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
Say that one day you are sitting in your hotel room in NYC with an energy of one unit. Not moving, just sitting and watching CNN on the monitor. Then the next day you are sitting in LA in your hotel room with an energy of one unit. Not moving, just sitting and watching CNN on the monitor. What is the difference? Same energy, different day.
See, there is hope. The location is different, yet the energy is the same. What has changed? In the meantime there was a great burst of motion and noise, but all that is dissipated along with part of your bank account. You are back to the same energy level. If there were a way to slide soundlessly along a Willie_Green maglev track between NYC and LA, you could have gotten there for next to nothing and just as fast. Friction losses, air resistance losses, all gone, and you just move sideways for free.
The hope is that this idea could be extended to travel between planets and stars. There are levels of identical potential energy and they are equivalent. We should be able to move between these equivalent energy levels with next to zero loss of energy, no noise, no scurrying around. We're here, then we're there, it's the same.