I don't
keep a Top 40 list of SETI questions, but if I did, this one would be
perennially on the charts: "Could our experiments pick up Earth?"
In other
words, if the best of our SETI setups were suddenly transported lock, stock,
and spectrum analyzer to some star system a few tens of light-years away and
turned our way by snoopy aliens, would it be sensitive enough to detect any of
our terrestrial transmissions? Could it successfully eavesdrop on our
television, radio, radar, or cell phones?
Well, the long answer can be found in a previous
SPACE.com article, but the nutshell answer is "yes." At least for
some of our earthly emanations. In particular, the most powerful terrestrial
radars would be visible at this distance and more. Consider: When the Arecibo antenna uses its megawatt radar transmitter to blast away in the direction of an
asteroid, that signal is so strong that our most delicate SETI experiments
could detect it from 500 light-years away.
So SETI
practitioners routinely assume that at least a few garrulous aliens are
wielding really big antennas to beam really big signals in our direction. Alas,
this optimistic assumption is compromised by the so-called "synchronicity
problem" — they've got to be aimed our way when our receiving antennas are
aimed their way.
For a truly
large antenna
like Arecibo, whose beam focuses on merely 0.00001 percent of the sky, the
synchronicity problem is hardly trifling. It would be "like two hunters
accidentally hitting one another's bullets," as a correspondent put it.
Well, here's
a scheme that might beat that rap, and it depends on Earth being an obvious
target for some hunters.
Those who
have followed the ongoing search for extrasolar planets know that the tally of
worlds around other stars is nearly 300 (a number seemingly in a race with gas
prices). Most of these planets are found by observing stellar wobbles, but
others have been discovered via transits: noting the slight dimming of a star
when it is (partially) eclipsed by a planet. Now imagine turning that idea
around, and considering whether ET might see transits
in our solar system. About one percent of the cosmos lies within a
quarter-degree of the path of the Sun through the sky, and any galactic
neighbors who happen to be situated in that lucky stripe could — if they've got
the telescopes, the patience, and the funding — detect Earth's transits (they
last up to 13 hours) taking place every year. These transits constitute a
celestial clock. A clock that both we and the aliens could read.
So if an
alien society that's found our planet has an itch to get in touch, they might
send us a ping timed to arrive during the half-day or so when this mini-eclipse
occurs. And since they would be pinging only one star system at a time, they
could do so rather economically. Yes, they need to know the distance from their
world to ours with an accuracy of a few light-hours, but that's merely a matter
of good astronomical measurement. (To forestall aggressive e-mails, I note that
it's also necessary to know our Sun's motion to or from their star system, to
account for the slight displacement that takes place while their broadcast is
on route. But that is a simple thing for even modestly talented aliens to
measure.)
How would
this affect our SETI searches? Well, we could make ourselves receptive to such
deliberate pings by simply aiming our SETI antennas in the direction opposite
to the Sun. That means pointing to the east right after sunset, and tracking
the sky to the west by dawn. During the daytime, we can read a good book, or
(better yet) turn over the observing to a second SETI team on the other side of
the world.
So
eschewing all the technical mumbo-jumbo for a moment, this scheme offers
something both interesting and useful: It tells us where to point the antenna, and
when. It can mitigate the huge improbability of bullets intercepting bullets.
A proposal
to use the nascent Allen Telescope Array to search for signals coming from at
least some of the ecliptic plane has been submitted by scientists in Baltimore and elsewhere, and you may have seen stories about it in
the news. (In the interests of full disclosure, I note that I am part of
this proposal as well.) There are no guarantees that such observations will
turn up a sign of extraterrestrial presence, but — just as with international
diplomacy — being able to second-guess the motivation and behavior of others
might give us a leg up.
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