A common theme in science fiction is the existence of parallel universes, where different versions of reality play themselves out. In films such as the Back to the Future trilogy, the hero Marty has to change events in his past in order to return to the parallel universe that he started off in.
In this particular case, photographs are posited as having a semi-permanent record of alternate realities, and it is only by having one of these that Marty knows that reality is different to what it should be. Without this convenient plot device, Marty would have no incentive to change the future.
In the acclaimed Star Trek episode, ‘City on the Edge of Forever’, Captain Kirk and Mr Spock have to decide whether the social worker Edith Keeler should die or not. Depending on their answer, they will either prevent her from dying and consequently changing the future that they want to restore and return to, or she will die and the future reverts to what it ‘had’ been before they were stranded in the past. Behind this conundrum is a machine-like being, the Guardian of Forever, that plays the role of an exophysical observer, relative to which existence and time are defined and can be seen to change.