Swimming pool reactors glow visibly blue by Cherenkov radiation.
And if any of these schemes were feasible, they would (he repeated) have been perfected by intelligent ETs thousands or millions of years ago. At 5% of light speed, there has been ample time to visit every single star in the galaxy, using self-replicating 'von Neumann' robot probes. If intelligent species are common (and we are the latest on the block as Carl Sagan pointed out) there should be a traffic jam of probes all over the galaxy. In particular we should have been visited multiple times and there is no reason such visits should not be continuing. Yet we see no evidence (such as aliens landing on the White House lawn) of such visits.
Hence, he repeated, either we are alone or none of these schemes is feasible.