tion about the potential obsolescence of
radio waves, optimism persists. Instead
of packing up their radio telescopes,
SETI researchers are reassessing what
they look for and how they search. While
signals from space are scarce, ideas for
alternative strategies have been proliferating like new HD programming on TV.
SETI efforts are expanding the search
to a wider range of electromagnetic channels, both at radio and optical frequencies.
Some researchers propose ne w detection
possibilities based on forecasts of what
technologies extraterrestrials hundreds
or even a million years ahead of Earthlings might possess. Other alien hunters
argue for casting an even wider net, challenging scientists to open their minds to
any unexplained signals as possible subtle
footprints of life from beyond.
After all, now would be an unfortunate
time for SETI to die. Evidence for potential life outside the solar system is piling
up in the form of planets orbiting other
stars. At the time of Drake’s first search,
no one knew whether such planets
existed. Astronomers have since found
more than 400 exoplanets, and NASA’s
Kepler mission is scanning the skies to
find any that resemble Earth.
“All of astronomy has come to
embrace this idea that there must be
life out there,” says Gerald Harp of the
SETI Institute, a nonprofit organization
in Mountain View, Calif., that has been
the powerhouse behind much of SETI.
The challenge: “Out there” is a pretty
big place.
seth shostak
The Allen Telescope Array, in California,
will scan the sky across an unprecedent-
ed range of frequencies in searching for
signals from extraterrestrials.
Drake Equation
Soon after his first search for extraterrestrials, Frank Drake proposed an
equation to estimate the number of civilizations sending detectable signals.
N the number of civilizations in the milky way galaxy with which communication might be possible
N=R*xfpxnexfℓxfixfcxL
R* the average rate of star forma- tion per year in the galaxy
fp the fraction of stars in the galaxy that have planets
ne
the average number of planets
that can potentially support life
per star with planets
fℓ
the fraction of those planets
that actually go on to develop
life at some point
fi
fc
L
the fraction of those planets
that develop intelligent life
the fraction of intelligent civiliza-
tions that develop a technology
that releases detectable signs
of their existence into space
the length of time such civiliza-
tions release these detectable
signals
The ET lottery
There are roughly 300 billion stars in
the Milky Way. Any life around any one
of those stars could be broadcasting
radio waves at frequencies ranging from
30 kilohertz to 300 gigahertz.
Covering that kind of ground — or in this
case space — requires a lot of listening.
In 1960, Drake’s Project Ozma (named
for the queen in L. Frank Baum’s Land
of Oz) used a telescope in Green Bank,
W. Va., to monitor two stars at 1.42 gigahertz, the radio frequency emitted by
the neutral hydrogen atom. Drake tuned
to this channel because, he thought, ET
would be aware of water’s significance for
life. Turns out it’s practical, too: The frequency falls within a quiet range that has
little interference from human or natural
sources. Since 1960, searches that target
individual stars and those that sweep the
entire sky have been conducted in a systematic way at frequencies between 1 and
3 gigahertz, but not much lower or higher.
That’s like trying to find a movie on
HBO by randomly flipping to just one
out of more than a hundred channels
—assuming that your cable package
includes HBO, the volume is loud enough
and you’ve picked the right time for the
movie you want to see.
“It’s a real long-shot bet,” Drake says.
One sure way to improve your chances
is to try more channels. A new project
using that strategy, the Allen Telescope
Array, is under construction northeast of
San Francisco. Of its 350 proposed tele-
scopes, 42 are operational. The array will
simultaneously monitor more than a bil-
lion channels from 0.5 to 11 gigahertz.
Beyond the radio realm
Radio telescopes, though, aren’t the only
eyes on the skies. Some searches have
been looking out into the darkness for
pulses of light, which have frequencies
higher than those of radio waves.
When Giuseppe Cocconi and Philip
Morrison suggested the possibility of
interstellar communication via electromagnetic waves in a 1959 paper in Nature,
