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Page 137: Adieu SG-51 
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Post Re: Page 137: Adieu SG-51
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?

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Fri Jul 20, 2018 6:31 am
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Post Re: Page 137: Adieu SG-51
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.

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Fri Jul 20, 2018 8:27 am
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Post Re: Page 137: Adieu SG-51
GabrielGABFonseca wrote:
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).

RedDwarfIV wrote:
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.


Fri Jul 20, 2018 8:41 am
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Post Re: Page 137: Adieu SG-51
Victor_D wrote:
(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",....)

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Fri Jul 20, 2018 10:41 am
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Post Re: Page 137: Adieu SG-51
Krulle wrote:
Victor_D wrote:
(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
Spoiler: show
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.


Fri Jul 20, 2018 12:01 pm
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Post Re: Page 137: Adieu SG-51
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.

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Fri Jul 20, 2018 12:42 pm
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Post Re: Page 137: Adieu SG-51
GabrielGABFonseca wrote:
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.


Sat Jul 21, 2018 9:47 am
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Post Re: Page 137: Adieu SG-51
hi hi

Absalom wrote:
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.


Sat Jul 21, 2018 1:55 pm
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Post Re: Page 137: Adieu SG-51
icekatze wrote:
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.


Sat Jul 21, 2018 3:06 pm
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Post Re: Page 137: Adieu SG-51
hi hi

Absalom wrote:
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.)

Absalom wrote:
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.


Sat Jul 21, 2018 3:32 pm
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Post Re: Page 137: Adieu SG-51
And FTL through shortcuts is directly equivalent to FTL through inflation, by which I mean that it isn't FTL at all.


Sun Jul 22, 2018 1:19 pm
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Post Re: Page 137: Adieu SG-51
hi hi

Absalom wrote:
And FTL through shortcuts is directly equivalent to FTL through inflation, by which I mean that it isn't FTL at all.
What do you mean by FTL through inflation?

Does FTL through shortcuts mean that some object disappears and is never seen again? That's not very functional as a method of travel between two points.

Does FTL through shortcuts mean that some object goes from point a to point b and takes more time to do so than it would take a beam of light to go from point a to point b without using the shortcut? Because you're right, that isn't FTL at all.

Granted there is plenty of disagreement among scientists about cosmological inflation, but I don't see where you are getting the idea that it allows for FTL travel. Not without negative mass, it doesn't, and it would still violate causality in that case.

Warping spacetime with wormholes implies violating causality.


Sun Jul 22, 2018 4:46 pm
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Post Re: Page 137: Adieu SG-51
Quote:
Your average physicist holds Relativity quite strongly. It has been tested again and again with an accuracy of many decimal places. They hold onto Causality even tighter. Without Causality the entire structure of physics crumbles. Causes must preceed effects, or it becomes impossible to make predictions. If it is impossible to make predictions, it would be best to give up physics for a more profitable line of work.

It occurs to me that this is not an argument for why causality must exist. It is an argument for keeping physicists in a job.

It states that relativity has been tested again and again, accurately. But it does not say the same about causality.

Therefore, if we can build FTL drives... then all we have done is put the physicists out of work. So long as effect before cause is limited to people who can't actually do anything with the information, no paradoxes will occur, and the universe carries on as normal.

Just don't build an instantaneous ansible.

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Sun Jul 22, 2018 7:26 pm
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Post Re: Page 137: Adieu SG-51
hi hi

RedDwarfIV wrote:
It states that relativity has been tested again and again, accurately. But it does not say the same about causality.
Actually, it does.

Quote:
Your average physicist holds Relativity quite strongly. It has been tested again and again with an accuracy of many decimal places. They hold onto Causality even tighter.
Emphasis mine. Scientists really have put a significant amount of effort into looking for ways to break causality, especially with quantum mechanics. But even proponents of retrocausal quantum theory recognize that information cannot be sent back in time, even if a future event changes the random quantum state of entangled pairs. (See Bell's Theorem)

RedDwarfIV wrote:
Therefore, if we can build FTL drives... then all we have done is put the physicists out of work. So long as effect before cause is limited to people who can't actually do anything with the information, no paradoxes will occur, and the universe carries on as normal.
It is not an argument for why causality must exist, but it isn't an argument for keeping physicists in a job either. It is an argument for why you can't have Causality, Relativity, and FTL travel at the same time, and an argument for why scientific method breaks down without cause and effect.

The reason why scientific method breaks down is because people can do things with that information. If cause can come after effect, then every experiment, every experience you witness, could be due to untestable and invisible forces from the future.


Sun Jul 22, 2018 7:45 pm
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Post Re: Page 137: Adieu SG-51
I'm sure such tests have been done, the point I was making was that the wording ProjectRho was using either suggested it hadn't, or that physicists getting to keep their jobs was more important.

The highlit part about holding on to causality even tighter was not specific. If they meant they held onto it because of scientific research, then they should have said that, instead of making a joke. The site does intend to be educational, after all.

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Sun Jul 22, 2018 9:19 pm
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Post Re: Page 137: Adieu SG-51
I've been trying to understand the http://www.physicsmatt.com/blog/2016/8/ ... ime-travel blog post but I think it's going over my head.
If the light of a later action arrives later than the event of an earlier action then causality is violated? Does that mean watching a movie played in reverse violates causality?


Fri Jul 27, 2018 12:50 am
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Post Re: Page 137: Adieu SG-51
hi hi

boldilocks wrote:
Does that mean watching a movie played in reverse violates causality?
No. :|

If you played a movie before it was filmed, then you'd be in a violation of causality, but the universe doesn't care what time stamp we put on our recordings of events.

You can set your computer's calendar to 1990, but that's not violating causality. And the light from a series of p-n junction diodes attempting to imitate the light from an earlier event, is not the same thing as the light from that event. It is its own event.

The digitally encoded data at the beginning of a movie does not cause the digitally encoded data at the end of the movie to exist.


Fri Jul 27, 2018 6:40 am
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Post Re: Page 137: Adieu SG-51
icekatze wrote:
hi hi

boldilocks wrote:
Does that mean watching a movie played in reverse violates causality?
No. :|

If you played a movie before it was filmed, then you'd be in a violation of causality, but the universe doesn't care what time stamp we put on our recordings of events.

You can set your computer's calendar to 1990, but that's not violating causality. And the light from a series of p-n junction diodes attempting to imitate the light from an earlier event, is not the same thing as the light from that event. It is its own event.

The digitally encoded data at the beginning of a movie does not cause the digitally encoded data at the end of the movie to exist.


I guess I'm having problems understanding how you can observe something before it happened.


Fri Jul 27, 2018 7:49 am
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Post Re: Page 137: Adieu SG-51
boldilocks wrote:
I guess I'm having problems understanding how you can observe something before it happened.

You can't. That's kind of the point.

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Post Re: Page 137: Adieu SG-51
Arioch wrote:
boldilocks wrote:
I guess I'm having problems understanding how you can observe something before it happened.

You can't. That's kind of the point.


But for FTL to be impossible you'd have to be able to. So it ends up as "FTL is impossible because if not you'd be able to do this other thing which is impossible."


Fri Jul 27, 2018 10:50 am
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Post Re: Page 137: Adieu SG-51
boldilocks wrote:

I guess I'm having problems understanding how you can observe something before it happened.


The point is, as I tried to show in my pictures on the preceding page, that there is no such thing as a universal "now" time. It's relative for each observer. That's all fine if you're not allowed to travel or communicate information faster than light. But once you introduce ftl, you can create arrangements where some observers will perceive that effect precedes cause. And that cannot be because no point of view is more valid than any other. The speed of light limit ensures that for all observers in any frame of reference, effect will never precede cause.

If this were not true, then you'd be able to produce the paradox I described in the pictures. It arises because for both observers in that scenario, an FTL ship coming from the other location is going backwards in time.


Fri Jul 27, 2018 10:57 am
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Post Re: Page 137: Adieu SG-51
Victor_D wrote:
boldilocks wrote:
I guess I'm having problems understanding how you can observe something before it happened.


The point is, as I tried to show in my pictures on the preceding page, that there is no such thing as a universal "now" time. It's relative for each observer. That's all fine if you're not allowed to travel or communicate information faster than light. But once you introduce ftl, you can create arrangements where some observers will perceive that effect precedes cause. And that cannot be because no point of view is more valid than any other. The speed of light limit ensures that for all observers in any frame of reference, effect will never precede cause.

If this were not true, then you'd be able to produce the paradox I described in the pictures. It arises because for both observers in that scenario, an FTL ship coming from the other location is going backwards in time.


Which time is it going backwards in, if time is relative for each observer? The time they observe or the time of the people "transmitting"? At the end of the day, the photons it picked up in order to "observe" something would have to have been set in motion for them to be received, right?


Fri Jul 27, 2018 11:18 am
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Post Re: Page 137: Adieu SG-51
icekatze wrote:
boldilocks wrote:
Does that mean watching a movie played in reverse violates causality?
No. :|

If you played a movie before it was filmed, then you'd be in a violation of causality, but the universe doesn't care what time stamp we put on our recordings of events.

Instant Cassettes, anyone?
(Where did they go?)

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Fri Jul 27, 2018 11:29 am
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Post Re: Page 137: Adieu SG-51
boldilocks wrote:
Arioch wrote:
boldilocks wrote:
I guess I'm having problems understanding how you can observe something before it happened.

You can't. That's kind of the point.


But for FTL to be impossible you'd have to be able to. So it ends up as "FTL is impossible because if not you'd be able to do this other thing which is impossible."

Well, what this implies for the real universe is that FTL drive is impossible, which is perhaps a good explanation of why we don't see starships zipping by every day. However, To say "any FTL travel breaks causality" is not really a true statement, because there are instances in the real universe in which phenomena happen faster than light, but the universe is structured so that such phenomena can never transmit meaningful information in violation of causation.

For example, there is an effect of quantum entanglement (see double-slit experiment) in which two entangled particles can be separated by great distances, but performing a measurement on one can force the other into the same state. This would seem to be information exchange at faster than the speed of light, but the universe is structured in such a way that this effect cannot practically be used to transmit information from the future to the past (see this explanation by PBS Spacetime). Another example of real FTL effects is the expansion of spacetime faster than light in the early moments of the Big Bang, and in the potential exponential expansion of spacetime in the future (if the rate of Dark Energy continues to increase exponentially). However, because the effect goes only in one direction, it can't be used to transmit information back in time.

However, when it comes to storytelling, all that really matters is that the system is internally consistent and doesn't have any obvious holes. Going back to what started this whole digression, the rules of a fictional universe can be structured in a way so that FTL travel is limited so that causal violations can't happen. In Outsider, this is done by limiting FTL travel to hyperspace, requiring that travel is not instantaneous, and requiring the endpoints of the jumps to be so far away from each other so that no meaningful information can be transmitted backward in time.

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Post Re: Page 137: Adieu SG-51
hi hi

I find that it is a really interesting field of research right now, and one where the answers haven't been definitively figured out. "Spooky action at a distance," is still the subject of questions and experiments. There's even some evidence that suggests that contextuality and non-locality are two different parts of the same quantum resource.

I wish time travel was possible though, then someone from a few million years in the future could come back and give us some hard observational evidence on whether the universe really is expanding exponentially or not. :lol:


Fri Jul 27, 2018 1:42 pm
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