Good Timing

I don'tkeep a Top 40 list of SETI questions, but if I did, this one would beperennially on the charts: "Could our experiments pick up Earth?"

In otherwords, 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 andturned our way by snoopy aliens, would it be sensitive enough to detect any ofour terrestrial transmissions? Could it successfully eavesdrop on ourtelevision, radio, radar, or cell phones?

Well, the long answer can be found in a article, but the nutshell answer is "yes." At least forsome of our earthly emanations. In particular, the most powerful terrestrialradars 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 anasteroid, that signal is so strong that our most delicate SETI experimentscould detect it from 500 light-years away.

So SETIpractitioners routinely assume that at least a few garrulous aliens arewielding really big antennas to beam really big signals in our direction. Alas,this optimistic assumption is compromised by the so-called "synchronicityproblem" — they've got to be aimed our way when our receiving antennas areaimed their way.

For a trulylarge antennalike Arecibo, whose beam focuses on merely 0.00001 percent of the sky, thesynchronicity problem is hardly trifling. It would be "like two huntersaccidentally hitting one another's bullets," as a correspondent put it.

Well, here'sa scheme that might beat that rap, and it depends on Earth being an obvioustarget for some hunters.

Those whohave followed the ongoing search for extrasolar planets know that the tally ofworlds around other stars is nearly 300 (a number seemingly in a race with gasprices). Most of these planets are found by observing stellar wobbles, butothers have been discovered via transits: noting the slight dimming of a starwhen it is (partially) eclipsed by a planet. Now imagine turning that ideaaround, and considering whether ET might see transitsin our solar system. About one percent of the cosmos lies within aquarter-degree of the path of the Sun through the sky, and any galacticneighbors who happen to be situated in that lucky stripe could — if they've gotthe telescopes, the patience, and the funding — detect Earth's transits (theylast up to 13 hours) taking place every year. These transits constitute acelestial clock. A clock that both we and the aliens could read.

So if analien society that's found our planet has an itch to get in touch, they mightsend us a ping timed to arrive during the half-day or so when this mini-eclipseoccurs. And since they would be pinging only one star system at a time, theycould do so rather economically. Yes, they need to know the distance from theirworld to ours with an accuracy of a few light-hours, but that's merely a matterof good astronomical measurement. (To forestall aggressive e-mails, I note thatit's also necessary to know our Sun's motion to or from their star system, toaccount for the slight displacement that takes place while their broadcast ison route. But that is a simple thing for even modestly talented aliens tomeasure.)

How wouldthis affect our SETI searches? Well, we could make ourselves receptive to suchdeliberate pings by simply aiming our SETI antennas in the direction oppositeto the Sun. That means pointing to the east right after sunset, and trackingthe 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 ofthe world.

Soeschewing all the technical mumbo-jumbo for a moment, this scheme offerssomething both interesting and useful: It tells us where to point the antenna, andwhen. It can mitigate the huge improbability of bullets intercepting bullets.

A proposalto use the nascent Allen Telescope Array to search for signals coming from atleast some of the ecliptic plane has been submitted by scientists in Baltimore and elsewhere, and you may have seen stories about it inthe news. (In the interests of full disclosure, I note that I am part ofthis proposal as well.) There are no guarantees that such observations willturn up a sign of extraterrestrial presence, but — just as with internationaldiplomacy — being able to second-guess the motivation and behavior of othersmight give us a leg up.

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