Project META's Control RoomI mentioned this in Ppodcast #138 and wanted to provide more info. Here are the relevant paragraphs from Sagan's book, "The Pale Blue Dot."
"Of course, there's a background level of radio noise from Earth-radio
and television stations, aircraft, portable telephones, nearby and more
distant spacecraft. Also, as with all radio receivers, the longer you
wait, the more likely it is that there'll be some random fluctuation in
the electronics so strong that it generates a spurious signal. So we
ignore anything that isn't much louder than the background.
Any strong narrow-band signal that remains in a single channel we take
very seriously. As it logs in the data, META automatically tells the human
operators to pay attention to certain signals. Over five years we made
some 60 trillion observations at various frequencies, while examining the
entire accessible sky. A few dozen signals survive the culling. These are
subjected to further scrutiny, and almost all of them are rejected-for
example, because an error has been found by fault-detection
microprocessors that examine the signal-detection microprocessors.
What's left-the strongest candidate signals after three surveys of the
sky-are 11 "events." They satisfy all but one of our criteria for a
genuine alien signal. But the one failed criterion is supremely important:
Verifiability. We've never been able to find any of them again. We look
back at that part of the sky three minutes later and there's nothing
there. We look again the following day: nothing. Examine it a year later,
or seven years later, and still there's nothing.
It seems unlikely that every signal we get from alien civilizations
would turn itself off a couple of minutes after we begin listening, and
never repeat. (How would they know we're paying attention?) But, just
possibly, this is the effect of twinkling. Stars twinkle because parcels
of turbulent air are moving across the line of sight between the star and
us. Sometimes these air parcels act as a lens and cause the light rays
from a given star to converge a little, making it momentarily brighter.
Similarly, astronomical radio sources may also twinkle-owing to clouds of
electrically charged (or "ionized") gas in the great near-vacuum between
the stars. We observe this routinely with pulsars.
Imagine a radio signal that's a little below the strength that we
could otherwise detect on Earth. Occasionally the signal will by chance be
temporarily focused, amplified, and brought within the detectability range
of our radio telescopes. The interesting thing is that the lifetimes of
such brightening, predicted from the physics of the interstellar gas, are
a few minutes-and the chance of reacquiring the signal is small. We should
really be pointing steadily at these coordinates in the sky, watching them
for months.
Despite the fact that none of these signals repeats, there's an
additional fact about them that, every time I think about it, sends a
chill down my spine: 8 of the 11 best candidate signals lie in or near the
plane of the Milky Way Galaxy. The five strongest are in the
constellations Cassiopeia, Monoceros, Hydra, and two in Sagittarius-in the
approximate direction of the center of the Galaxy. The Milky Way is a
flat, wheel-like collection of gas and dust and stars. Its flatness is why
we see it as a band of diffuse light across the night sky. That's where
almost all the stars in our galaxy are. If our candidate signals really
were radio interference from Earth or some undetected glitch in the
detection electronics, we shouldn't see them preferentially when we're
pointing at the Milky Way.
But maybe we had an especially unlucky and misleading run of
statistics. The probability that this correlation with the galactic plane
is due merely to chance is less than half a percent. Imagine a wall-size
map of the sky, ranging from the North Star at the top to the fainter
stars toward which the Earth's south pole points at the bottom. Snaking
across this wall map are the irregular boundaries of the Milky Way. Now
suppose that you were blindfolded and asked to throw five darts at random
at the map (with much of the southern sky, inaccessible from
Massachusetts, declared off limits). You'd have to throw the set of five
darts more than 200 times before, by accident, you got them to fall as
closely within the precincts of the Milky Way as the five strongest META
signals did. Without repeatable signals, though, there's no way we can
conclude that we've actually found extraterrestrial intelligence.
Or maybe the events we've found are caused by some new kind of
astrophysical phenomenon, something that nobody has thought of yet, by
which not civilizations, but stars or gas clouds (or something) that do
lie in the plane of the Milky Way emit strong signals in bafflingly narrow
frequency bands.
Let's permit ourselves, though, a moment of extravagant speculation.
Let's imagine that all our surviving events are in fact due to radio
beacons of other civilizations. Then we can estimate-from how little time
we've spent watching each piece of sky-how many such transmitters there
are in the entire Milky Way. The answer is something approaching a
million. If randomly strewn through space, the nearest of them would be a
few hundred light years away, too far for them to have picked up our own
TV or radar signals yet. They would not know for another few centuries
that a technical civilization has emerged on Earth. The Galaxy would be
pulsing with life and intelligence, but-unless they're busily exploring
huge numbers of obscure star systems-wholly oblivious of what has been
happening down here lately. A few centuries from now, after they do hear
from us, things might get very interesting. Fortunately, we'd have many
generations to prepare."
Our book: "SolderSmoke -- Global Adventures in Wireless Electronics"http://soldersmoke.com/book.htmOur coffee mugs, T-Shirts, bumper stickers: http://www.cafepress.com/SolderSmokeOur Book Store: http://astore.amazon.com/contracross-20
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