Is that only supposed to work within our own galaxy, or is it supposed to be intergalactic? I would guess that the distances between galaxies have to be greater than the distance between any two stars within our own galaxy.
Also is there not some third assumption in the picture, i.e. that the robot probes would be able to travel vast distances without suffering damage from debris, dust, and radiation?
Also is there not some third assumption in the picture, i.e. that the robot probes would be able to travel vast distances without suffering damage from debris, dust, and radiation?"
Good questions. The original idea applied only to the Milky Way Galaxy. The age of the galaxy and the probable ages of advanced ET civilizations allow ample time for every star to be visited at relatively (sorry) modest velocities. 0.05 c is "assumed" to be within their capabilities. Other galaxies are MUCH further away and 0.05 c would be a stretch. On the other hand, their technology might be much more capable. (and, BTW, there is nothing prohibiting alien life forms which live for centuries or millennia, making slow trips much easier than for mayflies like us).
Regarding robot damage, of course. Remember though we are dealing with advanced technological civilizations 10,000 or 100,000 years more advanced than we. Self-repairing robots, nanotechnology, and other measures we cannot imagine could be applied to the problem. Plus, as I noted, you expect attrition, so instead of 100 probes maybe you launch 1000.
A civilization with technology 100,000 years beyond our own would probably not notice the difference in cost between launching 100 and 1000--or 10,000 such probes.
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If Drexler's projections for nanotechnology on Earth are even 25% correct, eventually economics will cease to be relevant. Literally anything could be made for almost nothing. In such a society, the only valuable commodities would be energy, creativity, and originality...
--Boris