Isaac Asimov's Science Fiction Magazine 8(September, 1984):30-44.
Note: This web version is derived from an earlier draft of the paper and may possibly differ in some substantial aspects from the final published paper.
Only problem is-I've just figured out they can't exist!
Lemmings are also incredibly feisty animals-restless, courageous, pugnacious, everything a vigorous frontiersman could admire. For instance, when a resting lemming is suddenly disturbed, instead of fleeing it sits up, back staunchly against a rock, hissing and showing fight in a determined manner. The little guys won't take no for an answer, And they're quite the travelers-during each exodus they roam many hundreds of miles, and they've been sighted on arctic ice floes more than 30 miles from land.
Too bad they can't exist! Here's my reasoning:
Okay, let's try passenger pigeons. Birds breed pretty fast, and can fly all over the world. Here's my reasoning again: (1) If passenger pigeons exist, they should be here. (2) If here, we should see them. (3) We don't. (4) Hence they're not here. (5) Hence they don't exist. Well, we can believe that-the last known passenger pigeon died in 1914 at the Cincinnati zoo, and they're now thought to be extinct. But this conclusion should bother you just a little, since passenger pigeons were around for years and years until humankind killed them all off.
Now let's do extraterrestrial interstellar colonists: (1) If the aliens exist, then They should be here. (2) If They are here, then we should observe Them. (3) We do not observe Them. (4) Hence They are not here. (5) Hence They do not exist. Haven't we heard all this somewhere before?
Come to think of it, maybe I have seen a lemming or two. These are the folks who like to espouse the above line of reasoning, known as the "Fermi Paradox." It's getting to be quite a popular bandwagon to jump onto, these days. But, like the lemmings' relentless death-march to the sea, it may also be the equivalent of intellectual suicide, for the Paradoxers have committed a very fundamental error.
The error is that the "should" in steps (l) and (2) is not a logical operator at all. "Should" is only a subjective judgement, tainted with assumptions, prejudices, unknowns, hidden agendas and chauvinisms. "Should" is not "must." "Should" is barely maybe. Consequently, the arrow of logical implication cannot validly be reversed, given the fact of null evidence.
So can we really conclude, after hearing out the Fermi Paradoxers, that aliens, like lemmings, cannot exist? Or that aliens, like passenger pigeons, can never have existed? Obviously not.
Scientists can readily explain why my garden isn't crawling with lemmings, even though these rodents exist on this continent and could theoretically get here in numbers rather quickly. And if some were around, but I hadn't looked too hard for them, or didn't know what they looked like exactly, I might not have spotted them at all. Similar notions may explain why extraterrestrials might not be here, or might be unobservable, even if they are around.
Let's think this through. Why might extraterrestrials not be here, even though they exist somewhere else in the Galaxy? Perhaps there are billions upon billions of planets teeming with zoologically fascinating, but non-technological, life. We'd never know unless we went there in person to check it out.
Why do we migrate, colonize, and settle? Mainly, to seek new opportunities to become rich, to escape tyrannical governments or persecution, to escape crushing population pressures, and so on.
Well, to get richer you need materials, energy, and factories. Advances in
robotics will soon make possible totally automated, unmanned factories-which are
capable of replicating more factories ad infinitum--even here on
backwards Earth. If more materials and energy are needed, there are plenty of
uninhabitable star systems that can be pillaged. Why exploit life-bearing solar
systems, like ours, which is inherently more dangerous, raises thorny ethical
issues, and can't possibly help profits? Most likely They'll keep their distance
As for overpopulation, there are many ways to deal with it that are far cheaper and easier than shipping people off to the stars. Like birth control pills, for one. Or execution. Or space colonies, for another-if Jupiter's mass is reassembled in 10-kilometer-wide spheres 10 meters thick, the total interior surface living area is about a billion Earths. Arthur Clarke's Diaspar in The City and the Stars presents an idea in tune with the modern urge to recycle everything, and there are countless other solutions.
Also, genes are environment specific survival instructions, so transmission of pure genetic information seems pointless unless the target planet is terraformed as a prelude to colonization. This is unlikely to be done in our system, which is already inhabited, for reasons noted earlier. Interstellar ovum arks, generation ships, and automated bio-regeneration only make new independent competitors for exactly the same limited galactic resources. Where's the profit for the senders?
Like the bottle-babies of Huxley's Brave New World, there will be little parental attachment or sense of community with cloned societies whose cultural interaction has 100,000-year information feedback loops. This is comparable to the timescale of speciation. By the time the spurt of human germ plasm has flooded the Galaxy, our distant descendents might have evolved into intelligent raccoons. Or, biological engineering may permit travelers to change genes like jeans to suit the local environment at each of many planetfalls. Eventually the "root form"-- man -- might be forgotten.
Yet--surely out of all the millions of alien races evolving in the Galaxy, at least one will become, as physicist Freeman Dyson puts it, "a cancer of purposeless technological exploitation." All it takes is a single rapacious civilization somewhere, anywhere, anytime, and whoops! -- the whole galaxy (including us) is colonized in a cosmic eyeblink. We don't see this, hence no aliens. , This point is often raised as the coup-de-grace by the Fermi Paradoxers. So please bear with me as I carefully expose it for the nonproblem it really is.
First of all, I could just take the easy way out and claim we don't know if we've been colonized or not. Archeologists and paleontologists have sifted only a minute fraction of Earth's crust for clues to our past. Much has been destroyed by time.: Much remains undiscovered. Our chances of having detected an extraterrestrial intervention millions (or more) years ago, even by now, are nil.
But instead of copping out, let's take today's null data at face value. Let's just assume there really is no evidence of a massive wave of alien colonization sweeping through our Solar System now or anytime in the past. What does this tell us?
Unfortunately, not much. Science, like Sherlock Holmes, works by excluding various possibilities, one by one, until what remains cannot be ruled out by any known test and so is presumed true. And our null data? All we can say for sure is
-that large-scale, rapacious civilizations can be excluded. We don't see them, and their rapacity means they must be visible, hence they don't exist at this time. But this still leaves (I'm guessing) 99% of all possible civilization types which are not rapacious by nature and cannot yet be excluded by the scanty observational evidence.
Apparently galactic civilizations which make extensive and exploitative use of highly visible, very advanced technology are rare or nonexistent. Why might this be? Extinction as a natural phenomenon is quite common on Earth, where biological life has a 99.9 percent species-extinction rate. Maybe there can be no interstellar "bad apples," because they self-destruct before damaging any others, or because they have never survived long enough to complete their galactiforming program and leave any major observable effects. Indeed, social pessimists might argue that the almost instinctual voracity of humans for violence, coupled with our technological capability for self-destruction, may eventually lead to the annihilation of our planetary civilization.
So I have no problem accepting an as-yet unspecified selective mechanism which results in the "cosmic censorship" of those few galactic civilizations who exploit, glaringly obvious, rapacious technologies to the utmost. Do you?
Mind you, I'm talking about really huge civilizations. Of alien planetary or solar system cultures, or small galactic civilizations that are just starting out, we 'can know next to nothing about them. We can only see out along the galactic plane a few kiloparsecs because interstellar gas and dust obscure the view. .Even the most precise radio telescopes (these can penetrate the haze) cannot resolve a Dyson Sphere the size of Earth's orbit beyond a range of a few kiloparsecs. And no sky searches have yet been made to this 'extreme resolution, anyway.
High technology activities by extraterrestrials might not be immediately
obvious at interstellar distances. Certainly a major fraction of the Galaxy
could have been colonized by generation ships without us being able to observe
it. Giant circumsolar factories could be busy chugging away near any but the
nearest stars, and we'd see nothing from here.
Well, how do we know this isn't happening? Astronomers have found that unless the galaxy is about ten times heavier than it appears, there's not enough gravity to hold it together. If it does hold together, then where is the invisible "missing mass" hiding, perhaps 90% of the total? Could it be dammed-up star-stuff, stored in giant depots near Dyson spheres surrounding the remnants of the vast majority of stars in the Milky Way? At present, we have no way of knowing.
Of course, there are other reasons why extraterrestrials, though nearby, might not be observable by us or might not have contacted us. Maybe they don't want us to find them, or are waiting for us to pass a cosmic entrance exam, or are keeping us isolated in quarantine or in a galactic wilderness preserve or interstellar zoo. Since theirs is the superior technology, we must assume they'll succeed in hiding from us if that's their wish.
There's really no reason why They should not keep silent. It is anthropocentric to assume, for example, that alien spacecraft entering the Solar System on a mission of reconnaissance or self-replication will feel the obligation to announce their presence to us or to request permission to proceed. Probes will probably just ignore us and go on about their business. It is entirely conceivable that some extraterrestrials may not particularly care whether we find them or not, or may be interested in communicating with us but are waiting for us to speak up first.
Or, if life isn't especially rare in the universe, then rather than treating us like an endangered species, the aliens may adopt a much more casual approach. They may have tremendous confidence in their ability to manage first contact events, based on numerous previous successful encounters. In this case, humankind might rate neither ultraconservative wildlife management nor heedless astrophagic exploitation. The most likely response would be careful and unobtrusive observation, with no special effort to conceal the alien presence. Base sites would be chosen for reasons of efficiency, maintainability, and low environmental risk. And if we come knocking on their door, they'll answer.
The Paradoxers are truly up against the wall now. The only extraterrestrials they can positively exclude are rapacious galactic civilizations. Even those vigorous societies that do colonize or exploit may not come here, and, with present-day astronomical equipment we couldn't see much of this activity anyway. Lesser civilizations and nontechnological worlds are entirely invisible, unless they happen to be our next-door neighbors.
Well, just to be sporting, and for the sake of argument, I'll tie both arms firmly behind my back and assume that "should" equals ,"must" in the Fermi Paradox argument-even though we now know this is not true. Thus, if aliens exist, they must be here and we must be able to observe them, provided we look.
Well, Freitas, you've thrown it for sure, now.... Oooof!
Forgot I could kick, huh?
Don't forget Fermi Paradox step (3): Fact--we don't observe Them in the Solar System. From this the logic of "must" winds us inexorably back to the conclusion that we are alone. So--is there any such "fact"?
Virtually all past and recent discussions of the Fermi Paradox blithely assume that the absence of extraterrestrials or their artifacts on Earth or in the Solar System is an undisputed datum. Totally bogus! Indeed, the vastness of our ignorance in this area is rarely appreciated. For me, it is this fact which most nullifies any persuasive force the Fermi Paradox might once have had.
No, I'm not a closet UFOholic. I'm not talking about UFOs. In fact, it's just fine with me if we ignore the Earth's surface, Earth's atmosphere, and UFOs altogether for the remainder of this discussion. Fair enough?
What I am talking about is an interstellar probe that aliens might have sent here to reconnoiter our star system and its environs. It's not implausible. Didn't Pioneer 10 just exit the Solar System? My alien device would be something like our own interstellar probes, the Pioneer and Voyager spacecraft, but a trifle more sophisticated.
A typical alien probe might be 1-10 meters in size -- this is large enough to house a microwave antenna to report back to the senders, wherever they are, and to survive micrometeorite impacts for millions of years, but still lightweight enough to cross the interstellar abyss without consuming untold amounts of energy.
Okay, then. Where might it be?
Well, almost . . . anywhere, centered on the Sun, enclosing the orbit of Pluto. That’s where alien artifacts might be hiding. Roughly 260,000 cubic AU (astronomical units, the mean Sun-Earth distance) of mostly empty interplanetary space, plus 100 billion square kilometers of planetary and asteroidal real estate. To be able to say for sure that there is no extraterrestrial presence in the Solar System, you have to have carefully combed most of this space for artifacts.
Lets review the facts. The ability of a telescope to detect faint objects is measured by its visual magnitude limit. The unaided eye can see down to sixth magnitude. The sky is exhaustively and repeatedly surveyed by amateurs to, at best, magnitude +14. The Palomar Schmidt Sky Survey extends to +21, but these plates are just snapshots of patches of sky and don't count as a search. The best telescope on Earth reaches only to magnitude +24.
The three latter magnitude limits correspond very roughly to spotting an unmoving, mirror-shiny, optimally-oriented 10-meter object orbiting 0.01, 0.25, and 1 AU form Earth, respectively. An object that's smaller, moving, black or canted at a different reflection angle is even harder to see.
So we can only scan the nearest cubic AU of space for probes, but we have 260,000 cubic AU to search. Even if Mount Palomar was employed exclusively to look for alien artifacts (don't hold your breath!) it could only scan one-millionth of the necessary volume and would take thousands of years. Orbital space, in other words is at least 99.9999 percent unexplored for 1-10 meter objects.
How about probes parked on planetary surfaces? Of the 0.1 trillion square kilometers of Solar System territory other than Earth, less than 50 million has been examined to 1-10 meter resolution. So 99.95 percent is still virgin territory as far as a serious search for extraterrestrial artifacts is concerned.
The surfaces of most bodies in the Solar System, have only been mapped, if at all, to a resolution of 10 kilometers or worse. Astronomers have difficulty seeing even one-kilometer asteroids flying by unless they pass pretty close to Earth. So who can say if there are 1-100 meter extraterrestrial artifacts lurking about somewhere nearby, quietly performing their mission--or not?
You see? All we know for sure is how ignorant we are.
If objects are buried or floating in a jovian atmosphere, there is zero chance we could have found them yet. Even huge 1-10 kilometer artificial alien habitats lumbering around in the Asteroid Belt could not be distinguished from asteroids by terrestrial observers, and the Belt population itself is poorly catalogued. So it is exceedingly unlikely that we'd have spotted an extraterrestrial artifact anywhere in the Solar System yet, unless it was desperately trying to signal our attention. And why should it bother to do that?
Detecting an operating self-replicating machine system is only marginally easier to observe. Likely sites are the Asteroid Belt and the outer-Jovian and Saturnian moons. Recent technical studies suggest individual replicating systems may be 100 meters in diameter, or less, so a factory system for building probes should not exceed 0.1-1 kilometer in size, again well beyond our ability to see it except on the moon and portions of Mars. Ignition of fusion rockets to propel daughter probes out of the Solar System could be spotted using amateur equipment, but the observation window is very small and of very short duration. Self-reproducing probes should be able to replicate a whole generation in 1000 years or less, and be quickly on their way, so only mining pits and small debris may remain at this late date. Again, probably unobservable.
The total mass of probes needed to explore even the entire Galaxy is astonishingly small. I did a technical study a few years ago which showed that a self-reproducing probe, patterned after the Daedalus starship (designed by the British Interplanetary Society a few years before) but capable of entering orbit at its destination, could have a fully-fuelled mass of about 10 billion kilograms. If such a device makes 10 replicas and reproduction continues for 11 generations, that's enough to "bug" every star in the Galaxy. This eats up the mass of Ceres, the largest asteroid.
So even if our Solar System played nursery for all 100 billion probes, how would we ever know if one Ceres-size asteroid had once been consumed way out there in the Asteroid Belt?
And let's take this argument a step further. Assume that one million extraterrestrial civilizations each pillage the Solar System for materials to build and launch their own million independent spaceprobe spy networks, each covering every star in the Galaxy. The total resource requirement is still only about the mass of Jupiter. I doubt we could say for certain if even this much matter had been stolen away sometime in our remote prehistory. (Is the Asteroid Belt Just tailings from some past gigantic alien industrial operation?)
More likely, starfarers won't be so greedy and may reqt1ire us to supply no more than one new generation of replicants. This is only 100 billion kilograms, enough to fill a one-kilometer crater 40 meters deep or to make one 400-meter-wide asteroid. Probably we'd never notice anything was missing.
Even more likely, the aliens would program their automata to erect self-replicating probe factories only in uninhabitable star systems and just send nonreproducing exploratory probes here to sniff around. Why make the natives restless? No local mass would be missing in this case, and there'd be no telltale surface debris either.
So! The Fermi Paradox is just a toothless tiger. It is all bark and no bite. The Paradoxers can go stew in their sterile universe. There's aliens Out There, yessir, and I, for one, am going hunting for them. We haven't looked nearly hard enough, or long enough, to spot any of them yet. But we will.
And maybe sooner than you think.