Forget the idea of "other dimensions" for a moment. Perhaps Jacques Vallee's proposed "psychic thermostat," while a well-intentioned attempt to reconcile UFO observations with their psychosocial effects, isn't needed to encompass the weirdness of alien visitation. Forget, also, the idea that aliens are necessarily from space. (Sure, some might be, but I'll deal with them another time.)
Instead, let's assume for adventure's sake that we're sharing the planet with a flesh-and-blood offshoot of the human species. As I've tried to demonstrate, the prospect isn't as absurd as it initially seems; indeed, I expect it will seem much less so when we've learned more about our world and our relatively brief tenure here. (It bears mention that eminent primatologist Jane Goodall has defended the scientific search for "Bigfoot," a cryptohominid commonly described as enormous. Assuming a gigantic and purportedly foul-smelling primate can successfully lay low, it may be substantially easier for an intelligent technical society, with a tested capacity for stealth and a full repertoire of disinformation tricks, to dodge our radar.)
Astrophysicists discern black holes -- the invisible corpses of collapsed stars -- by detecting their gravitational influence on neighboring phenomena. Similarly, the search for extrasolar life hinges on the belief that technological civilizations -- regardless how advanced -- will necessarily betray their existence via electromagnetic emissions. Freeman Dyson, for instance, has suggested hunting for alien megascale engineering by looking for its distinctive energetic signature.
We can apply the same basic principles to the search for nonhuman intelligences here on Earth. If some UFOs are indeed the work of an indigenous race, we ought to be able to detect the inevitable "signature" it's imprinted on the planet. This confirming evidence can take many forms: anomalous fossils, genetic traces, "mystery" transmissions, and even inexplicable artifacts.
Our technology-driven world is effectively shrinking at a pace that threatens to obliterate remaining wilderness areas. At the same time, we continue to map the continents and oceans (not to mention the surfaces of other planets) with ever-improving instruments. It stands to reason that the "Ultraterrestrial Hypothesis" is testable. In other words, no matter how addicted to seclusion, a parallel society will eventually betray its existence.
But maybe they don't
want to be found. Maybe they'd prefer to observe from the balcony, unseen and unsuspected, while we go about our blundering affairs onstage. If so, then they've almost certainly noticed the hazard we pose to their maintained stealth. And while they might be our technological superiors, one couldn't blame them for being at least a little concerned.
Whitley Strieber has remarked that his "visitors," the subject of the best-selling "Communion" and subsequent books that delve into the ufological, accomplish their agenda largely through stealth and cunning; their technology, as enviable as it may be, is secondary. Strieber attributes the fall-back in his encounters with nonhumans to the fact that he no longer resides in his isolated New York cabin, but in the busy community of San Antonio. Apparently the "visitors" (whoever they are) are daunted by the ubiquity of modern civilization, able to exist among us for only limited periods -- and even then assisted by considerable disguise and technical savvy.
In many ways, this would be an appalling predicament for our hypothetical ultraterrestrials. For most of human history they would have enjoyed unimpeded dominance. Humans, without a global media infrastructure, would have been easier to fool (and perhaps to exploit) than we are now. (Or do I err on the side of overconfidence?)
In almost any event, the "others" would have been compelled to misdirect us in order to maintain cultural coherence. I suspect that the prevailing notion that they hail from outer space originates from an overarching disinformation campaign with roots that predate humanity as we know it. For millennia, we've interpreted them according to the disguises they adopt, each tailored to mesh with the given paradigm. Even a cursory overview of world folklore indicates that this ability is extraordinarily well-honed; it may be their most zealously guarded secret.
However, I suggest that our abrupt transformation into a global, intricately networked society poses a grave challenge to what has traditionally been a routine effort. We may be on the threshold of some oblique form of contact; alternatively, this contact may have begun in modern times, marked by the emergence of the contemporary UFO phenomenon and the equally alarming epidemic of so-called "alien abductions."
Jacques Vallee has remarked, somewhat famously, about the possible futility of trying to look behind the curtain; what might we be confronted with? Given the opportunity, could we even comprehend what we're seeing?
Like the origin of the "aliens" themselves, this sense of existential humility may prove to be a clever construct designed to limit our perceptions.