Several explanations have been offered over the years:
- Life is actually quite rare and humanity is the first species to become advanced enough to search for other civilizations.
- Intelligent species have emerged regularly throughout the history of the cosmos but end up destroying themselves through misuse of technology (e.g. nuclear war, environmental catastrophe, misguided genetic engineering).
- Advanced civilizations are common and aware of us but keep themselves hidden so as not disturb our culture (cf. Star Trek's prime directive).
If there are no aliens out there,any eﬀorts at communication were obviously wasted. Thus we can assume for the sake of discussion that there are aliens out there likely to receive the messages at some point. The relevant parameter, then, is not the probability of our messages being received by aliens who might potentially do us harm: it is the conditional probability of the aliens who receive the messages doing us harm, given that the messages are indeed received (and understood to be messages). Can we really say that this probability is so negligible, bearing in mind that any such aliens appear to have made no reciprocal attempts to advertise their existence? . . . it often seems to be implicitly assumed, and sometimes is explicitly argued, that colonising or otherwise exploiting the resources of other planets and other solar systems will solve our problems when the Earth’s resources can no longer sustain our consumption. It might perhaps be worth contemplating more seriously the possibility that there may be limits to the territory we can safely colonise and to the resources we can safely exploit, and to consider whether and how it might be possible to evolve towards a way of living that can be sustained (almost) indeﬁnitely on the resources of (say) our solar system alone.Which leaves me wondering at what point the precautionary principle morphs into paranoia.