Good Timing

June 12, 2008
by Seth Shostak, Senior Astronomer

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 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.