I have never met an extraterrestrial. Neither have you or anyone you know (alien abduction folklore notwithstanding).
This is a somewhat surprising thing given that as planets go the Earth is really nothing special or unique. In fact, there appears to be a substantial number of very similar planets in our galaxy alone—one estimate puts the number at around 6 billion. Considering that life materialized very soon after the Earth formed, it seems highly likely that many if not most of these Earth-like planets are also populated with living creatures, and that several of those are inhabited by intelligent beings capable of space exploration. So, where are they? if the galaxy is teaming with intelligent life, then how is it that you and I know only Earthlings?
That question forms the substance of what has been called the Fermi paradox.
There are several potential rejoinders to this paradox. Until recently, the easiest solution was to claim that the Earth is unique in the universe, and that life is an extremely unlikely thing to happen anywhere. Both science and simple logic suggest otherwise, however, and the absence of evidence is not evidence of absence.
A slight variation on the uniqueness explanation is to suggest that perhaps the universe is infused with life, but intelligence like ours is a rarity. This explanation, however, smacks of hubris, reinforcing common but grossly misguided opinions about human superiority and implying that humans serve as some kind of benchmark for what counts as intelligence.
A humbler and frankly more interesting possibility is that intelligent life—whatever the benchmark—is a fairly common occurrence, but there is only a narrow window of time in a planet’s lifespan in which technologically-advanced beings appear on the scene. The Earth was crawling with life for almost 4 billion years before humans came along, and humans themselves have been around for a couple million years, but it’s only in the last hundred years or so that we have been engaged in technological activities detectable by someone outside our solar system. So, even if intelligent life is common throughout the universe, it may be statistically unlikely for two or more technologically-advanced interstellar species to be operating at the same time. Maybe the window is narrow on both ends, and the particular kind of intelligence that leads to space exploration inevitably results in catastrophic extinction shortly after it appears. Consider how we have very likely guaranteed our own species’ extinction by failing to respond to global climate change, for example.
I personally find the narrow-window idea compelling. But there is a far more parsimonious explanation that involves a slight reworking and extension of the original uniqueness argument. The Earth itself may not be all that unique as planets go. And the emergence and evolution of life might be a rather pedestrian occurrence, perhaps inevitable once certain minimal conditions are met. But the specific way in which life has evolved on Earth is unique, expressing forms and patterns that are idiosyncratic to the point of being entirely unrepeatable anywhere else in the universe. Communities of self-replicating multiple-cell organisms might show up everywhere there is liquid water, but there is only one place that herds of bison could ever happen. Self-aware intelligent creatures might be as common as snowflakes, but planet-consuming metropolitan congregations of linguistically-endowed tool-using primates are a one-off event in the history of the universe.
There are at least three “uniqueness filters” that render human technological civilization as we know it an entirely distinct and unrepeatable phenomenon, one that is extremely unlikely to have even a remote analog anywhere else. Ever.
The first filter is natural history. The human species is a chance result of 4 billion years of intersections and interactions among an uncountable number of very specific biological, climatic, and geographical events, many of which are unique to the point of being unrepeatable on Earth—let alone something that could happen in the exact same way anywhere else. One stray asteroid more or less, a different sequence of volcanic eruptions, the survival of a now-extinct species of insect, a slight deviation in the pattern of continental drift, an unlucky solar flare, a difference in the timing of glaciation, a random virus mutation, and humans never happen. The natural history of life on another planet would be likewise unrepeatably unique to that planet, and even if the other planet started out as an exact physical duplicate of earth, any resulting intelligence would be housed in creatures very different from humans.
The second filter is the appearance of technological civilization. Humans are a foraging species. For hundreds of millennia, all humans everywhere were hunter-gatherers living in small nomadic or semi-nomadic communities. In the wake of the last ice age glacial period, some of these groups became larger, more sedentary, and increasingly dependent on domesticated food sources. But the transition to agriculture was not an inevitable event in human social evolution. It was a chance result of a unique confluence of climatic, geographic, and social conditions.
It turns out that some (at least one) kinds of domestication-based lifestyles have a viral quality to them, and spread quickly (and violently). Once dependence on agriculture exceeds some minimal level or degree of spread, civilization might be an inevitable eventuality. Even so, the fact that the first civilizations appeared when they did instead of a hundred thousand years previously or yesterday or not at all has nothing to do with the nature of humans as a species. Rewind time 12,000 years, change something as seemingly trivial as the pattern of glacial ice melt in the north Atlantic that altered the ocean currents affecting climate in northeast Africa and the Levant, and civilization would never have happened. At least not when and where it did.
The third filter is the shape of historical events once civilization appeared. Civilization, because it happened when and where it did, took on specific forms that set the stage for future versions of civilization. The modern global technologically-advanced world still carries systemic traces of physical, social, economic, and political forms and patterns that emerged in the earliest cities of Sumer and Egypt and Babylon. Technology itself “advances” in unpredictable ways and in response to unlikely events, and might rightfully be considered a fourth uniqueness filter. Add to this the fact that the details of our present circumstances are entirely contingent on the details of uncountable and arbitrary past events, and that our values, interests, beliefs, and ideas are linked intimately with a specific historical past that didn’t have to happen the way it did.
All of this means that popular interest in space exploration is not necessarily an inevitable consequence of human intelligence and curiosity. Nor is its form independent of the specific colonial and technological history of modern civilization. There is a direct through-line connecting Enlightenment ideas of progress, nineteenth century notions of manifest destiny, and present-day ideas about space colonization—ideas that may have never occurred to anyone had our social history been somewhat different.
These three filters stack upon each other, with each layer adding an order of magnitude to the unrepeatable uniqueness of the present human situation. The universe might very well be packed to the brim with intelligent lifeforms that share virtually nothing in common with each other, and the reason I have never met an alien is that the motives of civilized humans are not galactically generalizable and I’m the only one even remotely interested in the possibility.