Summary (Aug 05, 2003): For those who suggest that SETI detection seems speculative research, astronomers consider the balance between experimentation and guesswork: their conclusions suggest that science is just about 'doing it'.
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Speculation and Experimentation by Seth Shostak, Astronomer, Project Phoenix
We seem to have the Galaxy to ourselves. At least, that's the obvious conclusion from the apparent lack of aliens in the neighborhood.
But this conclusion might be a bit too obvious, and possibly wrong. In previous articles, we've considered why extraterrestrial intelligence - even if common - would have restrained itself from spreading to every half-decent star system in the Galaxy. It's possible that the aliens have done cost-benefit analyses that show interstellar travel to be too costly or too dangerous to warrant ambitious colonization efforts. An alternative suggestion that would explain our apparent solitude is that the Galaxy is urbanized, and we're in a dullsville suburb.
 | The farthest known planet ever discovered is a strange world indeed. Whizzing around its star every 29 hours, it is shrouded in clouds made not of water droplets but of iron atoms. This is a world of iron rain.
Credit: David A. Aguilar, Harvard-Smithsonian Center for Astrophysics |
Yet another resolution for the so-called Fermi Paradox is that we've been singled out for special treatment: we are an exhibit for alien tourists or sociologists. Our world may be known to the extraterrestrials, but they observe us through a sophisticated type of one-way mirror.
While there's no evidence to give credibility to this last idea (known as the "Zoo Hypothesis"), many would argue that evidence does exist for another possibility - namely, that the Paradox is just a red herring because the aliens are in the neighborhood. In fact, they're in our back yards, or just above them.
Many thousands of sightings of unidentified flying objects (UFOs) are reported each year, and polls show that one-third to one-half of the population believes that at least some of these aerial apparitions are alien spacecraft. The presence of aliens on Earth would neatly resolve the Fermi Paradox.
But while this is a prevalent idea among the public, the evidence for alien visitation has failed to sway most scientists. To convince researchers, who are inherently skeptical, unambiguous and repeated detection of flying objects by satellites or ground-based radar would be required. Better yet would be some indisputable physical evidence, such as the landing lights from an alien craft. In other words, something better than witness testimony is necessary, since such testimony isn't good enough, no matter how credible the witness.
Consider the fact that lots of people claim to have seen ghosts, and will be pleased to tell you what they saw. But the case for the existence of these shrouded spirits isn't what you would call convincing. You don't read a lot about the parameters of ghosts in scholarly journals.
Until and unless better evidence is collected, few scientists are inclined to accept the premise that the Fermi Paradox can be resolved by the claim that aliens are either soaring through the stratosphere, or are stashed away in meat lockers at Area 51.
Of course, there's no doubt that aliens in the neighborhood would be dramatic news, and that's part of the appeal of such claims. But the opposite circumstance would be similarly startling. What if we have failed to espy the extraterrestrials simply because there aren't any?
After all, the evolution of intelligence may be a rare occurrence, even if biology is common. Could it be that in the enormous reaches of the Milky Way, ours is the only planet with thinking beings? That would neatly solve the puzzle posed by Fermi. And no matter how discouraging (or otherwise) the thought of being unique may be, we still haven't the proof that it isn't true.
While possible resolutions of Fermi's Paradox are as plentiful as gas stations, we still have no idea which, if any, is correct. Perhaps the universe is teeming with societies so subtle we can't prove their presence. Or haven't yet.
On the other hand, maybe we're alone.
It's all a bit perplexing, but in fact there's hope. SETI experiments offer the promise of relegating the Fermi Paradox to the dustbin of historical curiosities by proving that other intelligence is out there. So while it's interesting and instructive to consider the pros and cons of galactic colonization, we should also make sure that we do some careful observing.
In science, speculation is desirable, but experiment is definitive.
It's one thing to search for intelligent aliens, but it's another to actually talk to those we might find. This isn't 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 don't have a name for yet? Until we pick up a signal, we really haven't 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, there's another problem that's 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. There's always a bit of a delay between question and answer. This isn't 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. That's 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). We've 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. That's tedious.
But it leads to a provocative thought. If the aliens are altruistic (they just want to beam information into space, and don't care about chatting), then the long turn-around time doesn't matter. But if they are "pinging" nearby stars with a giant laser in the hope of waking up their galactic neighbors, then it's reasonable to assume that they won't blast away at targets that are so distant that they can't 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.
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 there's no point in flashing the neighbors unless you believe there's 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 it's 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.
It's 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.
Related Web Pages
SETI Institute
Extrasolar Planets Encyclopedia
Planet Quest (JPL)
Kepler Mission
Eddington Mission
Darwin Mission
Herschel Mission
Space Interferometry Mission
Note: Phoenix
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