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.
Cotta and Morales take a different tack by studying how automated probes sent ahead of the colonization could explore the galaxy. Obviously, this could advance much faster than the colonization wave front. The scenario involves a civilization sending out eight probes, each equipped with smaller subprobes for studying the regions that the host probe visits.
This is not a new scenario. One previous calculation suggests that in about 300 millions years, those eight probes could explore just 4 percent of the galaxy. The question that Cotta and Morales ask is this: what if several advanced civilizations were exploring the galaxy at the same time? Surely, if enough advanced civilizations were exploring simultaneously, one of their probes would end up visiting the solar system. So that fact we haven't seen one places a limit on how many civilizations can be out there.