The metaphysical possibility of time travel
Nikk Effingham, University of Birmingham.
Are fictional stories about time travel possible? By that, we don’t mean physically possible—when Hermione uses magic to travel back in time in Harry Potter and the Prisoner of Azkaban, that’s obviously inconsistent with the laws of physics! Rather, the question is whether it is logically possible? Some stories involve contradictions; the question is, did J. K. Rowling end up writing one of them?
A famous philosophy paper argues that some time travel stories are possible, namely those in which time travellers don’t change the past. This would include Prisoner of Azkaban (other examples you may have seen are Twelve Monkeys and Los Cronocrímenes). The trick is that if you try and change the past, some strange coincidence prevents you. Try and kill Hitler and your gun will jam ten times in a row. Try and save JFK and you’ll randomly have a heart attack.
What about fictions in which time travellers can change the past? In Back to the Future, Marty changes things so his parents become rich and successful. In The History of Time Travel, it’s Russia who first gets to the moon. Are those stories possible?
There are philosophical theories suggesting they are logically possible. But their possibility requires something weird: two dimensions of time, ‘time’ and ‘hypertime’. Just as a cup of tea is hot at one time and cold at a later time, the entire past is one way at one hypertime (e.g. America got to the moon first) and a different way at another hypertime (e.g. the Soviets beat them to it!). Time travellers travel back into the past of regular time but always into the future of hypertime. That is how the past can change.
The ‘hypertemporal model’ struggles with fictions that apparently have inconsistent rules of time travel. Consider long running TV series like Star Trek, Red Dwarf, or Doctor Who. In some episodes, time travellers can change the past willy-nilly. In other episodes, they find their every attempt thwarted—often by unlikely events and coincidences. And this seems nonsensical. If the past can be changed in some cases, why not in every case?
To remedy this, we can introduce a ‘mixed model’. Sometimes, time travel takes you forwards in hypertime. In those cases, you can change the past. But in other cases, time travel takes you back only within the hypertime you are already at. In those cases, you’d be like the characters of Prisoner of Azkaban, unable to change anything from how it originally played out. Given this mixed model, we can see that those TV series are telling non-contradictory stories.
There are also stories where time travellers can change the past, but not freely. They can change some events, such as those events with no great consequence to history, but they can’t change ‘big’ events that play a major historical role. A good example is Doctor Who’s ‘fixed points in time’. Whilst the Doctor can change many things, certain events are sacrosanct and he’s prohibited from meddling with them. A mixed model can also account for these fixed points in time—as long as a narrative is suitably embellished, even stories involving fixed points in time are logically possible.