Where are the alien ham radio operators beaming scientific secrets or extraterrestrial poetry? Why no mysterious engineering projects out among the stars? Where’s our invitation from the Galactic Council? As the great physicist Enrico Fermi once asked, “Where is everybody?”
Maybe the Great Filter got them, Dr. Hanson proposed. The Great Filter is a civilization-scale event or circumstance that would prevent a species from colonizing space or ever meeting other species — perhaps of even continuing to exist.
The filter could be a chemical bottleneck that prevents the formation of RNA that jump-started evolution, or a geophysical roadblock to the production of oxygen, which enabled multicellular creatures. But the filter could also be nuclear war, or a world-destroying asteroid, or global warming, or a malevolent artificial intelligence gone amok. Or, even, a vicious pandemic.