Hi, Dave Goldberg here, and I enjoyed your post. I just wanted to address a couple of things. First, I hope you do enjoy the "User's Guide," but you won't find detailed calculations in there. I did, as you note, do a later post where I showed the math behind the energy calculation. If there is interest, I might also do a technical post on the relations between acceleration, travel time as seen by the earth, and travel time as experienced by the astronauts.

You're right, of course, that we might not be limited to the energy available on earth by the time we make such a trip. I just wanted to put the energy requirements in perspective. And I agree that a light sail powered by an earth-based laser would, indeed, get around some of the energy requirement issues. It does creates others, however.

For instance, you'd need to be able to collimate and aim the laser with ridiculous precision, which would require doing so above the atmosphere.

There's also the fact that as the ship approaches the speed of light, the beam would appear more and more redshifted from the ship perspective, meaning that it would pack less of a wallop than it should. I'd have to do the calculations in detail, but in essence, it would mean that the laser would have to increase in power tremendously in mid-flight.