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The primary mechanism of “faster than light” travel in my story is via artificially formed wormholes that can be created to transport a craft to a reasonably precise location. Would the existence of the ability to create an artificial wormhole mean that time travel would also be possible, or even a side-effect of this method of travel? I’ve found seemingly conflicting information regarding this, so I would greatly appreciate someone’s help understanding the implications for this.

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    $\begingroup$ I just answered your other question and I'll mention something from it here. Please don't get caught in the Religion of Science trap, meaning the blind belief that because humanity has toyed around with mathematically modeling something, it must be real and obviously limited by the boundaries described in the mathematics. The factual answer to your question is, "we don't know." And it isn't helped by the fact that, without some empirical evidence to judge the mathematical models by, we really don't know if our models are right or not. This is why this stack focuses on imaginary worlds. $\endgroup$
    – JBH
    Commented Apr 17, 2023 at 7:35
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    $\begingroup$ There is no such thing as wormholes, so you can make them behave however you wish. $\endgroup$
    – Tony Ennis
    Commented Apr 17, 2023 at 20:20
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    $\begingroup$ @TonyEnnis It is possible to coherently theorise about things that haven't actually been created in real life. Nuclear reactors and bombs didn't exist in the past, but research upon them worked out their nature and capabilities. The fact that "there's no such thing as X" in no way implies that X will "behave however you wish". Wormholes that are particular solutions to the Einstein field equations are constrained to behave in certain ways, for example. The OP didn't specify magic, after all. $\endgroup$ Commented Apr 18, 2023 at 15:27
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    $\begingroup$ The Stargate franchise opened wormholes all the time without time travel. $\endgroup$
    – RonJohn
    Commented Apr 18, 2023 at 17:13
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    $\begingroup$ @ths it's a colloquialism. I go to the convenience store store down the road all the time, but it's not the only place I go. $\endgroup$
    – RonJohn
    Commented Apr 18, 2023 at 18:03

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Be clear to distinguish two things: FTL effects and non-simply-connected space.

Given a wormhole network subject to some chronology protection (see Logan's answer on that subject) there isn't necessarily any actual FTL going on at all... everything is moving at or below the speed of light at all points, but space-time is no longer simply connected and you can travel take the shortcut between points A and B without exceeding C. There are a lot of restrictions that your protected wormholes have to comply with regarding the shape of your wormhole network (probably a tree rather than a grid) and how the wormhole mouths are deployed (you need to fly them to other stars at slower-than-light speeds), but so long as you follow them you'll be fine.

However.

Your wormholes, as described in your other question If a civilization had the technology to create portals (wormholes) for FTL space travel, what physics favor their use for ships over small weapons? would appear to violate causality:

If a civilization had the technology to create wormholes for travel, why wouldn’t they use this technology to open wormholes right above their targets and launch a weapon through it?

Opening wormhole mouths at arbitrary points in space is Bad News. If you can open a wormhole between two points A and B in less time than it takes for light to travel from A to B, then what you've done is created an FTL transport and signalling system and suddenly All Bets Are Off, because relativity will have already come back and bitten you in your past light cone. The perils of superluminal signalling under relativity were described in the tachyonic antitelephone thought experiment and I won't repeat it here, but it breaks causality.

would that necessitate the existence of time travel?

Are you writing a hard-scifi setting that tries to keep as close as possible to physics as it is currently understood? Then yes, magically and instantaneously opening a wormhole at a distance means some form of time travel.

If this interferes with your plans for your setting, then I can heartily recommend not going the hard-scifi route. Hyperspace and teleporters and ansibles are all nice things to have. If you need some or all of them, and don't want to worry about time travel, then handwave them in. No-one will mind. You can waffle a bit about preferred reference frames if you like, but I suspect that most people won't care very much.

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  • $\begingroup$ Thank you for this answer, I appreciate the info! 👍🏻. I’m starting to think that perhaps I should set my story in a fantasy universe rather than our universe in the far-distant future. This would give me the leeway you’re talking about, and I could probably just handwavium some of these aspects of my plot, though I still don’t want to completely break probability or jar people out of my story by things that are completely unrealistic. As long as I sell it with some realism, a fantasy setting could possibly solve some of these issues and give me much more freedom than hard-sci-fi. $\endgroup$
    – Kal Madda
    Commented Apr 18, 2023 at 16:16
  • $\begingroup$ "If a civilization had the technology to create wormholes for travel, why wouldn’t they use this technology to open wormholes right above their targets and launch a weapon through it?" Read Schlock Mercenary. (Short version: inventor invents a wormhole device, intends for it to be used for travel. Some people get the idea to abuse it this way. In response, the same inventor immediately invents a wormhole-detection-and-interdiction device that will collapse any wormhole being opened nearby before anything can come through it. Now it can basically only be used for travel again.) $\endgroup$ Commented Apr 18, 2023 at 18:40
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    $\begingroup$ +1 for "bitten you in your past light cone". $\endgroup$
    – jdunlop
    Commented Apr 18, 2023 at 20:42
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    $\begingroup$ @KalMadda it can be our universe just fine. If you want you can add a comment about how scientists of the 20th and 21st centuries were all wrong about spacetime. No biggie. We don't know all that much about spacetime. Most of it is conjecture. $\endgroup$ Commented Apr 19, 2023 at 10:20
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    $\begingroup$ @Hobbamok Stephen Baxter's 2013 well-regarded novel Proxima features a hatch found on Mercury that leads to a planet orbiting Proxima Centauri. Travel for the users is instantaneous, but they eventually discover it's at lightspeed to the outside world, so 4.2 years pass during every trip, which means it takes 8.4 lean years to get a response from the solar system. $\endgroup$ Commented Apr 19, 2023 at 20:19
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No. It implies that you could try to set up a time travel loop, but not that you could succeed. The Chronology Protection Conjecture suggests that, if you try to arrange a loop of wormhole paths which permits time travel, quantum field effects will result in at least one wormhole in the circuit collapsing before anything can actually traverse the circuit.

So, just declare this conjecture about quantum gravity to be accurate in your universe, and you don't have to worry about wormhole time travel.

The result is that you will end up establishing an effective standard reference frame for FTL travel artificially, based on how you construct your wormhole network, even though the universe does not come with any preferred frame built-in. It also implies that you will have to establish struct regulations on where wormhole-bearing ships can travel to avoid risk to other parts of the network, and militaries and terrorist groups could exploit attempted loop construction to probabilistically destroy other parts of the network. And if wormhole placement starts consistently failing at the edge of the expanding network, that's a good signal that you might be getting close to the edge of another expanding alien civilization which has established a different "Empire Frame" for their wormhole network.

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    $\begingroup$ +1 for mentioning the Chronology Protection Conjecture $\endgroup$
    – Seggan
    Commented Apr 17, 2023 at 1:46
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    $\begingroup$ The consequences here are worked out in depth in the classic Traversable Wormhole FAQ. $\endgroup$
    – gilgamec
    Commented Apr 17, 2023 at 8:42
  • $\begingroup$ Does that mean you can only go one way, and not back superluminally? $\endgroup$
    – kutschkem
    Commented Apr 17, 2023 at 13:13
  • $\begingroup$ @kutschkem No. You just have to come back through the same wormhole you left through, or along a circuit that takes long enough that you arrive after you left. $\endgroup$ Commented Apr 17, 2023 at 14:29
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The mechanism which makes faster than light travel possible is irrelevant. Whether FTL is achieved by means of warp fields, space distorsion, worm holes, subspace, hyperspace etc. does not matter. If a ship or a signal can travel from point A to point B faster than light would travel from point A to point B then the ship or the signal has travelled faster than light; this is all that matters.

Whether faster than light travel, by whatever means, does or does not automatically imply time travel depends on the specific details of what exactly is meant by faster than light travel, and not on the mechanism of faster than light travel.

In particular, if faster than light travel is only allowed with respect to a particular, fixed and universal frame of reference then faster than light travel does not imply time travel. See section 9.5.4 in the most excellent Relativity and FTL travel by Jason W. Hinson.

For the curious: this is not a loophole in Einsteinian relativity. To the best of our knowledge, there is not and there cannot be such a thing as a fixed and universal frame of reference. (Mostly because there is no way that we know of to measure the time coordinate of an event so that any arbitrary observer will agree on it; plus, of course, the existence such a fixed and universal frame of reference is forbidden by the fundamental postulates of the theory of relativity.) But if such a fixed and universal frame of reference will be found to exist then all time-travel-related objections to faster than light travel will become solvable. As a bonus, physics will have to be rebuilt from the ground up, providing lifetime employment to generations of physicists. Win-win.

The usual way of introducing such a fixed and universal frame of reference in the kind of science-fiction which is mostly soft but still attempts to be at least a tiny little bit hard is to assume the existence of a special field which extends $\rightarrow$uniformly$\leftarrow$ throughout the universe, and which is measurable so that anybody at any point and at any time can measure their velocity with respect to that field. But this is pure creative detail a.k.a. handwaving.

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    $\begingroup$ To me, the facts of, "the existence [of] such a fixed and universal frame of reference is forbidden by the fundamental postulates of the theory of relativity... But if such a fixed and universal frame of reference will be found to exist then all time-travel-related objections to faster than light travel will become solvable" indicates that the best answer to the question is, "Yes, FTL implies time travel according to current real-world physics understanding". $\endgroup$ Commented Apr 17, 2023 at 2:05
  • $\begingroup$ @DanielR.Collins Imagine you are floating in the middle of the ocean on a cloudless day. Wind is going one way, but even if you can see the sun you have no ability to determine how fast the wind is going because your speed is also affected by the current. Yes there is a fixed frame of reference (the earth) but there's no way for you to perceive it. $\endgroup$
    – aslum
    Commented Apr 17, 2023 at 13:58
  • $\begingroup$ @aslum A sextant measurement angle between the sun at its height and the horizon gives you latitude. Dropping objects from 1 meter up and 2 meters up and very (very) precisely measuring the acceleration due to gravity should let you calculate the distance from center of mass. And plumb bob in a vacuum chamber gives your orientation relative to the surface of the sphere with that radius. Still missing longitude, but that's more than enough to deduce the existence of that reference frame. (And if you also bring an interferometer, you can measure your movement relative to the aether.) $\endgroup$
    – Ray
    Commented Apr 18, 2023 at 22:27
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    $\begingroup$ I agree with Daniel that that point could have been emphasized differently (and light cones mentioned somewhere). But "If a ship or a signal can travel from point A to point B faster than light would travel from point A to point B then the ship or the signal has travelled faster than light; this is all that matters." is a critically important point that isn't being stated clearly enough by the other answers, which is worth a +1 by itself. $\endgroup$
    – Ray
    Commented Apr 18, 2023 at 22:31
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    $\begingroup$ "the existence such a fixed and universal frame of reference is forbidden by the fundamental postulates of the theory of relativity." I thought relativity just said no such frame could be privileged, not that no such frame could exist. "assume the existence of a special field which extends →uniformly← throughout the universe, and which is measurable so that anybody at any point and at any time can measure their velocity with respect to that field. But this is pure creative detail a.k.a. handwaving." Doesn't such a thing exist in real life, namely the frame in which the CMBR is isotropic? $\endgroup$ Commented Apr 19, 2023 at 1:29
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Both wormholes and time travel are hypothetical things. It is not established that either one of them actually do exist in real life. All mathematics, all science regarding them is completely speculative, not even remotely tested or (as of now) even testable in practice.

Time travel requires that time is a true dimension like the three known spatial dimensions, but science has not found a single shred of evidence of its existence. Even potentially existing retrocausality can be explained without it: it is just same causality as normal physical causality, only incorporating a lot more things from a lot broader area. No one has ever truly peeked even a fraction of second into the future or past to confirm that there is nothing more than the eternal now, the only moment whose existence we can be sure of. It is possible that there is no time dimension at all, and therefore there is no time travel either.

Wormholes require that fabric of space can be manipulated in a way that two otherwise distinct locations are actually the same. This is a bit more plausible thing given that it can "be same" as quantum entanglement, but at an enormously larger scale. However, still: quantum entanglement is a far cry from wormholes.

So, if you incorporate either in your world, you are stepping outside of boundaries of hard science, and therefore you can define the rules as they suit your story.

Or, to answer your question: wormholes can exist in a world where there is no time dimension. Connecting two locations, or making them identical, does not make it necessary to have any more dimensions apart from the three spatial ones.

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I would say no, FTL travel does not necessarily imply time travel.

Both ends of the wormhole are "moving" normally through time - forward through time. You are taking a shortcut, not actually moving faster than light.

Therefore, causality won't be violated.

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    $\begingroup$ When it comes to time travel, relativity doesn't care how you get from point A to point B, only that point B is somewhere outside of point A's forward light cone. Given a pair of such A-to-B routes, you can construct a causality-violating system, letting you go from A to B to a point somewhere in A's backward light cone. $\endgroup$
    – Mark
    Commented Apr 18, 2023 at 1:38

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