This book goes to show you the different attitudes prevailing in SF compared between the fifties when this book was written and the ideas today. Today, if someone took the concept of twins being separated by relativistic effects (ie one ages faster than the other . . . go read a physics textbook for the equations) it'd be a massive complex novel detailing the cultural shock and changes, the emotional cost, and mostly have a really downer attitude about it, treating it, for the most part "realistically" whatever that's supposed to mean in science fiction. But that's now and we're talking about then. Here Heinlein takes two vastly different concepts, telepathy between twins and the travelling between the stars in ships moving at barely sub-light speeds. The focus here is on both SF adventure and what it means to leave everything you know and have not only your entire family grow old and die while you age much slower but have the whole world change (Heinlein's future slang is a gas), while you stay the same. It's a fun story, and the main character, the twin that goes, narrates his story somberly but still with a dash of zest in his voice, for all the regret he feels at being left behind, he's having a grand adventure and seeing things that no one else has ever seen. Not that it's all fun and games and a lot of things happen before the mission is over, some good and some bad. Like I said, it's a light story, you won't find anything totally deep here but it's all handled capably, as you'd imagine a master like Heinlein dealing with this . . . but please don't read it as a gender study guide and keep in mind that it was written in the fifties, when women weren't expected to do all that much. Asking permission from your husband for just about everything was the order of the day mostly . . . just don't let it bother you. Other than that, it's good solid fun, though the ending comes totally out of left field (unless I missed something), but it's a great example of the golden age of SF, when anything seemed possible. For those who want a slightly darker take on the same subject . . . go read Joe Haldeman's The Forever War and that should knock the depress-o-meter right off the scale. Still a good book though. They're both good. Read them both, now!