The Chrono series has an absence of the Grandfather Paradox. If you recall, the Grandfather Paradox addresses what would happen if a time traveler went back in time and shot his mother before he was born. Would he cease to exist on the spot? The Chrono series has a knack for shunning paradoxes, and this case is no different. Let's view some examples.
Crono takes the Jerky to the Porre Mayor of 600 A.D. to make his 1000 A.D. counterpart give up the Sun Stone. So Crono's past now reflows through the new timeline; history is changed and the mayor is happy. Crono has no need to go to the past now to give the 600 A.D. mayor the Jerky because his 1000 A.D. descendent is now generous and gives him the Sun Stone. If Crono doesn't go back, the 600 A.D. mayor won't get the Jerky. The grandfather paradox is conveniently brushed aside. Another one -- Crono defeats Lavos in 1999 A.D., making the future bright and happy. In this new timeline, Crono's history through time is changed. The future he must warp to now is not derelict and ruined. So does he still find an Epoch just waiting for him there? There won't be one, so he can't do half the stuff he's supposed to in order to kill Lavos! Will he see the Day of Lavos recording, the event that motivated him to go on his quest to defeat the alien parasite? Certainly not; Lavos never destroyed the world in this happy future. You can take this idea and apply it to nearly every single instance of changing time in the game!
Now, how does Chrono Trigger avoid the Grandfather Paradox is these examples? The answer is simple: time travelers are immune to the changes they make in the past, and the Grandfather Paradox simply does not exist. Crono can change the future so that he will never see the Day of Lavos recording and never be motivated to change the future, but this does not matter. Crono exerts his own free will to change history; he remembers each act he performed, even if these acts would preclude his necessity to perform them. While non-time travelers will always see the bright and happy future, Crono and his group can clearly remember the ruined future and their actions to remedy it. Crono's actions are preserved in time; if you went back to 600 A.D. a little before Crono came in to give the Jerky to the elder's wife, you could clearly observe him emerge from a Gate and give the elder's wife Jerky. No matter how time travelers drastically change history (even their personal history), they are exempt from the effects. Consider the Principle of Discarded Timelines above; the very presence of Crono in the past would change history, if only by his miniscule exertion of gravity on the particles of the world. Yet Crono remains the same person while traveling through time!
The unifying principle behind Time Traveler's Immunity is that once an entity travels through time, he or she is disconnected from the original timeline by changing history (past or future), and causality and consequences are resultingly broken.
Time Traveler's Immunity has other ramifications; namely, the Emergence Preservation Corollary. Imagine that you go back in time to 999 A.D. and murder Crono a full year before he can travel through time. What occurs at 1000 A.D.? Does the ruined future magically come back into place now that Crono cannot conceivably go back in time to 600 A.D.? It would not; under Time Traveler's Immunity, a time traveler's actions are preserved in the past. Think of it like this -- when Crono enters a Gate to 600 A.D., it does not take him to a specific timeline, but merely deposits him somewhere spatially at the time of 600 A.D. in the current timeline. Let's make an example based on a few occurrences in chronological order, all hypothetical.
- Time Traveler Locke goes back to 600 A.D.
- Locke plants a tree in fertile ground, and goes back to the present.
- Time Traveler Ethan changes prehistory to make the world have a desert climate.
Now, when Locke comes through the portal, will he see fertile ground? Certainly not; he'll observe the desert, as the timeline containing fertile ground has been relegated to the Darkness Beyond Time. The same would happen if someone went back in time and killed Locke after he had already time traveled. Locke would still emerge in the Gate of 600 A.D., as the act of time traveling separated his relationship of causality from the timeline by relegating his historical timeline to the Darkness Beyond Time. This means that Crono and the others' actions as time travelers are preserved in time, and even though they might preclude their birth from ever happening, they still exist even if they return to a 1000 A.D. in which they were never born. Time Traveler's have immunity from their own actions or the change of timelines.
At this point, two questions are probably on your mind. Firstly, what about the incident at the beginning of the game where Marle is explained by Lucca to have disappeared due to the grisly fate of her ancestor, Leene? That will be addressed in the Issues section of this article, which is next in line after this section. Secondly, you may be wondering, what happens to the new versions of time travelers? That is, if Crono goes back in time and changes history so that the climate of the world is now desert, what happens to the version of Crono who grows up in this desert world? And what happens to this boy when the time arrives that the original Crono went back in time? Does he simply disappear, or is he free to live? These are addressed in the Time Bastard theory.
Thought, chrono eric, Eske
Time traveler's immunity also applies for dimensional travel, which can be thought of as "horizontal" time travel along timelines rather than "vertical" travel back and forth on a timeline (compare to traveling to areas of Time Error, like the End of Time). Thea absence of TTI for dimensions (which some might call "Dimensional Traveler's Immunity"), a paradox could arise. Eske provides an example:
- Professor A invents a Time Travel "gate key" and can open time gates anywhere.
- Then, 10 minutes later, he invents a machine that creates an Angelus Errare that exists in all time periods (like CTDS) and is fixed in place, and can only exist if the machine is operational.
- Professor A can lay time gates perfectly on top of the Dimensional Vortex to use both simultaneously.
- Professor A indeed uses his devices simultaneously, immediately after both are invented.
- From Dimension 1, Time X, Professor A travels to Dimension 2, Time X-10.
There, he tells a resident, Person B, to traverse dimensions to Dimension 1 and kill Professor A before all means of extra/other spatial travel were invented.
- Person B at D2: Time X-10 travels to D1: Time X-10 and kills Professor A just seconds after Time Travel is invented. Dimensional Travel is never invented.
Now lets add "1" to Time Error and play this through again.
- A (time) gate will open in Time X-10 no matter what. But where? Time portals will always open no matter what because of TTI. Which means Professor A (who, by TTI, will always remember inventing dimensional travel as well) will appear 10 minutes into the past, meet the counterpart of Person B and tell him to cross dimensions to kill Professor A. Professor A will be shocked to see that the dimensional vortex no longer exists. (If the machine is rendered inoperable, the vortex ceases to exist in all time periods. like how the CTDS portals only exist in all time periods for Time Error Y+1 after they are created at Time Error Y.)
- That means that no version of Person B crosses dimensions (NOT time) to kill Professor A. If DTI did not exist this would create a loop because if Person B can't kill Professor A, then dimensional travel will exist -- but then Person B would now be able to kill Professor A, negating the existence of dimensional travel - and so on.