Causal wizards and DeLoreans

Explore the link between causality and different concepts

By Callum Gill


*Spoilers for Back to the Future and Harry Potter and the Prisoner of Azkaban*

Time travel is featured quite heavily in popular fiction. Many different mechanisms of time travel have been devised which have caused much analysis and debate between the consumers of these stories. However, what’s also interesting is how these various stories handle the concept of causality and the very outlook of the timeline is underpinned by causality, whether it be explicit or not. But first, what is causality?


Causality simply means that every effect has a cause, without which it cannot occur. However, certain events cannot influence others if the information that these events have taken place does not move fast enough to produce the effect in question. This is because the fastest possible speed information can travel at is the speed of light. If you shoot a laser beam at the moon, the event in which the light has reached the surface of the moon depends on how long it takes for the laser light to reach it. You cannot immediately cause that event. Realistically, each event is caused by a hundred of different events, making causality extremely complex. This is one of the reasons why it is an avenue so many science fiction writers love to explore. 


In considering time travelling to the past, there are two distinct types of situations that could arise due to causality. There’s the inconsistent case in which the act of travelling back in time causes a new branch in the timeline, as the act of time travel had not occurred in the original timeline. This results in a butterfly effect: influencing a past event can create cascades of branches. You will often find that most stories depicting time travel will use this as a key plot point.


Alternatively, there’s the self-consistent case, in which the act of time-traveling simply causes us to travel back in time and live out history as it originally played out. Therefore, it causes no deviation from the original timeline.


In the film Back to the Future, teenager Marty McFly accidentally travels back in time in a modified DeLorean to the year 1955 and meets his parents. Whilst in the past, he prevents his parents from becoming a couple, thus inadvertently creating a new timeline where he slowly becomes erased from history. Since this timeline is inconsistent with the original, Marty’s existence begins to be erased since no event has occurred to allow him to be born. However, by getting his parents together, he would create a cascade of cause and effect that eventually results in his birth.


Harry Potter and the Prisoner of Azkaban, by J. K. Rowling, features a self-consistent timeline. Here, Harry and Hermione decide to travel back in time to find out the identity of an unknown figure who saved them from dementors. Once they arrive at this past event, they discover that no-one is there to save them. Harry then realises that he’s the one who in fact saved them. This makes logical sense from a causality point of view, since the event of being saved had already happened for the time-travelling Harry, so simply performing the act of saving himself in the past doesn’t alter the timeline in any way. It had all already happened, both for the past Harry and time travelling one. Despite carrying out the act of saving him and his friends, Harry doesn’t have any free will whilst travelling in the past. He had to carry it out, otherwise the timeline wouldn’t be self-consistent.


It’s clear that travelling to the past depends heavily on causality, potentially limiting our experiences there. These stories highlight that either we can influence history to play out differently or we are merely a part of the history that is already set in stone, and no amount of time travel would change it. What if we are, in fact, experiencing a set of events caused by a time traveller? What if time travellers live among us and this is how history is meant to be? What if our past, present and future are constantly changing, and we are completely oblivious, much like Marty’s parents? Are these all wonders of the human mind that will only ever be discussed within popular fiction?


From Issue 16

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