[Physicist]

Quite obviously, a lot happens between the point at which he satisfies himself that he has gone far enough, and the point at which he actually turns around. A lot of "events" get telescoped into that interval.

Precisely. The solution to the twins problem is sometimes referred to as the "simultaneity gap", but I consider that a misnomer, because there isn't an actual gap, unless you contrive something like my acceleration-free example. But during the turn-around, there's a hell of a "fast-forward" that takes place back on Earth, from the traveller's point of view.

[NicknamedBob]

I had a question about this statement;

"Accelerating a 1-ton payload to 90 percent of the speed of light requires an energy of at least 30 billion tons of TNT."

I'm assuming they're talking about chemical energy here. I'd like to think that there's enough energy in matter to produce the propulsion necessary to get close to the speed of light, and also to slow down again.

Presumably fusion power will be the manner of our acquiring and using this energy. It has always for me been a question of how do we get the thrust necessary to get to such speeds.

Do you see a problem with using synchrotrons to accelerate our propellant to maximize our thrust?