Robb. Knight.

I was recently^[1] asked by Zoe the following question:

What's your preferred type of time travel (like singular timeline where things always where, multiverse when you go back, some other shit)

That is quite the question. We should start with the different types of time travel to even be able to answer this. I'm not covering every possible version from media, just the ones I care or know about. Notably, I have not included Dr Who anywhere here because 1. I've never watched it 2. John and Kate in the Relay discord said it does whatever is needed for the plot and isn't consistent.

Types

Self-Correcting/Paradox

This is almost anti-time travel, the commonly-known "Grandfather paradox". If I travel back in time to kill my grandfather, I would never have been born and therefore couldn't have gone back to kill him in the first place. Anyone I went back to kill (assuming I chose someone I knew of) would self-correct because by them not existing I wouldn't know about them so would never have gone back.

This one also has the honour of being the least fun. Barely even time travel.

Single, Changeable Timeline

Examples: Back to the Future, Futurama "Roswell That End Well", Hot Tub Time Machine, Blackadder Back and Forth

This is the most common version seen in media and the one I'm most familiar with, having watched the Back to the Future movies at least once a year for the past 30 years. You go back, do something, and on your return things have changed. Maybe you go back and accidentally stop your mum meeting your dad, come up with an elaborate plan to get them back together even though she's into you now, turn your dad into a completely different person, play Johnny B. Goode for your parents classmates, then your mum names you after a guy she had the hots for in school and your dad says nothing about this. Or something less...weird.

The second movie explains these changes further as the timeline "skewing"; to get back to the "original" future you came from you need to fix whatever change you made in the past.

Inconsistencies

• Do Marty's parents just not notice their kid looks a hell of a lot like that guy from when they were at school? Why did they name their third kid Marty and not their first one?
• When Marty goes to the future in Part 2 he would get there to find his kids don't exist because he's been missing for 30 years, surely? Or does time travelling create some kind of clone so you keep existing in the timeline? It makes no sense.

Single, Changeable, Always-Happened Timeline

Examples: Harry Potter and the Prisoner of Azkaban, Bill and Ted

This is almost identical to above with the exception that the things that someone might do in the past already and always happened. So if you make a decision to go back in time and throw a rock at yourself, you already did.

This also happens in Bill and Ted when they need the keys to the police station cells. They think about doing it and the keys are there.

Notably, there isn't really anything in Bill and Ted that seems to change the timeline at all - even taking the princesses from medieval England doesn't change anything, at least that we know of from the movie.

Inconsistencies

• At the point Hermione and Harry decide to time travel they've already experienced some of the things they will do when they travel; What happens if they decide not to go or will they 100% go because it's happened already?

Multiverse/Branching

Examples: Avengers: Endgame

This one is the easiest to explain away from a plot point of view. You go back, kill baby Thanos, and now you have two timelines: yours where Thanos is alive and a new one where Thanos is dead. Every action taken in the past creates a new branched timeline.

Also worth noting here that every decision made, even in the present, can also create new branches, it's not exclusive to when you're in the past.

Inconsistencies

• None that I can think of. It's a very "clean" version of time travel.

Stable Time-Apparatus ("The Box")

Examples: Primer

As far as I know the only example of this is in Primer, the 2004 movie. I will try and explain the short version here but Wikipedia has a handy diagram too.

The Box is turned on by A with a 15 minute delay at 11:45am. A then hides (so as not to affect events and collect useful information like stocks) away until 6pm and which point he enters the box, and waits 6 hours to come out of the box...at midday (11:45 + the 15 minute delay). A is now B, with the original A still hiding away collecting stock info. If you understood this, you won't once you watch the movie.

This method only allows travelling back to the point when the box was turned on, no going back to punch Hitler here. Lots of opportunity for personal gain with this.

Inconsistencies

• None

Time Loop

Examples: Groundhog Day, Palm Springs, Russian Doll Season 1

Usually a day, the character is stuck in a loop of the same period over and over, never being able to change what happens or get out of the loop. This isn't always considered as time travel but the consequences for the characters in the movies/shows are similar: they know what's going to happen and they can change things using their knowledge of how the loop will play out albeit for only for the current "loop" they're in.

The exit methods for the loops are usually vague like "become a better person" except in the case of Palm Springs where they use a bomb.

Inconsistencies

• None

Forward or Backwards Only

Examples: Futurama: "The Late Philip J. Fry" (forwards) and Futurama: "Bender's Big Score (backwards)

Futurama features two machines that only go one way: one for forwards only and one for backwards only.

We're all doing the forwards one right now anyway it just happens to take one second to travel one second into the future. But if you want to go faster the "Forwards Time Machine" from Futurama does the trick.

Not sure of the usefulness of this one. In the Futurama universe if you go to the end of time the universe just starts again as it was before (but a few feet lower than before) so this could be used for backwards time travel but it's would be annoying to do.

The backwards-only version is mostly useless for humans unless your goal doesn't include coming back to the present. In the episode, Bender is the one who does it so he can just wait it out on account of being a robot. This does have a paradox of sorts that all the Benders waiting around to come back are all in the same place at once but that gets corrected evenutally.

Inconsistencies

• None

My Favourite

Picking a favourite here is hard and would entirely depend on my goals.

Multiverse is kind of terrifing that every little action is creating a brand new timeline with untold horrors based on my decisions. It's a handy version if your goal is to steal something without any consequences to your own timeline.

Primer's box is useful only in the way the movie presents: for short-term personal gain. I won't spoil the movie but there are big consequences for making your days 36 hours long instead of 24. I'd want to use this once every now and again but I'm not sure I could be trusted.

I don't like the predetermined nature of the already-happened-no-matter-what timeline so I think my favourite, despite having the most paradoxes, is single timeline, Back to the Future style. There's a reason this is the one that appears more in media than any other: despite the problems, it's easy to explain quickly without need to sit down and write notes and diagrams (looking at you, Primer).

Time Travel Miscellania

I am a sucker for time travel media and I'll pretty much watch anything where it's involved. I can't get enough of them like Timeless and Legends of Tomorrow even if they are cheesy as hell.

I would like to see a movie where someone creates a time machine based on the Back to the Future "science" and travels to the future only to find out about the paradoxes and hilarity ensues. Get on it, Hollywood.

The DeLorean is obviously the coolest time machine.

This Minute Physics video on the topic is excellent.