Why is it that when describing time dilation, we say that the object (the thing observed) slows down relative to the subject (the observer), rather than vice versa? If the subject were to speed up, everything around them would appear slowed down, correct? Let me take a crack at this myself before I open the floor (gratefully) for your input: The subject reports that the object has slowed down, and since relativity is all based on the frame of reference of the observer, we must therefore report that the object has slowed down, since there is no evidence from the observer's perspective that they (the observer) has sped up. The observer always appears "normal" from their own frame of reference. As always, thank you.