Some of the most interesting ideas are based around the colonization wave front.
From the Technology Review article:
- Various analysis 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.
Speculation aside, Carlos Cotta and Álvaro Morales from the University of Malaga in Spain, take a slightly different path to explain the curious absence. Cotta and Morales' suggest that the automated probe idea may be closer to the truth.
Again from the article:
- The scenario involves a civilization sending out eight probes, each equipped with smaller subprobes for studying the regions that the host probe visits.....this could advance much faster than the colonization wave front.
Cotta and Morales ask, why do we assume that only one intelligent race exist at any one time in the galaxy. Why not several and each sending out probes. Ahh but now it gets interesting.
Say each major probe had a lifespan of 50 million years and any evidence of their visit to a solar system lasted 1 million years, the figure that comes up is no more than 1000 intelligent races in the galaxy. The bad news is, that if each sub-probe left evidence in each visited solar system that lasted for 100 million years there would be no more than 10 civilizations out there.
Not good news at all huh? Read complete article in Technology Review