From Technology Review:
The absence of alien probes visiting the solar system places severe limits on the number of advanced civilizations that could be exploring the galaxy.
The Fermi paradox focuses on the existence of advanced civilizations elsewhere in the galaxy. If these civilizations are out there--and many analyses suggest the galaxy should be teeming with life--why haven't we seen evidence of them?
Today Carlos Cotta and Álvaro Morales from the University of Malaga in Spain add another angle to the discussion. One consideration is the speed at which a sufficiently advanced civilization could colonize the galaxy. Various analyses suggest that using spacecraft that travel at a tenth of the speed of light, a colonization wave could take some 50 million years to sweep the galaxy. Others have calculated that it may be closer to 13 billion years, which may explain why we have yet to spot extraterrestrials.
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