Where Could the Filter Sit?

The filter may sit at one stage, or it may be spread across several hard steps. Pinning down where the biggest barrier lies changes everything.

If the hardest barrier sits early - at the origin of life or the jump to complex cells - then the worst may be behind us. If it sits late, the danger is still in front of civilizations that reach our level. The debate is really an argument about which steps are rare enough to explain the silence.

Finding a dead civilization — the ruins of a technological species on another world — would be the most alarming discovery possible. It would suggest that civilizations routinely arise and routinely die, which would point to a filter still waiting ahead of us. No such ruins have ever been found.