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| + | ==== Where Are They? ==== | ||
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| + | 7.1 Extraterrestrial life is rare or non-existent | ||
| + | 7.2 No other intelligent species have arisen | ||
| + | 7.3 Intelligent alien species lack advanced technology | ||
| + | 7.4 Water world hypothesis | ||
| + | 7.5 It is the nature of intelligent life to destroy itself | ||
| + | 7.6 It is the nature of intelligent life to destroy others | ||
| + | 7.7 Periodic extinction by natural events | ||
| + | 7.8 Intelligent civilizations are too far apart in space or time | ||
| + | 7.9 Lack of resources to spread physically throughout the galaxy | ||
| + | 7.10 Lack of desire to live on planets | ||
| + | 7.11 It is cheaper to transfer information for exploration | ||
| + | 7.12 Human beings have not existed long enough | ||
| + | 7.13 We are not listening properly | ||
| + | 7.14 Civilizations broadcast detectable radio signals only for a brief period of time | ||
| + | 7.15 They tend to isolate themselves | ||
| + | 7.16 Colonization is not the norm | ||
| + | 7.17 Outcomes between all and nothing | ||
| + | 7.18 They are too alien | ||
| + | 7.19 Everyone is listening but no one is transmitting | ||
| + | 7.20 Earth is deliberately not contacted | ||
| + | 7.21 Earth is purposely isolated (planetarium hypothesis) | ||
| + | 7.22 It is dangerous to communicate | ||
| + | 7.23 They are here unacknowledged | ||
| + | |||
| + | (via Fermi' | ||
nik