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Accelerando

FTL Drives and Time Travel; how can you prevent the latter? Open question to K^2 & other physicists

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In short: Do wormholes and/or warp drives allow FTL travel that doesn't also cause time travel into the past?

This is a reply to one of K^2's posts from the Control of the Higgs Field thread, but it's open to anyone who's interested in replying. My post wasn't completely on-topic, and it seemed like an interesting line of thought in and of itself so I decided to give it its own thread. I understand that my understanding of relativity and physics in general is not great, so please correct me.

Minkowski isn't Eucledian. Their metrics have different signatures. Both are flat, yes, but you shouldn't confuse the term "Eucledian" to simply mean "flat". The space-time of General Relativity is a Pseudo-Riemannian Manifold. That means that while it isn't necessarily flat, you can always take a small enough region of space that locally it behaves just like Minkowski space. This is why any principle that applies in Special Relativity globally must apply to General Relativity locally. Speed of light is a global limit in SR, but a local one in GR. Causality is globally enforced in SR, but only locally in GR. And so on.

With partial regards to locality vs global-ness of causality in SR vs. GR, I've been meaning to ask you something. As a hard-SF worldbuilder, I've been searching around for information to make sense of an FTL system that doesn't cause time travel, as opposed to simply not violating causality, but I've only been able to find scattered information and the best lead I've had so far has been the Orion's Arm universe with its interpretation of wormholes.

From what I understood through Orion's Arm, wormholes are the only realistic FTL method that can prevent time travel when traveling back and forth between two points in space, to an extent; because the wormhole creates sort of a single link/tunnel between two points in space and time that stays open - if that makes sense - while an FTL "drive" system such as Alcubierre would need to "tunnel" both ways, once by activating the drive to get to the destination, and once by activating the drive to return to the spacecraft's origin.

The wormhole's advantage here is limited by the possibility of moving one or more mouths so as to create a tunnel into the past, but this is prevented in the OA universe by the wormhole immediately destroying itself by self-detonation if it becomes a time machine.

While I understand from a reply you wrote last year that using an Alcubierre drive to travel faster than light technically doesn't violate causality, you seemed to indicate that using it could still send its user into the past, albeit an alternate one.

I've read it suggested that a hyperadvanced system might be put into place so as to specifically target, "grab", and prevent FTL vessels from traveling in a way that would cause time travel; the author Charles Stross uses it in his Singularity Sky universe, in the form of the "Eschaton", a superintelligent computer and FTL traffic controller. But barring handwaves like this, is there any way that an FTL drive could be built or operated - other than, perhaps, traveling only "outward" from one's origin - so as to prevent backwards time travel? And would a wormhole actually permit travel between two points without also causing time travel?

EDIT: And if possible, could you explain why? I have a lot of difficulty visualizing these concepts in my mind and I'd love to be able to create a realistic-FTL resource for myself and for other interested people and SF writers to refer to.

Thank you.

Further reading:

Orion's Arm Wormholes FAQ

A Layman's Guide to Wormholes, Orion's Arm Encyclopedia Galactica

EDIT: Oops, I wrote the title wrong. Changed that!

Edited by Accelerando
added links for clarification

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It's not time travel you should be afraid of but of corrupt causality. I cannot prove that of course, but any FTL principle might lead to a potential causality paradox since it technically allows you to pass the information back in time. The question is - would the universe allow you to do that or not.

Of course, if Multiverse interpretation is correct you simply cannot return to your own past timeline and thus the causality will be protected.

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It's not time travel you should be afraid of but of corrupt causality. I cannot prove that of course, but any FTL principle might lead to a potential causality paradox since it technically allows you to pass the information back in time. The question is - would the universe allow you to do that or not.

Of course, if Multiverse interpretation is correct you simply cannot return to your own past timeline and thus the causality will be protected.

That's what I mean, too, yeah - I want to know if traveling back and forth between points with an FTL drive, and/or creating a "link" between two points using a wormhole, explicitly forces you to send information to the past, alternate or otherwise. Not just allows, but forces.

To be clear, I'm not entirely concerned about causality in and of itself; it's whether or not using an FTL drive doesn't, for example under the multiverse interpretation, send you to a parallel universe, that I'm interested in.

Mainly because I'm not sure if people would want to use a device that irrevocably sends you to an alternate universe instead of your own, without exception, every time you use it to travel somewhere and "back". Even if it's functionally the same, I feel like it would suck to maybe have to leave behind the people in your original universe...

If the FTL method in question doesn't cause backwards time travel in all FTL situations involving travel back and forth between two points, then for science fictional purposes I can handwave a mechanism that prevents FTL when the drive or wormhole would become a time machine. In Orion's Arm, this happens via the "Visser Effect", by which a wormhole collapses when it is arranged so as to permit time travel. Although I'm curious about the realism of such a thing, too...

Edited by Accelerando

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In short: Do wormholes and/or warp drives allow FTL travel that doesn't also cause time travel into the past?

Yes, you don't HAVE to travel in time. You can use it without noticing any weird time travel situations.

But wathever FTL mechanism you use, it will ALLOW for time travel situations.

So even if YOU don't use it for time travel because you fear causality, somebody else could exploit the same mechanism to kill his late grandfather.

EDIT:

The prevention of time travel through some "handwave" mechanism for your story may be very difficult.

Whether a situation "involves" time travel or not is dependent on the observer.

The most realistic "prevention" mechanism would just prevent all FTL entirely.

Alternativly, you could have a powerful entity (god, advanced civilization, mediclorians) that activly discourages time travel, and will use force to prevent someone from exploiting FTL.

Edited by N_las

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Mainly because I'm not sure if people would want to use a device that irrevocably sends you to an alternate universe instead of your own, without exception, every time you use it to travel somewhere and "back".

We're getting in the realm of philosophy here. Would they see the difference? Would they care? How do YOU presently know that you wake up in the same reality every morning and your memories are NOT just your imagination? How do you know you are even real? What exactly IS being 'REAL'?

Reality for us is what we BELIEVE reality is, nothing more.

Even if it's functionally the same, I feel like it would suck to maybe have to leave behind the people in your original universe...

Those people would still be in your new timeline. They should in theory even remember your fictional 'past' (from your point of view). Oh, this is too complex...

If the FTL method in question doesn't cause backwards time travel in all FTL situations involving travel back and forth between two points, then for science fictional purposes I can handwave a mechanism that prevents FTL when the drive or wormhole would become a time machine.

That's the point. ANY FTL device is by design also a potential time machine with no possible exceptions. There is a wikipedia article that suggests a workaround though.

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Yes, you don't HAVE to travel in time. You can use it without noticing any weird time travel situations.

But wathever FTL mechanism you use, it will ALLOW for time travel situations.

So even if YOU don't use it for time travel because you fear causality, somebody else could exploit the same mechanism to kill his late grandfather.

That's what I was hoping to hear. Although I'm also interested in knowing the details of how and why, so that I can better visualize it to develop a system for myself, and for other SF writers to reference.

In Orion's Arm, for instance, I don't really understand how you can conceivably build and connect wormholes together so as to create a system that prevents time travel. There's a page on their website which I interpreted as indicating that by having a wormhole's second mouth situated outside of the light cone of the two mouths' shared origin point H, the mouths can be connected without causing backwards time travel. However, when I tried to draw a space-time diagram myself of two relativistic spacecraft that "move" the wormhole mouths apart from the origin H, I didn't understand if there was a way to get either of the mouths to be "outside" H's light cone and thus prevent backwards time travel. For any light you all could shed on the matter, I'd be infinitely grateful.

EDIT:

That's the point. ANY FTL device is by design also a potential time machine with no possible exceptions. There is a wikipedia article that suggests a workaround though.

I'm not here to argue the philosophy; I'm only interested in whether or not it's possible to in some way prevent time travel. I understand that the FTL methods in question are always potential time machines in the sense that you can always think of a way they can be used/travel in spacetime so as to send them into the past. My question is whether or not an FTL drive will force you, in all circumstances, to send information back in time; and if so or if not, then why? The first question has been answered, but not the second. If you say there might be a workaround, then there isn't "no possible exception" for the purposes of effectively preventing time travel in a realistic-ish universe. Orion's Arm does assume chronology protection, which is why I brought it up.

Although I must say I am also curious about whether or not a chronology protection system that prevents multiversal travel in this way for, say, wormholes, would also still allow for some method of traveling between universes.

Edited by Accelerando

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For any light you all could shed on the matter, I'd be infinitely grateful.

Only slightly relevant: http://imgur.com/a/9x5pV

A while back I drew an explanation for how warp drive allows time travel.

The diagramm you linked to explaines than a wormhole connection between H and A can only exist if A is outside of H's light cone. It doesn't mean that H has to be outside of H's and A's origin point.

The wormhole F is a THIRD wormhole, also conected to the wormhole H. It is inside of H's light cone, so it is (in the logic of this universe) not possible.

But even this "Visser effect" is a bad chronology protection system. It can be easily broken by two sets of worholes. Just preventing the two ends of any particular wormhole to come to close to each other isn't enough.

EDIT: To ensure protection the ends of any two worholes (even if they are not connected to each other) aren't allowed to be in each others light cones.

Do you wan't this "chronology protection" in your story? If you don't care about it, your protagonist can just arrange their wormholes in a way that they personally don't experience time travel.

Edited by N_las

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Only slightly relevant: http://imgur.com/a/9x5pV

A while back I drew an explanation for how warp drive allows time travel.

The diagramm you linked to explaines than a wormhole connection between H and A can only exist if A is outside of H's light cone. It doesn't mean that H has to be outside of H's and A's origin point.

The wormhole F is a THIRD wormhole, also conected to the wormhole H. It is inside of H's light cone, so it is (in the logic of this universe) not possible.

But even this "Visser effect" is a bad chronology protection system. It can be easily broken by two sets of worholes. Just preventing the two ends of any particular wormhole to come to close to each other isn't enough.

EDIT: To ensure protection the ends of any two worholes (even if they are not connected to each other) aren't allowed to be in each others light cones.

Do you wan't this "chronology protection" in your story? If you don't care about it, your protagonist can just arrange their wormholes in a way that they personally don't experience time travel.

Thank you for the diagram. I might not have a lot of time to read it and post at the moment, but I'll definitely read through it soon, EDIT and I'll formulate a reply to your answer WRT my wormhole light-cone question soon.

I do want chronology protection, and I'm curious about how Visser can be broken by two sets. Orion's Arm canon does mention this, in the form of a "Roman" loop, in the Wormholes Layman's Guide:

3) Chronodynamic instability: Under certain circumstances a wormhole can become a time machine' date=' resulting in its immediate destruction through the so-called Visser Effect. Whenever a Closed Timelike Curve is formed, a Cauchy horizon will also appear which destabilises the wormhole and causes a collapse. The procedure for creating a CTC is:

· Create a wormhole

· Induce a time shift between the mouths

· Bring the mouths close enough together so that the distance through the simply-connected region ("normal space") is less than the time shift.

The simplest way to induce a time shift is to move one mouth at relativistic velocity. This is the usual course of events in deployment of a wormhole gate between systems. However, once brought to the target system, the wormhole is inflated and remains in far orbit around the star, so a normal wormhole will not create a time machine.

General relativistic means for inducing time shift exist (e.g. orbit around massive objects), but they are of no engineering concern.

Of far greater concern is the possibility of a "Roman" configuration (named for an Information Age physicist who first considered the idea) involving multiple wormholes. Such a configuration results when a set of wormholes by themselves are not time machines, but form a network that does produce a time machine.

For the simplest two-wormhole configuration, there are essentially 3 requirements:

•.Net distance traversed through the wormhole as measured in asymptotically flat space exceeds distance traversed in flat space to the mouths (trivially satisfied)

•.The wormhole time shift is anti-parallel

•.The distance between mouths is shorter than the overall time shift.

To avoid Roman time machines, one of two criteria are sufficient:

• The wormhole network consists only of directed, acyclic graphs

• Wormhole linelayers (ed note: a kind of starship) carry wormhole mouths from core systems to exterior systems only; no "backtracking" networks allowed.

As a concrete example, a linelayer with 1g of acceleration will achieve .7c in 10 months. Neglecting acceleration and turnaround (which is a small fraction of the total trip), the travel time as measured by the home system will be ~14 years; the linelayer will measure ~10 years. Upon arrival, if a second linelayer is sent back to the original system, it will generate a CTC when ~5 ly distant. The Chronodynamics significantly restricts wormhole placement and the overall structure of the Wormhole Nexus and Known Net.[/quote']

Does this system satisfy the "not in each others' light cones" condition? It seems like it doesn't, so I'm curious how Orion's Arm considers it a plausible system if that's the case. Their page on the Wormhole Nexus - the in-setting term for the human network of wormholes - mentions, under "The Topology of the Nexus", that there are "a few weeks stopover in each system at most," although I'm not sure what that means about travel distances and times between two non-connected wormhole mouths. Specifically:

The number of wormholes one needs to traverse varies according to the trip taken. In case of travel from Sol to Kepleria, the Lonely Galaxy Guidebook gives the following advice:

From Sol take the venerable Einstein Bridge to Tau Ceti, the Akiyoshidai to Djed, and then the Kepler Arc to Kepleria. Just three jumps. The trip would mostly involve resting in light stasis in a shuttle connecting to grapeships moving through the wormholes - it is only a few weeks stopover in each system at most.

The page on Wormhole Termini might have more illuminating information, but I'm not sure.

Edited by Accelerando

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FTL flight in one coordinate system is time-travel in another. It's really as simple as that. Now, constructing a loop, which you need for practical time travel, might be a bit more complicated, but these are purely practical limitations. If FTL is possible, reality you live in must at least allow for time travel. And physics as we know it does allow for time travel. Global causality is not a requirement at all.

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FTL flight in one coordinate system is time-travel in another. It's really as simple as that. Now, constructing a loop, which you need for practical time travel, might be a bit more complicated, but these are purely practical limitations. If FTL is possible, reality you live in must at least allow for time travel. And physics as we know it does allow for time travel. Global causality is not a requirement at all.

Thanks for the response.

I'm aware that some time-dickery happens no matter how you use your FTL drive, but I'm aiming to limit the "grandfather paradox"-esque scenarios (even if they don't technically violate local causality by virtue of sending the traveler into a parallel universe or something of that nature) or at least complicate the creation of such to something requiring more thought than moving a wormhole or two into a general configuration. Is there a way, in that sense, to make chronology protection, or something like it for practical purposes, make sense?

OA mentions that their wormholes require some sort of active stabilization in order to stay open; does this make sense, and could something similar - i.e. some physical phenomena creating the need for active stabilization - also complicate attempts to carry out grandfather paradox-style scenarios? Might it be something else? Why? Or if not, why not?

Edited by Accelerando

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Not sure exactly what mechanism you're trying to protect in your plot, but if you're afraid someone might kill your grandpa, you can use the Novikov self-consistency principle.

"The Novikov self-consistency principle, also known as the Novikov self-consistency conjecture, is a principle developed by Russian physicist Igor Dmitriyevich Novikov in the mid-1980s to solve the problem of paradoxes in time travel, which is theoretically permitted in certain solutions of general relativity (solutions containing what are known as closed timelike curves). The principle asserts that if an event exists that would give rise to a paradox, or to any "change" to the past whatsoever, then the probability of that event is zero. It would thus be impossible to create time paradoxes."

"We shall embody this viewpoint in a principle of self-consistency, which states that the only solutions to the laws of physics that can occur locally in the real Universe are those which are globally self-consistent. This principle allows one to build a local solution to the equations of physics only if that local solution can be extended to a part of a (not necessarily unique) global solution, which is well defined throughout the nonsingular regions of the spacetime."

"In response, another physicist named Joseph Polchinski sent them a letter in which he argued that one could avoid questions of free will by considering a potentially paradoxical situation involving abilliard ball sent through a wormhole which sends it back in time. In this scenario, the ball is fired into a wormhole at an angle such that, if it continues along that path, it will exit the wormhole in the past at just the right angle to collide with its earlier self, thereby knocking it off course and preventing it from entering the wormhole in the first place. Thorne deemed this problem "Polchinski's paradox".[3]:510–511After considering the problem, two students at Caltech (where Thorne taught), Fernando Echeverria and Gunnar Klinkhammer, were able to find a solution beginning with the original billiard ball trajectory proposed by Polchinski which managed to avoid any inconsistencies. In this situation, the billiard ball emerges from the future at a different angle than the one used to generate the paradox, and delivers its younger self a glancing blow instead of knocking it completely away from the wormhole, a blow which changes its trajectory in just the right way so that it will travel back in time with the angle required to deliver its younger self this glancing blow. Echeverria and Klinkhammer actually found that there was more than one self-consistent solution, with slightly different angles for the glancing blow in each case. Later analysis by Thorne and Robert Forward showed that for certain initial trajectories of the billiard ball, there could actually be an infinite number of self-consistent solutions."

Lol. The Princeton university website links to the Wikipedia, so here it is.

http://en.wikipedia.org/wiki/Novikov_self-consistency_principle

Edited by Aethon

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Actually, Novikov's principle has a great potential to exploit in sci-fi. Imagine that 'hand of doom' intervenes and changes things in ways opposite to protagonist's liking. ;)

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Actually, Novikov's principle has a great potential to exploit in sci-fi. Imagine that 'hand of doom' intervenes and changes things in ways opposite to protagonist's liking. ;)
Not sure exactly what mechanism you're trying to protect in your plot, but if you're afraid someone might kill your grandpa, you can use the Novikov self-consistency principle.

That's true, and with regards to what kind of protection I'm aiming for, it's mostly along the lines of keeping the theme of "travel between parts of an interstellar civilization" without having to venture far immediately into the territory of "exploring the implications of effective 'time travel' and/or being able to access parallel universes for the same cost as running a starship". Basically, I'm interested in knowing how or if I can write a realistic story where FTL works in the way most people think of FTL - as a method for traveling between "places", in the lay person's idea of the concept, rather than between "times".

At the same time, I also think the concept of Novikov self-consistency and similar chronology protection concepts are endlessly fascinating in their own right, and time travel muchly deserves a far more deeply written treatment in space SF and SF- and space opera-type works in general. The idea of the hypothetical ball being able to encounter itself at an angle sufficient to set itself on its original trajectory is wonderfully interesting and I'd love to understand how such a situation might work when taking into account the uncertainties in real-life interactions, as well... I've also always been curious if there's perhaps some convoluted way that time travel into, and altering or at least interacting with, "the past" could ever make any kind of sense, perhaps even barring multiverses or outright chronology protection.

But yeah - travel between places, rather than between times. While I understand that these are the same in physics terms, I want to be able to write a realistic setting where by the layperson definitions of these terms I can have an FTL system that only moves me between places, and to know why I can, or if not, why not. I'll compose a more detailed reply and wrap my head around N_las's diagram when I wake up again, too.

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I'm of the opinion that it is a false paradox to begin with. By the very nature of there BEING a faster than light propagation method, light no longer is the fastest frame of reference, and thus preventing causeless effects via traveling faster than light.

Consider an FTL regime instead in the same way a blind person regards events that are supersonic. Getting hit by a sniper bullet from a mile away, for instance, would appear to be a paradox as the bullet hits before the soundwaves reach the individual in question. So too would an FTL ship APPEAR to be causeless, when in fact that is entirely untrue.

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@CptRichardson

Relativistic effects aren't simply caused by signal delay.

The Person feels the bullet before the soundwave, but that is just the effect of signal delay.

In relativity, two events actually can happen in different moments for different observers, even when they take signal delay into account.

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I'm of the opinion that it is a false paradox to begin with. By the very nature of there BEING a faster than light propagation method, light no longer is the fastest frame of reference, and thus preventing causeless effects via traveling faster than light.

Not how it works.

Any FTL guarantees that you can travel into the past and change it. Can be impractical, but no FTL scheme makes it impossible. Time traveling paradoxes must resolve themselves. And they do in field theory. It is a non-issue. Grandfather paradox is no more paradox than EPR is.

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So how are the paradoxes resolved?

By not having events. You have fields that satisfy certain conditions locally, which gives you local causality. But global structure is determined by the Lagrangian and boundary conditions. In the big picture, there isn't a cause-and-effect, because the entire structure is already predetermined. If you have a flat space-time, you can use the local causality to build up a causal timeline, which is basically what we're doing. But having exceptions to that simply isn't a contradiction. Timelines become path-dependent. Your timeline might not agree with everyone else's, but that isn't a contradiction either.

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OK fine but what does that mean in practice, IE with the classic go back in time and prevent yourself from being born type examples? I thought field theory was a quantum thing not a macro scale thing. What does the macro scale reality look like?

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Maybe a computer failsafe shuts down the FTL drive if it detects a time travel event that's too big?

Anyways, if you go back in time and do something, you have no reason to go back in time, so you end up not going back in time at all.

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OK fine but what does that mean in practice, IE with the classic go back in time and prevent yourself from being born type examples? I thought field theory was a quantum thing not a macro scale thing. What does the macro scale reality look like?

On macro-scale, you get self-interference, which in practice, reduces probability amplitude. Arrival of the time traveler becomes a random event conditional on that random event, and time traveler's own time-line obviously has an opposite outcome. How you interpret it is up to you. From perspective of MWI, time traveler arrived from alternate reality. Other interpretations will look at it differently, but the effect is the same.

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