Adam said:

"I know you think the collision problem is insuperable above ~ 0.01c, but what does that do for interstellar prospects if that's the limit and yet the average distance between suitable planets is ~ 30 ly or more?"

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I don't have concerns about the collision problem. The vehicle would require a Whipple shield to protect against interstellar grains and surface heating due to free molecular gas collision. A practical starship propulsion system would almost certainly be based upon nuclear fusion (antimater propulsion is too hard to do). I don't think aneutronic controlled fusion is practical due to the high ignition temperatures required. However deuterium fusion seem practical. The energy loss due to neutrons limits the top speed to around 0.01c.

"Conventional" human interstellar travel is impossible due to cosmic radiation. However humans could still be transported to other star systems as information. The starship's autopilot could have human DNA sequences stored in its memory. The autopilot could reconstruct human beings based upon this DNA information after arriving in a new star system and discovering an inhabitable world. The autopilot could even modify the information based upon differences between the newly discovered world and the Earth. The science fiction argument that the Earth was seeded by ET probably does not wash because we have too much commonality between our DNA and lower life forms. Of course the counter argument is the lower life forms were seeded by ET and we evolved from those earlier creatures. The fact that vertebrates and invertebrates are so radically different gives some strength to that argument. One could carry the argument even further that ET comes by every 10 million years to tweek terrestrial evolution towards a preferred direction. Again, where are the artifacts for this? I'd expect to see some designer initials on our DNA along the lines of "Blade Runner".

Gary