December 16, 2003
A long-held theory
comes to a shocking end.
Many young people who study science come away
with the impression that all the important questions have been answered,
and that it would be difficult or impossible to contribute to such
a well-researched body of knowledge.
It ain't so. In fact, what we don't know is
much more impressive than what we do know. And even what we do know
is always subject to revision or refinement.
There's a good reason to revisit old knowledge.
The difference between Newton and Einstein is hardly noticeable
on typical scales, but philosophically, the two theories couldn't
be farther apart. If tiny problems with Newtonian physics hadn't
cropped up, Einstein would never have developed the theory of relativity
that would lead, ultimately, to atomic power. As measurements get
better, we often find that a beloved theory, like a comfortable
but ancient sweater, is becoming somewhat threadbare and ill-fitting.
Such is the case with theories of lightning.
You may have read that lightning is formed when an electric field
builds up between positively charged clouds and the negatively charged
earth. Just as rubbing your feet across the carpet leads to a build-up
of electrons in your body, the turbulent air of a thunderstorm "rubs"
electrons off. And just as touching a door handle causes those electrons
to painfully pop out of your finger in a spark, lightning provides
a similar shocking way to get rid of the built-up charge.
Time-lapse photography captures lightning
in Norman, Oklahoma.
Photo by C. Clark, NOAA Photo Library, NOAA Central Library; OAR/ERL/National
Severe Storms Laboratory (NSSL)
That's a good theory, since there's a 300,000
volt difference across this atmospheric "battery." But there's a
problem: To create lightning, the theory requires a field ten times
stronger than what can be measured. Scientists have scratched their
heads over this for years, and have thought that there must be a
problem with the measurements. After all, it's not easy to get precise
measurements of the electric field in the middle of a thunderstorm.
Not too many people can be convinced to fly a kite in a storm anymore
(it's a miracle that Ben Franklin wasn't killed).
So scientists have resorted to firing rockets
into the thunderclouds. Behind them, these rockets trail a thin,
Kevlar-coated wire (for strength over such long distances), allowing
the scientists to probe the electrical nature of the storm. Sure
enough, the field they measured is just too weak to provoke lightning
strikes - at least according to the standard theory.
In the world most people inhabit, a failure
like this would be an extremely distressing event. But in science,
it's the exception to the rule that generates the greatest excitement.
Whenever there's an incompatibility between theory and measurement,
it's time to come up with a better theory. And that, in turn, typically
bears even sweeter fruit. In science, failure is just as useful
as success as long as it helps to distinguish between competing
So it was back to the drawing board. The standard
theory predicted that x-rays of a certain energy should accompany
lightning, so Joseph Dwyer at the Florida Institute of Technology
(which enjoys a lot of lightning) decided to look for this signature.
But what he saw was surprising. Instead of the x-ray energies he
expected, he saw something else: x-rays that are usually associated
with cosmic rays.
But cosmic rays come from outer space. Can it
be that lightning is actually triggered by exotic cosmic events,
perhaps billions of light-years away? Dwyer realized that his conclusions
gave newfound credibility to a theory advanced in the 1990s by Alexander
Gurevich of the Lebedev Institute in Moscow. Gurevich proposed that
cosmic rays could initiate a kind of electron chain reaction. When
a cosmic ray hits an atom in the air, it can knock off an electron
which flies off at high speed. It can hit another electron, which
can hit another, etc., analogous to billiard balls on a pool table.
But just like billiard balls, the electrons ultimately lose their
energy (typically to heat) and then everything settles down again.
But now imagine that we lengthen the pool table,
and tilt it very slightly. The felt holds the balls in place at
first, provided we don't tilt the table too much. Now, when you
hit the balls, they continue moving for as long as there is a table,
and in fact they accelerate since they're going downhill. When they
hit other balls, there is an increasing cascade of faster and faster-moving
balls. For electrons, substitute an electric field for the tilt
of the table, and you have what Gurevich called a "runaway breakdown."
In fact, all that's needed is about a tenth of the electric field
required by the standard theory. Oddly enough, that's just the field
that we have.
This is not to say that the story is over and
that we now know everything we need to know about lightning. There's
plenty of room for improvement, and besides lightning, there are
recently discovered electric phenomena fancifully called sprites
and elves that require even more theories. But already, we have
gained a much greater appreciation for how the earth is affected
by its cosmic neighbors - and just how much there is still to learn.
Copyright © 2000-2013 by Scott Anderson
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Here are some other suggested readings about weather