So, what could the Great Filter be?
Well, perhaps abiogenesis (life arising from lifelessness) is wildly uncommon. Perhaps the extreme rarity of this event is in fact the Great Filter. Alternatively, perhaps it’s common for life to spontaneously arise, but the overwhelming majority of life never progresses beyond simple single-cell organisms. Maybe the universe is teeming with bacteria — but bacteria don’t build starships.
Alternatively, the Great Filter might be a consequence of technology itself. Perhaps advanced civilizations usually eradicate themselves via some sort of technology run amok, such as malevolent artificial intelligence, nanotechnology, or a doomsday machine. Humanity is already more than capable of destroying itself via global thermonuclear war. And sadly, it’s possible that such extinction events are virtually inevitable throughout the cosmos.
The Great Filter could also be a purely outside event that is not dependent on the species its testing, regardless of how advanced they may be. For instance, the impact of a giant asteroid or rogue planet, a nearby gamma-ray burst, or an intrusive supernova could potentially annihilate all life on Earth — or any other planet for that matter. No technology in our arsenal today could stop these events from occurring, even if we had forewarning.
Another possibility is that more than just one step of the Great Filter is extremely unlikely to occur. This would exponentially increase the difficulty of a civilization achieving the level of technology required to master interstellar travel.
Has humanity passed The Great Filter?
If the Great Filter is behind us, though, it bodes well for humanity as a species; the universe may be ours for the taking. If, however, the Great Filter still lies ahead, we may be doomed.
On the bright side, some have interpreted our apparent aloneness in the universe as a good sign — a blessing even — as it indicates we’ve safely made it through the bottleneck. Strange as it may seem, we may be the first species to have passed through the Great Filter (after all, someone has to be first).
On the other hand, if we were to detect a signal from a super-advanced technological species that makes us look primitive, it might imply that the Great Filter still lies ahead. Humanity could be destined to take a surprise cosmic test, one that we don't know what to study for.
The Great Filter is only a theory — yes. But from a logical perspective, it’s an appealing idea on many levels, offering a plausible explanation to the Fermi Paradox. So, although the question of “Where are they?” still remains unanswered, the Great Filter theory offers one of the best guesses we can dream up. Unfortunately, that doesn’t answer whether the Great Filter is already in our rearview mirror.
Doug Adler is the co-author of From The Earth to the Moon: The Miniseries Companion