From: LARRY KLAES (ljk4_at_msn.com)
Date: Tue Oct 28 2003 - 16:41:27 PST
MYSTERIES OF WOW
By Seth Shostak
>From Astrobiology Magazine
21 October 2003
Of the many "maybe's" that SETI has turned up in its four-decade
history, none is better known than the one that was discovered in
August, 1977, in Columbus, Ohio. The famous Wow signal was found as
part of a long-running sky survey conducted with Ohio State University's
"Big Ear" radio telescope. The Wow signal's unusual nomenclature
connotes both the surprise of the discovery and its sox-knocking
strength (60 Janskys in a 10 KHz channel, which is more than 50 thousand
times more incoming energy than the minimum signal that would register
as a hit for today's Project Phoenix.)
But is the Wow signal's notoriety merely the triumph of marketing over
substance? Could this momentary cosmic burp have really been ET, or was
it just random terrestrial interference dressed up with a sexy moniker?
For a decade, Robert Gray, a long-time, independent SETI researcher from
Chicago, has been trying to find out. Gray, like many others, was
attracted by an intriguing feature of the Wow signal: the manner in
which it rose and fell over the course of 72 seconds.
Why is this interesting? Just this: the Ohio State survey kept the
telescope fixed, letting the Earth's daily spin rotate the heavens
through its narrow beam. The "beam," of course, was the elongated patch
of sky to which the telescope was sensitive--the direction from which it
could pick up cosmic signals. The sensitivity was greatest at the
center of the beam, falling off to either side.
So as a celestial radio source passed by, it first rose in apparent
intensity as Earth's rotation brought it into the beam, reached a peak
in the beam center, and then faded away. Given the size of the Ohio
State beam, this rise and fall should take 72 seconds. And for the Wow
signal, it did.
Now contrast this with what you'd expect if the telescope had merely
been briefly flooded by an interfering terrestrial signal. The
intensity would suddenly switch full on, and then, sometime later,
switch off. Even if the interference was due to a low-Earth orbit
satellite, a source that might cause a rise and fall in intensity, you
wouldn't expect it to fortuitously last for 72 seconds. For these
reasons, the Wow signal gets high marks for being a credible candidate
for SETI.
On the other hand, there are some aspects of this seductive signal that
nudge it toward a lower grade. The Ohio State telescope actually used
two beams, situated side-by-side on the sky. Any cosmic source would
therefore be seen first in one (for 72 seconds) and then--roughly 3
minutes later--in the other (also 72 seconds.) The Wow signal failed
this simple test. It came on gangbusters in one beam, but was a no-show
in the other: suspicious and disheartening.
But as Gray and others have realized, this odd, one-beam behavior could
be caused by an alien transmission that simply went off the air during
the 3 minutes between beams. Maybe ET went on vacation, or took an
extended lunch break. If the putative aliens permanently shut down
their transmitter, then there's no chance of ever hearing the Wow signal
again. Like a single sighting of the Loch Ness monster, we would never
be able to prove what it was. But if the signal is periodic--if, for
example, the aliens are using a rotating radio beacon that sweeps the
star-studded strata of the Milky Way once every five minutes or every
five hours--then we could hope to find it by just looking again.
Robert Gray has looked again... and again. In the last decade, Gray
and his colleagues have used the Harvard META SETI system and then the
Very Large Array (VLA) to search for a reappearance of the Wow signal.
The experiment at the VLA, in particular, was an impressive effort, as
it was far more sensitive than the original Ohio State equipment and
covered more of the band. Neither attempt succeeded in retrieving the
signal, however.
Gray realized that he might be the victim of insufficient patience. The
longest of his re-observations had been 22 minutes. What if the aliens'
beacon flashed less often than once every 22 minutes? What if their
transmitter was fixed to the home planet, rotating (and flashing) once
every 20 or 30 hours?
In The Astrophysical Journal, Gray and Simon Ellingsen, of Australia's
University of Tasmania, report on new observations (partially supported
by the SETI Institute) designed to test this idea. Their new try was
made at the 26-meter radio telescope in Hobart, Tasmania. This southern
hemisphere instrument could continuously follow for most of a day the
patch of sky (in the constellation of Sagittarius) where the "Big Ear"
was pointing when it found the Wow signal. They made six 14-hour
observations, and even though their telescope was rather smaller than
the venerable Ohio State antenna, they still had sufficient sensitivity
to find signals only 5% as strong as Wow's 1977 intensity. They also
covered five times as much of the radio dial as the original "Big Ear"
telescope.
Bottom line? No dice. To quote from their article, "no signals
resembling the Ohio State Wow were detected..." Of course, if the
signal's repetition cycle were much longer than 14 hours, then even this
careful experiment could have easily missed it. But as Gray and
Ellingsen point out, if the signal were really this infrequent, then the
chance to have found it in the first place was very slim.
So was the Wow signal our first detection of extraterrestrials? It
might have been, but no scientist would make such a claim. Scientific
experiment is inherently, and rightly, skeptical. This isn't just a
sour attitude; it's the only way to avoid routinely fooling yourself.
So until and unless the cosmic beep measured in Ohio is found again, the
Wow signal will remain a What signal.
Read the original article at
http://www.astrobio.net/news/article641.html.
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