Hence sending out 100s of colonies per year is no more significant to them than flying a 747 is to us. Solutions (?) To the Hart (or Fermi) paradox They all self-destruct They are here: UFOs They were here: Ancient astronauts (von Dnikan) They are on the way: Colonization is like diffusion, i.e. it takes a while Zoo hypothesis: we are in a zoo Reserved space for expansion Proxmire scenario: Politicians will say too expensive Maybe slow interstellar travel is not possible. The interstellar medium (ISM) is full of iron basketballs?