Ripple Effect Indicator

In many Time Travel stories in which a character in the past is changing the future, there will be an object, or sometimes a person, from the future that is directly affected as a specific consequence of the Time Traveler's actions.

The main effect is that this object, or if it's a photograph, the subjects on the photo, will vanish or reappear as changes are made to the timeline. This is done typically to show whether what the protagonist in the past is doing the right or wrong thing.

In short, a Ripple Effect Indicator is an object, originally from the future, that fades away or otherwise changes based on actions in the present changing the timeline.

See also Ret-Gone, which can be an effect of this. See also Delayed Ripple Effect and Ripple-Effect-Proof Memory. This is usually seen in "overwriting the timeline" in the Temporal Mutability scale.

Examples of Ripple Effect Indicator include:

Anime and Manga

The Divergence Meter in Steins;Gate measures the difference between the current timeline and the reference alpha timeline. However, major changes in the timeline can make it Ret-Gone.

Comic Books

In the first story arc of Booster Gold Vol 2, Booster's JLA membership certificate changed to reveal alterations to the timeline, for instance becoming The Flash's death certificate.

Film

Marty's photo of himself and his siblings in 1985 serves as this in the original Back to The Future. As the time to get his parents falling in love draws closer, his siblings from the eldest on down vanish from the photo and reappear once he succeeds. Variations of this trope occur throughout the series enough that by Back to the Future III, Marty has gotten Genre Savvy about it, taking a photo of the grave of the person whose death he's trying to prevent before going to the past.
- In the Telltale Games sequel, Marty uses newspapers and a photo of George for the same purpose.
In Frequency, Frank (John's father) needs to give an indication to John that he is still alive, and so burns a few words into his desk in the past. In the present, John sees these words appearing on the desk at the same speed at which Frank is seen burning them, as if they're being burned in the present. Other examples include events in the past triggering sudden, sometimes disorienting changes in John's memories and that of his peers, averting or causing people's deaths that would have been avoided before, and giving a new, nicer appearance to the decorations in the house.

Literature

In one of the Charmed tie-in merchandising novels the sisters battle an ancient goddess that's not restricted to a single time so she can hop from era to era, dimension to dimension. She starts messing with a few of the Halliwell Family's ancestors and the effects in the present are changes to the family house's interior decoration, changes in Phoebe's appearance (her hair becomes long and dark) and Piper's son, Wyatt disappears because he was never born. Phoebe also passes out after getting a barrage of visions of her family in the future and the past being attacked by the ancient god.
In the short story "Abe Lincoln in McDonald's" by James Morrow, the titular American president visits a version of the 20th century in which slavery remains legal in the South. His decision to sign the Emancipation Proclamation, made at the moment a slave is shot to death, causes that slave's body to be replaced by a robot.

Live Action TV

Al often served this role in Quantum Leap. For the most part, he merely reported the timeline changes to Sam as relayed by Ziggy, however, one episode, "A Leap for Lisa," in which Sam had Leapt into a young Al shows changes such as Al being temporarily replaced by another person entirely.
In Voyagers Phineas Bogg's device showed a red light when history had to be changed and showed a green light when it was set right.
Another example would be the premise of Early Edition, in which a man literally receives tomorrow's newspaper today, and is expected to change the negative headline/lead story into something more positive before it happens. When he is successful, the headline/story changes accordingly.
In an obvious Shout-Out to Back to the Future, Jin's photograph of himself and his fiancée Miki in Jin. Unlike the simple fading of the BTTF picture, Jin's photo changes dramatically -- clothing, poses, even location -- and when he makes a dramatic change to the past, it vanishes entirely.

Video Games

In Mortal Kombat 9, Raiden's medallion cracks when he receives his first visions from the future. As the story progresses, the medallion shows only more signs of damage every time Raiden fails to prevent certain future events from happening...until the very end, when Shao Khan is finally defeated for good.
Marle's vanishing and return early in Chrono Trigger is used to demonstrate both how past events are affecting the future and how you know you succeeded in repairing the time line.
Achron uses various symbols on the timeline to indicate various events happening.

Western Animation

In the My Little Pony Friendship Is Magic episode "It's About Time", Twilight Sparkle herself is the Ripple Effect Indicator. After being (unsuccessfully) warned by a haggard-looking and supposedly battle-weary version of herself from the following Tuesday, the Twilight of the present day focuses her efforts to avert that outcome. As her exploits show the bad future goes unobstructed because her appearance grows to become that of her future self's until the fateful Tuesday arrives and she looks exactly the same. Ultimately it's all for naught as the "disaster" ends up being her worrying for nothing...so she tries to go back in time to correct her self from one week earlier not to make the same mistake...but can't get the message across, setting into motion the whole affair all over again.

Web Comics

In the General Protection Fault Surreptitious Machinations saga, the time traveling protagonist Todd uses himself as the Ripple Effect Indicator. With that, he's able to show that he has succeeded in breaking the Big Bad's Stable Time Loop.