As important as dreaming about exotic EM engines—perhaps even more so—is thinking about various more feasible technologies. After all, those are the technologies we’ll actually develop first.
Astronomer Alastair Reynolds explores these questions in an article for Reuters:
That said, any civilization willing to contemplate an interstellar expedition at close to the speed of light might also settle for something half as fast, or a quarter as fast. It would just be a question of waiting a bit longer for the news. At 10 percent of the speed of light, an expedition could reach the nearest star within 50 years. Such a mission could be achieved using fusion technologies which aren’t too far beyond those now on the drawing boards, although slowing down at the other end does add to the difficulty. Still, 50 years is a long time by any measure. The astronaut Scott Kelly has just embarked on a one-year expedition to the International Space Station, and no one has yet spent longer than 14 consecutive months in space. Clearly we have some way to go before we can contemplate decades-long interstellar missions.