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Page 137: Adieu SG-51 
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Post Re: Page 137: Adieu SG-51
Nathan_ wrote:
Arioch wrote:
Victor_D wrote:
(I wonder, Arioch, if you had given some thought to possible causality violations brought about by the existence of FTL; in a large part, these violations become far less likely if no FTL communications and no "free" FTL travel exists)

The system is constructed so that causal violations are essentially impossible. The entry and exit points of the jump are light years apart, so even if you did end up going backwards in time a fraction of a second, no one would notice.


While it isn't time travel, it is still a causal violation since the ship arrives long before the light of the ship making the jump gets to where it is. Whether this can be exploited to cause any wierdness is another matter.


It breaks relativity as all FTL must; my question was aimed at whether or not the Outsider-verse FTL is exploitable as something that can cause serious paradoxes relevant for the characters. As far as I am aware, all these schemes showing FTL completely breaking sanity by producing unsolvable paradoxes are predicated on the existence of effective FTL communication (which Outsider doesn't have) and unlimited FTL (in the sense that you can travel arbitrarily, like, say, with warp drive in Star Trek). Even then, they usually need to involve a ship moving at relativistic velocities for time paradoxes to occur when this ship perceives effect preceding the cause and communicating this via FTL communication.

In Outsider, a ship moving at relativistic speeds in relation to local space (say, the Leido-Sala theatre) may perceive significant causality violations occurring all around, but it has no means of communicating anything it sees back to base in time to make a difference, hence why this cannot be used e.g. to warn the Loroi fleet of Umiak invasion before it happens and cause other kinds of weirdness.


Sat Jul 14, 2018 1:54 am
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Post Re: Page 137: Adieu SG-51
Image


*fffffffffff-


Mon Jul 16, 2018 6:28 am
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Post Re: Page 137: Adieu SG-51
A good indicator usually is whether Arioch has posted a WiP.

Oh well, I'm looking forward to the double update next week. ;)


Mon Jul 16, 2018 7:18 am
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Post Re: Page 137: Adieu SG-51
"Do not my friends, become addicted to weekly updates! They will take hold of you, and you will resent their absence!"


Mon Jul 16, 2018 7:48 am
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Post Re: Page 137: Adieu SG-51
Askaris wrote:
"Do not my friends, become addicted to weekly updates! They will take hold of you, and you will resent their absence!"

Today were hauling Aqua cola! Immortan Jim!

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Mon Jul 16, 2018 7:57 am
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Post Re: Page 137: Adieu SG-51
Victor_D wrote:
It breaks relativity as all FTL must; my question was aimed at whether or not the Outsider-verse FTL is exploitable as something that can cause serious paradoxes relevant for the characters. As far as I am aware, all these schemes showing FTL completely breaking sanity by producing unsolvable paradoxes are predicated on the existence of effective FTL communication (which Outsider doesn't have) and unlimited FTL (in the sense that you can travel arbitrarily, like, say, with warp drive in Star Trek). Even then, they usually need to involve a ship moving at relativistic velocities for time paradoxes to occur when this ship perceives effect preceding the cause and communicating this via FTL communication.

In Outsider, a ship moving at relativistic speeds in relation to local space (say, the Leido-Sala theatre) may perceive significant causality violations occurring all around, but it has no means of communicating anything it sees back to base in time to make a difference, hence why this cannot be used e.g. to warn the Loroi fleet of Umiak invasion before it happens and cause other kinds of weirdness.

The old saying is "FTL, Relativity, Causality, pick two." It's relativity that introduces the causality issues. The FTL scheme in Outsider isn't anything special in that regard. "Unlimited FTL" as you call it is not required to violate Causality if Relativity holds. A loop of fixed wormholes is enough.

FTL travel is FTL communication, unless the FTL travel does something like scramble everything into completely random stuff, in which case it's kind of useless as a method of travel.

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*fffffffffff-

*waves cane* I still consider a year with a single update to be a good year. Having stuck around here for well over a decade, through the Great Hiatus, I have been Pavlovianly conditioned to always be pleasantly surprised when Outsider updates.

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Mon Jul 16, 2018 9:13 am
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Post Re: Page 137: Adieu SG-51
How does FTL as it works in Outsider break causality?


Mon Jul 16, 2018 10:09 am
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Post Re: Page 137: Adieu SG-51
I suppose you can see your past self or receive radio transmissions from your past self, but that's about it.

If you do a hyperspace jump into another star system, say, 5 LY away, and after 5 years look at your origin system with a powerful enough telescope, you will see your ship as it is about to jump. Same way you will be able to receive any radio transmission your past self sent out if it's strong enough and aimed correctly. Not sure if that counts as a causality violation, but it definitely is a form of weird FTL side effects.


Mon Jul 16, 2018 11:08 am
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Post Re: Page 137: Adieu SG-51
That can't be it. Then going supersonic and allowing the sound to catch up would also violate causality.


Mon Jul 16, 2018 11:12 am
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Post Re: Page 137: Adieu SG-51
Seeing yourself through a telescope 5 years ago does not break causality in any meaningful way. Causality is broken when you can see events from the future or influence events in the past.

Technically, any faster than light travel through spacetime breaks causality because the speed of light is actually the speed of causality within spacetime. However, this doesn't mean it's impossible for two objects to move relative to each other faster than light; in fact, the current theory of of the Big Bang (specifically, Inflation theory) requires that spacetime itself expanded much faster than the speed of light in the early fractions of a second after the Big Bang. Faster than light travel within the confines of flat spacetime is impossible, but you can travel faster than light away from another object if the spacetime itself between you stretches faster than light (as in cosmic inflation), or if you locally warp spacetime (as in an Alcubierre warp drive or a wormhole), or if you leave spacetime altogether (as in hyperdrive or jump/fold drive).

The problems for causality happen when you pair relativistic velocities with some sort of instantaneous communication or teleportation. For example, if you have two locations connected by an ansible which can communicate instantaneously, and one location is aboard a ship that is traveling at a high fraction of the speed of light, then the two points experience different rates of time despite being in communication with each other, and potential causal hijinx ensue. However, the way jump drive in Outsider works avoids such problems. Travel through hyperspace is not instantaneous, and ships in hyperspace do not experience extreme time dilation, and ships in hyperspace are blind to events in real spacetime, and the endpoints of the jump are so far away that there can be no meaningful causal interactions between them on the time scales of the jump.

And no, there's nothing unique about this FTL scheme.

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Mon Jul 16, 2018 11:26 am
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Post Re: Page 137: Adieu SG-51
hi hi

The issue with FTL travel and causality is in large part because of the phenomenon in relativity that is commonly described as, "Simultaneity is Observer Dependent."

The way FTL works in Outsider does a pretty good job of avoiding paradoxes. Even though it might still be possible in some extreme cases, as long as the time it takes to complete a jump increases as the relative velocity and/or distance between start point and end point increases, then it should at least be impossible to create a paradox at the tech level of the current civilizations. (Not counting telepathy, which does not follow the FTL rules.)

If you take for example, a jump between Sol and Barnard's Star, the jump would need to take about 2.11 seconds, to prevent the possibility of a ship jumping one way, sending a message to a ship on the other side, and having that ship immediately jump back prior to the first ship's departure.

Spoiler: show
The minimum distance that you could jump instantly to another object, send them a message via laser, have them jump instantly to your start point, and send a message via laser, without risking a paradox, in a two reference frame model, is = (y-1)*distance between the two. Where y = 1/Sqrt(1-(Velocity_difference^2/Speed_light^2))

Barnard's Star is 5.958 light years away, and traveling at 142.6 km/s relative to Sol, for example.

We can derive from this the amount of time it would take a laser to bridge the distance as the amount of time the jump would need to take in order to jump directly to the object without risking a paradox.


Mon Jul 16, 2018 2:52 pm
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Post Re: Page 137: Adieu SG-51
Hi. I don't know if there were forums the last time I dropped you a line....

The snooty dark-blue-haired one with the twisty-plaits.... she forgot to switch her suit to nightlight-mode at the bottom of the page.


Mon Jul 16, 2018 9:01 pm
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Post Re: Page 137: Adieu SG-51
Contraltissimo wrote:
Hi. I don't know if there were forums the last time I dropped you a line....

The snooty dark-blue-haired one with the twisty-plaits.... she forgot to switch her suit to nightlight-mode at the bottom of the page.

So I did, thanks. Should be fixed.

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Mon Jul 16, 2018 9:40 pm
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Post Re: Page 137: Adieu SG-51
I'd argued with people over similar complaints, and never got a satisfactory answer.

According to the argument, all FTL travel breaks the universe.

My view: A paradox requires that you go backwards in time. If all frames of reference (start point, starship, and destination) are moving forwards in time, then there is no way for a warp drive starship to go to another star system, return to its point of origin, and have arrived before it left.

ProjectRho gives the example of a relativistic ship going between Earth and Proxima Centauri. Earth sends Proxima a message by FTL comms. The relativistic ship sees the message arrive at Proxima before it is sent from Earth. Fine, that looks kind of weird.

But then it says:
Quote:
if the ship had a FTL phone set up in the right way, they could call Earth before Earth placed the call.

I don't get it. They see the call arrive. The FTL phone doesn't send messages backwards.

This is what I'm imagining:
Spoiler: show
Image


This is what they're saying:
Spoiler: show
Image


I can see how the second one might be right if the FTL communicator was near instantaneous, as the earth-proxima line suggests... but how does an FTL drive that goes at a certain speed relative to the universe fit into that?

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?




2:30 AM is way too late at night for me to be thinking about time travel.

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Tue Jul 17, 2018 6:23 pm
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Post Re: Page 137: Adieu SG-51
hi hi

RedDwarfIV wrote:
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.

RedDwarfIV wrote:
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.


Tue Jul 17, 2018 8:58 pm
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Post Re: Page 137: Adieu SG-51
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.

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Wed Jul 18, 2018 12:17 am
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Post Re: Page 137: Adieu SG-51
icekatze wrote:
hi hi

RedDwarfIV wrote:
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.

RedDwarfIV wrote:
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.


Wed Jul 18, 2018 7:43 am
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Post Re: Page 137: Adieu SG-51
hi hi

GeoModder wrote:
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.

Sweforce wrote:
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


Wed Jul 18, 2018 8:37 am
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Post Re: Page 137: Adieu SG-51
Time Travel makes my head hurt.


Wed Jul 18, 2018 9:55 am
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Post Re: Page 137: Adieu SG-51
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.

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Wed Jul 18, 2018 10:51 am
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Post Re: Page 137: Adieu SG-51
icekatze wrote:
GeoModder wrote:
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).

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Wed Jul 18, 2018 3:34 pm
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Post Re: Page 137: Adieu SG-51
hi hi

GeoModder wrote:
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.

GeoModder wrote:
Besides, it decided to power up its FTL *because* of something received by its eventual target (Earth thus).
That is the paradox.


Wed Jul 18, 2018 3:43 pm
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Post Re: Page 137: Adieu SG-51
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.

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Wed Jul 18, 2018 4:28 pm
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Post Re: Page 137: Adieu SG-51
hi hi

GeoModder wrote:
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.


Wed Jul 18, 2018 5:05 pm
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Post Re: Page 137: Adieu SG-51
@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:

Spoiler: show
Image


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:

Spoiler: show
Image


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).

Spoiler: show
Image


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.


Thu Jul 19, 2018 1:32 am
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