Why the Cosmos Is Silent: Two Lives of One Forest ✛ ✎ ✦
The Galaxy is a forest where birds don't start singing right away. First, star-trees grow, then planet-nests appear on their branches, and only after billions of years do the first songs ring out — signals from intelligent civilizations. The old Drake equation simply counted how many birds are singing now, but it never asked how the chorus has changed over time.
The transition from silence to chorus happens like an avalanche when the birth rate exceeds about 1.35 civilizations per year. But the reverse transition, from chorus to silence, requires a much stronger decline: the forest is reluctant to fall silent. This effect is called hysteresis. Previously, we didn't know that a population of civilizations could depend on its own history. Now we see: the fact that we don't hear signals doesn't necessarily mean we are alone. Perhaps the Galaxy is currently in an "era of silence," and we should be looking not just for civilizations, but for signs of which phase the forest is in.
In the nonlinear model, the critical rate of civilization emergence at which the phase transition occurs is about 1.35 civilizations per year — meaning that if at least a couple of communicative civilizations emerge annually in the Galaxy, then after some time the entire Galaxy could find itself in a state of contact saturation.
The idea of bistability in the galactic population echoes the concept of the 'Great Silence' from Liu Cixin's novel 'The Three-Body Problem': civilizations may hide because active contacts are dangerous, and the Galaxy may be in a state where everyone is silent even though intelligent life is widespread.