Hello Physics Forums! I'll try to cut right to the chase. I'm studying both length contraction and time dilation in my physics class and it seems these two occurrences are mutually exclusive. That is, in any given situation, only time will be dilated or length will be contracted. This doesn't really make sense to me though. For one, "how do it know?" whether to contract or dilate length or time respectively? Is it true that its one or the other, but never both?