A spaceship is initially at rest in the lab frame. At a given instant, it starts to accelerate. Let this happen when the lab clock reads t’ = 0 and the spaceship clock reads t0 = 0. The proper acceleration is a. (That is, at time t’ + dt’, the spaceship is moving at a speed α dt’ relative to the frame it was in at time t’.) Later on, a person in the lab measures t and t0. What is the relation between them?