DR. HAWKING: If there is a government conspiracy to suppress the reports and keep for itself the scientific knowledge the aliens bring, it seems to have been a singularly ineffective policy so far.
Furthermore, despite an extensive search by the SETI project, we haven’t heard any alien television quiz shows. This probably indicates that there are no alien civilizations at our stage of development within the radius of a few hundred lightyears. Issuing an insurance policy against abduction by aliens seems a pretty safe bet.
Why haven’t we heard from anyone out there? One view is expressed in this Calvin cartoon. The caption reads: “Sometimes I think that the surest sign that intelligent life exists elsewhere in the universe is that none of it has tried to contact us.”
More seriously, there could be three possible explanations of why we haven’t heard from aliens. First, it may be that the probability of primitive life appearing on a suitable planet is very low. See: NASA’s 50th Anniversary Lecture By Professor Stephen Hawking
Dialogos of Eide 