Any given time traveler moving form one time to another at a rate greater than the local entropic norm, or on a reverse entropic vector, has to move outside normal space-time along a pathway called a Feynman curve. Now, if the past the traveler goes to is not disrupted, the Feynman curve retains its integrity and, provided the traveler can find it again, the way is clear to return to the starting point. However, if the timeline is significantly disrupted, the Feynman curve collapses, because its end point-that is, the traveler's starting point—no longer exists. It's like cutting the end of a rope bridge.
Though strict causality did not exist at the most fundamental levels of the universe, it was the defining characteristic of macroscopic existence. Indeed, that was one of the chief reasons why the warp drive and time travel took so long to be discovered by emerging cultures. Even though both concepts were rather simple, requiring little more than a basic atomic-age engineering capability to demonstrate, the ideas of faster-than-light travel and time-like curves independent of space could not easily be grasped by minds narrowly conditioned by primitive Einsteinian physics—any more than Newton could have conceived of relativistic time dilation.
A long time ago, before anyone thought time travel was possible, theorists imagined a situation in which a time traveler could go back in time and kill his grandfather before his father was conceived. No father meant no son. No son meant no time traveler. But no time traveler meant that the grandfather hadn't been killed, so the father was born, the son became a time traveler, and the grandfather was killed.
That can happen. There's nothing to prevent it.
The difference between what the Einsteinian-era physicists thought and what we know today, from actual experimental demonstrations, is that no paradox results.
"How's that possible?"
Two solutions are suggested, but neither is testable—so both have equal validity. One solution is that if you, say went back and killed your grandfather, a temporal feedback loop would be established that would collapse into a hyperdimensional black hole, cutting the loop off from any interaction with the rest of the universe. The end result would be as if the events leading to that feedback loop never happened. The second solution states that the instant you killed your grandfather, you'd create a branching timeline. That is, two universes would now exist—one in which your grandfather lived, and one in which he died.
If he died, you can't come back and kill him. Not from the new timeline. But since you came from the old one, there's no paradox. However, because the Feynman curve you followed no longer exists, you are trapped in the new timeline you created, with no way to get back. In effect, you're a large virtual particle that has tunneled out of the quantum foam.
A few years ago, when Worf was on the Enterprise, he encountered a series of parallel universes that were extremely similar to our own. Some researchers suggest that those parallel dimensions have actually been created by the manipulation of past events by time travelers.
