Al writes: "The Fermi Paradox focuses on the existence of advanced civilisations elsewhere in the galaxy. If these civilisations are out there--and many analyses suggest the galaxy should be teaming with life--why haven't we seen them? Carlos Cotta and Álvaro Morales from the University of Malaga in Spain add an another angle to the discussion about the speed at which a sufficiently advanced civilisation could colonise the galaxy. Various analyses suggest that using spacecraft that travel at a tenth of the speed of light, the colonization wavefront 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 ET's absence. Cotta and Morales study how automated probes sent ahead of the colonisation could explore the galaxy. If these probes left evidence of a visit that lasts for 100 million years, then there can be no more than about 10 civilisations out there."