The Ultimate Long Distance Call

Dec. 27, 2001

by Seth Shostak - Senior Astronomer

Its one thing to search for intelligent aliens, but its another to actually talk to those we might find. This isnt just a matter of what language (if any!) to use, or even how to encode the information. Encoding for mutual understanding is merely a daunting technical challenge. For example, should we broadcast messages using pulse code modulation, AM radio, spread-spectrum techniques, or something we dont have a name for yet? Until we pick up a signal, we really havent a clue.

As for what language to use, better minds than mine are wrestling with that particular adversary. Personally, I vote for pictures. But aside from these difficulties, theres another problem thats as obvious as Mae West: it takes time for signals to traverse interstellar distances. The speed of light, fast as it is, is finite. Watch as CNN news anchors talk with their correspondents in Afghanistan. Theres always a bit of a delay between question and answer. This isnt because the correspondents are slow-witted, but is simply a consequence of the time it takes the signals to ping-pong up to the communications satellites and back down to Earth. Annoying, but not devastating.

Of course, conversing with extraterrestrials is going to be more than merely annoying. The nearest star, Alpha Centauri, is roughly 4 light-years distant, as every school child can remind you. Thats an 8 year delay between query and response. Three exchanges into the conversation, and your kids have already graduated from college.

But the aliens are unlikely to be hanging out at Alpha Centauri (which has been fairly carefully scanned for signals already). Weve noted in a previous article that if there are 10,000 broadcasting civilizations in the Galaxy, then the nearest one will be 500 to 1,000 light-years away. The resultant conversational delay will be measured in millennia. Thats tedious.

But it leads to a provocative thought. If the aliens are altruistic (they just want to beam information into space, and dont care about chatting), then the long turn-around time doesnt matter. But if they are pinging nearby stars with a giant laser in the hope of waking up their galactic neighbors, then its reasonable to assume that they wont blast away at targets that are so distant that they cant expect a response within an alien lifetime.

This prompts a simple, but interesting calculation: for any given lifetime, how many star systems can an alien reasonably ping? To make the computation, we need to know the average space density of stars. Big stars, those heftier than the Sun, have a density of 0.0004 stars per cubic light-year. Smaller stars, the ones we believe are better candidates for hosting sophisticated life, are (thankfully) more plentiful, checking in at 0.001 stars per cubic light-year. Wielding that number and a bit of middle school geometry, you can work out the following:

N = 0.0005 t3,

Where t is the lifetime of an alien broadcaster, and N is the number of stars that can be pinged with the hope of having an answer before death. Readers who have misplaced their pocket calculators can use the table below to look up N for various alien lifetimes.

t (years)N
5065
100520
2004,100
1,000500,000
5,00065 million

For example, if your lifetime is a hundred years and you want to chat, then there are 520 good stars within range.

What can we conclude from this? Optical communications are more likely to be deliberately targeted. But theres no point in flashing the neighbors unless you believe theres at least a decent chance of getting a reply. Even being relatively optimistic about the number of savvy societies in our Galaxy, most astronomers suspect that only one star system in a million or so is likely to host thinking beings. Bottom line? If we find flashing lights in the sky, then its probable that the guys behind the high-powered lasers at the other end have managed to engineer themselves to have lifetimes of thousands of years or more.

Its a speculative thought, but an interesting one. If we hear from ET, not only can we expect his civilization to be an old one, ET himself may be quite long in the tooth.