“Intelligent species might reasonably worry about the possible dangers of self-advertisement and hence incline towards discretion” — the “Undetectability Conjecture,” put forth by Beatriz Gato-Rivera, a theoretical physicist at the Instituto de Fisica Fundamental (previously Instituto de Matematicas y Fisica Fundamental) of the CSIC (Spanish Scientific Research Council) in Madrid.
According to Gato-Rivera, we may find ourselves in a universe in which there exist intelligent technological civilizations but they have chosen to be undetectable, camouflaging themselves mainly for security reasons (because advanced civilizations could also be aggressive).
Stephen Hawking says “there ought to be many other stars, whose planets have life on them. Some of these stellar systems could have formed 5 billion years before the Earth. So why is the galaxy not crawling with self-designing mechanical or biological life forms?”
Why hasn’t the Earth been visited, and even colonized? Hawking asks. “I discount suggestions that UFO’s contain beings from outer space. I think any visits by aliens, would be much more obvious, and probably also, much more unpleasant.”