快猫短视频

A YOUNG couple is sitting at a picnic table in a pavilion in upstate New York
during a summer storm. As the thunder roars all around them, the couple suddenly
notice something very strange about 30 metres away. It is a glowing ball of
light, about the size of a tennis ball. And it鈥檚 heading their way.

As the ball floats into the pavilion, the transfixed couple hear it sizzling
like a freshly struck match, although there is no heat coming from it. Rising
and falling, it floats out of the pavilion, strikes the ground鈥攁nd is
gone.

Almost 40 years later, Graham Hubler of the Naval Research Laboratory in
Washington can still vividly recall his encounter that summer evening with one
of the most mysterious of all natural phenomena: ball lightning. And he鈥檚 still
hunting for a decent scientific explanation for what he saw: 鈥淚t remains an
enigma to modern science.鈥�

There was a time when scientists like Hubler would have kept stories of such
encounters to themselves. Although well-documented reports of ball lightning
date back to the Middle Ages, it was long dismissed as imaginary.

Not any more though. The sheer weight of evidence for ball lightning has
convinced many scientists of its existence鈥攏ot least because much of the
information comes from within their own ranks.

Mind-bending

The easy bit is accepting it exists鈥攏ow all they need to do is explain
it. And that is proving to be bafflingly, frustratingly, mind-bendingly
difficult. For ball lightning is a floating bag of paradoxes. It glows like a
100-watt light bulb, yet has no obvious power supply. People say it is linked to
thunderstorms, but it has rarely been linked to a lightning strike. It emits
little heat, yet can melt glass windows. It floats like a ball of gas, yet hangs
together like a blob of liquid. To have a hope of explaining the mystery of ball
lightning you must first square all of these circles.

Now two researchers, one in New Zealand, the other in the US, have taken up
the gauntlet with rival theories each believes can do the trick. And both are
now planning to put their theories to the ultimate test: the first-ever
unequivocal creation of ball lightning.

While there are big differences between the two theories, both mark a major
shift in the attitude of scientists to ball lightning. After decades of guesses,
speculation and half-baked theories, it鈥檚 become clear that you can鈥檛 crack the
mystery of ball lightning with a spot of quick-and-dirty science. It鈥檚 time to
dig in鈥攁nd dig deep.

Ask most scientists what ball lightning is, and chances are they will say
that it is some sort of extremely hot ionised gas鈥攁 plasma. Seems
plausible? After all, the thousands of observations of ball lightning reported
over the last 200 years show that it is almost invariably linked to that most
prolific source of Earth-bound plasma, the thunderstorm.

Packing tens of thousands of amperes and capable of creating temperatures in
the order of 30 000 掳C, lightning storms are tailor-made plasma factories.
And with millions of bolts searing the skies each year, it鈥檚 hardly surprising
that the occasional freak is born.

That, at least, was the rationale behind an attempt by one world-class
scientist to solve the ball lightning enigma. In 1955, the Nobel-prizewinning
Russian physicist Pyotr Kapitsa came up with an explanation for ball lightning.
Kapitsa proposed that during storms, standing waves of ultra-high frequency
electromagnetic radiation can form between clouds and the ground, transforming
lightning bolts into a tight ball of plasma.

But why should such a standing wave form at all? Still, it makes more sense
than many of the strange ideas dreamed up since: lightning-triggered nuclear
reactions, vortices of luminous air and anti-matter from outer space.

鈥淭here have been so many specialists come to this phenomenon with their own
way of looking at the physics or chemistry that what looks silly to one is not
silly to another,鈥� says John Abrahamson of the University of Canterbury, New
Zealand. The silliest he has come across is that ball lightning is an
intelligent form of life, intent on collecting samples of human DNA: 鈥淭hat was
sent to me recently in all seriousness.鈥�

Abrahamson and his colleague James Dinniss can expect sack loads of such
stuff, having attracted world-wide media coverage in February for their own
explanation of ball lightning, published in Nature. It has earned the
respect of other ball lightning researchers, like Hubler, who think it may be
the best explanation yet for all those paradoxes that have bedevilled previous
explanations.

Abrahamson and Dinniss鈥檚 theory draws on a lot more than basic
electromagnetism seasoned with a bit of plasma physics. In particular, it
features that most bog-standard source of heat and light: chemical
reactions.

For Abrahamson, the theory marks a return to some pretty old attempts to
explain ball lightning. 鈥淭he chemical reaction approach has been around for more
than a century, and goes back to the French scientist Dominique Arago.鈥�

Where it scores over the usual theories, Abrahamson says, is in offering a
familiar source of energy for ball lightning to carry around, and one that can
pump out a decent glow for the typical lifetime of the ball鈥攁nything up to
a minute or so.

The question is, which chemical reaction? In the early 1980s, researchers in
the Soviet Union started kicking around the idea that ball lightning might be
due to chemical reactions occurring on chains of tiny metal particles floating
in the air during lightning storms.

Searing heat

As ever, it was all a bit touchy-feely: for a start, what metal? But then
Abrahamson came across a paper published in Science(vol 234, p 189) in
1986, describing the effect of a lightning strike on soil.

The bizarre effects of such a strike are familiar to lightning experts, who
have found strange root-like formations of fulgurite, a glassy mineral caused by
the searing heat of lightning, at the site of strikes. The Science
paper reported the formation of something else around the roots of a tree
blasted by a lightning bolt: the metal silicon. To a scientist like Abrahamson,
this made sense; silicon is one of the commonest elements in the Earth鈥檚
crust.

But as a chemist, the very mention of silicon rang a particularly loud bell
for Abrahamson, for silicon is unstable at high temperatures, and
oxidises鈥攔eleasing chemical energy as heat and light.

The glimmerings of a new explanation for ball lightning started to form in
his mind. It goes something like this鈥攁 bolt of lightning strikes the
soil, turning the silica into pure silicon vapour. As the hot vapour cools, the
silicon condenses into a gossamer-like aerosol of nanometre-sized silicon
particles that float in the air. Electrical charges created in the heat gather
around the surface of the aerosol, binding it together and the resulting ball
begins to glow with the heat of silicon oxidation. Hey presto鈥攚e have ball
lightning.

On the face of it, this sequence of events looks like solving some of the
toughest enigmas of the phenomenon. Lightning provides the raw materials,
electric charge the glue, and oxidation chemistry the glow.

It might even account for one of the most baffling mysteries surrounding ball
lightning: if it is so hot, why does it float near the ground, rather than
shooting skyward like a hot air balloon? The answer, according to the 鈥渇luff
ball鈥� model, is that the silicon particles have about the right density to
counteract the upward buoyancy, and you get extra downward force from the
electric fields created by the lightning storm acting on the charges.

Even on fine details the calculations look promising. Abrahamson and Dinniss
found that the rate at which the fluff ball would lose heat by convection was
around 30 watts, fitting in with reports that ball lightning emits little heat.
Better still, they estimated that the brightness of a silicon fluff-ball 30
centimetres across was equivalent to a 100-watt light bulb鈥攔ight on the
mark.

Perhaps most impressive of all, they found that if the fluffball was formed
at relatively low temperatures鈥攁round 1200 掳C鈥攊t would only
start glowing towards the end of its short life. This could explain those cases
of ball lightning popping into existence 鈥渙ut of thin air鈥�: although created
straight after the bolt struck, they only later become visible.

All very neat鈥攂ut Abrahamson and Dinniss knew that there were any
number of plausible theories that disintegrated as soon as they moved off the
backs of envelopes. And, sure enough, they ran into problems when they began
putting the fluff-ball theory to the test in their lab.

Rigging up a miniature lightning discharge and firing it into soil samples,
they found they could create an aerosol of tiny particles that stuck together.
The only thing missing was the ball lightning.

Blasted earth

Small wonder: as they stepped up the power of the lightning bolts to
realistic levels, the shockwaves blasted the soil clean away, before decent
amounts of silicon vapour could be formed.

Then Abrahamson had a brainwave: real lightning blasts a metre or more into
the earth: witness the long, straggly fulgurites occasionally unearthed after a
strike. 鈥淎 fulgurite cavity can provide a sheltered storage for hot vapour until
after the shock wave has dissipated,鈥� says Abrahamson. He believes the vapour
might drift to the surface, and gather itself into the classic glowing sphere of
ball lightning (see Diagram).

The key word there is 鈥渕ight鈥�. For while their theory has impressed
researchers like Hubler, Abrahamson and Dinniss are the first to admit that
their theory is far from watertight. 鈥淭he weakest feature of the theory is the
mechanistic details of the chains of particles coming together,鈥� says
Abrahamson. However, Abrahamson and Dinniss have unearthed the results of an
experiment carried out in the late 1970s by Soviet scientists who claimed to
create glowing balls of light for a few milliseconds from vapour-filled mock-ups
of fulgurite cavities. What鈥檚 not clear is whether the real soil will oblige and
perform the same trick.

鈥淚t鈥檚 a very good example of not quite getting there,鈥� says David Turner, a
British physical chemist now living in retirement in Huntingtown, Maryland. As a
veteran ball lightning investigator, Turner has seen lots of theories come and
go. Even so, he rates the fluff-ball model higher than most. 鈥淚鈥檓 quite sure
aerosols and chemicals are involved,鈥� he says. 鈥淭o get a stable, long-lived
ball, there must be some sort of chemistry going on.鈥�

But exactly what sort of chemistry? Turner has been pondering this problem
for years, and his ideas appeared in Physics Reports in 1998 (vol 293,
p 1). They focus on what happens when strong electric fields interact with humid
air.

His theory begins conventionally enough, with an electric storm creating
pockets of ionised air. It is when the ions combine with water molecules in the
surrounding humid air that something amazing occurs, says Turner.

His thermodynamic calculations reveal that if the ions attract sufficient
water molecules, they can produce a cooling effect. The result, he says, would
be a central core of searing hot plasma, wrapped up in a cool coat of ions and
water molecules, all bound together into a ball.

Turner says his theory resolves many of the paradoxes of ball lightning, from
its buoyancy to the lack of radiant heat鈥攐n paper, at least: 鈥淚 do not
know of any reliably reported property which my model cannot explain.鈥� But, like
Abrahamson, Turner has yet to produce the stuff in the lab. So he has plans to
put this right, with the help of what he believes are crucial clues from an
unusual source: an account of events that took place 250 years ago.

Turner has discovered an intriguing report of electrical experiments carried
out in 1757 by William Constable, a gentleman-scientist and fellow of the Royal
Society. In his home lab at Burton Constable in Yorkshire, Constable was working
with Leyden jars, glass vessels coated with foil that were used to store static
electricity.

During these experiments, however, Constable found something else in his jar
besides electricity. 鈥淗e saw two glowing balls emerge, each about 2 centimetres
across,鈥� says Turner. 鈥淚n both cases the jars were destroyed, but in one case,
there was enough intact glass left to show a hole about the same size as the
产补濒濒.鈥�

Turner believes Constable was the first person to create genuine ball
lightning artificially. Many others have made the same claim since鈥攐ften
accompanied by fuzzy pictures of glowing blobs lasting a few milliseconds (See
鈥淔lash, bang, wallop鈥�).

There may be vital clues lurking in Constable鈥檚 report, Turner suspects. He
is planning experiments focusing on the chemistry that can take place when ions
created by electric fields combine with water molecules鈥攊n other words,
when electric storms hit humid air. 鈥淭he electrochemistry of humid air has not
been studied for over a century鈥攁nd what is known is absolutely
minuscule鈥�, he says.

Abrahamson is drawing up his own plans to put his theory of ball lightning to
the test. 鈥淲e are actively preparing for further experiments this year, both
with soils and with metals,鈥� he says. A particular target is creating ball
lightning from electric discharges striking the types of metal used in aircraft
construction. One of the most famous of all sightings of ball lightning occurred
in March 1963, when a glowing sphere floated down the aisle of an Eastern
Airlines aircraft. One witness was Roger Jennison, a physicist at the University
of Kent, whose subsequent report in Nature prompted many scientists to
take ball lightning more seriously.

If Abrahamson鈥檚 theory is correct, he could also tidy up some aspects of
normal lightning鈥攚hat happens when a strike passes through porous
materials like brick, for example. It could shed light on 鈥渄usty plasmas鈥�
too鈥攃louds of hot, charged particles found in space.

Perhaps in as little as 12 months, a glowing ball will float serenely around
a lab, signalling the end of a centuries-old conundrum. Only the brave would put
money on the answer emerging so easily, however. Ball lightning is likely to
prove reluctant to give up its distinction of being the original scientific
riddle, wrapped in a mystery inside an enigma.

WHILE theorists still argue over the nature of ball lightning, reports of it
being created in the lab still rear their heads from time to time.

Around the turn of the last century, the pioneer of electrical technology
Nikola Tesla used a lightning machine to create ball lightning, apparently on
demand. During the 1990s, Yoshi-Hiko Ohtsuki of Waseda University and his
colleagues regularly reported making glowing balls of plasma several centimetres
across during experiments involving electric discharges and microwaves.

鈥淔ireballs have also been produced by direct current, alternating current and
microwave discharges in gases,鈥� says veteran ball lightning investigator Stanley
Singer of Athenex Research Associates, Pasadena. 鈥淚gnition of very dilute fuel
gases in air has also formed them.鈥�

So if it鈥檚 easy to create ball lightning in the lab, why are theoreticians
still concocting more or less incredible explanations for it? Because, says
Singer, there is far more to ball lightning than a glowing ball of light. When
the home-made examples are put to the test, they either don鈥檛 behave like the
real thing, or vanish too quickly to prove that they do.

鈥淥ne of the major properties found in observations of the natural
object鈥攖hat of slow motion in the atmosphere over an extended
path鈥攈as not been reproduced satisfactorily in the laboratory,鈥� says
Singer. Even so, lab reports can help sort the wheat from the chaff. John
Abrahamson points to reports of balls of plasma being created in all kinds of
current arcs and discharges.

鈥淭he common features between them are heating of the materials present to
high temperatures, an electrical field and electrodes,鈥� he says. 鈥淭he first two
features have been focused on by plasma theorists, but they have missed the
possible contribution of material coming from the electrodes.鈥�

And that, says Abrahamson, could result in the creation of the fluffballs he
needs to prove his own theory. A detailed study of these experiments could
tighten up the theory still further, and explain why some researchers seem to
create ball lightning regularly, while everyone else fails.