Is the apparent absence of extraterrestrial technological civilizations down to the zoo hypothesis or nothing?

The 'Fermi Paradox' refers to the mismatch between a widely held expectation that advanced technological life should be common in the Universe — recently given impetus by the discovery that other planetary systems are common — and the absence of any evidence for it. In this Perspective, we briefly review attempted solutions to the paradox and conclude that either (i) extraterrestrial technological civilisations are extremely rare (or absent) in the Galaxy; or (ii) that they exist but are deliberately hiding from us, a scenario generally known as the 'Zoo Hypothesis'. In this sense, we propose that the answer to the Fermi Paradox is 'the Zoo Hypothesis or nothing'. We argue that, given a strong commitment to the continued exploration of the Universe, humanity may be able to distinguish between these two alternatives within the next half-century.

Ian A. Crawford, Dirk Schulze-Makuch