In the 19th century, a long-standing problem in cosmology was that the sky was curiously dark at night. If the universe is infinitely large, and contains an infinite number of stars, the end point for any part of the night sky should be bright, illuminated by light shining from a distant star.
The problem, called Olber’s Paradox, has since been resolved by theories about cosmological radiation: light from stars that are too far way are “red-shifted” to lower wavelengths that eventually become invisible.
In the 21st century, cosmologists are plagued by a different problem: there are hundreds of billions of galaxies in the observable universe, which is itself billions of years old, plenty of time for alien lifeforms to have developed the necessary technological civilization to make long distance travels in outer space, but conspiracy theorists aside, few evince the belief that we have concrete proof of extra-terrestrial life.
The conundrum, called Fermi’s Paradox, has spawned numerous grand theories purporting to explain why humanity has yet to encounter aliens, ranging from the counter-intuitive—the Earth is the only planet in the universe with the proper conditions for life—to the paranoid—aliens do exist, but hide themselves from humans, who are observed as animals in a zoo.
One of the most popular theories available, one that neither posits the uniqueness of Earth nor the existence of alien overlords, is the idea of a Great Filter. It states that as civilizations become more technologically advanced, the possibility of self-destruction also heightens, eventually converging on 1, or absolute certainty.
It’s easy to see why the Great Filter theory, first proposed by the economist Robin Hanson in the 1990s, has accrued so much currency in cosmology. Since the invention of the atomic bomb, the possibility of human extinction has never been far away from from the popular imagination, a possibility that was nearly realized during the Cuban Missile Crisis.
The Cold War has since ended, and elementary school students no longer practice bomb drills to prepare for an atomic attack, but new dangers lurk on the horizon. The Great Filter could finish humanity off not with a bang, but with a whimper—as climate change continues apace, ice shelves thousands of years old are disappearing in Antarctica.
Fortunately, cosmologists aren’t the only people concerned about the Great Filter. Elon Musk, the founder and CEO of SpaceX, started his company with the explicit goal of creating the technology needed for humans to colonize the rest of the Solar System and, eventually, leave the Solar System.