Page 137: Adieu SG-51

[GeoModder]

But the moment it arrives at Earth, its frame of reference adjusts to Earth's frame of reference, where the message *has* been sent in its past. Kinda making the point mood if you ask me.

[Sweforce]

hi hi

certain speed relative to the universe

This is sort of the point, there is no universal constant, no privileged reference frame, every reference point is relative to every other reference point. There is no universal "now." What is happening right now, from one person's perspective, could be happening a week ago, from someone else's perspective in a different reference frame.

Relativity of simultaneity is something that science has tested extensively, and has very solid evidence.

Say the relativistic ship sees the message, powers up a warp drive and decides to go to Earth. Why does that result in it arriving before the message left, if its all going forwards?

From the relativistic ship's point of view, it is going to Earth "now," for it's definition of "now," but the message it received hasn't been sent yet, in it's frame of reference.

It is perfectly normal for people to return home before the postcards they sent back home arrive, it is hardly a causality problem, just the mail being a bit slow. I see it the same way with a ship that make an near instantaneous jump ahead of the slow light speed radio message sent just prior to the jump.

[icekatze]

hi hi

But the moment it arrives at Earth, its frame of reference adjusts to Earth's frame of reference, where the message *has* been sent in its past. Kinda making the point mood if you ask me.

In which situation are you saying that this happens? In the Outsider universe, there is a time delay for FTL travel that could avoid paradoxes, but in RedDwarfIV's question, that is not necessarily the case. (Also, it becomes even more complicated when more that two reference frames are brought in to the equation.)

In RedDwarfIV's hypothetical example of a transit between Earth and Proxima Centauri, it is also important to point out that there is no single "Earth's reference frame," but rather there are a series of reference frames over time, none of which is preferred over any other. Time is a series of events in space-time, the progression of which depends on the observer's frame of reference.

It is perfectly normal for people to return home before the postcards they sent back home arrive, it is hardly a causality problem, just the mail being a bit slow. I see it the same way with a ship that make an near instantaneous jump ahead of the slow light speed radio message sent just prior to the jump.

This has nothing to do with the problem at hand. The matter is not people arriving home before their postcards arrive, but their postcards that they sent from their destination arriving back home before they leave in the first place.

Relativity of Simultaneity is not an optical illusion, it isn't merely an issue of people experiencing latency, it is a fundamental quality of how the universe operates with regards to the speed of light.

Here's an animated explanation that might make it more clear

[Witty_Username]

Time Travel makes my head hurt.

[Siber]

Personally, while I rarely have enough grasp on the relativity issues to explain it, I take it as a general rule that FTL will enable time travel if you try hard enough, though some schemes may make it very hard to create testable breakage. I suspect with Outsider's setup you'd need to drag stars up to dialation-inducing relative velocities and then jump between them to really get spectacular results, which puts the bar for time travel pretty high. I never rule out someone clever than I coming up with an easier exploit, though.

[GeoModder]

But the moment it arrives at Earth, its frame of reference adjusts to Earth's frame of reference, where the message *has* been sent in its past. Kinda making the point mood if you ask me.

In RedDwarfIV's hypothetical example of a transit between Earth and Proxima Centauri, it is also important to point out that there is no single "Earth's reference frame," but rather there are a series of reference frames over time, none of which is preferred over any other. Time is a series of events in space-time, the progression of which depends on the observer's frame of reference.

RedDwarf's example.

That relavistic ship clock still adjusts to Earth's frame of reference once its goes down from its high relative velocity relative to Earth. It's simply not practical to maintain its own clock once it joins another objects frame of reference. And since in this case it is the smaller object so to speak, it should abide to Earth's.
Besides, it decided to power up its FTL *because* of something received by its eventual target (Earth thus).

[icekatze]

hi hi

That relavistic ship clock still adjusts to Earth's frame of reference once its goes down from its high relative velocity relative to Earth. It's simply not practical to maintain its own clock once it joins another objects frame of reference. And since in this case it is the smaller object so to speak, it should abide to Earth's.

Every object always has its own frame of reference. That frame of reference may agree with another frame of reference nearby, but there is no special connection. Size isn't relevant, time measuring devices aren't relevant, only velocity and position are.

It doesn't matter if the FTL ship matches velocity with the Earth after it arrives, because it has already arrived before the message was sent in the first place. The only thing that changes when it agrees with the Earth's frame of reference is that it now seems to have traveled from its own future.

Besides, it decided to power up its FTL *because* of something received by its eventual target (Earth thus).

That is the paradox.

[GeoModder]

It can only arrive before that message was sent as seen from within its own timeframe, but to me that timeframe becomes irrelevant the moment it enters Earth's timeframe again.

We simply see it different on that point, Icekatze.

[icekatze]

hi hi

but to me that timeframe becomes irrelevant the moment it enters Earth's timeframe again.

Earth does not have a single, objective time frame. It is relative. That is how the science of relativity works, it's not just a matter of opinion.

We're not obliged to have this out here and now, but anyone can do the math and run the proofs themselves.

[Victor_D]

@RedDwarfIV, @sweforce, @Geomodder, @Siber, @Witty_Username

We're talking about this, more or less (if I understand it correctly, which is not given). If everyone were in the same frame, FTL would pose no issues at all (Fig. 1). But that's sadly not the case:

SpoilerShow

The relative movement is vastly exaggerated to illustrate the point. Observers on Earth and Barnard's Star have different "now" because Bernard's Star is moving relative to Sol. This means that if Sol sends a ship to Barnard's via an INSTANTANEOUS ftl jump (Fig. 2), the ship will have arrived from the "future" from Bernard's point of view (Fig. 3) Now, if there is a ship waiting at Bernard's which jumps back to Sol IMMEDIATELY when the ship from Sol arrives, the ship will have travelled into Sol's past:

SpoilerShow

See the paradox? You may think this is a trick, and it sort of is, but the point is that for relativity to be correct, and we know it is correct because of numerous observations, this paradox WILL occur: although both observers believe they're communicating instantaneously in the "now", they're in fact communicating with the future/past of observers in different frames.

So, what happens if the arrangement depicted above happens and the ship from Bernard's arrives before the ship from Sol even departed, and tells the ship from Sol: "don't jump!!!". A paradox will have occurred: why did the ship from Bernard's jump if the ship from Sol hasn't even departed yet?

To understand what the diagrams represent, watch:

The Geometry of Causality
and
The Race to a Habitable Exoplanet - Time Warp Challenge
and
Superluminal Time Travel + Time Warp Challenge Answer


Now, this is why I asked my original question to Arioch whether he intentionally set up the FTL system in Outsider-verse in order to prevent paradoxes from occurring. If the jump isn't instantaneous and if there isn't an FTL radio, and if the ships need some time to get ready for a jump, and if the stars' relative movements are within the realm of normalcy, it is next to impossible to arrange a sequence of events that would lead to the paradox described above. The ships are still jumping from one star system to the past of other star systems, but practical difficulties prevent this from being exploited to cause a causal paradox (i.e. moving back in time and preventing the event that made you travel back in time in the first place).

SpoilerShow

The ship from Sol makes a non-instantaneous FTL jump; it signals the waiting ship at Barnard's to jump to Sol. The Barnard's ship takes some time to spool up its FTL drive and reach the correct jump speed. When it jumps to Sol, it will arrive after the Sol's ship from Sol's point of view. Paradox avoided, no problem.

I quite like this solution.

[RedDwarfIV]

Makes more sense now that I've had sleep and you're using diagrams that aren't full of confusing and unlabelled lines.

I usually use warp drive in my fictional settings... which is not an instantaneous drive. It takes time to get places. So that would avoid the paradox, yes?

[GabrielGABFonseca]

No matter how hard you try, every single FTL method will create paradoxes.

You can even use clever tricks to avoid it from yours and your target's Reference Frames, but there will always be another frame of reference, somewhere, where it will be breaking causality.

It sucks.

[Victor_D]

No matter how hard you try, every single FTL method will create paradoxes.

You can even use clever tricks to avoid it from yours and your target's Reference Frames, but there will always be another frame of reference, somewhere, where it will be breaking causality.

It sucks.

Of course, but since there is no FTL radio and no unlimited, free, instantaneous FTL travel, you cannot easily use it to create a major causal paradox. That's all that matters to the story. (Well, if somehow the Farseers could communicate ideas and specific information, this could potentially be considered an "ansible", but Arioch made it clear it doesn't work that way).

(I loved what Alastair Reynolds did about this in his Revelation Space universe; sends a chill down your spine).

Makes more sense now that I've had sleep and you're using diagrams that aren't full of confusing and unlabelled lines.

I usually use warp drive in my fictional settings... which is not an instantaneous drive. It takes time to get places. So that would avoid the paradox, yes?

I wouldn't worry about it that much. As others have said, there is no way to reconcile relativity, FTL and causality. Something always has to give. If we're talking about something that's not supposed to be ultra-hard science fiction, nobody will care. If you do, though, I'd avoid FTL radio like the plague since the most obvious paradox-creating arrangements use it. That applies to wormholes too. Also, relativistic speeds coupled with FTL are also an explosive combination.

[Krulle]

(I loved what Alastair Reynolds did about this in his Revelation Space universe; sends a chill down your spine).

But the timescales his stories pass immediately show why many SF authors decide to have FTL available...
Spacetravel is simply a one-way ticket to the future in his stories (one dark universe, his revelation space, but I like it because he is one of very few authors having no way to cheat Einstein's postulation that nothing can be faster than light. But Revelation Space is explicitly built around a paradox, because that's how the Conjoiners got to know how to build their Conjoiner drives - I liked it better before he gave explanations how they work, or before he implemented the "computers calculating heat away",....)

[Victor_D]

(I loved what Alastair Reynolds did about this in his Revelation Space universe; sends a chill down your spine).

But the timescales his stories pass immediately show why many SF authors decide to have FTL available...
Spacetravel is simply a one-way ticket to the future in his stories (one dark universe, his revelation space, but I like it because he is one of very few authors having no way to cheat Einstein's postulation that nothing can be faster than light. But Revelation Space is explicitly built around a paradox, because that's how the Conjoiners got to know how to build their Conjoiner drives - I liked it better before he gave explanations how they work, or before he implemented the "computers calculating heat away",....)

He even has

SpoilerShow

FTL, which is what I've been pointing at. It is implied, or at least that's how I read it, that the Universe sort of "rearranges" to "edit out" anyone who breaks causality too much. Those who tamper with the inertia-suppressing technology could simply vanish and no one will remember them because as far as they're concerned, these people died years ago in a freak accident; only their colleagues exposed to the same field remember the old timeline where they existed (this happens twice in Redemption Ark). The Inhibitors hint that those who try to create true faster-than-light travel meet the same fate and that whole species have been "edited out" of existence.

I love that idea. I mean, its terrifying for some reason, but very interesting as well.

Don't read this if you don't want spoilers, obviously.

[Krulle]

I'll have to re-read redemption ark then, cannot remember that detail.

A different author I once read a short story from had a similar idea. FTL was possible, but due to time running backwards in the universe outside the time field (compared to ship time), it was impossible to determine an exit point/time from the FTL trip, and basically all who tried landed in the big bang, when time was warped to a degree that FTL worked differently, thereby annihilating them...
Alas, cannot remember the story name nor author... (the story itself was about a very, very patient alien race (more precise, their emmissaries), who seeked out other intelligent races to try if their intuition cooked up some theory of how that problem could be solved.... Humanity found them sleeping at the border of our solar system, woke them up,...)
Similar base idea to Michael McCollum's Life Probe.

[Absalom]

No matter how hard you try, every single FTL method will create paradoxes.

You can even use clever tricks to avoid it from yours and your target's Reference Frames, but there will always be another frame of reference, somewhere, where it will be breaking causality.

It sucks.

That's only true for "true FTL". To understand what I mean, think of jump space from Babylon 5. Now imagine that jump space is literally just a higher-energy dimension of real-space (the fifth dimension or whatever) that only happens to have shorter distances because it's higher-energy state has caused space-time warping somewhat analogous to that of space-time close to a black hole: the physical paths are shorter, and literally nothing else. Travelling through that does not necessarily create paradoxes, because not only can you travel through jump space, but so can causality, so it appears to you that you've traveled faster than light, and you have arrived faster, but only because you took a shortcut. Two equivalents might be useful:

1) You are in a space ship, beside a space station. The station fires a laser at a mirror in orbit around Pluto, and your ship goes into FTL. You arrive at Pluto before the laser, turn around, and return to other side of the station. You beat the laser back, but when it does arrive, it is received by a detector on the side of the station that you returned to. This can break causality, because you might have used a method that allows you to travel faster than the "causality cone".

2) You are in a space ship, beside a space station. The station fires a laser at a mirror in orbit around Pluto, and your ship starts accelerating with ordinary thrusters. You maneuver around the station, to the other side, before the laser reaches Pluto. You beat the laser to the detector, but the laser still is received by the detector on the side of the station that you maneuvered to. This cannot break causality, because you explicitly did not use a method that allows you to travel faster than the "causality cone".

Every causality violation system falls into scenario 1, but there can be "FTL" systems that fall into scenario 2 instead, the trick is just that causality has to be allowed to pass by that same system. There are consequences to this, of course: I haven't run any math, but just as an example, thinking about it has lead me to suspect that if wormholes are actually traversable in the real universe, then there must be some sort of inertia preservation mechanism going on (thought experiment: you have both ends of a wormhole following an identical ballistic path, but they're 180 degrees out of alignment, so that anything that goes in one end will be travelling the opposite direction when it comes out the other; how is momentum conserved?), which I assume would manifest itself in wormholes either having mass (in which case I would expect perfectly elastic collisions in the example case), or forcing a reshaping of space-time such that momentum would be preserved seamlessly (in which case I suspect that the example case is impossible, either through forcing the wormhole to collapse, or requiring an energy investment to rotate the wormhole that approximates that of a massive particle reaching the speed of light).

Regardless, not all "FTL" systems can actually be said to violate causality + relativity, because the term is broad enough that some of what it covers should instead automatically let causality + relativity traverse it like ordinary space-time.

[icekatze]

hi hi

That's only true for "true FTL". To understand what I mean, think of jump space from Babylon 5.

Actually, it still applies even in the case of higher dimensions, wormholes, and warped space. Jump Space from Babylon 5 hand-waves relativity by inventing a privileged reference point, which does not exist in reality.

If a wormhole is opened between two stars, ten light years apart, it doesn't matter if the start point and the end point agree that the space ship only traveled 3 meters through a shortcut. A third observer watching 5 light years between the start point and the end point still sees the ship move 10 light years. There is no privileged reference point, so long as any observer can see them changing positions faster than light, then causality is broken.

Velocity is a primary limiting factor in accelerating faster than light, but change in position is all that is required to break causality.

The problem of causality breaking with FTL is not a matter of a ship leaving with a message and returning to the start point before the message arrives back, it is a matter of the ship leaving with the message, and returning to the start point before the message is sent in the first place.

[Absalom]

If a wormhole is opened between two stars, ten light years apart, it doesn't matter if the start point and the end point agree that the space ship only traveled 3 meters through a shortcut. A third observer watching 5 light years between the start point and the end point still sees the ship move 10 light years. There is no privileged reference point, so long as any observer can see them changing positions faster than light, then causality is broken.

Velocity is a primary limiting factor in accelerating faster than light, but change in position is all that is required to break causality.

The problem of causality breaking with FTL is not a matter of a ship leaving with a message and returning to the start point before the message arrives back, it is a matter of the ship leaving with the message, and returning to the start point before the message is sent in the first place.

Ah, but the ship doesn't have to be able to return before it started for the travel method to be described as FTL. That is why I gave those two examples that I did: for all conceptual forms of FTL to run afoul of causality violations also means that all non-FTL movement methods should fall afoul of the same. After all, topology is not identical to the reference frame of anything passing through it. In Babylon 5 they opened a wormhole (or whatever those were: it's been years since I watched it), but if it was just a method of temporarily reshaping space-time to allow travel through a dimension that is normally too constrained for a ship to pass through, then marking it as a causality violation would be directly equivalent to causality violations arising from moving through any gravity field.

Or, to put it another way, if moving through an already-existing shortcut dimension to produce an emulation of true FTL is identical to traveling at FTL from the perspective of causality, then there is little to no reason to consider causality relevant at all. After all, universal inflation produced that very effect already.

[icekatze]

hi hi

for all conceptual forms of FTL to run afoul of causality violations also means that all non-FTL movement methods should fall afoul of the same.

Non-FTL travel is perfectly capable of reconciling different frames of reference, regardless of how many there are or where they are.

(Time dilation due to gravity is not the same thing as time dilation due to velocity. In time dilation due to velocity, both observers will see the other's clocks moving slower than their own, but in time dilation due to gravity, both observers will agree that the high gravity clock is moving slower than the low gravity clock.)

After all, universal inflation produced that very effect already.

Universal inflation is not equivalent to faster than light travel. Relativity does not permit the propagation of information to exceed the speed of light. When the universe expands faster than the speed of light, no information is propagated faster than the speed of light, and those portions of the universe that do expand faster than the speed of light from the point of view of some observer will never transmit information to that observer. This is part of why there is a limit to the observable universe, and why if the universe keeps expanding, the observable universe will shrink over time.