It’sone thing to search for intelligent aliens, but it’s another to actuallytalk to those we might find. This isn’t just a matter of what language (ifany!) to use, or even how to encode the information. Encoding for mutualunderstanding is merely a daunting technical challenge. For example, should webroadcast messages using pulse code modulation, AM radio, spread-spectrumtechniques, or something we don’t have a name for yet? Until we pick up asignal, we really haven’t a clue.
As for whatlanguage to use, better minds than mine are wrestling with that particularadversary. Personally, I vote for pictures.
But asidefrom these difficulties, there’s another problem that’s as obviousas Mae West: it takes time for signals to traverse interstellar distances. Thespeed of light, fast as it is, is finite. Watch as CNN news anchors talk withtheir correspondents in Afghanistan. There’s always a bit of a delaybetween question and answer. This isn’t because the correspondents areslow-witted, but is simply a consequence of the time it takes the signals toping-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. Thenearest star, Alpha Centauri, is roughly 4 light-years distant, as every schoolchild can remind you. That’s an 8 year delay between query and response. Threeexchanges into the conversation, and your kids have already graduated fromcollege.
But thealiens are unlikely to be hanging out at Alpha Centauri (which has been fairlycarefully scanned for signals already). We’ve noted in a previousarticle 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 resultantconversational delay will be measured in millennia. That’s tedious.
But it leadsto a provocative thought. If the aliens are altruistic (they just want to beaminformation into space, and don’t care about chatting), then the longturn-around time doesn’t matter. But if they are “pinging”nearby stars with a giant laser in the hope of waking up their galacticneighbors, then it’s reasonable to assume that they won’t blastaway at targets that are so distant that they can’t expect a responsewithin an alien lifetime.
This promptsa simple, but interesting calculation: for any given lifetime, how many starsystems can an alien reasonably ping?
To make thecomputation, 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 hostingsophisticated life, are (thankfully) more plentiful, checking in at 0.001 starsper 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 isthe lifetime of an alien broadcaster, and N is the number of stars that can bepinged with the hope of having an answer before death. Readers who havemisplaced their pocket calculators can use the table below to look up N forvarious alien lifetimes.
| t (years) | N |
| 50 | 65 |
| 100 | 520 |
| 200 | 4,100 |
| 1,000 | 500,000 |
| 5,000 | 65 million |
For example,if your lifetime is a hundred years and you want to chat, then there are 520good stars within range.
What can weconclude from this? Optical communications are more likely to be deliberatelytargeted. But there’s no point in flashing the neighbors unless youbelieve there’s at least a decent chance of getting a reply. Even beingrelatively optimistic about the number of savvy societies in our Galaxy, mostastronomers suspect that only one star system in a million or so is likely tohost thinking beings. Bottom line? If we find flashing lights in the sky, thenit’s probable that the guys behind the high-powered lasers at the otherend have managed to engineer themselves to have lifetimes of thousands of yearsor more.
It’s aspeculative thought, but an interesting one. If we hear from ET, not only canwe expect his civilization to be an old one, ET himself may be quite long inthe tooth.
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