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How to avoid blatantly time traveling or breaking causality in a big way when getting my characters to places quickly (Faster than light)?
The method used is a tunnel in some sort of different space with set entrances that will get you somewhere faster then light would get there.
I don’t mind getting hand wavy or just completely ignoring it. If it’s too hard to dumb down something complex I understand. It’s not a feature I plan on focusing much on. Just a tool to get places quick.
TL;DR: Decide whether any part of your system needs actual FTL (and wormholes do not require this by themselves, if the mouths get moved at sublight speeds). If you do need FTL, handwave a preferred reference frame. If you don't, handwave in chronology protection, and then just concentrate on the interesting aspects of your setting instead.
Hawking offered the Chronology Protection Conjecture, suggesting that it is in fact impossible to travel backwards in time (and so form a closed timelike curve). On the face of it this seems at least slightly plausible, because there don't appear to be many time travellers here.
A suggested mechanism for this, with wormholes at least, was that vacuum fluctuations around the wormhole mouths would rapidly build in strength as you attempted to form a time machine with them, causing the wormhole to collapse before a CTC arose (possibly with a bang, possibly turning into a black hole, posisbly both, depending on how your wormholes are made). In Luke Campbell's Verge Worlds RPG, he used this as a basis for warfare against wormhole networks... a nice example of how things that might seem like unreasonable constraints initially can bring in new and interesting ideas.
The method used is a tunnel in some sort of different space with set entrances that will get you somewhere faster then light would get there.
With wormholes, no-one actually travels anywhere faster than light. Its just that space is no longer simply connected, and there are now two routes between any two points, one of which passes through the wormhole and one of which is the scenic route.
Things like Orions Arm and Verge Worlds have wormhole networks that are deployed at sublight speeds and form trees rather than graphs with cycles (because cycles are Roman Rings), so there's only one route between two points in the wormhole network, and that puts some significant restrictions on how your setting can work, which doesn't always play nicely with the sort of universes people want to write about or play in.
The critical thing is that the mouths of the wormholes are never moved at FTL speeds. If you can open a wormhole mouth at an arbitrary point in space, instead of having to fly it there the hard way, then that's FTL magic, and you start running into issue like the tachyonic antitelephone (or Tolman's Paradox). And that's where most FTL flight/communication/teleportation issues arise. FOr more informnation on this sort of thing, various related answers on this site reference Sharp Blue: Relativity, FTL and causality, which explains that even if you can't FTL into your own past with one jump (or message), with a second FTL event All Bets Are Off.
Ways around that restriction will tend to make things handwavy and soft-scifi-y, and I'd like to point out that these aren't always bad things, so long as they let you make the setting that you want. The excellent suggestion by Someone Else in the comments (referencing this answer by JDługosz) describes what a "safe" FTL system might look like but doesn't explain the why, but you can just handwave in some Chronology Protection that means that trying to do FTL that results in a CTC causes your jump drive or teleporter or remote-wormhole-mouth-opener to go foom. You can invoka preferred reference frame, ensuring there's a point of view from which physics still looks fine and that's the one that everyone can agree on. Another of JDługosz's answers also covers this subject.
The universe may have an in-built mechanism for causality protection, preventing wormholes specifically from forming time machines.
Wormholes as we understand them don't always form time machines. They can be arranged safely and even into complex chains/networks while remaining stable. The trick is to balance them spatially and temporally.
Let's look at an example where two wormholes form a time machine.
Like everything else, wormholes experience relativistic effects like time dilation. Let's create 2 wormholes, A & B, side by side and synchronized, and send B off to some distant star 100 ly away at near the speed of light, while keeping A here at home. Measuring B's transit with Earth clocks, we'd expect it arrive in 100 yrs. However, according to clocks traveling with B, the trip from Earth to the distant star takes only 1 year. There's nothing strange about that. That's basic time dilation. However, this is a wormhole, and we have the other end point to consider. A and B contain the same set of points in space-time. Looking through A, situated on Earth, we see B arrive in 1 year. After only 1 year of flight, astronauts step through wormhole A and visit the star system 100 ly away.
You might think that's impossible. After all, we can look through a telescope on Earth and still see wormhole B in flight to the star, with 99 ly to go. But that's relativity. If you attempt to catch up to B, you'll never reach it before it reaches its destination. In its frame of reference, the 100 ly distance is length contracted to only 1 ly. In its frame, the star isn't 100 ly away, but only 1 ly, and its frame is just as valid as any other.
Anyway, wormhole B is now 99 years in our past, and any astronaut stepping through arrives 99 years in our past. They will have "time traveled". However, with the current arrangement, there's no way to communicate with Earth before the wormhole's departure. Lightspeed messages through space from the star still take 100 years. The astronauts may be situated -99 years temporally, but they are situated +100 years spatially.
That's the rule. A wormhole's temporal offset must be less than its spatial offset.
If one wormhole endpoint sits 1 second into its counterpart's past/future, then the two cannot come within 1 light-second separation without forming what's known as a closed time-like curve (CTC), which is what time machines are made of.
So, how does the universe exhibit an in-built safeguard?
A CTC is a loop. It is a trajectory through space-time whose endpoint lands on its own start point in both space and time.
Imagine a photon traveling along this path. It travels the entire path and constructively interferes with itself in the past, back at the starting point. The new doubled photon then travels the same path and again constructively interferes with itself in the past, quadrupling, and so on. The photon energy ramps up towards infinity in zero time.
This resonating effect isn't limited to light. According to Matt Visser, "Lorentzian Wormholes: From Einstein to Hawking" (1996), if you look at what happens in semiclassical quantum gravity, you will get a buildup in the amplitude of quantum fluctuations on the cusp of forming the time machine, an effect called "vacuum polarization". These fluctuations are expected to build sufficiently to destroy the time machine, before a CTC can form.
So, when you try to bring two descynchronized wormholes to within their hard spatial-temporal separation limit, this effect destabilizes and destroys the topology, rendering them no longer wormholes. Maybe they dissipate harmlessly as gravitational waves, or maybe they collapse into black holes and dump a few gammas out while they're at it.
Because of strangeness, it's also possible that the wormholes "bounce" off each other, so as to prevent time machine formation.
You could just say that time travel doesn't happen. No, really - this isn't a cop-put. Sometimes nature is just like that.
Suppose you are looking at the Andromeda nebula. If you have good sight and it is dark, you can see it with the naked eye. Light is both a particle and a wave. The light has been spreading out as a wave for the last 2.5 million years. It could be anywhere on a sphere with a 2.5 million light-year radius. But if it is detected as a particle, all the energy is dumped within one cell in your retina, and the rest of the giant sphere gets nothing.
How does nature organise this? Does the fact that your head got in the way of the photon just here have an instantaneous effect over the rest of this giant sphere? It seems it does. The light wave does not seem to contain hidden information on where in the wavefront the particle actually is. You cannot use this to pass useful information faster than light. Well, some people hope a way may be found, but I doubt it.
For a bit more on what light does, read about Bell's theorem. By analogy with Bell, the no-time-travel clause could be called Wossname's Theorem, or the Wossname Inequality. That would fit with there being a hypothetical limiting process, but no-one knows how it works.
The easiest way to ensure that there isn't any time travel is to make sure that there are no loops in the wormhole network. They need to form a tree-like graph with only one path from any place to any other place.
With no loops, if you travel from A to B, then the only way to get back from B to A is along the same path in the opposite direction, which would undo any of the time changes.
You may end up with travelers aging at different rates, but nobody will arrive anywhere before they left.
One way to explain the no-loop requirement is that there is a single source from which the network is projected, and something horrible would happen if you projected two wormholes to the same planet.
I am familiar with the problem you're worried about: it plays an important role in the novel Exultant by Stephen Baxter. But I think it's a misconception.
The reason FTL can't result in time travel is that remote frames can't impose their own timelines on each other.
Consider this thought experiment: physicists think that time might actually run backwards inside black holes. So, pretend you have a spaceship that lets you go into a black hole, hang out for two weeks, and then come back out. What would happen?
Now let's consider a less absurd scenario: two star systems whose individual frames have been drifting part in space and time for billions of years: time is running forward in both, but each has a different age-since-the-big-bang because their clocks have been running forward at different speeds.
Your heroes pop into a ship and travel from one to the other instantaneously. They spend two weeks playing Space Poker, and then flit back home.
Again, the reason this cannot result in time travel is because the frame back home has its own clock that has continued to tick at its own rate throughout. Now, it's probably true that the amount of time that has passed for Earth during your space-poker trip is different than Space Vegas, but Earth still moved forward in time. Even if Space Vegas ran backward in time, that just means Space Vegas's physics has been running backward, not that it has become connected in spacetime to an earlier segment of Earth's world line.
You can't use FTL to reach an earlier time in any single frame. It may be true that calendars in Space Vegas say 1975 while Earth says 2023, but when you come back from Space Vegas, the calendars in Earth will still say 2023+. That's true whether you return faster or slower than light.
You can't use FTL to reach an earlier time in any single frame. Right, but with unrestricted FTL (the ability to perform FTL phenomena in any frame), you can easily build time machines, which necessitates some kind of preferred frame, either natural or artificial. Because depending on the arrangement, a closed topological loop can form. A wormhole 1 second in the past, closer than 1 light-second to its other endpoint, forms a loop.
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I'm going to re-post a modified version of an answer I made to a similar,previously asked question:
The simplest solution is to assume that the 'laws of physics' including those pertaining to the creation of wormholes (WH) prevent anyone from using WH that violate the chronology protection conjecture (CPC). As as a result the CPC is no longer a 'conjecture' it is a fundamental principal of physics.
The basic idea would be that all WH start off as small, unstable sub atomic 'seeds' that have to be spun up to usable sizes using exotic matter. This makes WH expensive to bring up to sizes large enough for useful travel. As a result they are precious assets. Also and only if you want you can make it a requirement of your universe that the one end of a WH has to be 'towed' to its final destination point by an automated probe. As a result you only have FTL travel from point to point once it reaches that destination.
Want to travel 1000 LY? Sorry, you have to wait however long it takes for your probe to get to your destination and drop off its precious cargo while it travels at a % of the speed of light. (Annoying isn't it.)
You can also write into the background of your story that shortly after WH were first discovered or created experiments were conducted to determine if they could be used to violate causality. These experiments proved conclusively however that any attempt to do so caused the WH involved to 'collapse' the instant the attempt was made. You simply can't send meaningful information back in time through one. And in this context mass is also 'information'.
It doesn't have to be a violent explosion (though it can be if you want it to be!). The end result is the same though. The instant a particle or photon enters a micro-sized WH that potentially violates CPC the WH falls in upon itself and radiates out its tiny mass as sub-atomic particles.
You can choose any description of the process that best suits your purposes. But the result is a world where only non-CPC defying WH are stable and useful for the transfer of information and cargo. No time machines.
Well, let's break this down a bit.
According to special relativity, kinetic energy is asymptotic as you approach light speed. This means that traveling at light speed requires infinite energy. And contrary to contemporary usages of the term "infinite", what this really means is you can never gather enough energy. If you brought every object in the universe to a single point, at a standstill, and robbed it entirely of thermal energy, you would still not have enough energy to accelerate a baseball to lightspeed. And faster than that is simply excluded from the domain of the equation. Asking what happens when you exceed lightspeed is like asking what happens if you make a triangle with sides of lengths 2, 3 and 40. Well, if you have those segments, you don't have a triangle.
But let's handwave that.
The "speed of light" is kind of a misnomer. First of all, light doesn't travel at a constant speed. It travels at different speeds depending on the medium it's traveling through. And the difference in these speeds is responsible for the phenomenon of refraction. The universal speed limit really has nothing to do with light; light is just an easily observable and easily understandable example of it in work. Light travels at light speed in a vacuum because, well, that's as fast as it can go.
The real cause of the universal speed limit is causality, the idea that causes precede their respective effects. Special relativity is really an observation that causality depends not only on time, but on space. A better term for the universal speed limit than "speed of light" might be "gradient of causality." In order for one event to result from another, the effect must be within the region of spacetime where its delta time is greater than its delta space times the gradient of causality. In other words, it takes time for a cause to propagate through space to trigger an event, and the relationship of the amount of time to the amount of space is the universal speed limit.
For example, if a star burns out ten lightyears from Earth, that's a cause, and us observing this is its effect. Because the cause and effect are ten lightyears apart, a minimum of ten years must pass before we can observe it. Anything faster would be a violation of causality.
Now, if we look at the passing of time as a series of events, we can see how these events have to "slow down" as we speed up. For example, The time must be 12:00 before it can be 12:01, so these two events share a causal relationship. So if we were to, at exactly 12:00, accelerate a clock all the way to lightspeed (which is impossible, but remember, we're handwaving that), and send it ten lightminutes away and abruptly bring it to a halt, it would take ten minutes to get there, but so would the propagation of the 12:00 event, so it would not show 12:01 until one minute after it arrived, even if the clock is functioning normally. If we then sent it back to Earth at exactly 12:01, again at lightspeed, the clock would still say 12:01 when it arrived, even though the time on Earth is now 12:21. Here's a diagram of what it would look like if we sent a clock ten lightminutes away and immediately back, both FTL:
Following the time axis to the right, we first send the clock at 12:00 (blue line) and the event begins propagating (red line). Then it arrives at the point ten lightminutes away, and begins to return (blue line), while the event begins propagating (red line). Then the clock arrives back at Earth (depending on the actual speed the clock moves at, this could be before or after 12:10, but in the diagram it comes before). At 12:10, the Earth departure has had time to propagate 10 lightminutes and it begins to "echo" back to us (green line). A little later (some time after 12:10), we can observe that the clock has arrived 10 lightminutes away. Then finally, at 12:20, we can observe that 10 lightminutes away could have observed the clock leaving Earth.
Now, aside from at the initial point, what time the clock says is anyone's guess. It can't be after 12:00 because 12:00 hasn't happened yet. But there's also no reason for the clock to tick backwards. Whatever it says is a violation of causality (but then again, so is the whole thing). We also have some unexplained oddity in terms of our causal echoes. But in our frame of reference, it doesn't actually arrive before it left.
Looking into causal echoes and the reverse observation of events when FTL happens could be interesting, but just like audio echoes, I don't think anyone would complain if you just didn't mention them.
In theoretical physics, there is a concept of "universal now." People who don't actually understand relativity insist that there is no universal now because they heard the phrase "no privileged frame of reference," and think that the latter prevents the former. This is not the case. What it really means is that the rate of time in one location isn't beholden to the rate of time in another location. Neither is the "right" or "correct" rate of time. Neither is privileged with that designation.
A light cone is an expanding sphere of space within light-speed distance of any event. The functional rule of causality is that nothing outside of an event's light cone can effect the event that the light cone is expanding from.
For a light cone to exist, there has to be a consistent flow of time in all locations. It might not be happening at the same rate, but everything experiences now at exactly the same instant.
Causality violations are less specific than a light-cone violation. They state that you can't go backwards in the universal now. If you can teleport, it basically turns the cone into a flat plane. If you have good enough telescopes, you can literally see into the past of the other location. This is not a causality violation because you can't actually visit that past.
Teleportation creates a local violation of thermodynamics by transferring your state from one place to another, but it isn't a causality violation.
If you travel back in the past, you are just no longer executed by the simulation that runs the universe. Its like beeing memory pasted into amber. You float forever preserved in some past flickering second, with not consequence whatsoever rippling into the future.
It sounds like you are referring to a wormhole.
Think of space like a piece of paper. Ordinarily it may take quite some time to traverse from point A to point B on the paper. But if I were to bent the paper so that point A and B were to touch, then I could move, effectively instantaneously, from point A to B. The equivalent in our three-dimensional universe would be a wormhole.
It sounds from your question that these wormholes are fairly permanent (a tunnel to "set entrances"). With a sheet of paper, it would be hard to fold it where several different points are in contact but not the rest of the sheet, but that's inconsequential. Maybe spacetime fabric is more stretchy/flexible.
Personally as a reader, it'd be fine just to call it a 'teleporter', 'Doorway,' 'portal' or 'wormhole.' Naturally, I'd want an explanation as to why it worked, who built it, etc., but if you sculpt the scene right, you can get away with just calling it a wormhole without having a 15-page scientific breakdown.
