Voted for
- It is the nature of intelligent life to destroy itself
- Periodic extinction by natural events
- Intelligent civilizations are too far apart in space or time
- Human beings have not existed long enough
- We are not listening properly
- Civilizations broadcast detectable radio signals only for a brief period of time
It is a conundrum. We currently have only one known case of any life arising on a planet. We need more to get any idea of how common it is. The Drake equation has too many unknowns.
In a few decades we have just managed to get a probe outside our solar system. We need to avoid wiping ourselves out to explore further, but reaching other stars will take centuries. The exoplanet search may reveal something.