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Thread: FTL and Violating Causality
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2018-01-18, 10:03 AM (ISO 8601)
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Re: FTL and Violating Causality
How? Photo was taken on *T(1) from Ship [T(1)]. It arrives on Earth [T(2)]; who looks at the current ship's position [T(-1)]. Therefore sends the message to current Location T(-1), where they perceive ship is (just as the ship is looking at "past" Earth, Earth is looking at "past" Ship).
But as it turns out, when the message is dispatched, it cannot arrive Ship on T(-1); because ship was always located at T(1) all this time... Point being, just because light is a mirror of the past, that doesn't mean "instantaneous" travel of information would arrive on either the "future" or the "past". That is irrelevant, because those definitions about time only involves the ship/Earth local frames of time. AFIK, "time" isn't an absolute reference, hence why it's very complicated to explain properly how causality can actually be affected.
*T would be a short for "current time and space"
tl;dr: unless somehow Earth can create a channel to your own past (barring all "time cones" shenanigans); then causality isn't actually violated. If you send a message to "someone else's past"; that doesn't mean his/her past is the same actual time/space reference you are dealing with. As I understood it (and I don't claim that I actually understood it too well, nor that I am able to explain it just as good); if causality can be violated, nothing should prevent you from violating your "own" perception of causality.
The funny thing is, OP's proposed situation couldn't actually violate any causality, unless FTL travelling for ships was possible (and I think he mentioned it wasn't). So nothing would prevent any ship being destroyed, unless you could communicate with the same ship before they exploded.
And causality problems are fun and all, but don't make any actual narrative sense without resorting to multiverse travel, as far as RPGs go.(sic)
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2018-01-18, 10:08 AM (ISO 8601)
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Re: FTL and Violating Causality
Near-c sci-fi would be a very interesting field. At 0.9c there are 8 systems that can be reached within 10 years, 35 system that can be reached within 15 years, and probably (extrapolating) over 60 systems that can be reached within 20 years. Which is probably well over 200 terrestrial planets and who knows how many spherical moons. (Though this ignores accelerating and decelerating at the start and end of a trip.)
This allows for sci-fi that is actually scientisfic and not just pure fantasy in space.
The (currently extrapolatable) realities of space battle with real physics are also really fascinating. (Love the idea from one series where chases come down to which ship is willing to keep pushing down on the accelerator longer before the crew gets crushed by the g-forces.)We are not standing on the shoulders of giants, but on very tall tower of other dwarves.
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2018-01-18, 10:11 AM (ISO 8601)
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Re: FTL and Violating Causality
My current understanding:
For literal, direct, in-universe FTL it is, as I understand it, an inherent result of moving faster than light. The math gets weird and time objects strongly.
For things like "warp drives" which allow a vessel to arrive at a destination faster than light without actually breaking the "light barrier", it's possible with the right setup to end up back where you started before you left.
For more speculative (some will same "fanciful") ideas, such as "hyperspace", even if you can't end up back where you started before you left, there is potential for data to get back to where you started before you left given the right set of "moves".It is one thing to suspend your disbelief. It is another thing entirely to hang it by the neck until dead.
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2018-01-18, 10:14 AM (ISO 8601)
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Re: FTL and Violating Causality
But wouldn't that just make the messages come in slower or faster?
I've heard the example of moving wormholes before, and I get that. If one wormhole is moving faster than the other time there passes slower as seen from the rest of the universe. But if you can fly into one hole and out the other without breaking your ship into a thousand pieces because the particles that have already gone through travel at a different speed than the ones still in the first part of the universe (or some similar but way weirder ****) the wormholes must be coupled in such a way that part B going in exactly a second after part A comes out exactly a second after part A, which means that when a thousand years have passed at one end of the wormhole a thousand years have passed at the other. Since time runs objectively slower at the fast end, because it's fast, stepping in to the slow end of the hole means traveling forward in time, and the opposite.
What I don't get is how this applies to sending messages that don't have any history they must be coupled to. If I send you two instant messages an hour apart while you're flying really fast you're just going to receive two messages an hour and twenty minutes apart right? Which means both of them arrive just after I send them, not before.
I get from the amount of people agreeing in the thread that I'm wrong, but I don't get why yet. I think I should just reread the explanation more carefully at some point in the next few days.The Hindsight Awards, results: See the best movies of 1999!
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2018-01-18, 10:14 AM (ISO 8601)
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Re: FTL and Violating Causality
It can't. There is no such thing as universal "current time".
The photo is taken at Earth time -1 year from the PoV of the planet, and at (say) Earth Time -8 months from the PoV of the ship. The planet sends the picture, which arrives at Earth 1 year before the event, and Earth sends it to the ship, again 1 Earth year before the event. But because of time dilation, the ship is not due to arrive at the event until 4 Earth months later, since it will have experienced the event at Earth time -8 months.
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ETA: by the way, my variation of the example requires the planet to be immobile with respect to Earth. So it's probably an artificial refueling planetary base or something dragged in position for reasonsLast edited by Grey_Wolf_c; 2018-01-18 at 10:30 AM.
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2018-01-18, 10:37 AM (ISO 8601)
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Re: FTL and Violating Causality
For the message-passing construction, sending messages forward quicker than the speed of light doesn't immediately give you the causality violation. It's when you send a message forward, have the receiver change (subluminal) velocity, and then send a message back that you get the loop. That velocity change is necessary to the construction, since that's what gives you the time dilation/length contraction factor you need to generate an offset.
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2018-01-18, 10:39 AM (ISO 8601)
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Re: FTL and Violating Causality
Near-c space travel is certainly a thing in fiction. Most series I believe tend to assume a drive that instantly accelerates you to 0.9999999...c and then instantly decelerates you to effectively motionless so that the jump is instant from the traveler's perspective. There's also fiction that uses ramscoops, but they have fallen out of fashion because accelerating up to the speed where you can use them is impractical.
I think the only series I've seen do non-ramscoop relativistic travel 'legitimately' is Revelation Space, where the super-intelligent group of humans invented reactionless engines which they then stuck on a bunch of gigantic (~4km long) ships, everybody else only has interstellar travel because they sold a limited number of these ships centuries ago. Travelling between inhabited star systems can take a ship a decade from their own perspective and half a century from the perspective of their start and end points. There's still trade between systems and contact, but it's certainly a case of some finished goods and information, not raw materials. All other ships use reaction drives.
So one of the ideas I've had has been to ignore the idea of ships having FTL drives, instead everybody uses reactionless drives (most ships ones that only provide 1-2g, warships might get up to 5-6g but only use full power for a relatively short period) and colonisation consists of packing a massive ship with people and supplies then flying for a decade. The PCs are a group of interstellar traders who accept the fact that they'll essentially never return home and fly people, objects, and information from one system to another. I'm a bit sad to get rid of delta-v tracking, but for any kind of interstellar gameplay we've got to break physics in at least one way.
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2018-01-18, 10:58 AM (ISO 8601)
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Re: FTL and Violating Causality
I used "current" in reference to their respective's POV. I was trying to avoid all implications of "Universal Time".
But if the ship is sending the message, why is it using the "time dilation" effect of 1 year separating them, instead of the 8 months apart? AFIK, time dilation is an effect of gravity, not actual distance. Of course it involves speed; but since the message is "instantaneous", why is it assumed the message applies to either spacetime scale? That is the part I have issues with: for what I understand, Earth is currently both at 1 "lightyear" and 8 "lightmonths" distance from the planet (or the ship); depending what time reference you use. But obviously, Earth can not properly be at two different places "at the same time"; so it is either one of them (or none of them, but at least one Earth does exist, somewhere).
I have this question for a long time for this topic; since those examples of "violation of causality" seem to me to imply that various (nay, infinite) "past/future Earths" DO actually exist, somewhere, somehow and simultaneously* (if the scales are pushed to even more absurd levels).
*since it is "simultaneous information" what is actually communicating them.
If it is fixed from Earth's perspective, shouldn't Earth ETA (estimated time of arrival) for the ship to the planet be 8 months too, so everything matches in the end? Mind you, the information doesn't actually reach Earth before the event, but rather, before Earth has a chance to watch the event.
What Earthlings would experience then, is the ship reaching the planet "earlier"; because they would just need to adjust the clock in the photo to match it (the adjustment needed for anything affected with time dilation). Time dilation is what is really causing the "apparent time travel" when information is sent "instantly" but it is still "fixed" by the difference in how experience and the order of events are experienced from either PoV.
tl;dr: what I read in the past about simultaneity, time references, and time dilation (and FTL issues) is that the most accepted consensus is that even when depiction of events can appear distorted depending on the PoV; simultaneity of the universe (the fact that there is no absolute past or future) still prevents anything from actually violating causality. Then again, there's a chance I am mixing two completely different issues here...
Btw, sorry if I mixed it up with the numbers and such, hope you can still get what I'm trying to say here. Brain still affected by "synaptic dilation" from insomnia. I don't mean there is a calculation error on the problem you and the other poster brought up; and I still think those are great for thought experiment (like that uncertain cat).Last edited by Lord Joeltion; 2018-01-18 at 11:01 AM.
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2018-01-18, 10:59 AM (ISO 8601)
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Re: FTL and Violating Causality
This, I think, is the part of Douglas' post that I'm not understanding. How is "one year before the light beam hits earth" in the past for the moving ship? The stationary ship is one light year from earth. The moving ship was one light year from earth, at the point where it passed the stationary ship. How many millions of miles did they travel in the time it took me to read the quoted paragraph? Isn't the difference in time accounted for by distance traveled? As soon as ftl isn't instantaneous, it falls apart. Or at least, I'm not seeing how it holds together.
Can you give an example using real times and distances where ftl is c+x, or even 2c for simplicity, where that is the case?
How would changing the reference frame (to the ship, I assume) change anything in this example? If ftl is c+x, then the ship could have crashed into the planet and ceased to exist before the picture of it even reaches earth.The first chapter of The Book of Svarog
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2018-01-18, 11:17 AM (ISO 8601)
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Re: FTL and Violating Causality
The ship never sends a message. The planet does. The ship receives the message from Earth.
No, it is an effect of movement. The planet is not moving respect to Earth, the ship is. Therefore, the planet is experiencing time at the same rate as Earth, the ship is not.
No, Earth can and does exist at two different relative times, because of time dilation. The same light beam will take 1 year to arrive to Earth from the PoV of the planet, and only 8 months from the PoV of the ship, because they both measure the beam to be moving at c. The beam will arrive at the same time to Earth, and it is traveling at the same speed towards Earth, but from the PoV of one of the observers Earth is "getting closer" (and thus the distance to be traveled by that beam is different between observers) and from the other's PoV it is not, then something has to give. And as it turns out, what "gives" is time. The beam will take less time to arrive to Earth from the PoV of the ship than from the PoV of the planet.
No. The issue is that you are not accounting for time dilation. Despite what you say, you are implicitly assuming that time passes exactly the same for all observers.
No, from the PoV of the planet, light traveling at c will reach Earth in 1 year, by their clocks (planet clocks and Earth clocks move at the same rate).
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2018-01-18, 01:01 PM (ISO 8601)
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Re: FTL and Violating Causality
And that is where you are wrong. Both time references are correct, "at the same time". They are in different reference frames, but both reference frames are equally valid.
You are correct that there is a definitive single definition of "where/when Earth is" that all reference frames agree on, but it requires bringing in additional concepts such as Lorentz transformations and it still doesn't produce any agreement on what the time difference is between that coordinate and "now".
What prevents anything from actually violating causality is that all reference frames always agree that if light could travel from event A to event B then event A happened before event B. As long as nothing travels faster than light speed, this is enough to guarantee that all causes happen before their effects regardless of reference frame.Like 4X (aka Civilization-like) gaming? Know programming? Interested in game development? Take a look.
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2018-01-18, 01:24 PM (ISO 8601)
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Re: FTL and Violating Causality
Still no example that doesn't use at least two reference frames, one of which ALSO has a time and/or spatial subset. Still no resolution for the assumption of FTL as instantaneous.
I'll clarify the problem: why do people build up these examples with numerous references to relativity being inviolable and reference frames being really important to keep track of because they interact strangely, and then assume that FTL inherently disregards any disparity in frame of reference, using that in a circular argument of "well it doesn't conform to relativity, so it must be impossible!" having of course defined FTL by that yardstick.
Basically, I don't accept the idea that FTL fails to concord to the reference frame that includes it. If we're using max's hyperspace wormholes, why do you get to have one that connects different reference frames without necessitating that they agree?
Your examples only end up with alleged time travel because you specifically assume that FTL moves across reference frames without being concordant with them, creating time-holes (all of which seemingly only go backwards in time for reasons nobody has yet explained). I do not accept that assumption.
So here's my example: planets L, C, and R are connected in series by orbital instant-travel wormholes 1 and 2, with wormhole 3 connecting L to R. L and C share the exact same reference frame, but planet R is moving relative to all of L, C, and 1-3. NOTE: wormhole 3 is NOT moving relative to any frame except R, and R is the only reference frame in motion from any other point of reference. None of the wormholes or their openings are moving with respect to eachother or L or C.
L, C, and R are however arbitrary thousands of LY apart that you like. Please explain how any of them can send messages back in time.
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2018-01-18, 01:33 PM (ISO 8601)
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Re: FTL and Violating Causality
You do realise that everything in the universe is moving at different speeds, correct? That the assumption that L and C could be "in the exact same reference frame" is completely unrealistic, and only used to make the example easier, right? Saying that the example should work for a situation in which everything is in the same exact frame of reference is completely unrealistic for any actual scenario where the objects are not on the exact same planet.
Fine. In Douglas' example, FTL takes 1 day to travel 1 ly. So instead of the picture arriving to the ship 4 months before the ship gets to the planet, it arrives 4 months minus ~2 days.
I have to wonder if you picked those letters to maximize confusion. I can't even figure out what you mean. You say that L connects to R through 3, but R is moving and 3 is not. Therefore, you can't get to R from L through 3.
Also, what exactly is wormhole 2 for, and what does it connect?
ETA: Also, Is R moving towards any of the other objects, or away from them?
GWLast edited by Grey_Wolf_c; 2018-01-18 at 01:51 PM.
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2018-01-18, 01:40 PM (ISO 8601)
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Re: FTL and Violating Causality
If all of your FTL devices (wormholes, stargates, warp drives, etc.) operate forwards in time (or instantaneously) in some single universal reference frame, then there is no implied time travel.
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2018-01-18, 01:55 PM (ISO 8601)
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Re: FTL and Violating Causality
Yes, and i'm presenting the example as having only two frames of reference for that convenience.
Fine. In Douglas' example, FTL takes 1 day to travel 1 ly. So instead of the picture arriving to the ship 4 months before the ship gets to the planet, it arrives 4 months minus ~2 days.
I have to wonder if you picked those letters to maximize confusion. I can't even figure out what you mean. You say that L connects to R through 3, but R is moving and 3 is not. Therefore, you can't get to R from L through 3.
Also, what exactly is wormhole 2 for, and what does it connect?
ETA: Also, Is R moving towards any of the other objects, or away from them?
GW
You can't get to the surface of R merely by going through 3, you get to the proximity of R from either holes 3 or 2(depending if you are at L or C).
Let us assume that R is moving directly away from C, along the ray LC.
edit: confusing redundancyLast edited by exelsisxax; 2018-01-18 at 02:10 PM.
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2018-01-18, 02:12 PM (ISO 8601)
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Re: FTL and Violating Causality
Yes, I can see that. So your own example doesn't even fulfill your own requirements of proof. You can't complain that the example doesn't use a single frame of reference, and then present an example that also doesn't use such.
Again: your set-up doesn't allow you to travel FTL to R.
Instead, the way to describe it is that C connects to two points: one is where L is, 1 ly away from L's perspective (and C's perspective), the other where R is when it is 1 ly away from C from R's PoV.
And now we are back to Douglas' example: R passes very near to L. An L-ling takes a picture, sends it to C, who sends it to R, who gets the picture back when it was 1 ly away from C, and 4 months before it is due to pass by L.
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2018-01-18, 02:29 PM (ISO 8601)
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Re: FTL and Violating Causality
I am assuming that because it is one of the core postulates of relativity - no matter what reference frame you pick, the laws of physics will stay exactly the same in all ways. Even if you switch from one reference frame to another part way through, all that requires is that you apply the right transformation at the point where you switch. The existence of FTL travel does not change that.
I did part of the calculation in one reference frame, switched to another while applying the appropriate transformation, and finished the calculation in the new frame. Incidentally, the switch is where the time discrepancy is introduced. This time discrepancy is physically real, it's an actual thing that happens in reality, but other factors that I left out of my explanation for simplicity make it resolve by the time anything travelling at light speed or slower could reach whatever observer you care about.Like 4X (aka Civilization-like) gaming? Know programming? Interested in game development? Take a look.
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2018-01-18, 02:30 PM (ISO 8601)
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Re: FTL and Violating Causality
Corrrect: if you use my example I can't complain about the example being bad. However, when you completely change the example as below, I do get to complain.
There are 2 reference frames so that you can attempt to demonstrate that FTL causes time travel because different reference frames exist. That was not the complaint. The complaint is when, like excelsior's example, you ALSO have a third observer at both a different time and space that you use as an intermediary because you NEED it. I'm not letting you have it for free.
Again: your set-up doesn't allow you to travel FTL to R.
Instead, the way to describe it is that C connects to two points: one is where L is, 1 ly away from L's perspective (and C's perspective), the other where R is when it is 1 ly away from C from R's PoV.
And now we are back to Douglas' example: R passes very near to L. An L-ling takes a picture, sends it to C, who sends it to R, who gets the picture back when it was 1 ly away from C, and 4 months before it is due to pass by L.
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2018-01-18, 02:34 PM (ISO 8601)
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Re: FTL and Violating Causality
On a second thought, I think I just mixed up two different ideas in my mind. And got too hung up on the word "simultaneous". Now I feel dirty for making a fool of myself. To myself.
Damn you Lorentz! Btw, yeah, I was thinking about Lorentz (among other things, apparently). The best answer to that issue (that I found), is that the numbers agree with causality being thrown out the window; but looks more like a cheat code than anything (the part about travelling to your own past, that is).
Ok, probably I lack research/jargon to discuss those things here. And sleep Now that I think about it, violating sleep causality would be the best invention ever.Last edited by Lord Joeltion; 2018-01-18 at 02:38 PM.
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2018-01-18, 02:35 PM (ISO 8601)
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Re: FTL and Violating Causality
No, it is not. R is moving away from L and C, and therefore it is moving away from the wormhole exits that are in the same frame as L and C. R is only "right there" at one very specific point in time. After that point, R leaves the wormhole exit behind, never to be near it again.
ETA: Let me try Douglas' example again, with your wormholes:
No, I can't make it work. R is simply not connected via wormhole to anything. It's wormhole needs to move with it.
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2018-01-18, 02:44 PM (ISO 8601)
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Re: FTL and Violating Causality
...Because pretty much everything in the universe is in its own reference frame? When it comes to interplanetary discussions, having two things in the same reference frame is an extremely rare situation, so it's vital to understand how reference frames work. A ship in FTL has its own reference frame, it's not in the same reference frame as objects near it, because it's moving relative to them. I mean, I'm not in the same reference frame as the satellites in orbit of Earth right now, so why should we start with the assumption that I'm in the same frame of reference as someone moving faster than light?
Also yeah, FTL travel only violates causality if relativity is true. FTL, Causality, Relativity, pick 2. If you want to throw Relativity out the window then yeah, you can absolutely have FTL travel without Causality violations. Have fun. But we have mountains of evidence that say Relativity is accurate and that causality is preserved, so FTL is the one that's highest on the chopping blocks.
So here's my example: planets L, C, and R are connected in series by orbital instant-travel wormholes 1 and 2, with wormhole 3 connecting L to R. L and C share the exact same reference frame, but planet R is moving relative to all of L, C, and 1-3. NOTE: wormhole 3 is NOT moving relative to any frame except R, and R is the only reference frame in motion from any other point of reference. None of the wormholes or their openings are moving with respect to each other or L or C.
If you want a simple communication-only FTL violation that could use any means of FTL communication, there's a great one with diagrams here: http://www.theculture.org/rich/sharp...es/000089.html . Just imagine the ansible they're using is a wormhole and that the ships are planets with the wormholes in orbit.
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2018-01-18, 02:47 PM (ISO 8601)
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Re: FTL and Violating Causality
Please read my example again, where I explicitly note that R is moving with respect to the rest of the system, and no wormhole is moving with regard to any other, nor any points of entry with regard to any other. The wormholes are nearby the initial locations of the planets, not remaining on their surfaces.
Again, the wormholes are not ON L, C, and R. They are nearby each at t=0, and none of them move with respect to eachother/themselves.Last edited by exelsisxax; 2018-01-18 at 02:50 PM.
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2018-01-18, 02:51 PM (ISO 8601)
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Re: FTL and Violating Causality
Last edited by Grey_Wolf_c; 2018-01-18 at 02:55 PM.
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2018-01-18, 03:03 PM (ISO 8601)
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Re: FTL and Violating Causality
I am, like you, douglas, and the several linked sources posted throughout this thread, assigning certain starting conditions with some arbitrary "start from this" event in order to communicate with people.
R isn't exactly where R was slightly after that and slightly before that, but that's because R has a relative velocity, not because you can't go through a wormhole to get to its proximity. Because, once again, the wormhole doesn't put you on R itself, only nearby. I understand that R will immediately stop being the same distance from everything else, that's why I mentioned, on many occasions now, how R is moving relative the rest of the system and the rest of the system is NOT moving relative to any of its parts.Last edited by exelsisxax; 2018-01-18 at 03:09 PM.
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2018-01-18, 03:09 PM (ISO 8601)
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Re: FTL and Violating Causality
If the wormholes are the only means of FTL communication, and all wormholes are stationary relative to each other, then the rest frame of the wormholes functions as effectively an absolute reference frame, which has already been mentioned as the one way to resolve FTL without introducing time travel.
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2018-01-18, 03:21 PM (ISO 8601)
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Re: FTL and Violating Causality
Maybe. But you are doing so by trying to establish is a Universal timeframe, despite the fact that R is not in the same frame as the other objects. It doesn't work. I.e. you can say at point x the objects are in those positions, but the moment we run time forward to generate a scenario that creates a paradox, R moves away from the wormhole and you can no longer say that you can travel via FTL to R. Douglas might be able to come up with a new example under those conditions, but I can't: you've removed the possibility of causality violation by disabling FTL travel, as far as I can see.
Compare this to Douglas' event, which happens both at 1ly away from Earth (from the planet's perspective) and 8 lm away from Earth (from the ships' perspective). Douglas doesn't need to establish a T=0 for his example. Just a common event happening at different times for different observers with respect to a common reference point.
Essentially, your answer to the problem is "I will only allow FTL travel between points in the same frame of reference", which in the universe is "nowhere", because there are not two points in the same frame of reference other than on the same planet. The wormhole "to R" only is so for a blink of an eye in the entirety of the lifetime of the universe. Not what I'd call "a wormhole to R".
Grey WolfLast edited by Grey_Wolf_c; 2018-01-18 at 03:23 PM.
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2018-01-18, 03:31 PM (ISO 8601)
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Re: FTL and Violating Causality
Dougas's ships are also not in the same frame of reference. My example is no different. His starting point is based on an apparent simultaneous ray of light, I'm not using one because I haven't enforced a phenomenon to base the time travel on.
bold: I... win?
For the 5th or 6th time, the wormhole goes to a location in space that is proximal to R at the starting conditions, and never at any point in time intersecting R itself. Unless R is moving at a significant fraction of C, the other planets have plenty of time for sightseeing. If this prevents the alleged causal breaks, I'll call that a win.
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2018-01-18, 03:47 PM (ISO 8601)
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Re: FTL and Violating Causality
If your goal was to create a hypothetical scenario where relativity doesn't apply, then you definitely succeeded, as far as I can tell.
Remember, FTL only necessarily violates causality when relativity applies. If, in a hypothetical scenario, relativity doesn't matter/exist, then FTL won't necessarily break causality.Favorite sports:
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