Above I mentioned a basic view of the grandfather paradox. Another possibility is that when one actually travels back in time and changes something, it creates a new time line to branch off. The original one continues to exist unchanged while the new one starts with that change and continues forward.
my education officially goes no further than American public High School. In other words, All this is based on my understanding of extremely complex theories of which I have no formal training or education in. So I could be completely wrong.   See 19 other replies

David S. I will start with you. if you assume that past present and future exist together each of which is continually happening right now then maybe, but they are not jumping through earths history they are passing through dimensions, Now is always going to be now. even if 5 minutes go by it is still now. History can progress as it has but when they come in and change something, bringing medicine to a world that has none for example creates no paradox simply because it is introduced at the now in that dimension. The grandfather paradox is introduced when one travels to the past in order to effect a change in the time line of that universe. The change created is only possible if one is able to be there to create that change. If one changes something then there is no need for one to travel to the past to change said outcome. If there is no reason to travel back in time then there will be no traveling back in time. Effectively it would be as though you never made the trip back in time, causing everything you changed to never have happened in the first place. I think it is string theory that, at its basics, states that time branches off infinitely from specific points in history causing multiple time lines. Each one offering an alternate history and progression of events from past to future. Each branch continues forward while the original time lines stays intact and continues forward continually branching off creating new time lines. Each one of these time lines, each branch of time, represents a different series of progressions effectively creating an alternate reality.   See 19 other replies

No it's not, a paradox is something that is logically contradictory. "interfering in the natural evolution of sed state" does not compromise a logical contradiction.   See 19 other replies