Robert Garriott retweeted
Let's suppose we have a spaceship with a plasma drive that never runs out of fuel. It can accelerate at 1G. On a trip to Alpha Centauri, the ship accelerates at 1G to the halfway point then does a 180 and starts reducing speed at 1G until it reaches Alpha Centauri. The trip takes just 6 years, but due to time dilation, only 3 1/2 years pass for the crew.