I have been asked about time dilation which I thought I understood, but maybe you can help me provide an answer? If a spaceship is travelling at 0.866c then time will slow by a factor of 2 due to the speed the spaceship has. Why? If it is alone in space, light years away from any other body, how can that speed affect the clock? What is the speed talked of so glibly actually relative to? If two spaceships were travelling in opposite directions at 0.433c (lorentz factor = 1.109) then for an observer midway between their clocks would each be running slow at 0.901 x a standard clock, i.e. keeping the same time, yet each spaceship would measure the other's time as being slow by a factor of 2. So how can one say the clocks are actually running slow, rather than just being measured to run slow by the moving observers?