It is presumptuous to assume that we are worthy of special attention from advanced species in the Milky Way. We may be a phenomenon as uninteresting to them as ants are to us; after all, when we’re walking down the sidewalk we rarely if ever examine every ant along our path.
Our sun formed at the tail end of the star formation history of the universe. Most stars are billions of years older than ours. So much older, in fact that many sunlike stars have already consumed their nuclear fuel and cooled off to a compact Earth-size remnant known as a white dwarf. We also learned recently that of order half of all sunlike stars host an Earth-size planet in their habitable zone, allowing for liquid water and for the chemistry of life.
Since the dice of life were rolled in billions of other locations within the Milky Way under similar conditions to those on Earth, life as we know it is likely common. If that is indeed the case, some intelligent species may well be billions of years ahead of us in their technological development. When weighing the risks involved in interactions with less-developed cultures such as ours, these advanced civilizations may choose to refrain from contact. The silence implied by Fermi's paradox (“Where is everybody?”) may mean that we are not the most attention-worthy cookies in the jar.
As a first approximation to what humans look like, it is reasonable to glance at the mirror. This approach relies on the unremarkable assumption that each of us shares a common genetic ancestry with all people. But this might not be the case for life that developed independently on other planets. For example, animals and vegetation on the nearest habitable exoplanet, Proxima Centauri b, could be shockingly different from those on Earth. In particular, the animals might possess strange-looking eyes, optimized to detect the infrared radiation emitted by Proxima Centauri, a dwarf star with half the surface temperature of the sun.
Since Proxima b is 20 times closer to its star than the Earth is to the sun, we expect it to be tidally locked, showing the same face to its star at all times, as the moon always shows the same face to us. The species residing on its permanent dayside may be completely different from those on its colder nightside, exhibiting distinct patterns of enforced sleep. Any vegetation on the planet’s surface would adapt to harvesting infrared light, showing a “red edge” at a longer wavelength than plants on Earth. As a result, the grass in our neighbor’s yard may be dark red and not green like ours.
It is even more difficult to forecast what technologies that are billions of years old would look like. In searching for them, we must flag anomalies seen through our telescopes and not sweep unexpected signals under the rug of conservatism. If our instruments are not sensitive enough or our search techniques are inadequate, we will not discover technosignatures. Processing data without suitable machine-learning algorithms may resemble casting an ineffective fishing net that never catches fish because its holes are too big.
We design our searches based on what we see in the mirror. After radio communication and lasers were invented, we started searching for radio and laser signals from outer space; search considerations progressed similarly with the technology of light sails. As we imagine new technologies, we may ultimately find the one that would allow us to detect many other species who use it.
However, we should take caution with anecdotal observations that are not up to the standards of quantitative scientific evidence. This includes conspiracy theories without supporting evidence, which appear with some regularity, or reports about unidentified flying objects (UFOs), which do not stand up to the scrutiny of reproducibility—the prerequisite for being counted as credible scientific data. UFO reports provide clues that are always on the borderline of detectability. Since our recording devices have improved considerably over time, one would expect that a fuzzy photo taken by an old camera from 50 years ago would turn into a crisp image in today’s advanced cameras, thus providing conclusive evidence beyond any reasonable doubt.
But the clues are always marginal, implying that UFOs are most likely artifacts in our instruments or natural phenomena. To achieve scientific credibility, any finding of an unusual object must be followed by studying it or other objects of its type quantitatively through well-documented scientific procedures. Scientific evidence restricts our imagination and brings salvation from far-fetched ideas.
Fermi’s paradox is pretentious in that it assumes we humans have some sort of cosmic significance. The reality may be that we are ordinary and doomed to perish, just like the dinosaurs, consequent to some catastrophe. Why would our galactic neighbors care about how green our grass is? Given that dwarf stars like Proxima Centauri are much more abundant than the sun, most habitable planets might be covered with dark red grass, which would be as soothing to the infrared eyes of most exo-vacationers as green grass is to us. As a result, interstellar tourist agencies may find Proxima b to be a more attractive destination than Earth. We could wonder, as Enrico Fermi did, why no exo-tourists have shown up to admire us. But better yet, we could get in touch with Proxima b and entice the locals to visit and share a water-based drink with us.