Aliens Apart

For yearsscientists have wrestled with a puzzling fact: The universe appears to beremarkably suited for life. Its physical properties are finely tuned to permitour existence. Stars, planets and the kind of sticky chemistry that producesfish, ferns and folks wouldn't be possible if some of the cosmic constants wereonly slightly different.

Well, there'sanother property of the universe that's equally noteworthy: It's set up in away that keeps everyone isolated.

We learnedthis relatively recently. The big discovery took place in 1838, when FriedrichBessel beat out his telescope-wielding buddies to first measure the distance toa star other than the sun. 61 Cygni, abinary star in our own back yard, turned out to be about 11 light-years away. Forthose who, like Billy Joel, are fond of models, think of it this way: If youshrank the sun to a ping-pong ball and set it down in New York's Central Park,61 Cygni would be a slightly smaller ball near Denver.

Thedistances between adjacent stars are measured in tens of trillions of miles. Thedistances between adjacent civilizations, even assuming that there are lots ofthem out there, are measured in thousands of trillions of miles – hundreds oflight-years, to use a more tractable unit. Note that this number doesn't changemuch no matter how many planets you believe are studded with sentients – theseparation distance is pretty much the same whether you think there are tenthousand galactic societies or a million.

Interstellardistances are big. Had the physics of the universe been different – if thegravitational constant were smaller – maybe suns would have been sprinkled farcloser together, and a trip to your starry neighbors would have been no morethan a boring rocket ride, kind of like cruising to Sydney. As it is, no matterwhat your level of technology, traveling between the stars is a toughassignment. To hop from one to the next at the speed of our snazziest chemicalrockets takes close to 100,000 years. For any aliens who have managed to amassthe enormous energy reserves and ponderous radiation shielding required for relativisticspaceflight, the travel time is still measured in years (if not for them,then for those they've left behind).

This hassome obvious consequences (which, remarkably, have escaped the attention ofmost Hollywood writers.) To begin with, forget about galactic "empires"or more politically-correct "federations." Two thousand years ago,the Romans clubbed together an empire that stretched from Spain to Iraq, with a radius of about 1,200 miles. They could do this thanks to organization andcivil engineering. All those roads (not to mention the Mediterranean) allowedthem to move troops around at a few miles an hour. Even the most distant Romanrealms could be reached in months or less, or about one percent the lifetime ofyour average legionnaire. It makes sense to undertake campaigns designed tohold together an extensive social fabric when doing so requires only a percentor so of a lifetime.

In the 19thcentury, steamships and railroads increased the troop travel speeds by a factorof ten, which extended the radius of control by a similar amount. The Britishcould rule an empire that was world-wide.

But here'sthe kicker: Even if we could move people around at nearly the speed of light,this "one percent rule" would still limit our ability to effectivelyintervene – our radius of control – to distances of less than a light-year,considerably short of the span to even the nearest star other than Sol. Consequently,the Galactic Federation is a fiction (as if you didn't know). Despite beingwarned that Cardassian look-alikes were wreaking havoc and destruction in thegalaxy's Perseus Arm, you couldn't react quickly enough to affect the outcome. Andyour conscripts would be worm feed long before they arrived on the front linesanyway.

In otherwords, aliens won't be getting in one another's face.

There's asimilar argument to be made for communication. We seldom initiate informationinterchange that takes longer than months (an overseas letter, for instance). Moregenerally, we seldom begin any well-defined project that lasts more thantwo or three generations. The builders of medieval cathedrals were willing tospend that kind of time to complete their gothic edifices, and those who burytime capsules are occasionally willing to let a hundred years pass before thecanisters are dug up. But what about a project that takes several centuries,and possibly millennia? Who's willing to do that? Only Stewart Brand's "LongNow Foundation" seems to have the guts for this type of enterprise,proposing to build a clock that will keep time for ten thousand years.

Clearly,these simple observations must have implications for SETI which, as we noted,involves transmissionsthat will be underway for hundreds to thousands of years. In particular, ifthere are signals being bandied about the galaxy for purposes of getting intouch, either (1) the aliens are individually much longer-lived than we are,which – if you're a fan of circuit-board sentience – implies that they'reprobably not biological. Or (2) we're missing some important physics permittingfaster-than-light communication, and extraterrestrial signaling efforts don'tinclude burping light and radio waves into space.

Manyreaders will, in a display of endearing perversity, choose (2). Maybe they'reright, but that flies in the face of what we know. And what we know arguessomething worth bantering about at your next cocktail party – namely, that thetime scales for travel and communication are too long for easy interaction withbeings whose lifetimes are, like us, only a century or less. So while thecosmos could easily be rife with intelligent life – the architecture of theuniverse, and not some Starfleet Prime Directive, has ensured precious littleinterference of one culture with another.

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Seth Shostak
Senior Astronomer, SETI Institute

Seth Shostak is an astronomer at the SETI (Search for Extraterrestrial Intelligence) Institute in Mountain View, California, who places a high priority on communicating science to the public. In addition to his many academic papers, Seth has published hundreds of popular science articles, and not just for; he makes regular contributions to NBC News MACH, for example. Seth has also co-authored a college textbook on astrobiology and written three popular science books on SETI, including "Confessions of an Alien Hunter" (National Geographic, 2009). In addition, Seth ahosts the SETI Institute's weekly radio show, "Big Picture Science."