The key problem is that as velocity approaches the speed of light, the relativistic mass approaches infinity. The fuel mass also approaches infinity, but derivable energy does not because it is related only to the rest mass. However the energy required to drive it is related to the relativistic mass. Therefore, energy requirements increase dramatically. This has been revived recently, in the assumption that an advanced civilization can use lasers more effectively than we do.
Strategy is very important.
Noise will overwhelm radio astronomy soon. Started 1973, ended 1979. 70 stars within 45 light years vii. Others will be mentioned if time permits. Perfect zoo hypothesis 2.