FTL without paradoxes?

[Travis Woodward]

I have a story I'd like to tell which requires both relativistic space travel and some form of FTL travel, but I want to avoid the possibility of someone creating a paradox, and want to minimise the number of arbitrary restrictions I need to employ.

Wormholes are the obvious answer, but I'd like to use something a bit more flexible, and a bit different. I have come up with a tentative idea, but I'd like a 2nd (or 3rd, or 4th) opinion on whether it could still lead to a paradox.

I'm thinking of having a series of objects (let's call them gateways for now) which allow instantaneous travel to any point within a field surrounding that gateway, and that field is of a fixed radius (e.g. 1 lightyear) relative to that gateway's frame of reference. The only restrictions are that (a) the fields of the gateways interfere with each other, such that any point in space can only ever be in range of one gateway at once, and (b) any change to the field (e.g. if a gateway is switched off) propagates at the speed of light.

I think (a) + (b) prevent paradoxes. If the system is static, then (a) means that information from a traveler's arrival can never reach the traveler's point of origin before the traveler left - at best it could arrive at simultaneously with their departure. The traveler's arrival and departure would happen out of order for someone travelling at relativistic speeds, but once they'd observed the arrival it would be too late for them to affect the departure.

I've added (b) to prevent two of the gateways being used sequentially to get around restriction (a), while still allowing for the network to be reconfigured periodically.

Thoughts/ criticisms/ alternatives? And does anyone know of examples in sci-fi where something similar has already been done well?

 

[Brian G Turner]

The TV series Babylon 5 had "space gates" (presumably using wormholes, or similar) that could be used between "fixed" points, but the travel time was not instantaneous - it could take hours or days to move between space gates. Presumably this prevented the problem of any paradox.

 

[Onyx]

The simultunaeity you describe is paradoxical. There will always be a paradox if information can move faster than the maximum speed in the universe.

 

[goldhawk]

The simultunaeity you describe is paradoxical. There will always be a paradox if information can move faster than the maximum speed in the universe.

No, there won't. It's not a paradox because in the inertial frame of the cause and effect, they're always in order.

The reason nothing can travel faster then light is because that would imply a preferred frame of reference. Travel faster than light would be faster than light in this frame of reference. If you want FTL travel in your story, you just have to come up with some handwavium to explain why physicists have discovered it before. Once you do that, FTL is easy.

 

[Onyx]

No, there won't. It's not a paradox because in the inertial frame of the cause and effect, they're always in order.

The reason nothing can travel faster then light is because that would imply a preferred frame of reference. Travel faster than light would be faster than light in this frame of reference. If you want FTL travel in your story, you just have to come up with some handwavium to explain why physicists have discovered it before. Once you do that, FTL is easy.

Unfortunately, that's not true. If you can travel at FTL, you can travel to someplace where an observer in a different reference plane due to their velocity has witnessed something that is essentially in the future of the place you left. If you return with that observation, you have created the paradox.

FTL would be paradox free if there was only a single reference frame - every object in the universe essentially static to each other. But the universe is actually full of stuff that are in completely different reference frames, and if you jump to some other place you are likely to encounter information that is out of sync with the reference frame you'd like to return to.

Another way of looking at it: There is no place in the universe that could agree on when you arrived. What would be simultaneous to the "static" observer would be in the past of an accelerated observer.

This is classic light cone problem stuff. And while we like to worry about killing grandpa, paradoxes can simply be stopping a single photon that already fell on our destination because we intercepted its out of reference plane origin.

Why FTL implies time travel


Alastair Reynolds had an interesting take on FTL - when a wormhole was opened between the Milky Way and Andromeda, we stopped being able to see Andromeda - it simple went black. His idea was that the mechanism of FTL might exist, but causality was preserved by the travel being one way and Andromeda information being unable to interact with our reference frame any more. This seems a little neat until you start thinking about what that means to a particle moving between them at 99%C.

 

[goldhawk]

Unfortunately, that's not true. If you can travel at FTL, you can travel to someplace where an observer in a different reference plane due to their velocity has witnessed something that is essentially in the future of the place you left. If you return with that observation, you have created the paradox.

There would be a paradox in our universe because there's no preferred frame of reference. If there was a preferred frame, it would be impossible for you to travel back where you came with future knowledge. By the time you got back, that knowledge would also be in the past for everyone there. The maximum speed you could travel would not be a constant but there would be a limit that depended on the distance between your start and end points.

 

[Onyx]

There would be a paradox in our universe because there's no preferred frame of reference. If there was a preferred frame, it would be impossible for you to travel back where you came with future knowledge. By the time you got back, that knowledge would also be in the past for everyone there. The maximum speed you could travel would not be a constant but there would be a limit that depended on the distance between your start and end points.

I can't tell if you are now agreeing with me or not.

 

[Parson]

The easy answer to preventing paradoxes is .... don't talk about them. It's your story and if you don't bring up the paradox few of your readers will consider it. A good share of S.F. has F.T.L. and the obvious time travel and paradox is never considered. If you are doing hand wavem and have F.T.L. might as well wave your hand and ignore the problem.

 

[Cathbad]

No matter how you do it, some science nerd is gonna complain!

 

[Onyx]

The science nerds are only going to complain if you try to sell them some bad science. Hand waving is the bread and butter of SF. But if you do want to get into the nitty-grits, a solution that ends up being the equivalent of a sailboat with a fan mounted on the deck to blow the sail is going to bother readers.

Several authors, like Reynolds and Ken MacLeod, have dealt with the paradox problem by erecting a barrier to information moving back in time. Other 'solutions' to time travel issues could also be applied.


Travis, some version of what you propose could be made to sound reasonable, just look at the light cone models to figure out what happens when a wormholes in a different reference frame going speeding by the first set and you have the ability to throw a message backwards.

 

[Dave]

Can someone explain to me how instantaneous FTL travel must involve time travel?

If I travel by instantaneous FTL and watch a supernova, and then I travel by instantaneous FTL back, then I can tell about it before it is observed, but I saw it when it actually happened. There was no time travel. I cannot see something before it happens.

If someone uses explosives on a quarry face. You can sometimes see the explosion seconds before you see it. Or, think of Thunder and Lightning. You hear it long after it happened, but that doesn't mean that you are time travelling the past. It just means that you heard it late.

I believe people here are confusing the relativistic properties of 'very close to light speed travel' with FTL. That is an entirely different thing. Of course, instantaneous FTL is likely impossible, and so it is indeed, all Handwavium.

 

[goldhawk]

I can't tell if you are now agreeing with me or not.

My point is that if there is a preferred frame of reference, then Relativity follows completely different rules. Some of those rules will create behavior similar to our universe and some will create behavior not possible in our universe.

 

[Joshua Jones]

Hmmm... not to stir the pot, but I am not fully convinced that such paradoxes could happen. I read through the article shared above pretty carefully, analyzing the charts, and he seems to make a mistake at charts 8-9 and the subsequent text. Specifically, he treats the spaceships frame of reference as the privileged frame of reference here and ignores what would happen to the FTL communication upon returning to Earth's frame of reference. Unless I am mistaken, Earth would receive the ship's message significantly after they sent the first message, rather than before. Again, unless I am seriously misreading this, it would seem to the ship that the FTL communication is moving toward Earth at a snail's pace, then the call goes out, then the FTL communication from the ship is received. Then, the Earth comm officer sends them a terse message about not wasting FTL communication time on relativistic illusions the Academy warned them about.

So, while I am not a professional physicist and am certainly open to be proven wrong, it seems the author of the blog, at least, didn't take into account the effects of the ship's FTL communication returning to Earth's reference frame. While the ship perceives the effect preceeding the cause, there is a difference between epistemology and ontology, and in this case, relativity seems to create an illusion, rather than an actual paradox.

Am I missing something?

 

[goldhawk]

Can someone explain to me how instantaneous FTL travel must involve time travel?

If I travel by instantaneous FTL and watch a supernova, and then I travel by instantaneous FTL back, then I can tell about it before it is observed, but I saw it when it actually happened. There was no time travel. I cannot see something before it happens.

You're assuming a preferred frame of reference. In this preferred frame, everything always happens in the same order. If you're in a different frame, you may observe events happening in a different order but you can always work out the order in the preferred frame.

With Relativity, time is flexible. For events outside of the other's light cone, the order of the events changes. In fact, for half of all possible observers, they have one order and for the other half, their order is reverse. But it is impossible to tell which half is correct. They are both correct.

If you make an observation of these events and decide to tell someone in the of half, you can send them a message using light (radio) or you can change your velocity and go to them. But it takes time for the messages to get to them and by the time it does, they will have already observe both events themselves. In other words, you can't tell them about their future.

FTL changes this; with FTL it would be possible to tell someone their future by going into their future. And if you can time travel to their future, you can time travel to their past. But the observers are no people; they are frames of reference. You can become any observer by changing your frame of reference. Which means you can travel to your future and you can travel to your past.

Added fact: FTL completely destroys the Laws of Thermodynamics.

 

[Ursa major]

I'm in the camp of those who say that if it isn't mentioned in the story, few if any people will be upset by this.

but I want to avoid the possibility of someone creating a paradox

The author is in control of the story, specifically the narrative, so if the author does not want to create a paradox in the story, then they should refrain from doing so in the narrative.

 

[Phyrebrat]

I'm in the camp of those who say that if it isn't mentioned in the story, few if any people will be upset by this.

The author is in control of the story, specifically the narrative, so if the author does not want to create a paradox in the story, then they should refrain from doing so in the narrative.

^this!!!

(Also, whenever these discussions come up I read through the theory carefully and still never manage to get a grip or understand any of it. I hate my ignorance!)

 

[Joshua Jones]

You're assuming a preferred frame of reference. In this preferred frame, everything always happens in the same order. If you're in a different frame, you may observe events happening in a different order but you can always work out the order in the preferred frame.

With Relativity, time is flexible. For events outside of the other's light cone, the order of the events changes. In fact, for half of all possible observers, they have one order and for the other half, their order is reverse. But it is impossible to tell which half is correct. They are both correct.

If you make an observation of these events and decide to tell someone in the of half, you can send them a message using light (radio) or you can change your velocity and go to them. But it takes time for the messages to get to them and by the time it does, they will have already observe both events themselves. In other words, you can't tell them about their future.

FTL changes this; with FTL it would be possible to tell someone their future by going into their future. And if you can time travel to their future, you can time travel to their past. But the observers are no people; they are frames of reference. You can become any observer by changing your frame of reference. Which means you can travel to your future and you can travel to your past.

Added fact: FTL completely destroys the Laws of Thermodynamics.

I may need some help in understanding how it would be possible to move into their past. I could see a one way trip into the future (I think Forever War dealt with this), but moving into the past seems to require the inverse function of this process, which would require moving at speeds significantly below -C. I can't begin to imagine what -C would look like, but it certainly isn't possible in spacetime, at least that I am aware of... What am I missing?

 

[Joshua Jones]

I'm in the camp of those who say that if it isn't mentioned in the story, few if any people will be upset by this.

The author is in control of the story, specifically the narrative, so if the author does not want to create a paradox in the story, then they should refrain from doing so in the narrative.

Absolutely. Forgive the expression, but physics in a story should be much like a bra; provides support and visible effects, but only shown directly to the most intimate of relations...

 

[goldhawk]

I may need some help in understanding how it would be possible to move into their past. I could see a one way trip into the future (I think Forever War dealt with this), but moving into the past seems to require the inverse function of this process, which would require moving at speeds significantly below -C. I can't begin to imagine what -C would look like, but it certainly isn't possible in spacetime, at least that I am aware of... What am I missing?

Yes, you are mixing classical mechanics with Relativity.

In classical mechanics, space is a ridged 3D manifold and time is governed by a universal clock. Time always flows the same way and by the same amount for everything in the universe. You can travel faster than light and everyone will observe you arriving before you left but you will always arrive after you left because of the universal clock.

But with Special Relativity, space and time form a ridged 4D manifold. When you change your velocity, you cause a hyperbolic rotation within space-time. Time between two events becomes longer while the distance between them becomes smaller. Or vice versa. For events in each other's light cone, time cannot go negative. The hyperbolic rotation prevents this.

If the events are outside of each other's light cone, it is possible for the time between events to go negative. That is, the order of the events reverses. The question: which order of events is the correct one? Or to generalize, which amount of time is the correct one? Because there is no preferred frame of reference, they are all correct.

If you can travel FTL, then you can jump from any point in space to any other without going thru space. And because space-time is a ridged manifold, it mean you can jump between any two points in time. And since there is no preferred order of these points, you can jump forward or backward in time. (Altho with Relativity, forward and backward only makes sense within a light cone.)

Also, if you can jump between any two points in space-time, you can jump between any two velocities. Speed is distance divided by time and you can change them arbitrarily. Which means FTL breaks the First Law of Thermodynamics. (Which is the biggest reason FTL is thought to be impossible.)

 

[Ursa major]

I can now see why so many people are upset by the use of FTL in stories: nearly everyone knows about ridged 4D manifolds....