Comment 1: If the human race does not destroy itself or encounter a cosmic catastrophe such as an asteroid, we will have to pack our bags and relocate eventually anyhow (or our descendants will). The Sun cooks everything in about 8 billion years.
Comment 2: "If any of these schemes were feasible, intelligent ETs would have reduced them to practise millions of years ago. We do not observe their traffic; hence either there are no intelligent ETs or none of these schemes are feasible."
Comment 3: Robert Bussard, in Acta Astronautica, described a fusion ramjet operating using the interstellar medium as propellant (rare hydrogen atoms) which potentially can reach very high fractions of "C". Nobody knows how to build a fusion engine--yet.
Comment 4: Neglecting Einstein, a kilogram of mass at "c" has 4.89 times ten to the 17th power joules of kinetic energy. It turns out that one "gee" acceleration is 1.03 light years per square year. If one could accelerate at one "gee" for one year, one would be "near" light speed and 1/2 light year from earth. A year is about 3.15 times ten to the seventh seconds. Thus the kilogram would require about 1500 megawatts delivered continuously for one year at 100% efficiency and directed into propulsive power to reach near "c". To account for various inefficiencies, call it 2000 megawatts. Roughly the output of two large terrestrial generating plants--per kilogram.
If one plans to take the propulsion along for the ride, the problem is to reduce these power plants to a small fraction of a kilogram in mass and volume. (Otherwise there is no room for payload, crew, structure). Scale up as necessary until you hit "Enterprise". Something like compressing the Sun into a small space.
Human beings are not (yet) able to deal with these energies, powers, durations.
Comment 5: One question I have saved up for the Almighty is: "Why the heck did you put everything so bleeping far apart?" It is almost as if the Universe is designed to prevent travel/contact/exploration...
--Boris
C.S. Lewis's answer was that if intelligent life has been created elsewhere, it has been kept far away to PROTECT THEM FROM US.
I rather doubt that things are going to go on for that long, but in any case we do not need travel at speeds anything close to c in order to colonize potentially teraformable planets. If the colonists are going to be taking a one-way trip anyway, then what's the rush? The exploration, teraforming, and colonization of other planets would be an enterprise spanning many centuries, if not millennia -- totally unprecedented in human history up to now. It would take a very, very long time before we would be up to it, but still just a fraction of your 8 billion years. And again, we get back to economics -- it isn't going to happen unless and until someone can figure out how to provide a "return on investment" to planet Earth for the interstellar colonization ventures. Probably the real need will be to find a way to send cargos of very rare minerals back to earth both quickly and inexpensively. There is probably a tradeoff between those two, and the realities of economics will probably put the premium on "inexpensively" rather than "quickly."