This hypothesis — somewhat similar to the Prime Directive in Star Trek — explains the lack of clear evidence of other civilizations by suggesting that they’re aware of us, watching us, but choose not to make us aware of their presence.

First proposed by John Ball (Ref 17: Ball, John A., The Zoo Hypothesis , Icarus, v.19, p.347, 1973). This simple hypothesis rests on three premises.

  • Whenever the conditions are such that life can exist and evolve, it will.
  • There are many places where life can exist.
  • We are unaware of any extraterrestrial forms of life.

As Ball himself says, the zoo hypothesis is pessimistic and psychologically unpleasant. It would be more pleasant to believe that extraterrestrials want to talk with us if they knew we were here. However, as Ball concludes, the history of science is full of examples of of psychologically unpleasant hypotheses that have turned out to be true.