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This is a question about the relationship between a character moving backward through time and her relationship with characters who move forward through time.

The character [Cate] travels backward through time at midnight every night. She's not doing it actively (no time travel machine or vehicle). It's happening organically to her.

At midnight, when everyone else moves forward one second into the next day, she moves backward 48 hours to the start of the previous day. (She then travels forward for 24 hours like everyone else until midnight when it all happens again.)

Simply put: When she goes to bed on Tuesday, she wakes up on Monday morning rather than Wednesday morning.

If today is October 29th, 2020, in 29 days she will be at October 1st, 2020.

When the backward travel happens, she ends up in the exact same spot. So if she is in a bed at midnight, she wakes up in that same bed (assuming the bed was there 48 hours earlier, if it wasn't- she ends up in the space where the bed was).

A character she knows [Charles] goes to bed on Tuesday and wakes up on Wednesday like everyone else in the world. He does no time traveling (outside the 1 second per second time travel we all do).

So here's my question:

Let's say Cate and Charles get into bed together on Tuesday and fall asleep. Let's also say that when Charles wakes up on Wednesday Cate is in bed with him. Is that a younger version of the same Cate he went to bed with or a different Cate entirely?

Or another way to ask this question:

When Cate wakes up on Monday morning. If Charles is in bed with her, is that a younger version of the Charles she went to bed with on Tuesday? Or a different version of Charles entirely?

If it is an entirely different version of the person they went to bed with: Then the assumption is that Cate and Charles can not change the future (or past for each other). They can only impact the timeline of the exact person they are with at the moment they are with them.

If it is a younger version of the person they went to bed with: Then the assumption is that Cate and Charles can change the future (or past) for each other.

Are there any other possibilities beyond these two that I'm not considering?

If these really are the only two possibilities, are both of them viable? Or is there some paradox or problem I should be aware of?

Ultimately, I understand this is a creative choice for me to make within fiction writing. But I'd like input before I dive too deep and end up locking myself into a problem I can't solve.

Thank you for any help you can offer...

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    $\begingroup$ Hello ElmCity, welcome to Worldbuilding. When you get a moment, please take our tour and read through the first two bullets of our help center. $\endgroup$
    – JBH
    Oct 29, 2020 at 15:02
  • $\begingroup$ Hi JBH, thank you. Can you help me reframe my question to be less subjective and more answerable? Ultimately, I'd like to know if the two outcomes are the only outcomes possible, and if there are any paradoxes that I should be aware of. Is that a legitimate question for this space? $\endgroup$
    – Elm City
    Oct 29, 2020 at 15:09
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    $\begingroup$ Have you seen Doctor Who? This sounds very similar to the entire character arc of River Song. Her encounters with The Doctor aren't at neat, precise 24 hour intervals, but roughly speaking (and with some exceptions), every encounter is an older Doctor and a younger River Song, or vice-versa (same thing from different perspectives). $\endgroup$
    – Matthew
    Oct 29, 2020 at 15:17
  • $\begingroup$ As I see it, there are at least three possibilities: (1) Time is linear, and Cate is going into her own past (and can possibly change the timeline) (2) Time is a tree, and when she goes back in time, she "splits off" a new branch that doesn't have a future to change, but which will develop in its own way organically (3) Cate is actually jumping to a "parallel world" that is 24 hours behind her original world, and doesn't have a future to change, but which will develop in its own way organically. You as the author/creator need to decide which model works best for your story. $\endgroup$
    – Jeff Zeitlin
    Oct 29, 2020 at 15:36
  • $\begingroup$ @ElmCity, :-) having just answered your question, yup, it's legitimate. We regularly help people develop the rules of time travel in their world. Your first question was pretty good. After reading those bullets in the help center, remember one thing: Stack Exchange is not a discussion forum. Ideally, you're asking one-specific-question and selecting one-best-answer. We recommend waiting 24-48 hours before selecting a best answer as we have users all over the world and you don't want to miss anything good just because human nature is to stop caring after the green check mark shows up. $\endgroup$
    – JBH
    Oct 29, 2020 at 15:48

2 Answers 2

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The Zeroth Rule of Time Travel: It's your world, so it's your rules. Time travel works the way you want it to work. There is no "right" solution (even given the current mathematical theories we have today).

So, what we're really doing here is pointing out inconsistencies with your rules so you can refine the rules of time travel in your world. Excellent! That's what we do here.

Cate lives to be an adult and at midnight on November 1, 2020 begins moving backward 48 hours every midnight. Our "target" Cate (call her "present Cate") has a continuous memory from early childhood through each time hop. Given enough hops (and ignoring a LOT of details), she eventually ends up flat on her back on bare ground because she reaches a time before the building exists. This is theoretically possible because she's moving backward in time at twice the rate she's moving forward in time. So, if she lives through this for 50 years, she travels backward 100 years.

Problem: How many Cates are in the bed at midnight?

Remember the Zeroth Rule of Time Travel: it's your world, you make the rules.

  • If you decide time is a continuous stream that coexists past, present, and future, AND you decide that Cate's time hops are the property of Present Cate and no Past Cate, then Present Cate arrives in the same space as Past Cate and a tremendous amount of dry cleaning ensues. Charles is scared for life as he must climb past offal equal to twice the body weight of Cate. Yuck. This isn't an interesting set of rules as it stops the time travel on the first hop and basically erases Cate from the time line.

  • If you decide time is a continuous stream AND that Cate's time hops affect all Cates past, present, and future, then there's no coexistence conflict as Present Cate arrives on the edge of time (having "spent time traveling" that'll cause a headache) just after Past Cate left. No dry cleaning, but Charles still needs counseling because the Cate he wakes up with knows things about the next 48 hours that she shouldn't. (If Cate moves far enough into the past, ignoring the fact that she's aging as she goes, she'll eventually be burned as a witch.)

This particular rule has a bit of a problem because all Cates are affected. That means baby Cate just hoped backward 48 hours — which will be a surprise to Past Dad and Past Mom, who hasn't gone into labor yet. That means this rule doesn't work well, either.

  • Let's decide time is NOT a continuous stream. You can believe that each "time stream" is an alternate or parallel reality or that they simply don't exist, the only stream that does exist being the one "Present Cate" is in. (It doesn't matter which you choose because it's all window dressing, the effects are the same.) If you do this, then the moment Present Cate begins to shift backward in time all of creation that isn't the target point in time 48 hours in the past cease to exist.

This is a great rule because it eliminates all of the paradoxes and all of the problems. Cate never arrives on top of her past self and her past self, having ceased to exist, doesn't travel. This means Baby Cate remains the memory she should be and Mom and Dad don't need counseling because every 24 hours someone that looks a bit like them shows up with increasingly capable language skills.

It also eliminates the problem of what to do with each point in the past. Think of it this way. Cate starts time hoping two months in the future compared to the day Charles started sleeping in the same bed with her. From the perspective of Charles on that first night, he wakes up each morning on the same day (that first day) to an increasingly older Cate who knows more and more about what's going to happen further and further into the future — except that from his frame of reference, each time he wakes up it's as if it never happened before. Yeah, I like this rule. It's neat and tidy.

But don't simply take my word for it. You might want the ugliness of one of the other rule sets. You'll probably get a headache keeping everything straight, but that's what being an author is all about!

Will Charles ever wake up with a younger Cate?

Nope. Charles always wakes up with an older Cate. Someone from the future.

Cate, on the other hand, always wakes up next to a younger Charles — until she passes the date when Charles started sharing her bed. One of the great problems of Time Travel is what happens spatially when someone travels? This is really important because the Earth is rotating, and orbiting, and the sun is moving... From a "realistic" perspective, Cate will never appear in her bed. She'll appear in a geospatial location where the Earth was. Depending on where she started, she'll either be underground or above the ground. (The odds of her being anywhere on the ground are so close to zero that they're not worth considering.) Go back far enough and she appears in space. At that point her hops must coincide with the orbital passage of the Earth, which creates ever-narrowing windows until any more distance plops you a long way ahead of where the solar system will eventually be.

This means your time travel rules allow for both a change in time and a change in position to guarantee she's always in the location of the bed.

Except that things change. Beds move. Houses are built. You'll either need to ignore those ugly details for the sake of your story, or you have to incorporate a limit to how many hops are made, or you get to worry about every little messy problem that could occur. Heaven help Cate if the cat decided to sleep on the bed on December 31, 2019, while she was up partying with Charles for New Year's Eve. Cat... Cate... Dry cleaning...

Finally, can Cate and/or Charles change the future?

Remember the zeroth rule of time travel: it's your world and your rules. If time in your world is a continuous stream, then somehow "time" won't let them change the future — or it will and that creates paradoxes, and how those paradoxes resolve depends on which of the three earlier examples of rules I gave you (or any other rules, if you so choose) are used.

Given your initial conditions, Cate will never meet herself. But she could (for gruesome example) kill some Past Charles. Now what? Does she remember the previous-timeline-memories she had of Charles? Does Present Cate cease to exist because the time conditions that brought her into being no longer exist?

It's your world, so you set the rules. Which rules make sense depend a lot on where you want to take your story. Just remember — there is no "right" answer. It's your world. What ultimately matters is your readers: they expect your world to be imaginative and consistent.

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  • $\begingroup$ Note: she's not living backwards "twice as fast" (at least as I understood); rather, she is literally "taking one step forward and two back". For every day she experiences, she jumps back two, for a net of -1. So, for her, "today" is 29/10, "tomorrow" is 28/10, 27/10, 26/10, etc. In five days her time, she moves backwards five days relative to when she started. (Also, as I understood it, she wakes up where she went to sleep, not the same place every time. She might wake up on the floor because the bed wasn't moved in yet, but probably won't wake up where a building doesn't exist yet.) $\endgroup$
    – Matthew
    Oct 29, 2020 at 17:48
  • $\begingroup$ @Matthew The phrase "living backwards" exists nowhere in my post. In fact, I make a point of the fact that she's getting older. At best we're in violent agreement. At worst you didn't read my post very well. We're also in violent agreement about the bed - the OP uses the bed as an example, but she certainly wakes up wherever she went to bed ... except the Earth moves, so the bed, or the floor, or the sofa, aren't where she went to bed after all. $\endgroup$
    – JBH
    Oct 30, 2020 at 16:13
  • $\begingroup$ You said "if she lives through this for 50 years, she travels backward 100 years." Maybe that's badly worded and/or my own misreading, but I read that as "if this starts in 2020, after 50 years from Cate's perspective, she will be in 1920". But that's not how I understood it; she would only be in 1970. Also, don't read too much into "living backwards"; that may be poor wording on my part. I only meant to note the apparent discrepancy how quickly she's "moving backwards in time". (Definitely agreed about the bed; I said as much in my own answer.) $\endgroup$
    – Matthew
    Oct 30, 2020 at 21:21
  • $\begingroup$ On an unrelated note, every CSI fan "knows" (disclaimer: I don't know if this is actually true) that dry cleaning makes, ah, the particular sort of mess we're talking about worse. You want (according to the "infallable" internet, anyway) hydrogen peroxide 😉. (Or fire. I'm not sure you can really clean up after, ah... that...) $\endgroup$
    – Matthew
    Oct 30, 2020 at 21:23
  • $\begingroup$ @Matthew :-) My dry cleaning line is a paraphrase from Steve Martin's movie My Blue Heaven. "Richie loved to use 22s because the bullets are small and they don't come out the other end like a 45, see, a 45 will blow a barn door out the back of your head and there's a lot of dry cleaning involved, but a 22 will just rattle around like Pac-Man until you're dead." Cute comedy! $\endgroup$
    – JBH
    Nov 1, 2020 at 3:03
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So... JBH already noted the Zeroth Rule and why reality makes things... messy... Let's assume you're hand-waving all that (because otherwise you're going to have a very short, very gruesome story).

There is, as noted, the "multiverse" version of this; every time Cate "jumps", she enters (or creates!) a new quantum universe in which she has never existed before (besides her past self) and will cease to exist in 24 hours. The good news is you don't have to worry about paradoxes, since each universe will continue on once she's gone.

Another option is that time is immutable. In this case, there is some point at which an older Cate springs into existence and dies later that day, at which point either she stops jumping backward, or else her dead body has existed for even longer. From the rest of the world's perspective, "current" Cate disappears every night and is replaced by a younger version that remembers her life except what happened to her in the prior 24 hours. The younger version may or may not appear in the same place, which will give the illusion that she sometimes teleports. Also, she's aging backwards. At some point, there is also a baby version of Cate that is normal.

Both cases will make for a very frustrating experience for Cate, as she has very limited ability to affect anything and can't see the long term results of anything she does. I expect her to very quickly become severely depressed.

As for the relationship between Cate and Charles... you need to watch the River Song episodes of Doctor Who. It'll be like that.

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  • $\begingroup$ Thank you, Matthew. $\endgroup$
    – Elm City
    Nov 2, 2020 at 19:22

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