I'm sorry, I am one of those idiots who just doesn't get it. I see this question has been asked in about three threads below, and I still am having trouble with it. Please be patient with me and allow me to ask it again in my own words. I am using Modern Physics by Taylor and Zalfiratos to self study the subject. They begin the subject of time dilation with the example of a man on a moving train, who has a light flash from the bottom, reflect off a mirror on the ceiling, and beep when it hits the ground again. The light travels a distance of twice the height of the train (in his reference). TWO observers on the ground see the light travel the two edges of a triangle with height h and base related to the speed of the train. Since the light is measured at the same speed in both reference frames, time must have dilated in the reference frame of the moving train. I will now quote the authors for a second, "This asymmetry may seem surprising, and even to violate the postulates of relativity, since it suggests a special role for the frame S'. . . . in our experiment the frame S' is special, since it is the unique inertial frame where the two events occured AT THE SAME PLACE. " That sort of makes sense to me. But then the authors go on to describe the 1971 plane experiment where sycrhonized atomic clocks where put on planes and observed to experience time dilation with the US Naval Observatory in Washington, D.C. A 273 +/- 7ns difference. Now to complicate matters, this must also take into account the affect of general relativity with the gravity of the earth, which the author's mention in a note, but they are useing it to prove the portion due to normal SR time dilation. Okay, finnally my question. Why did the plane time dilate with reference to the ground time? In the rest frame of the plane, the clock is ticking away AT THE SAME PLACE. BUT(!!!), if you ask the pilot and he looks down as he flies over Washington, he could just as well say the earth is moving past him at (whatever) speed it was, and the clock in Washington is ticking AT THE SAME PLACE. So he would measure the Washington clock as dilating. But when they got back together; the plane clocks was definitly behind. Now, IF the answer to this question has something to do with the acceleration and deceleration of the plane; wouldn't that imply that this IS NOT an example of SR after all, but of GR of some sort. Couldn't you have simply accelerated the plane to its full speed then decelerated immediatly and gotten the exact same result (no need to spend time in the air as part of a seperate inertial frame). I'm so lost.