I've really been trying to understand the concept of time being relative in special relativity. I keep hearing the example of a spaceship taking off from earth and traveling 99.999% the speed of light. It is said that possibly 200 years could pass for an observer on earth while only days could pass for those on the spaceship. This makes absolutely no sense to me. If 200 years have passed on earth and the spaceship hasn't returned, then it has been gone for 200 years. Can somebody please try and explain this to me.