A friend of mine is writing a hard sci-fi novel about interstellar colonisation. He wants to use the real Earth and the surrounding stellar neighborhood. The plot requires that the initial voyages take 50-60 years to get to a habitable planet (let's say up to 100ly from Earth) but that, over time, and in little increments, the travel time is reduced to around five years.
He does not want to resort to things like hyperspace, warp speed, or "jumps" (like in the Foundation series), he feels that these methods are too fantastic and the travel time is too short for his plot. He is also worried about trying to explain how relativistic effects were overcome.
My advice to him is to not explain the technology behind his story and rely on the readers' suspension of disbelief. But does anyone have some technical explanation that would fit these details? Thanks, much obliged.
Update: in the plot the amont of time passed on the planets is not important. It is the amont of time on the ships travelling that has to fit the above criteria.