our surprise, the results from long, repetitive pulses matched TMS fields astonishingly well,鈥� says Kendl.
They estimate that such apparitions account for about half of all ball lightning sightings. Hallucinations would explain how a fireball could apparently drift through a wall or window and leave it unharmed.
鈥淭he general notion that the electromagnetic fields of lightning might [affect] neural tissue is an interesting issue,鈥� says of Germany鈥檚 University of Ulm, who advised the team on the neurobiological aspects of the work.
Eye of the beholder
Kammer notes that the strong electromagnetic field is likely to cause neurons in the eye鈥檚 retina to fire before those in the visual cortex, since retinal neurons are sensitive to weaker electric fields. But it鈥檚 not clear whether this would make observers even more likely to hallucinate ball lightning, since the visual effects that retinal neurons produce are not well understood.
However, he doubts that sightings of ball lightning lasting longer than a few seconds can be explained by TMS hallucinations, since even repetitive return strokes don鈥檛 last very long.
What鈥檚 more, the serious burns and deaths attributed to ball lightning need a material explanation.
Plasma research suggests that dust-gas fireballs may be produced in a strike that hits sand or water.
And 鈥渂all lightning鈥� that doesn鈥檛 drift may be 鈥� a glowing plasma of excited electrons generated around the tops of pointed objects, such as a steeple or a ship鈥檚 mast.
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