It's the
mother of all earthly radio transmissions, a broadcast that's been on the air
for billions of years. However, and despite the long run, it's one radio
program that you'll probably give a pass: it sounds like Fast-Finger Freddie
twisting the shortwave dial at a few hundred RPM.
This
cacophony of radio static from Earth is known as Auroral Kilometric Radiation
(so-called because the wavelength of the emission is typically kilometers long).
AKR is generated when fast-moving particles boil off the sun, gush into space,
and then get manhandled by Earth's magnetic field. The same circumstance
accounts for the aurora borealis —
those ghostly celestial displays that quietly amuse bored Canadians and
insomnious polar bears.
But AKR,
which has been in the
news lately, has caught the notice of many space fans. They see it as just
the sort of signal that could tip off aliens about Earth's existence, a kind of
radio fingerprint of our world. And if that's possible, then perhaps we might
use our radio telescopes to detect the AKR billowing off ET's planet.
Adding to
the allure, AKR is no pipsqueak signal: The power involved is measured in
millions of watts.
So, could
aliens be tuning into this, the most powerful radio transmitter on Earth?
In
principle they could, but in practice this would be tougher than a three-dollar
steak. One problem for any extraterrestrial listeners is confusion with other
solar system emitters. Both the sun and Jupiter — each of which have magnetic
fields many thousands of times stronger than Earth's — belch more powerful
natural signals into space than we do. (Mercury, Saturn, Uranus and Neptune are
also members of this cosmic chorus.) The aliens would have to wield highly
directional antenna arrays to pick out Earth from this noisy crowd.
Of course,
the aliens may have such sophisticated instruments. But we don't. Our best
low-frequency radio telescopes couldn't find an alien Earth's AKR emissions.
Even so, radio astronomers are still hoping to detect the static from so-called
"hot Jupiters" around nearby stars. Being so close to their suns,
these blistering behemoths would be showered by charged particles. The
particles would twist and shout once they got embroiled in the planet's
magnetic field, producing signals that are many thousands of times more
powerful than the AKR from our own planet. But hot Jupiters are a special case.
So you
might wonder, if finding multi-megawatt natural transmissions from Earth-like
worlds is difficult, why do we bother to look for ET's radio broadcasts with
our SETI experiments? The answer is two-fold: First, deliberate transmissions
could be far more tightly beamed (thereby wasting less energy on a signal that
largely dissipates into empty space). Second, an intentional radio ping might
be narrow-band.
The last
point is important. Nature, lacking a degree in electrical engineering,
produces radio signals that are wastefully wide-band. Think of lightning: the
color is close to white (in other words, the light's spread over all
wavelengths), and if you have your AM radio on during a storm, you'll note that
the associated radio crackle is also wide-band. You can hear it no matter where
you tune. While there are exceptions to this general behavior (both
interstellar masers and the 21 cm emission from neutral hydrogen come to mind),
it's usually the case that nature's radio transmissions — from quasars
to pulsars to AKR — are spread all over the dial.
On the
other hand, ET, benefitting from higher education, could build a transmitter
capable of corralling a lot of energy into a very narrow band. This
would produce a signal that's far easier to detect than nature's broadcast
splatter. If you do the numbers, you'll see that — at light-years distance –
AKR, despite all the megawatts, is enormously harder to find than even a modest
radar installation.
So yes,
it's interesting that an Earth-like planet will have its own, thoroughly
natural radio signature. Indeed, as our radio telescopes improve, this is one
more phenomenon that legions of grad students will no doubt study. But it is
the deliberate, carefully-engineered signal — a signal able to stand head and
shoulders above this noisy background — that could reveal something far more
precious than the rowdy interplay of magnetic fields and charged particles: the
presence of intelligent life.
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