Transmitting to a Million Worlds

January 12, 2006
Seth Shostak, Senior Astronomer, SETI Institute

Hardly a week goes by without some keyboard correspondent reluctantly telling me that SETI has a honking big problem. “Even if the Galaxy is brimming with intelligent life,” they’ll gravely intone, “SETI has virtually no chance of success.”

Their reasoning is this: the probability that SETI’s receiving antennas are pointed at the aliens’ world just when their transmitting antennas are aimed at ours is vanishingly small. Since we lack a TV Guide for extraterrestrial broadcasts, we’ll never twist our telescopes to the right direction, at the right time, to pick up a signal.

Note that this is not the well-known “lifetime” problem (their civilization flowered and died a billion years ago, or a billion years from now). That’s hardly an issue if the Galaxy spawns technical societies on a regular basis – societies that manage, on average, to keep self-destruction at bay for at least a few thousand years. No, this is a synchronization problem. SETI researchers have typically looked at any particular star system (at a given frequency) for only a few minutes, at most. But what are the chances that an alien signal has been sent our way just at the right moment to splash upon our antennas during that brief interval?

Well, the chances are 100 percent if the extraterrestrials beam their broadcasts to the whole Galaxy (or at least a big chunk of it). If it’s All Alien, All the Time, then it doesn’t matter when we look, as long as we eventually observe the right direction at the right spot on the dial.

The problem with this scenario is that an omnidirectional radio broadcast really combusts the kilowatts. Consider an extraterrestrial transmitter halfway across the Milky Way, belching out Galactic Headline News with a strong enough signal to prick the digital ears of Project Phoenix, which was the most sensitive targeted SETI search ever made. The required transmitter power would be 100,000 trillion watts, or about ten thousand times the total energy consumption of Homo sapiens today.

That sounds like the mother of all utility bills. And, indeed it is, at least for the types of societies we can easily envision. On the other hand, that power is only one billionth the output of the Sun. So if, for instance, we undertook to carpet Mercury with wall-to-wall, high-efficiency solar cells, we’d have enough juice to take on a pan-galactic broadcasting project. Frankly, it’s not inconceivable that truly advanced civilizations are willing and able to field high-powered, omnidirectional beacons.

But there’s another approach to transmitting that’s far less costly – and one I think makes sense. It’s a “two-tier” strategy, and I recently presented it in some detail to a (mostly phlegmatic) audience at an astronautics conference in Japan.

It assumes that extraterrestrials wanting to get in touch will be at least a century or two ahead of us. This means they will have already built the sort of space telescopes that can find and analyze planets around other stars. If life is commonplace (still a big “if”), then one can imagine that the appendixes of alien astronomy textbooks will contain long, long lists of worlds known to have biology. After all, for two billion years, the oxygen in the atmosphere has been whispering to the Galaxy that there’s life on Earth, because bacteria produced that oxygen. Without doubt, our planet will be on those textbook lists of “bio-worlds.”

Any sensible alien signaling project would target its transmissions to bio-world-blessed star systems. If the extraterrestrials are on a fishing expedition, why not concentrate on those places where the fish are most likely to be? But there’s still the irksome synchronicity problem. Even though Earth has had detectable biology for two billion years, it’s had technically sophisticated life for only a century or so. Let’s assume that our modest attempts at civilization last for another 20 thousand years. Then, doing the desperately simple math, you’ll realize that a random transmission to Earth would have a roughly 0.00001 chance of arriving when its inhabitants have the technology for noticing.

Putting this another way, if you run an alien radio station, wielding a big antenna (in order to save power) for “targeting” worlds known to have life, you’d better ping at least 100 thousand of those worlds if you hope to garner even one listener.

So here’s the strategy. The aliens could decide to use a highly directional antenna (maybe an array) to beam a short, intense pulse to a long list of bio-worlds. They repeat this serial signaling project every 20 minutes, for example. If each pulse is one-thousandth of a second long, their list will comprise about a million targets, enough to provide a decent chance for including at least one star system sporting some nerdy creatures able to build big radio telescopes. That’s the “hailing” signal; the one whose only purpose is to get someone’s attention. It’s the first tier of the strategy.

The second step is even simpler. If we, or any other society, find that hailing signal (and it could be easily verified, as it repeats every 20 minutes in our example), you can bet that we’d pull out all the stops to examine the patch of sky from which it came, looking for additional signals. We’d reason that no one would send us an empty string of pulses; there must be a message somewhere. So the second tier of the fishing strategy, from the aliens’ point of view, is to have a relatively low-power (and therefore affordable) omnidirectional transmitter that provides the message. Their assumption – a good one – is that we would eventually build a big enough receiving setup to find it.

If any extraterrestrials are signaling with such a two-tiered scheme, we need to modify our traditional SETI observations to find them. First, we have to extend our “dwell” time on each target from a few minutes to, say, a few tens of minutes. Second, we have to change our SETI receivers to more easily recognize short radio bursts.

Neither adjustment is hard, and making the change might be worth the effort. After all, we don’t yet subscribe to the galactic TV guide.