+1 Time travel fiction is always contradictory because time travel must break our understanding of causality. I share frustrations of the "how does this make sense" variety, but I can understand why "timey-wimey" handwaving is a thing… it's not a personal trigger (in a way that kind of handwaving kicks narrative into fantasy, and I can more easily suspend my disbelief that way). What really gets under my skin is FTL, which is unavoidably time travel in any universe remotely like our own… it's getting so I can't even enjoy space opera anymore. :)

"Timey-wimey" is a thing because that's the level that writers/directors/showrunners are often working at. For Doctor Who, it was basically a statement of "We're a kids show. Don't expect us to have coherent resolutions to long-standing time travel paradoxes, let alone ones which are consistent over our 50 year run. Time travel works this way because that's what we need for this episode. Don't pick at the peeling paint, and just sit back and enjoy the ride."

wasn't "timey-wimey" originally uttered by the 5th doctor in the Time Crash mini-episode (where he met the 10th doctor) and just picked up by 10 from that encounter?

@R.M. So how did DW get away for all those decades without mentioning that phrase?

@PeterM They had the same sentiment, they just didn't voice it explicitly in a meme-able fashion. (BTW, I made my comment to point out that sometimes "timey-wimey" is the best you can get: if the writers/directors/showrunners didn't think things through in that fashion, it could be the time-travel paradoxes just aren't resolved and there isn't a coherent explanation aside from "that's what makes the plot work". I don't know if that's the case for Edge of Tomorrow, but that's why the sentiment pops up frequently.)

@Lexible - There a fair number of ways to get around the equivalence of superluminal communication being equivalent to time travel, at least if you are willing to allow space magic like FTL travel in the first place. Global FTL with local STL through spacetime bending (also the only kind of FTL that anyone thinks has even a small chance of working in real life), positing small-small violations of Lorentz invariance, or only allowing those specific trips that, while the order of events will reverse in some frames, will not allow an event to be its own cause (no "local" time travel).

@Adamant "Global FTL with local STL through spacetime bending (also the only kind of FTL that anyone thinks has even a small chance of working in real life)" Warp drives still equal time travel.

@Lexible - That's not so obvious to me, though admittedly relativity was not my specialization. Everett, in 1996, showed that the Alcubierre metric could be used to create closed time-like curves ("Warp drive and causality"), but it is not obvious that their argument demonstrates that any travel that relies on locally STL and globally FTL effects ("warp drives") would inevitably produce CTCs. In fact, in General Relativity and Quantum Cosmology, "Introducing Physical Warp Drives," Martire and Bobrick discuss a much broad class of metrics.

While their claims of being physical seem very premature, it certainly provides fertile territory for fiction, which is what is being discussed here. Traversable wormholes are the other globally FTL solutions; see Mattingly, 2020, which notes: "The most dangerous potential consequence is how easy it is to create a CTC by adjusting the relative velocities of the wormhole throats or the warp-drive bubbles." Of course, in fiction, one can simply impose a rule saying that such relative velocities are not realizable and get to writing about space wizards and laser swords or whatever.

Then there is of course the option of saying that one is actually totally fine with time travel, for that matter and adopting some kind of logically consistent theory while allowing for some causal weirdness. E.g., in terms of space opera, Star Wars actually has time travel.

I bet you're glad you said not to mention "timey-wimey" in your post. It's like the old "don't think of an elephant" trick.

@N.Virgo There's a difference between describing "time-wimey", and basing an answer on "time-wimey". I achieved the former and not the latter.

@PeterM fair enough, I was just trying to be funny

@Adamant 'FTL = time travel' does not depend on the mode of FTL operation, and the thought experiments illustrating this also work with Alcubierré drives… at some point you collapse the warp bubble, yeah? Why this is germane to fiction, and the motivation of my comments, are precisely because of the "timey-wimey" complaint: time travel always breaks causality, because causality depends on causes preceding effects in time; violate that and paradoxes will create trouble for suspension of disbelief. "Relax, it's just magic in a story" (aka "timey-wimey") is a way of confronting that trouble.

@Lexible - One needs to go beyond thought experiments and actually rigorously understand the physics involved. As I said, my physics specialization was not relativity, but it's still something that we had to study. Please read the papers in my comments carefully to understand the actual issues. The CTCs in Everett's paper only apply to the Alcubierre metric, and they can be created because of being able to create two bubbles at specific relative velocities. There are other "warp" metrics that may or may not have these problems, and in a fictional story, you can impose restrictions...

...on the relative velocities to disallow these modes of travel. "Globally FTL and locally STL trips are always equivalent to CTCs" is not quite correct. The correct statement is the one from Mattingly: in a wide class of these metrics, if you can adjust the wormhole or bubble relative velocities however you like, you can create a trip that is a CTC. (It's not clearly whether this applies to all such metrics).

As for the statement that time travel always breaks causality, that's true! But it's not true to equate that to the existence of paradoxes: there are theories of deterministic self-consistent time travel that avoid paradoxes, though not retrocausality (read the famous "Cauchy problem in spacetimes with closed timelike curves.") Again, that's why looking at the details of the physical

@Adamant "Here go read these things, they will back me up" is not an argument. Nothing you write speaks to my point about time travel fiction, and problems with suspension of disbelief (a la dissatisfactions with "timey-wimey"). Pretty sure we are done here.

@Lexible - No, it's directing you to the arguments, to avoid writing an argument here that would be five pages of LaTex and that, on top of that, someone else already made. That's what arguments look like in physics.