What happens when a time traveler goes back in history and changes time? An answer beyond "it simply changes" was not available until Chrono Cross elaborated on timelines themselves and how they react to change. The exact quotes speaking of timelines are these:
[Miguel] But in saving our planet from the death Lavos was going to bring about, they also changed the course of history... [Lucca] At that very moment, this whole future time axis ceased to exist. Call it cause and effect... It is just a problem of possibilities, after all... In this world, there are no such things as absolutes or certainties. [Marle] But the future that was supposed to have disappeared is about to be restored here... The future destruction of our planet is going to become a reality in this world once again...
These comments were made by the Ghost Children at the Dead Sea; they explained that the future Crono and his crew averted -- the 2300 A.D. seen in Chrono Trigger, where the world lay ruined by Lavos -- ceased to exist once Lavos was defeated, but was soon to come to pass again as the future of the Home World dimension. But where do these eliminated timelines fade? A few moments later at the Dead Sea, the Ghost Children provide explanation once more:
The Discarded Timelines Principle, in summary, states that whenever a timeline is altered by a time traveler, a new timeline is created as a result of these changes. The original timeline from that point onward is sent into the Darkness Beyond Time, a realm of frozen non-existense containing all discarded timelines. The creation of a new timeline does not imply that the substance of the universe is sent to the DBT and recreated out of nothingness. The new timeline represents the new state of the future light cone of the time traveller which is disturbed by his physical presence. This new state is affected by gravity and the other physical forces, even if only exerted on a subatomic level by the time traveler's mere presence in the past. The original state of the universe is preserved as information in the DBT. Nothing is created from thin air.
In summary, when Crono steps back to 600 A.D., his changes relegate the future beyond 600 A.D. to the Darkness Beyond Time and expunges the old timeline, as a new one is created with his changes intact (even if he just stepped on a twig somewhere and went back to the future). This accounts for Lucca's explanation that when Crono defeated Lavos, the ruined future went to the Darkness Beyond Time. Marle's revelation that the ruined future is about to return is somewhat cryptic, because she seems to suggest that the actual, distinct ruined future they avoided was on the way back. However, she was probably speaking figuratively, merely implying that Lavos was due to ruin the future of Home World as it had originally in Chrono Trigger before the heroes stopped it. One analogy draws on MSPaint. Have a blank picture, save it. This is timeline A. Now, draw a spot on it. That's timeline B. Now erase that spot. While it looks exactly like the first one (timeline A), it's still timeline C. As proof, exit MS Paint. It will ask if you want to save the changes (even though it looks exactly the same).
How is conservation of time, energy, and matter not violated?
Two problems have been raised with this theory:
- If the universe is a closed system, the departure of Schala and Lavos into the DBT represents the destruction of matter and energy in that system.
- Discarded timelines still retain information; a few pass by Serge as he fights the Time Devourer in transparent orbs. It seems every time a timeline is changed, information is created, violating conservation.
This theory can be resolved by considering the universe and the Darkness Beyond Time as two systems with an important relationship. The Compendium can theorize that the Darkness Beyond Time exists as a sort of finite "underworld" beneath the timeline, accessible physically through special Gates. When a timeline is changed, the past timeline is subsumed to this subterranean layer, the Darkness Beyond Time. Information from a discarded timeline is then broken down and recombined to form the new timeline as the time traveler creates it. There is a backlog of discarded timelines; the oldest in the sequence is what's taken to become the new timeline. As for the problem of physical matter (like Schala) entering the DBT and exiting the universe, her act of entering displaces gases in the DBT back into the universe, compensating for her departure in an even exchange of matter and energy.