We can imagine, probably, a nearly endless run of motivations for ETI meandering the stellar systems, but here we will briefly assess seven of the most discussed. We won't delude ourselves that we've covered the scope of possibilities, and we will hope the the discussion serves only to place ETI and the UFO phenomenon into useful alternative perspectives. The seven motivations are:
The first of the list has already been mentioned as the motivation most debated s1Hart 1975 s2Newman & Sagan 1981 s3Singer 1982 s4Fogg 1986ab. Although it is possible to envision "colonization waves" being driven by needs other than population growth, this is the factor which has dominated the discussion. This dominance is one more oddity in the discussion of ETI, as the choice of population pressure as a driver would seem to be one of the poorest choices we could focus upon.
If, as most feel, the moving of craft through interstellar space will involve a major resources and technology effort, then this is not something which will be done either casually or on a massive scale. A culture wishing for relief from population pressure will not find it by sending 300 citizens to the nearest star while 300 billion remain at home. Some other solution will be sought, like population control. Since on our own planet we have spotted the dangers of overpopulation even at this rudimentary stage of our development, and most of the advanced nations are vitally concerned with attaining stable population levels, it stretches credulity to think that advanced ETIs would not long ago have seen this problem and dealt with it. When you read the literature you get the intuition that the writers are using this particular motivation because it allows them to play "number games" (doubling times, filling times, expanding colonization waves) and so to make irrelevant "estimates" of how long it takes to saturate the galaxy based on a veneer of math and implausible assumptions. It reminds the reader of the drunk and the lightpole. The drunk spends all his time looking for his lost keys near the lightpole (despite the fact that he knows that he didn't lose them there), because it's the only place that he can see. The other more probable motivations do not lend themselves to the mathematical game, so they aren't often discussed.
Let us stretch the population problem scenario to its limits by assuming that the ETIs have developed some absolutist position such as a "sacred priority of propagation," and are, therefore, mindlessly spewing out citizens and somehow surviving all the crises this creates. Even this scenario does not demand colonization of all Earth-like planets or Sun-like systems in ways that require readily recognizable extraterrestrial presence. For instance, such a civilization would surely do the easier task of colonizing its own system thoroughly, prior to launching to the stars. In doing so, it would learn to live efficiently in space colonies or cities. Should such a civilization later decide to colonize other systems, eventually entering our own, such a colonizing group might easily choose to settle in space with the readily accessible solar energy and asteroidal minerals rather than at the bottom of a difficult gravity-well on our planet's surface. They might not even want to risk immersion in our alien biosphere any more than necessary. In short, they could have been here many times, and could still be in the solar system, without ever setting up housekeeping on Earth. And, at our crude level of solar system exploration, it could be many years in the future before we suspect what has been going on nearby s5Papagiannis 1978ab.
Following life's innate tendency to expand into every available space, technological civilizations will inevitably colonize the entire galaxy establishing space habitats around all its well-behaved stars. The most reasonable place in our solar system to test this possibility is the asteroid belt, which is an ideal source of raw materials for space coloniess6Michael Papagiannis, University of Boston, 1983.
The point of this speculation is that being absolutist about any of these scenarios make no sense. Many possibilities are readily imaginable. The second scenario, material gain or power, is really an analog of the population problem. If it is truly difficult and expensive to travel star-to-star, then this possibility makes even less sense than the first. Mass freighting of some relatively abundant universal constituent seems inconceivable, and the specialty freighting of some rare commodity (genes? humans?) seems a poor return on the investment if this is some economic game. And could some power-mad tyrant want to go out and conquer star-systems just for the kick of it? Maybe. But if such existed, how many would be required to saturate the galaxy? And, each would have to spawn generations of power-mad successors to keep the "power wave" expanding for several millions of years. And, how does one hold "The Empire" together with the most isolated chains-of-command imaginable? Most tellingly, we know that this bizarre idea is irrelevant for us anyway. Despite Hollywood, no conquerors have arrived.
A third possibility is threat-at-home. This we can divide into two: a specific threat prejudicial to a small group, or a cosmic threat against the whole system. Hi-tech pilgrims in their fusion-powered Mayflowers may leave the stifling repression of home worlds for freer spaces, but this is a piecemeal effect not likely to give us the sustained continuity of expansion necessary to cover the stars of the galaxy. And our space-faring pilgrims may also be no more interested in planetary surfaces than our generic colonizers discussed earlier. A more certain occurence would be the flight occasioned by rare but inevitable coincidences of an advanced civilization lying about an unstable sun. Would such a civilization meekly accept its end or make a heroic effort to reach safe havens in the stars? Of all the mass movement scenarios this seems the most necessary, although the cosmic coincidence needed to inspire it should be exceedingly rare. Such people would be a reluctant group of colonizers seeking a long-lived star, and stopping their expansions after one great wrenching jump. If one's own Sun did not happen to be the nearest stable neighbor to such a tragedy, there is little reason to expect visitors from such a cause s7San 1981.
What if we comprise a threat of some sort? Such may seem another bit of human egocentrism, but perhaps not.
We are constantly reminded that we are competitive, xenophobic, and violent. We are also curious, inventive, and
risk-taking. We understand nuclear power and the rudiments of space flight. We have been very fast to accelerate into
a high-technology lifestyle. How fast and how far will we go? Recently there has been talk of relativistic
rockets
, devices which might approach the speed of light. Science fiction? Maybe, but who knows when we will
"turn over the right rock" and discover the key secret to make it a reality? Such a device would participate in the
relativistic effects of objects moving at very high speed, including tremendously increased mass. Relativistic rockets
have been called planet crackers
, a doomsday weapon, the "gun" that makes all civilizations equal s8Pelligrino 1986.
If you were living around a nearby star, you might well want to know what we, your neighbors, were like. Once you found out, you probably would want to keep track of us, while keeping a low profile yourself. Depending upon your level of interspecies ethics, you might be sitting "out there" right now, weighing our existence in the balance, hoping that we learn how to behave properly, or just paranoically biding your time until you give up on us and pull the trigger. Many such paranoia scenarios might be possible, but they all call for one alien behavior: ultra-secrecy. The last thing a worried civilization wants to do is give itself away. A larger organization of civilizations might not feel as threatened, but still be concerned. In such a scenario more genuine concern over the survival of dangerous but fledgling species could be evidenced out of both self-interest and a sort of cosmic morality.
This leads us to a possibility of some galactic kinship group, oft termed the "Galactic Club" s9Bracewell 1975. Such an alliance is pictured as an association of advanced civilizations who oversee the maturation struggles of species such as ours. This overseership could be driven by anything from total self-interest to total "moral duty-to-others." Within that spectrum can be imagined any amount of overtness, ranging from nearly-total quarantine (the so-called "leaky embargo" hypothesis) to blunt intervention. Once again the point is: this possibility allows an ETI presence in the Solar system in a variety of levels of covert activity with, however, some purposeful interaction or manipulation s10Tough 1986.
Only certain extremes of alien motivation would demand overt display, and one such extreme relatable to the above is the sixth scenario: religious mission-work. It has been reasoned that if interstellar travel is as difficult as it seems it should be, then only extreme survival pressures or powerful "matters of the spirit" would motivate ETI to engage in the task. One of the things that has made blood run hot here on Earth has been religion and the desire to bring one's truth to others no matter what the sacrifice. Such an interstellar apostolate is quite conceivable, but it is difficult to conceive as other than an overt interactive mission. Since nothing like that is happening, we are left only with the unlikely situation of a "conversion by stealth" to an alien throught-system. Subtle persuasions through hidden means: an excruciatingly slow method for evangelization. This possibility, despite the claims of some UFO contactee groups, seems irrelevant to reality as we currently find it.
The last possibility is the one this author finds most congenial and most likely, hopefully on more than purely intuitive grounds. This seventh scenario is motivated by curiosity: the desire to explore. It is a motivation that strikes a responsive chord in most of us because it is the motivation which has primarily driven our own space excursions. There is little question upon listening to our spacecraft designers and "high frontiersmen" that if (when) Homo sapiens goes to the stars it will be because we want to know what's out there. Curiosity, for us, is a powerful "matter of the spirit" which is one of those irrational urges which disregards economics, security, and other practical values and plunges forward anyway. Curiosity is the driving force of Discovery. As such it would be the same motivator that pushed any technological civilization forward in the development of its elaborate tools.
But is there any reason other than intuition and the history of our own species to give better validation to this idea? Perhaps there is. First let's try logic. Imagine any life form in any situation. To be able to behave appropriately (to survive), the life form must have some means of either altering its situation to move toward (become more involved with) something, or of altering its situation to move away (become less involved with) something, or of maintaining its present situation. We might call such abilities "exploration," "flight," or "stasis" in common language, or, if we were psychologists, "novelty seeking," "harm avoidance," and "reward dependence." For an intelligent species, the triggers for these instincts would be located in the brain and serve as the foundation of behavior. It has been said, loosely and without any depth of analysis, that alien intelligence would never share any behavioral similarities with our species. Yet logic, simple deductive reasoning, indicates that the foundation stones of behavior must be the same three universals, one of which is closely related to, if not indentical with, curiosity s11Cloninger 1988.
Now that the tools of science have advanced enough to let us probe the physics and chemistry of the brain, psychologists are moving beyond the limits of external observation of behavior and are beginning to apply the physical sciences to their discipline. Some of these researchers have already shown that a chemical trichotomy serves to facilitate the three foundation stone behavioral drivers just described. These researches delineate a "Behavioral Activating System," related to impulsive and exploratory activity, driven by the critical consciousness-alerting hormone, dopamine. A second "Behavioral Inhibiting System" relates to caution and shyness, and is driven by the major sleep-state controlling hormone, serotonin. The third "Behavioral Maintenance System" relates to dependency and conservatism, and is inversely driven by the main energizing hormone, nor-epinephrine s12Cloninger 1988.
We have known that these three neurotransmitters (brain hormones) are vitally important to behavioral stability for some time. Imbalances in these chemicals have been accused of producing certain schizophrenias, depressions, hyperactivity, and neuroses. We are just now realizing how fundamental they are. They go to the roots of behavior and one of them is the activator of what we see as a biological essential relatable to the ETI story: curiosity, exploration, novelty-seeking. Species everywhere should seek novelty, avoid harm, and conserve the good. If we were to assume the absence of a powerful curiosity and exploration instinct in ETI, we assume that they are missing one of the three required instincts of life forms. Would their level of curiosity be strong enough to take them into the stars and ultimately to us? No one, of course, can say. But if they do come, they will come with curiosity and a sense of exploration among their other instincts.