Are there any ways to allow some form of FTL travel without allowing time travel?

Question

Faster than light travel is a really cool thing to have in sci-fi settings. It allows humans, in relatable time scales, to travel the galaxy and see a variety of worlds. It allows for conflicts spanning not just a solar system or perhaps a solar system and its nearest neighbors, but huge sections of the galaxy. FTL communication, meanwhile, allows governments and organizations to exist that span huge sections of space, something that might otherwise be impossible if communication between planets took decades, centuries, or even millenia.

The only problem with allowing FTL travel is that is also allows time travel. Most sci-fi settings with some form of FTL ignore this fact, choosing to leave it unaddressed and focus on the story. There is nothing wrong with this. However, it would be more satisfying as a science loving story teller to have some sort of explanation as to why FTL doesn't allow people to travel back in time.

Are there any ways to construct or limit FTL travel that would let people travel between points light years apart without allowing for time travel to take place? I'm fine with solutions that stretch our current understanding of physics, but would prefer to keep everything as believable as possible. Any form of FTL travel, be it wormholes, advanced engines, or some sort of hyperspace dimension is acceptable, so long as it doesn't permit the travelers to travel back in time.

Answer 1

As you said, any FTL drive is implicitly a time machine. Let’s review the reasons why. As a prerequisite you need to understand spacetime and the ideas of space-like and time-like intervals.

first you need…

We will make use of space—time diagrams which show that events (points in spacetime) have different x and t values depending on your reference frame.

I thought I had first learned of them through the Alternative View column in Analog, but I can’t find it. I think the illustrations are not archived here. But, a number of the essays involve wormholes, warp drives, and other related subjects, so it well worth going through his archive now. (I'll wait ☺)

A more recent tutorial is Sharp Blue: Spacetime and coordinates.

An observer in uniform motion traces out a straight line, called his world line. He is at rest in his own reference frame, so this becomes his time axis: his x value stays 0 and his clock ticks away. Meanwhile, his x axis—points of equal time—will appear to be angled by the same amount off the parent diagram’s axis. That is, it is squished. Furthermore, the scale of the tickmarks is different. Formally, you can note that the interval can be computed and is the same in any reference frame.

The particular squishing and scaling of the moving reference frame’s axis explains all the effects of special relativity we have heard about: time dilation, length contraction, and puzzles such as the pole-in-barn can simply be read off on such diagrams.

(from Wikipedia) Minkowski diagram of ladder paradox. The garage (barn) is shown in light blue, the ladder (pole) in light red. The diagram is in the rest frame of the garage, with x and t being the garage space and time axes, respectively. The ladder frame is for a person sitting on the front of the ladder, with x′ and t′ being the ladder space and time axes respectively.

time travel happens

Consider two planets around distant stars, A and B. Their world lines are drawn in the following diagram as vertical lines. Our traveler Charlie has a FTL drive that (through whatever sci-fi trope we choose) can jump from one planet to the other “instantly”. I will choose instantly because it is the simplest to show. Any speed faster than light and longer than instant will have the same effects. (And any speed faster than instant is explicitly time travel into the past so that case is trivial.)

Now I put “instantly” in quotes because time is relative. I drew a line of equal t at different positions, with the t axis as used by the reference frame of A. (And for simplicity, B is at rest relative to A; but see Coda below.)

In spacetime no reference frame is better or worse or different than any other. If my drive makes instant jumps, then we are saying that it is instant in the reference frame which the pilot is in when he engages it.

So, look at the point of view of Derral, who is passing near planet A at the time Charlie made his jump. Derral is moving along a line in the direction of B to A (since we are only showing one space dimension) at a noticeable fraction of the speed of light. The red line is D’s position at any time, so this is not just his path on the diagram, but his t axis: he's at rest in his own reference frame and the planets are moving, from his point of view. So, his x axis is also different, and that’s labeled in the same color, and called x_D in short.

In Derral’s reference frame, on this drawing points at the same time are those lying on lines parallel to x_D.

So, Derral engages his jump drive at the same time as Charlie, and ends up at point D₂, which beats Charlie's “instant” arrival! Derral has to slow down and then return to planet B, which he has plenty of time to do since such a maneuver stays within B’s past light cone. (Alternatively he could have overshot B’s world line and dropped back into normal space within B’s past light cone and let his high velocity take him back B.)

Thus, due to symmetry, the FTL drive functions as a time machine. You can choose your spacettime axis, jump far enough to amplify the difference between time axes of different observers, and travel into your past using multiple jumps or travel into the past of another traveler.

multiple FTL trips required

An intro recommended by some other WB beings is Sharp Blue: Relativity, FTL and causality. It covers this same introductory material, noting how a second superluminal signal causes a causality violation.

You will notice that Derral cannot return to the past at planet A by making normal spaceflight. We will illustrate this by drawing a diagram with the jump being a very extreme angle, near that of the light cone – i.e. the pilot was moving at highly relativistic speed when engaging the jump.

As we postulated earlier, the angle will always be to a space-like separated point outside of his light cone. If it was inside of the light cone, it would be either not-FTL or explicit time travel.

If he jumps any distance away, and then returns as fast as possible (the 45° angle of light on the diagram) he will always return later than he left.

Although any space-like jump can be viewed as time travel in some reference frame, only multiple jumps in different reference frames can cause a causality violation.

Break the Symmetry

The solution then is to disallow that. We don't want to disallow more than one jump ever since it would not be a useful drive. Instead, introduce a preferred frame of reference and have all jumps use the same x and t axes.

See also Jason W. Hinson’s presentation.

It fits with existing tropes

This is easily done as part of the drive description. For example, you access a subspace dimension, and there is an absolute or preferred reference frame associated with that subspace e.g. it has a medium filling it. More complex explanations are possible: our universe has a distinguished reference frame defined by the average motion of everything in it and easily seen from the microwave background. The stuff in the universe has specific reference frames, but spacetime itself and the laws of motion do not. You need to find a reason why some thing in the subspace universe matters to our ability to move through it, or how it connects to the spacetime of our universe.

Of course, if it wasn’t so, then we would not have the necessary limitations to make FTL travel possible without causality violations, so you could use an anthropomorphic argument that if it were otherwise we could not have accessed it.

It provides story potential

This also offers some interesting plot points that fit with it. In particular, suppose that the specific reference frame isn’t just the line that’s followed when doing a jump. Rather, the ship must be in that specific reference frame in order to engage the drive! Now this could be matching the CMB rest frame, or it could be something at an inconvenient relativistic speed compared to the stars and planets we are moving between.

Option A is that the drive won’t work unless at rest in “the” jump frame. Option B is that a ship that jumps while in a different frame will arrive all smeared out in space and time, since the ship was not simultaneous with the transit but interpreted different parts leaving at different times! Ouch.

Note that proof of being safe for causality required the light cones to be parallel in all places. What tips light cones is general relativity. So, we can avoid running into problems by postulating that the drive doesn't work in a strong gravitational field. Note that this is also an existing trope, and comes about naturally from option A. Warped space is not the same reference frame you require! So, ships must travel far from the sun first, and Earth-based observations (nor astrophysical effects involving stars themselves) would not ever show anything that would lead to its discovery! Niven’s known space universe used that effect to explain why Outsiders sold the tech to planet-hugging species like ours.

And finally, you can complicate matters by having this design break down. Normal use of the drive does not involve time travel. But what if you prepared a flat patch of spacetime that was itself deep in a gravity well?

What if there was not just one jump frame, but others waiting to be found? Perhaps there is a whole series, at successively higher velocities in our space, or things in the subspace dimension can be changed and manipulated.

In general, use the flaws and rough edges of this drive explanation for plot potential rather than cause to reject it as not being “hard” enough S-F.

---

Coda: A and B are always in different reference frames

See the Andromeda Paradox. Just two people passing in the hall will have different reference frames, and ever so slightly different planes of what they consider to be simultaneous. That slight difference in x axis (that is, all x where t=0) can be amplified by distance so that in another galaxy it is clearly a different time, a different day.

Planets around different stars will have relative velocities orders of magnitude greater than walking speed, just from the difference in orbits and the star’s movement within the galaxy.

How do I interpret this?

See: How does paradox-free FTL travel affect the details of my story or gameplay?

Answer 2

Yes, you could use traversable wormholes (which have a reasonable degree of scientific plausibility since they are solutions to the equations of general relativity, though they would require 'exotic matter' to construct) along with Stephen Hawking's chronology protection conjecture. Traversable wormhole can only be used for time travel when the two wormhole mouths have been arranged in certain configurations, and the chronology protection conjecture says that at the moment you cross from a configuration that doesn't allow time travel to one that does, you get a feedback loop of virtual particles between the two wormhole mouths that destroys the wormholes or causes some other unknown quantum gravity effect to kick in that prevents them from being used to send anything back in time. For more details on this see the wikipedia page on the conjecture or a book like Time Travel and Warp Drives. In configurations that don't allow time travel, traversable wormholes would allow for a type of effective FTL in which you could reach a distant destination and return home in much less time than it would take light traveling through the ordinary space outside the wormhole (though of course light that traveled along with you through the wormhole would still move faster than you as it passed you by, so a physicist would say that nothing moves 'locally' faster than light in this scenario).

This page has a good article by Michael Clive Price that explores some plausible science-fictional consequences of FTL travel via wormholes, assuming the chronology protection conjectures holds (he notes at the top that a revised version of the article appeared in Extropy #11, which is available online as a pdf here, with the article starting on p. 14). For example, one interesting consequence is that each civilization would probably send out wormhole mouths traveling at nearly the speed of light (while retaining the other mouth at their home system), and due to time dilation a clock traveling alongside a wormhole mouth might measure very little time to reach very distant locations, for example the clock might measure only 2 years to reach a star that is 100 light years away in the inertial frame where the Earth is at rest (in the clock's own rest frame the distance would be shrunk due to length contraction). This means that we on Earth would also measure a time of only 2 years before we'd be able to step through the mouth we kept on Earth and arrive at a destination 100 light years away--but we'd be arriving over 100 years in the future, as measured in a reference frame where the Earth is approximately at rest. But then if we stepped back through the wormhole, we'd arrive back on Earth very shortly after we left. So, the author of the page suggests interstellar empires using wormholes to reach distant locations would use some type of "Empire Time" that would be different than time in the Earth's rest frame, and would treat an event happening 100 light years away and just slightly over 100 years in the future (as measured in Earth's rest frame) as happening on the same Empire Time date as an event happening on Earth today. In effect, all events on our future light cone at the moment we first started sending out wormholes at close to light speed would be treated as happening on about the same date.

Another related consequence: if we ran into an alien interstellar empire that was also sending out wormholes, we would both agree that the event of our meeting happened a relatively short (on historical scales) time after we first began sending out wormholes, in terms of Empire Time. So, just as in many space operas like Star Trek, different alien civilizations would all encounter one another at approximately the same level of technological development, even if they evolved millions of years apart in ordinary non-Empire-Time.

Answer 3

Most forms of FTL travel have problems with time travel when either FTL signals are sent in opposite directions or the FTL vessel travels first in one direction and comes back again. Therefore, one solution to time-travel-free faster-than-light travel is that FTL travel can only happen in one direction.

This concept was developed by Adel Antippa in a series of papers that can be found here. Antippa and his co-authors were investigating the possibility of superluminal frames of reference. Everett apparently gave up when the model he was working on required three dimensions of time and he found it conceptually and psychologically too difficult to imagine what they related to physically.

In two dimensions Antippa's tachyon corridor only allows superluminal motion in one spatial direction that always increases. This is equivalent to the way in our sublight world it is time that always increases and in one direction. If FTL travel was confined to a two-dimensional submanifold of spacetime and in only one direction spatially this leads to one form of FTL travel with time-travel. This would allow a spacecraft travelling to, say, Sirius at FTL velocity, assuming Sirius lies along the tachyon corridor, but the spacecraft would to make the return at sublight velocities.

This does lead to a weird and wacky universe with FTL travel only one way in a specific direction and orientation. All other travel would be either purely sublight or a combination of sublight and with various stretches at FTL velocity along the tachyon corridor.

This model does assume there is a practical mechanism for transitioning from the bradyon (or sublight) domain to the tachyon (or superluminal) domain. Possibly this might be conceptualised as a form of quantum tunnelling. For example, Stanley Schmidt came up with something like that in his Kyyra SF novels where he had a not too unreasonable model for tachyonic FTL travel.

Traversable wormholes are almost scientifically feasible options, at least, at the conceptual level. However, if an interstellar civilisation set up transport networks with wormholes. Now because a trip through a wormhole across astronomical distances is also a trip into the future, this means stepping back through the same wormhole means going from the future back to the past. If there are two sets of pairs of wormhole mouths (basically two wormholes going in opposite directions between locations A and B) this means time travel becomes possible.

One solution is that if photons can pass through both wormholes and continue circulating back and forth in time this will amplify into a massive radiation flux that will eventually destroy the wormholes. Also, making any trips through the wormholes fatal.

Another solution is to set up wormholes in a series through a transportation so each wormhole is effectively in the future of the wormhole that you need to travel through to reach that system.

**For example, there is a wormhole connecting Sol to Tau Ceti. The next wormhole goes from Tau Ceti to Alpha Centauri, then Alpha Centauri and Sol are connected by a further wormhole. This wormhole network is arranged in an approximate circle. Travellers can only go forward and with no backtracking between a pair of connected systems, but a trip starting at Tau Ceti would have to go right around the network proper before returning to Tau Ceti. Undoubtedly, travel would be further safeguarded by Customs and Security.

Most people opposed to time travel are concerned with causality violation and rightly so. In the universe we see around us, common causality is almost invariably the case (I say this, because if there were any causality violating events their incidence is indescribably small, in fact, quite undetectable). This is despite the fact that most physical laws are time symmetric and this of itself should allow forms of time travel like processes, but this doesn't happen. Therefore, it is not unreasonable to assume our universe has what could be called a strong causality condition. This acts to 'suppress' any event that leads to causality violation.

There is no reason not to suppose that if there was auniverse where FTL travel, either tachyonically or by traversable wormholes or Alcubierre drive vessels that the strong causality condition would also apply. This would FTL travel but any FTL trip (and this includes FTL signals) could only happen in a manner that avoided time travel.

Edit: I had assumed, but failed to explain, that the ring of traversable wormholes are installed on after another and going around the ring itself. This means Sol to Taul Ceti, followed by Tau Ceti to Alpha Centauri, and then Alpha Centauri to Sol.

**: I am going to disagree with myself. I have decided to leave this example in my answer to show how easy it is to go wrong. Essentially a wormhole network works without causality violation if the network isn't closed. Also, it would work, at least, conceptually, if strong causality existed as an attempt to form a closed timelike curve would be blocked. Implicitly the ring of wormholes was structured as if weak causality was in place.

The example of the wormhole network is feasible if it isn't closed. As long as the wormhole isn't closed this prevents causality violation from happening.

Although in practical terms we have never to have worry about whether FTL travel leads to time travel (except at a theoretical level) because the technological requirements for FTL travel are too incredibly extreme to even remotely achieve.

Answer 4

The usual argument that "FTL implies time travel" is based on special relativity (SR). A crucial assumption is that whatever physics governs FTL still obeys the symmetries of SR — in particular, if you can travel at some coordinate velocity in one inertial reference frame (as defined by SR), then you can travel at that velocity in all inertial reference frames.

The way to defeat the argument is simple: don't require FTL to obey the symmetries of SR. This shouldn't be troublesome to do, since you have already broken the laws of SR simply by assuming tardyonic matter (i.e. you) can be accelerated to be faster than light.

So you pick out a comfortable Newtonian reference frame, explain that it turns out length contraction and time dilation really is the right way to think about things after all (which is convincing, since the "absolute" rest frame is something we can now detect through whatever physics govern FTL), and enjoy FTL without time travel.

The real problem is why you don't blow up the universe with an optic boom.

Answer 5

No

FTL inevitably leads to time travel.

Faster-than-light communication is, by Einstein's theory of relativity, equivalent to time travel. According to Einstein's theory of special relativity, what we measure as the speed of light in a vacuum (or near vacuum) is actually the fundamental physical constant c. This means that all inertial observers, regardless of their relative velocity, will always measure zero-mass particles such as photons traveling at c in a vacuum. This result means that measurements of time and velocity in different frames are no longer related simply by constant shifts, but are instead related by Poincaré transformations.

Any form of FTL (including those mentioned in other answers) will also lead to time travel and several interesting novels by physicists like Stephen Baxter (Manifold: Space) explain how they can be used in this capacity.

Example

Here's more information both as reference and a thought exercise.

Creating a "time machine" from a stable wormhole:

1. Create a stable wormhole.
2. Put one end on the Moon.
3. Put the other end in a spaceship.
4. Use it to resupply your spaceship (especially fuel and propellant).
5. Accelerate your spaceship to a very high velocity.
6. Fly your ship first away from and then back to the wormhole opening.
7. When your bring the wormhole ends back together, time dilation will have caused time to move more slowly for the traveling end than the stationary end (the amount depends upon the particulars of your trajectory but they could be years, decades, or any amount of time "out of sync").
8. If you enter the traveling end, you'll appear in the stationary end before stepping through.
9. Voila, time travel

Video Discussion

Here's a video news story discussing one scientist's thoughts and experiment.

Story trumps Science

If you need a form of FTL without providing for time travel, you could assume some sort of cosmic censorship or other plot mechanism that simply prevents it. If you do need to resort to this just make your rules consistent and try to think out the first order effects and take those into account for your Universe.

Answer 6

If you want a story that is self consistent and realistic, but does not necessarily correspond exactly to reality, the answer is simple:

Go back to Newtonian Physics

Simply state that in your universe, Newtonian physics are in affect instead of relativity. This means no black holes or time dilation, but it does mean you can go as fast as you want. Newtonian physics is internally consistent, and works the same as our reality at slow speeds.

(If you want to be nit picky, you can't technically go FTL since light is infinitely fast in a Newtonian, but the point is that there is no speed limit.)

Edit: Quantum Mechanics is okay too.

Answer 7

Well, The Alcubierre drive kind of does that. The principal is simply warping space time, exactly like a worm hole, but localized and specialized. instead of changing the distance of point A to point B to almost nothing for an extended period of time like a worm hole, It does so temporarily and on a lesser scale, and only for whatever is inside its small bubble of influence. Just like with worm holes, you never go FTL, It merely alters the distance between you and your destination as you travel, so that every step you take is on compressed space time which if uncompressed can be dozens, if not hundreds.

By doing so only as you travel, and undoing it right behind you, You get caught in a bubble that is unaffected by the manipulation, so time can flow roughly the same as if you are out of the bubble. At worst, you can "travel" to the future thanks to limited time dilation, but you can't go back in time because space time is allowed to expand and compress at almost any speed. The actual numbers seem to depend on where you look, but the the Universe was more than a light second big a second after the big bang, with some numbers be thousands of lightyears in some cases. As of right now, We can never reach certain galaxies simply due to the expansion of the universe making the distance between us and them increase by more than the speed of light.

Answer 8

Well, my solution to it is kinda simplistic, compared to some of the detailed answers that others have put into it - author fiat :D

More seriously, the way I have things working, FTL travel doesn't imply time travel because the universe itself disallows it. You just can't travel back in time. No slingshots around the Sun, or emergency cold starts of the drive, or super-advanced chroniton 29th century blah blah, nothing. Everything else is like reality unless noted.

If you don't particularly feel like going with that route, have FTL go through some other dimension where it's not an issue - subspace or hyperspace being the most common name for it, although you could do something really weird, like "nullspace" or "clownspace" or something. The important thing is, it's another space-time, in which stuff works differently, whether that be something about relativity, or the speed of light, or the distance between two points. For instance, hyperspace might be a place where the speed of light is infinite, or at least unreachable without months of acceleration. Or it might be a place where light-years are compressed down to mere light-minutes. Then, rather than FTL travel through normal space, ships jump over to hyperspace or whatever and just travel normally there. From the perspective of somebody in normal space, you disappear from normal space, and reappear somewhere else. From the perspective of hyperspace, you haven't done anything screwy, you have not disobeyed the physical laws of the land in the slightest.

Everybody wins!

Answer 9

FTL does not necessitate time travel.

Wormholes (Bose-Einstein bridges if you prefer the term, used in "Sliders"), as defined by Einstein and Bose were in "the quantum electrodynamics of gravity" are not violations of special relativity. They were short cuts between point A and B. You travelled this bridge at normal speeds. There is conjecture that quantum teleportation or strings seeming to be everywhere at once is just similar shortcuts/bridges/wormholes.

Alcubierre's drive hoped that gravity propagated faster than light for it to work. Recent studies of colliding neutron stars have proven gravity waves have the same speed of light limit as matter and light.

During the inflationary phase of the universe, time did not go backwards. The universe expanded faster than the speed of light and continued on. If your FTL somehow mimicked those conditions, I would expect a similar result.

If you are traveling near the speed of light, the shortening of space and lengthening of time makes it "feel" like you are going faster than light. Someone accelerating up to C using outside stars as a measurement of velocity would, if uncorrected for relativity, think themselves going much faster than light. And, in relative time, the trip would take as short a time as if they went faster than C. It would only be when travelers compared themselves to ground clocks that the discrepancy would appear.

Answer 10

The universe expands at a rate that is faster than light. The whole universe would be time-travelling if any FTL thing would mean time travel. Because this is very probably not the case, then FTL without time travel should not be implausible.

It is even possible that FTL travel (if you suppose it is possible) operates under the same principles that allows the universe to expand faster than light. Hence FTL might even mean that there can be no time travel at all as a consequence.