Basic Information
A Time Ripple occurs when changes in the past are not reflected in the future instantaneously. They "ripple" forward at some rate less than infinite and more than or equal to one Second per Second. In a typical story the changes can be averted or reverted by the protagonists if they can get to their Time Travel Machine in time and prevent the change from happening by travelling back in time. The consequences of a Time Ripple can be as simple as alteration of the time line with no one being aware the past had ever been different to a Time Quake that has a temporal epicenter from the point where the time traveller left from. If the rate of change propogates forward slowly enough the protagonists can have considerable time to prepare to avert the change. If the change is instantaneous, but the ripple in time can be avoided, the protagonists can still revert the change. This is true if they were in the time machine, and thus outside of normal time, at the moment the change was made.
Sources
Back to the Future (all): In the first movie, Marty McFly has his hand start fading while he was in the past until further changes in the time line can be made to avert an altered timeline. In the second movie the time line was altered, but Marty's life has been altered while his memories were unchanged to being in transit at the time.
Millenium (1989 based on the novel "Air Raid" by John Varley): The result of an inadvertant time change causes a Time Quake that has its epicenter at the Time Gate station in the year 3000.
Game and Story Use
- In a typical game using this trope, a deadline for averting the change must be figured out. This can be a d20 roll that gives some number of turns before the change causes a predertmined alteration to the timeline to become inevitable. Saving rolls can be used to determine the chances that the players character is unchanged by an instantaneous time ripple (using any of the "shielded by a temporal field" or "in transit" ideas that can be invoked).
- There's no reason why the time ripple has to propagate only forward, either. The change might work backward as well, fitting linear causality to its new form so that the change is "stable" and possibly setting off more ripples in the process. This might be part of an extra strength masquerade, a temporal paradox prevention mechanism, the workings of a belief based reality, or something stranger.
- Time ripples can also happen through space; Terry Pratchett's Mort has something similar where the titular protagonist creates a pocket reality where he saved someone, inside the larger reality where he didn't.
- Being the only people who remember the way things "used" to be can be a story hook, and seeing time ripples happen can be the first clue that someone's meddling with time.