Time Travel is a very, very fun thing to do. However, a lot of writers are unsure how to resolve certain paradoxes, and many writers will create plot holes via impossibilities and not even know it.

My purpose here is to describe the many (and I mean many) types of time travel that exist in fiction. If you're familiar with the many possible paradoxes, then this won't be too hard to grasp. But if you haven't thought too much about time travel, it may take a few read-throughs to really get the concepts described here.

After describing the types of time travel, I'll give a little advice regarding it, and that'll be it.

Anyway, here we go:

Time travel is separated into two separate categories:
1 - Immutable time travel (ImT for short)- Basically, time travel changes nothing.
2 - Mutable time travel (MuT)- Time travel changes things by creating a new timeline or replacing the old one.

All ImT situations create what are called "causal loops" (aka bootstrap/predestination/ontological paradox). Basically speaking, nothing in the timeline ever changes. If you travel into the past, then future you has already been where you are going, and nothing will change. In ImT, there are certain situations that are impossible, for example, something physical going into the past only to become itself (it would degrade more each loop), or someone going into the past to attempt to kill themselves or an ancestor to prevent them from living to be able to travel back in time and kill themselves or an ancestor (considering that time is immutable, that means that they already attempted to prevent their own life and failed, therefore, they WILL fail... no matter what). Basically, anything done in the past causes the same future that any traveler from the future was in. (Think 'Twilight Time' - s2e20, or perhaps Harry Potter 3) There are no paradoxes here because paradoxes are impossible to create.

However, in MuT, pretty much anything is possible. Every time someone travels back in time, a new timeline is created and the old one either exists elsewhere, is deleted, or is replaced. There are also no paradoxes here, just new timelines.

What's the difference between deleted and replaced? Well, traveling to the future becomes different. Here's a timeline:

————A———C—————————B———>

Say someone travels into the future from A to B and changes something, then leaves and travels back in time to C.

We have two situations:
Deleted/Exists elsewhere timeline - In this situation, once this person travels from B to C, everything from C onward is completely different and B doesn't exists on the new timeline; only on the old one.
Replaced timeline - In this situation, however, once the person arrives back at C, then he or she can simply wait until point B, where his or her past self will arrive in a timeline that is very different from the one that he or she originally encountered. At this point, there are now two of the time traveler, C (the time traveller that arrived at C) being older than B. Now B, if he or she meets C, can decided to stay or do something different. However, if B travels to C, then the current timeline that exists will be replaced once again, but this time with a different C.

Cool huh?

Now, there is a third type of time travel that I don't really like because it isn't logically sound, but because it is so flexible, it makes for good story telling. People use it when they create and attempt to resolve paradoxes, and it is the only type of time travel where paradoxes exist. It is the type of time travel found in Back to the Future, and other similar stories:

2.5 - Mutable time travel with echos (MuE)- This is the time travel where MuT tries to resolve itself into ImT. There are multiple methods for doing this, and each is different. For example when Marty goes back in time and has to get his parents back together. We see that he begins fading out of existence as the chances of his parents creating him get slimmer. (I say he should have faded sooner because chaos theory would dictate changing which of his dad's sperm reached the egg first, and it would be a very different Marty. Probably wouldn't even be named Marty (Martha?). But whatever.) This is also seen in movies like The Butterfly Effect (terrible movie, but hey, time travel), but most movies and books default to simple MuT (Terminator, Groundhog Day, etc.).

And in rare cases, a movie, book, or show will use MuT, but will allow the original timeline to be returned to (S5 finale, anyone?), and what that means is that it was most likely an "exists somewhere else" situation. The scary part about that kind of situation is that all of the timelines continue to exist simultaneously. So, unless Starlight's hacked spell somehow stored the original timeline somewhere and replaced/deleted all the others, all of the other timelines still exist.

D:

Finally, there are two types of MuT or MuE time travel that can occur on top of those:
1 - Branching Mu - Where the old and the new timeline both originate from the same point in time on the same timeline. If travelers from both timelines traveled to the same time before the split point, they would meet each other. (Back to the Future does this, I believe)
2 - Multiverse Mu - Where the old and new timelines do not touch anywhere. If travelers from both timelines return to the same time, no matter when, they will not meet each other as they are simply traveling into parallel version of their timeline, instead of back in time in their own. (In the season 5 finale of MLP, it becomes clear that this is the version that is occurring as they never meet themselves in the past.)

In summary, here are the possible time travel situations:
1 - ImT - Harry Potter 3, It's about time (MLP;FiM - s2e20), etc.
2 - MuT (MuT.a - deleted, MuT.b - exists elsewhere, MuT.c - replaced) - Terminator, Groundhog Day, MLP;FiM - s5 finale, etc.
2.5 - MuE (MuE.a - deleted, MuE.b - exists elsewhere, MuE.c - replaced) - Back to the Future, Butterfly Effect, etc.
And if it's MuT or MuE, it's either branching or multiverse.

Side note: If you are in ImT or in MuT, then paradoxes do not exist in any way, shape, or form. Paradoxes and their resolutions can only become plot points in MuE.

Those are the possible time travel scenarios. Pretty much all versions of time travel you will see will fit into one of these categories. Otherwise, they would be logically inconsistent.

Knowing which version of time travel you will be writing is important because you want to maintain consistency. If you are not consistent, then there will be major logical plot holes that will become obvious to the reader that pays attention.

Because of this, you should generally use the same type of time travel throughout your entire story unless you explicitly show that switching between them is possible. A good example I will use is from our beloved MLP.

In It's About Time, Twilight travels back in time, and causes herself... to travel back in time. It's an ImT situation. However, at the end of season 5, we see a multiverse MuT situation.

How is that explained? Well, what if Star Swirl designed his original spell to be ImT specifically because he didn't want to deal with the many issues (moral, physical, etc.) of MuT or MuE time travel? That would make sense. Nothing says that a spell can't dictate the type of travel. We can then easily say that Starlight modified the spell to be MuT specifically so that she could change the past and affect the future (something she couldn't do with Star Swirl's original spell). In the end, the elements of harmony said, "uh, hey... y'know... uh... this ain't right," and brought them back to the original timeline. Ta-dah! Logic!

In conclusion, if you're going to write time travel, you should know what kind of time travel you are going to be using, and if you're going to do funky things with it, you'd better have a logical explanation.

That's about it. I hope this has been educational.

If you have any questions about time travel, ask 'em here! Any others, try my classroom, the study hall, or PM me if the first two don't work.

Safe travels!

Blue Telephone Boxes,
PiercingSight

4972242

4972259
And that is why I avoided mentioning Dr. Who. The show switches between these types of time travel so much that trying to tie it down is next to impossible. It's mostly MuE on steroids (the whole "fixed points in time" shtuff), but sometimes it's other things. It just goes all out ham on time travel problems and makes no attempt to solidify any explanation.

That's probably why I love it, though. It can do anything it wants to stay interesting, and I can't say there are plot holes because there are no set rules.

4972261

The concept of Fixed Points was pretty much added to the revival to explain why there were times when the Doctor could do stuff and why there times when he couldn't. (I think it was only in The Aztecs of the original that the Doctor firmly said that time was immutable.)

Good post, by the way. Writing time travel properly is probably the trickiest thing you can do, and even media which gets it sorta right (Looper) still makes mistakes/generalisations.

4972264
Thank you. Finding problems in stories is specifically why I wrote it. Not to mention the fact that people don't usually attempt to separate ImT from MuT, and that's why paradoxes come up. If you're strictly in one or the other, paradoxes don't exist. MuE was pretty much invented just to make paradoxes a plausible plot point.

4972242 "Time travel" is extremely difficult to make sense of, since as we understand it in the usual sense that fiction usually depicts it, it appears to be impossible in the current physics understanding of time as a dimension.

For this and other reasons relating to the Grandfather paradox, I find mutable time travel stories nonsensical and frankly pretty much unacceptable since they're based on unintelligible premises. It's wasting my time as a reader to predicate a story on an impossibility. I can buy into almost anything, no matter how improbable, if the author does it really well, but violating and/or thoughtlessly disregarding the very foundational concept of causality is a line I have to draw. Without causality, events within a story literally mean nothing because any event can just arbitrarily be inserted and rationalized (since, remember, apparently nothing needs a cause), which erases any possibility of dramatic tension.

Physics indicates that immutable time travel might be possible within limited, probably fixed scopes using something like a closed timelike loop, but this could not be perceived as "time travel" per se by us, since to our frame of reference per the theory of relativity we wouldn't be able to tell the difference between information about an object from the "past" vs. the "future" and something or someone traveling along such a closed loop would just perceive time to be linearly progressing forward from their frame of reference.

4972287
Well, strict MuT doesn't have any of the logical paradoxes that you have mentioned. The limits of how MuT can be used is up to the writer. There are many really well written MuT stories that have very logical progressions (time loop stories are one of those). An MuT story that attempts the grandfather paradox would just result in the time traveler living in a timeline where he's never born. No logical issues.

MuE however, has many issues, and the paradoxes and how they are resolved are problematic, but if you want your story to have a paradox become a problem, then MuE is the way to go. Now, if you do MuE, it's good to have rules for how it works, but no matter how many rules you have, there will still be paradoxes and problems. It is, however, a good option for that type of story.

But from a purely mathematical, GR, physics perspective, I have to agree with you that ImT is the only option, and the oddities that occur with it. However, with ImT, the paradoxes are no longer a problem thanks to the fact that time can be considered simply another dimension. But just because MuT or MuE isn't possible, I wouldn't call it a waste of time if it makes a good story.

4972242

Well, I think Glimmer did say she altered Starswirl's Spell in "Cutie Re-Mark" didn't she?

Also, this is why I prefer alternate timelines. No grandfather pardox and free will is left intact. But the whole Free Will Vs. Fate thing is kind of another subject in and of itself. you ever though of writing about that btw in a thread?

THANK YOU for being one of the first people I've ever heard who actually understands ImT. I didn't know there was actually a name for it, though. My friends never understand me when I say "you can't change the past because you're already living in the effects of it". I know that other people like MuT, but logically, I just don't because ImT seems more realistic to me.

4972308
The Free Will vs Fate argument is null to me. Allow me to explain. Whether free will exists or not, I will still be conscious, I will still learn things, and I will still make decisions. It doesn't matter if I'm just a super complex computer, or an intelligence dwelling in a mortal vessel. Why doesn't it matter? Because I can still decide to be a good person, still decide to learn how to do things, still decide not to give into my instincts. I, as a consciousness, have control, and it doesn't matter how that control works.

So in the end, I don't care about the outcome of the Free Will vs Fate argument because it isn't going to change the fact that I'm still responsible for my own actions.

That's about all I have to say about that subject. Now, defining consciousness? Defining free will? That would be an interesting discussion, but the results would be the same.

So yeah. Not likely to be a thread about that as it has little to do with writing. It can however be a philosophical plot point in your story.

4972910
Heh, I just made up these names because the current names that exist for all these concepts have no organization and seem to be really bad at being described. I created this naming system and description so that it's easy to categorize a time travel situation and it's repercussions. :P

4973212

Yes, but if a timeline is static, aren't you "fated" to do something, regardless of "free will?" In other words, you will always make choice a in a timeline instead of b, c, etc?

4972242 Not everyone has seen that movie...

Or even heard of it

4974604
Pretty much. But you'll still choose to do that anyway. It's still you choosing.

4982883
Well, considering that he returned to the same seemingly unaffected time afterwards, I would assume ImT.

4984320
Ah, I though he went to the past at one point...

Well, it never shows any of the effects of his travels backwards. Time travel into the future rarely does anything, but it will matter if you stay in the same timeline. All we know is that he returned to the same timeline in the past as where he started. Pretty much everything except ImE or multiverse is possible. ImE is out because it would almost certainly affect the flowers he brought back. Multiverse is out because he almost certainly returned to the same timeline afterwards. But even those are still possible depending on what rules you use.

TLDR; in the story it can be pretty much anything because time travel wasn't a plot point, just a transport mechanism.

4972287 Yeah and magic isn't physically possible either. If you can't open your mind and love new stories, what are you doing on this site?

4997334 No, you misunderstand completely.

"Magic" as the show has it is entirely possible with a modified perspective on physics.

Many types of time travel, on the other hand, violate causality, which is entirely a different issue.

My mind is open, just not to complete nonsense. Nonsense isn't good storytelling.

4997572 People like you are why sci fi writers are afraid to write anything too "Out there".

Also multiverse theory is true, so time travel can easily be true. It just makes more timelines.

(Also you're sorta ok. I don't hate you a lot, I just think its annoying that you think time travel is nonsense.)

4972242 4972259 4997572


IDW Comics Issue 24.

THX Animations, Season 2, Episode 20

Hasbro seems to favor Type 1, but there isn't any specifics on the kind of time loop that Twilight reports as being stuck in at the end of EQG3.

4999988

It was the time loop from The Cutie Re-Mark that she was referring to.

5000002 I was referring to this without respect to any specific examples.

1 - Immutable time travel (ImT for short)- Basically, time travel changes nothing.
2 - Mutable time travel (MuT)- Time travel changes things by creating a new timeline or replacing the old one.

These are the two most general types of time travel. So I proceeded by showing time travel as it's represented by those who write the MLP canon.

5000009

Hasbro seems to favor Type 1, but there isn't any specifics on the kind of time loop that Twilight reports as being stuck in at the end of EQG3.

5000020 Ah! Thanks for the clarification. I guess I was being a bit ambiguous, and have not yet seen that episode but I'm definitely going to go watch it now.

5000025

Fair enough. I won't spoil anything for you, then.

4997572 In storytelling terms, could you explain how mutable time travel violates causality? Suppose a traveler starts on timeline A at time a and travels to time b on timeline B.

From the perspective of people on timeline A, the traveler casts a spell or uses a machine and disappears. There's a clear cause for that.

From the perspective of the traveler, the traveler casts a spell and moves from time a to time b. Again, everything that happens has a clear cause.

The only perspective where causality even starts to get iffy is in the perspective of timeline B, where the traveler appears out of nowhere. But even then, the traveler carries information from timeline A. If they wanted to, an observer could ask the traveler where she came from and why she was here, and determine the series of events that led up to her appearance.

Here's a version where causality is completely clear for everyone, though: Suppose you have a time machine or spell with two states: "send" and "receive". When the machine/spell is activated in the "send" state, it moves the user to a time where someone else activated the spell/machine in the "receive" state. Everyone is happy: people at the origin time see the traveler activate the spell and disappear. People at the destination time see someone activate "receive", causing a time traveler to appear. The time traveler sees himself activating "send" and appearing at "receive". May violate conservation of energy, but that's where magic comes in.

4999849

People like you are why sci fi writers are afraid to write anything too "Out there".

Actually, I'm not. Sci fi can be as "out there" as it wants and I'll enjoy it - as long as doesn't ask me to believe something logically impossible. There's a difference between improbable and impossible.

I just think its annoying that you think time travel is nonsense.

Not all time travel - closed timelike loops that don't break causality seem to be possible, and in fact, when stories like that are done well, I really enjoy them (for example, I loved "It's About Time").

Also multiverse theory is true, so time travel can easily be true. It just makes more timelines.

Experimental evidence doesn't suggest so. It suggests the opposite, that we're constrained to one timeline, even from frames of reference in which there appear to be retroactive changes (there really aren't any, but relativity leads to some strange observations) because causality can't be violated. Even experiments like the delayed choice quantum eraser can't break causality to do things like, for example, change the outcomes of their own experiments before they're conducted.

5000102 Among other significant problems (such as there being no evidence of multiple timelines and no particular reason why they should exist in contradiction to experimental evidence indicating that we only have one), it still seems to violate causality in the receiving timeline because it's an uncaused event from that relative frame of observation: there's no reason in that timeline for why a time machine / time traveler should just appear there at random out of nothing. I don't really think the information argument works because that's information from an apparent future, which isn't available in the past.

4972242 Whoo. Fun little post(s).
4972259 Represents my actual stance: we aren't the ones meant to walk on time, so just walk on it.
4974604 Fate is like believing A + B = C because no matter what esle in the universe, C will be if certain conditions are met: questionable.

Can a static timeline be travelled? Train wreck drama story? That "The Time Machine", movie I saw, 2002, was about it: he couldn't change the event that made him travel back in time, it just happened differently. While hardly explained, I think it was well pulled-off: They didn't focus on explaining what he could do, but on what he desperately did. Drama, hope and suspense: really liked the story.

Whether something is possible is subjective: our capacity to observe is never perfect, and we don't fully know the small things that make big stuff work.
4997572 Hence magic could be some super-evolution of something we don't suspect of: We don't have the technology to know the effective limits of physics. And thus,
4973212 Fate is indeed irrelevant: we can't predict perfectly what we don't know the results of. And messing with time does not change that: if we change something in the past, it's just messing around with the equation at a different point. And how could you mesure the exact consequences of the change? (Also... not going into weird maths.)

Often, it is simpler to wave 'alternate dimension' or "The matrix" then write something in how time-travel 'works'. Because how would a time traveler actually prove what they say? And authors explaining with word of god? Invent more rules, and it is easier to break them.

4972287 Yeah: One of the most 'fate' like ideas in physics is that if an object exists, it's constituent particles will, and have existed. Traditionally, it happens in a simple get-along way, too. Any actual transportation in time would mess up with that idea, royally. (Why focus on something fundamentally impalpable? For understandable and relatable, it's a thin straw.)
4997334 The existence of magic doesn't conflict with physics as long as it doesn't actually do the impossible.
4997572 Indeed: Magic in the show is rather consistent. The wildest effects are often temporary, too.

Also, the magic doesn't gizmo away: Most big events are clearly theatrical, happen around notable magic users / artifacts, and are affected by intuitive actions and emotional states of those involved. Somewhat consistent. Haven't really seeing wide scale permanent effects. Just booms and visions of true power unknown.

4999849 Yep, blunt positions on ambiguous things can be annoying. Especially when they feel wrong.

Which wraps it up: The question in storytelling is about appreciation of the telling. Of which believability. And time travel is easy to mess with too much.

TLDR: The benefit and cost of time travel is discrepancy: some people can know or react more effectively than normal, or stress (and laugh) at the lone task of figuring out something very difficult. Or it could be about the messing around with stuff like some masterful know-it-all that just isn't truly playing without risk. But the point is: you mess up the world setting with a strong xeno-factor. One that could often mess-up everything else if not applied with tact. Which is why its inclusion must be clear-cut, or beneficial to the genre.

I know few time-travel psycho-thrillers: minimalism, drugs, memory manipulations and virtual realities are more often used.

Anyhow, just random commentary on things. Not in chronological order.

Funny enough, writing yourself out into a dead-end in believability and interest is easy with time travel. Recommended that the source of time travel has conditions that make it unspammable: An undesired repeat event horizon, something half-understood, or something you want to avoid overusing / must avoid in the end. The mechanics of the world shouldn't prevent the story from having a decent end, ideally.

Anyhow, just love little conversations, generally have nothing to really add, though. Sorry.

5000209
Regarding your argument on causality, we still do no know for certain whether or not a multiverse exists, so saying there absolutely is or isn't one doesn't work as an argument.

As for your statement "it's an uncaused event from that relative frame of observation: there's no reason in that timeline for why a time machine / time traveler should just appear there at random out of nothing." I believe the key phrase is "from that relative frame of observation".

Just because one frame of observation doesn't know that the other exists, doesn't mean that there is no cause. In that case, the cause would be that something in A interacted with B. So far as we know, there is no KNOWN cause of the big bang, but we know there was a cause. What if it was theoretically the result of a gigantic sun from another universe collapsing into a black whole so dense that it 5+ dimensionally "stabbed" into our universe, shooting all of it's shredded material through? That's a direct cause; it doesn't violate causality.

Causality in this case of time travel would only be broken if there wasn't another universe in existence to cause the appearance of the traveler in B. But if there is, then causality is preserved.

5000209 Those are, at worst, contradictions with reality as we know it, not fundamental logical flaws. Fiction contradicts reality by definition. If every character who matters can see the causality behind the story's events, and so can the reader, why does it matter where that information comes from?

As an aside, what experimental evidence is there suggesting that there is only a single timeline? I was under the impression that all interpretations of QM are, at present, indistinguishable, which makes the Many Worlds interpretation just as valid as any other.

5000268 If frame of observation A doesn't know that frame B exists (meaning, there's no information conveyance possible), then B cannot influence causality in A. If it did, A would have that information and instantly know concrete states (rather than probabilistic superpositions) in B. In backwards time travel, this implies the past having certainty about information from the apparent future which it can then change to be some other future, which breaks causality because if that future is changed, it never exists to provide that information - suddenly you have uncaused information / events going on.

The special case of the closed timelike loop is a way around this, because causative information never moves "backwards" to any frame of reference such that it results in impossible uncaused events, but it also means that time travel can never actually change the past, because it was already always part of the past that provided the motivation to time travel in the first place. This is why "It's About Time" was a great episode, in my opinion - it did this right.

5000295

If every character who matters can see the causality behind the story's events, and so can the reader, why does it matter where that information comes from?

For literary purposes, it probably doesn't. I'm not saying no one should be able to enjoy movies like "Back to the Future" and such, because most people probably don't really care about how the mechanics might or might not relate to what's really possible. I'm just saying that it bugs me. I think about these things perhaps way too much and it has the unfortunate habit of breaking most time travel fiction. On the plus side, though, I really enjoy what few are left unbroken after the shakedown.

As an aside, what experimental evidence is there suggesting that there is only a single timeline? I was under the impression that all interpretations of QM are, at present, indistinguishable, which makes the Many Worlds interpretation just as valid as any other.

It's ambiguous and various possible interpretations may be valid simultaneously, yes. I think I sort of misspoke on that, sorry. I mostly meant that there don't seem to be experiments showing that there can be more than one timeline that humans can ever perceive due to the way our consciousness works.

If we built a machine that could travel around a Many Worlds conceptualization and somehow transport our consciousness unaltered between them, it actually just gets weirder: there as many indeterminate probabilistic superpositions (alternate timelines) of the past as there are of the future, so it becomes not a linear regression as most time travel fiction depicts it, but a selection from infinitely many equally possible pasts. You're not traveling in a deterministic fashion so much as you're picking a point rather arbitrarily in a many-dimensional holographic matrix. You can have literally any past, present, and future you like with equal ease, if (as this machine would require) you have some mechanism for choosing the results of collapsing all the probabilistic functions of all the possible particles that make up reality.

Indeed, in that scenario, "past", "present", and "future" don't mean anything anymore, I think, because those are just concepts we use to think about the fixed timeline we experience as an artifact of human consciousness.

5000393 What do you think of my proposed spell, where the "send" spell only allows the caster to travel to a place and time where someone else is casting the "receive" spell? Would that preserve causality to your satisfaction?

Just curious, but do you also take issue with Sci-Fi that allows FTL travel? Would you still object if the story took place in a universe that was explicitly Newtonian or, say, Aristotelian?

As for the travel between Many Worlds universes, that would depend on the machine itself, wouldn't it? Suppose the (made up) laws of physics in your story allow you to travel from time a in universe A to time b in universe B only if the state of universe B at b is identical to the state of universe A at some other time a'. This would give pretty much the same results as "normal" mutable time travel.

5000102

From the perspective of the traveler, the traveler casts a spell and moves from time a to time b. Again, everything that happens has a clear cause.

Not exactly. I've kept getting this weird feeling about this thread since this discussion was raised a week ago in TWG. Let my focus on clarifying my thoughts as to why causality gets thrown ass-first out the window when you travel in time.

>> 4986559

Personally I have two ideas regarding causality that are the reason I don't write time travel fics. If we consider augury--the ability to see the future--a form of time travel [a lesser extreme than actual time travel] then the question becomes: are we seeing the future in which we saw the future or the future in which we didn't.

If the future we see is the one wherein we saw the future, [then our future is in our past, and contextually our past determines the actions we will take from that point on.] We are compelled to act in a manner defined in part by personality, and in part by circumstance. This would indicate that future is inescapable. Our actions have no effect on the outcome as what we do is already

determined by who we are. Even the notion to make a choice to try to break free of that is itself an ingrained in our personalities--[that defiance is] part of who we are.

On the other hand, if the future we see is one in which we didn't see the future then it is an impossible future, as the present [the moment we are living] is already different from that future's past [and cannot lead to our future]. We can of course try to make that future occur but it can never be because that future is based on a different past than has occurred. Time travel itself is very much the same. Unlike augury however, time-travel has the added problem of going in more than one direction.[/quote]
5000102 [quote]The only perspective where causality even starts to get iffy is in the perspective of timeline B, where the traveler appears out of nowhere. But even then, the traveler carries information from timeline A. If they wanted to, an observer could ask the traveler where she came from and why she was here, and determine the series of events that led up to her appearance.[/quote] 

There is another possibility:
>> 4986559 
[quote]...there is funnily enough an idea that once you arrive in the past it ceases to matter from whence you came, or how you got there, because you exist then regardless of what comes after. There is speculation among scientists that objects in the fourth dimension can break, very much the same way that objects in the third dimension can... That is how I prefer to think of time travel. Not moving through time, but instead the breaking of our fourth dimensional self, and redistributing the pieces.  Thinking of a time traveler as one continuous fourth (or above) dimensional object is what leads to many of the paradoxes we see [so often] discussed.[/quote] If we consider many of the common, and/or famous paradoxes thought up, with regards to causality, we can easily see how an object breaking in the fourth dimension and redistributing itself along that axis may address the paradox of causality along a singular timeline. That is to say, changing the state of a fouth dimensional object in the fourth dimension breaks all of the paradoxes we consider when we talk about changing a fourth dimensional object from the third dimension. 

We don't need a reason to travel in time--we exist there as a part of it. We don't need a reason to be in the past, or even a way to get there, and we can't guarantee we'll exist in the future. Once our fourth dimensional self is shattered, all bets are off with what happens to those pieces. We, in the third dimension, can't see that break and may not even notice.

[hr]

4974604 I like to think of free will as a very complex illusion. It's something I wrote about in a story about the nature of free will, and it wouldn't spoil the story in the least for you to read that chapter first. That is, if you're interested enough to listen to a changeling called Madness explain  the basics of the idea.

[hr]

5000209 That's a lot of big words that basically boil down to: "time is not a dimensional axis, if it was, it could traveled along." It is possible that our universe uses imaginary numbers for probability, and the generation of a constant, or a subatomic particle, is the that thing moving from an imaginary axis to a real one. That is, mathematically, for every real axis there is an imaginary one wherein imaginary numbers are concerned. As for the future affecting the past, you might be surprised, as that's something scientists already believe is possible.

[hr]


5000268 It seems far more likely that the universe as a whole exists in cycles. That after the expansion phase ends, a crunch phase will begin, and everything will collapse into a single point. A point that will create a new big bang, and the cycle will endure. Nature has been pretty consistent in how much it fucking loves cycles.

5000313
>If frame of observation A doesn't know that frame B exists (meaning, there's no information conveyance possible), then B cannot influence causality in A.
Ah, now I see what you mean. What I was talking about was that both A and B exists, and while they may not be interacting, they still have the ability to interact. Meaning that there can be communication of information between them, even though there isn't any known communication right now. Make sense?

As for the causality of it all, if time is a 4th dimension (meaning that all of possible states of our 3d space all exist simultaneously and we're just viewing slices), then passage through a 5th dimension can allow changes to the state of the 4th dimension, just as passage through the 4th dimension allows changes in the 3rd dimension. Time travel would be 5th dimensional travel, and whether it is back into it's own timeline or into another universe, it can still change the status of the timeline.

Causality isn't limited to 3 or 4 dimensions. If there are more than 4 dimensions, then causality would exist in all of them, and thus, from our perspective, any changes caused by the 5th dimension or higher won't make any sense to us because we can't see those dimensions. Nevertheless it doesn't mean they aren't there. (Example: What if quantum "randomness" is actually deterministic, but can only be understood through 5th or higher dimensions? We're looking into that)

In the end what I'm saying is that we scientists do not know enough to say whether or not mutable time travel is possible or impossible. Most scientists are open to the possibility of it because of things like extra dimensions.

Mutable time travel certainly isn't nonsense to a physicist.

5000989
Well, even though it seems likely (and we really only say likely because it's the only thing that we can easily comprehend), that doesn't necessarily make other options impossible.

5000478

What do you think of my proposed spell, where the "send" spell only allows the caster to travel to a place and time where someone else is casting the "receive" spell? Would that preserve causality to your satisfaction?

That might actually be one of the more reasonable ways to do it, if we're assuming that the magic in question is linked together to form a closed timelike loop. If the future spell is providing for its own causality to motivate backwards time travel, then it might not break anything because everything has already happened and it's not really changing without cause.

Just curious, but do you also take issue with Sci-Fi that allows FTL travel?

It depends. I take major issue with propulsion of a physical object at FTL because that requires > infinite energy and the relativistic effect would be backwards motion in time, which has similar causality problems to time travel because, in that case, backward motion through time as a result of propulsion implies that the object is trying to move at some point before there was a causative event propelling it, which is self-contradictory.

On the other hand (hoof), maybe there's some leeway for weaseling around that by changing the conformation of space so that physical objects never have to actually move FTL.

Would you still object if the story took place in a universe that was explicitly Newtonian or, say, Aristotelian?

I suppose not. Newtonian classical physics assumes an infinite speed of light (because it's a special case Lorentz transformation where c -> infinity), meaning there's no real barrier to achieving any particular velocity.

However, since the speed of light is really the speed of causality, this assumption of infinity has some really strange implications for how stuff would interact in that kind of universe. For example, it's possible that the singularity causing the big bang might have never been able to expand into a large universe full of the "stuff" we see, since inflation at FTL might have been vital to overcoming the intense gravity of that amount of mass / energy in such a small space (since it can only gravitationally collapse back towards itself at however fast causality can transmit the influence of gravity, meaning light speed).

But it might also be possible for it to make complete sense, though. I hadn't really thought about it that much, so the most concise answer would be that I'm not sure. It'd be very interesting to see what someone can make of a universe like that, if they were clever.

Everyone here is really smart like me.

This is the smartest discussion I've ever seen.

I love bronies.

5231726
>So that means I can be able to use different types of time travel with my Doctor Who-oves series as long as I make it make sense?
Absolutely. Heck, you can play on the Dr Who "fixed points in time" thing to describe the ImT effects in the series and leave everything else as MuT.

4972242 Quick question do methods of time travel also have their own affects?