There are two separate clocks, each set in a plane and on the ground. Assuming inertial reference frames, how can this be? Perspective of the observer in motion: The observer in motion on the plane will have recorded some time duration. Since the the world outside the plane is moving at a velocity relative to the plane, time on the ground would have slowed down. Perspective of the observer on the ground: The observer at rest would have seen the clock inside the plane slowing down since it moving relative to the ground. Observer vs observer When the two observers do meet again, what will they have to say about each other's observations? Would they have measured the same relativistic effects? Would the pilot say that the ground observer's clock had slowed down, or would the ground observer say that the pilot's clock had slowed down?