Time is the 4th dimension we don’t think about as something other than “it goes on”. The thing about time is that it’s always moving forward relative to the individual. Science fact seems to agree. Because space and time are tied so closely, I believe this “law” holds even if you are hopping around to different points in time. Stay with me here.
Science fiction seems to have gotten it wrong. As a corollary to this law, it would not be possible to re-visit a timeline you’ve “passed through” before (even if you weren’t “present” in that “when”). This would mean the more you bounce around in time the more restricted your available landing points become. Here’s an illustrative example.
Let’s say your first time-travel experience happens when you are 20 years old, and you spend 3 years at your destination. You cannot return to when you left but instead must return to when you left, plus 3 years. Think of this as your “home” timeline. So assuming your family and friends aren’t time travelers themselves, they would have last seen you when you were age 20 and lived 3 years of their lives without seeing you at all. It’s essentially the same, from their perspective, as if you were across the world on a 3-year tour, out of contact. In this way, time and space separation appears the same to the observer who isn’t time traveling.
But perhaps you don’t want that. When your twin brother turned 21, you missed it because you were “somewhen” else. Now that you are back “home”, 3 years later, at age 23, you would like to travel again, this time back in time to your twin’s 21st birthday. But alas, due to the laws of space-time, you cannot, just as if you asked to be in two places at once. You can’t be in two places, or times, at once. When your twin turned 21 (and so did you), you were “somewhen” else but your home timeline was still moving forward without you. This will remain immutable throughout your lifetime no matter whether you are living in your home timeline or “somewhen” else.
This solves many of the time-travel paradoxes we see in mainstream science fiction. Though not all of them. The classic example is: how do you know traveling back in time won’t somehow mess things up where you are never born? Not being born would mean you don’t go back in time. But wait, if you never travel back in time, you’d be born and then could eventually travel back in time, doing something that prevents your birth. And so on. This recursive problem is difficult to solve gracefully and isn’t covered here. We’ve only set out to illustrate how the intimate connection of space and time would make them both behave with similar limitations, such as the “law” proposed here.