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This isn't entirely hard science but bear with me.

(Prologue) In the near future, a series of increasing heavy elements are synthesized building on the theory of islands of stability, incredibly strong compounds are made and all of the engineering benefits that come with them. All science progresses in turn, with various advancements made, allowing for fast, 0.8c-ish interstellar travel and so forth. This is then supplemented with the largest advancement made so far, atoms of which where thought to be impossible are made, elements with more neutrons and protons than 174 each are synthesized. A strange phenomenon occurs, where quantum waveforms of each atom result in waveforms which stretch out in such a way that they result in FTL transmission of information. Time progresses from then on, and it turns out that that's the only way to send information FTL, even with thousands of years of scientific advancement during which FTL transmission of information is possible, and so all FTL transmission lines have to be physical. Of course if you could lay these lines then interstellar civilization would be truly possible. Which begs the question, how do you lay light years worth of cable?

TLDR: FTL transmissions have to be over fantasy "fiber optic" (actually metal / elements with atom number > 174) cables, and so even with relativistic spacecraft, I'm not entirely certain how you could haul a cable that extends multiple light years. Even if you stagger it with segments and transmitters, that's still a lot of cable over a very long distance.

The cables have a radius of half a millimeter, and weight a tenth of a kg for every kilometer of distance covered. The cable has a relatively hight tensile strength, being made of ultra dense materials, let's say about 1300 gigapascals. Beyond this, the cable snaps, that being said, it can't take an extended beating from a relativistic medium, it is able to survive for a few minutes at 0.8 c, and conversely proportional for the relative energy levels of the velocity in turn, after which is ensuingly useless.

The spaceships in concern use exotic particle interactions to propel spaceships up to 0.8 c, using the production a generic dark matter particle at high energies for reaction mass so the exhaust wont toast the cable. The cable for example is being laid between two stars 3 light years apart.

If more technical information is needed I'll be happy to add it to the question.

The cables, within the solar system would be 100 to 200 au out, as to minimize gravitational forces experienced.

Jakav
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MegatheriumMegafauna
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    First question, to be clear, that I've made on this platform. – MegatheriumMegafauna Oct 30 '23 at 04:08
  • The cables will need to be a certain thickness, presumably, in order to carry the data and to have structural integrity. Given that no information has been provided on the spacecraft, can we assume that there are spacecraft large enough to carry and lay the mind-boggling huge volumes of cable that will be required? And assume that production of the cable is not an issue? – KerrAvon2055 Oct 30 '23 at 04:37
  • How thick a cable? Does it need shielding against EM spectrum? Does the question include dealing with the cable’s own internal stresses or just external stresses (like drifting near a star or black hole)? – SRM Oct 30 '23 at 05:10
  • Start between planets: https://worldbuilding.stackexchange.com/questions/64926/is-it-possible-to-build-a-bridge-between-planets/64931#64931 – SRM Oct 30 '23 at 05:16
  • What's the rout? Can it trace the boundaries of heliopauses between stars, or does it need to be "straight"? – Escaped dental patient. Oct 30 '23 at 05:50
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    Seeing the weight and mass required, it is more likely they will send probes with FLT to and from the ships. A km of cable is already big. A cable from here to only Pluto will require so much mass you can better have multiple ships where a few run back and forth than an actual cable. Space is *BIG*. – Trioxidane Oct 30 '23 at 06:50
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    Let me explain @L.Dutch's close. You're asking us to provide a feasibility study (aka, most "efficient" or "probable" method for a question that's entirely fantastic. We're OK with that, but it means you need to supply a lot of details that you ignored. (a) Between which two points in space? (b) Since you're limited by the speed of light, why would you use a slower solution like fiber optics (light travels more slowly through F.O.)? (c) Why wouldn't you use beam transmission with repeaters, which would require less material? (d) Exactly what are you pushing through the cable? ... – JBH Oct 30 '23 at 07:00
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    ... (data rates, multiplexing, etc.) (e) Should we use currently available fiber optics and assume (e.g.) an unbreakable support wire is included with the cable? (f) How much time are you expecting for this? (Unlikely you can unspool wire at relativistic speeds.) (g) And then there's the questions asked by others in comments. Finally (h) What, really, are you asking for? "Feasible" doesn't actually mean much. Are you expecting us to design the ship? Design the cable? Design the spool? Etc.? Remember, we'll help build the world, but not tell the story. – JBH Oct 30 '23 at 07:03
  • @JBH his space ships are limited to 0.8C, his cables have FTL transmissions available. – Demigan Oct 30 '23 at 10:18
  • @L.Dutch the question has been edited according to the guidelines JBH layed out, as well as the site guidelines. – MegatheriumMegafauna Oct 30 '23 at 18:55
  • With your edit it looks like you answered your own question. I'm unsure what you're looking for us to do. Please remember that we're not a discussion or brainstorming site. Is there some specific issue with your approach that you'd like help with? Try asking about that directly. – sphennings Oct 30 '23 at 19:14
  • Planets and stars move. Line-of-sight between planets is guaranteed to contain other material and be subject to differing gravitational forces. Any cable run from one planet to another will snap long before the cable-laying vessel is able to even complete laying it. Space is not an ocean and planets are not islands. Unless they are in your world, in which case you necessarily reject so much science that this cable can be anything you want. (Which is fine! Sci-fi does not have to be realistic to be good.) – Tom Oct 30 '23 at 19:40
  • @Tom, to be fair didn't clarify it, but I assumed they be 200 au, or the equivalent therove, where it takes thousands to years to complete an orbit. It seems like that would nullify forces to the degree that the risk of snapping is gone – MegatheriumMegafauna Oct 30 '23 at 19:48
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    Your cable weighs 950 million tonnes per light year. This may be difficult to produce from the relevant materials and to accelerate. – John Dallman Oct 30 '23 at 20:46
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    With those numbers, each light year of cable has a mass of nearly a billion tons and a volume of 7.4 cubic kilometers. Getting this to 0.8c requires 5.7e28 J (and dissipating the same again to stop), which is about 1.5x the kinetic energy of the moon due to its orbit around Earth or 147.3 seconds of the entire power output of the sun or about 102 million years of current human primary energy consumption.

    May I suggest using a Krasnikov tube instead? You can even use the same superheavy atoms as the magic unobtanium that enables them to be built.

     – BenRW Oct 30 '23 at 21:26
  • In the future, please consider using our Sandbox. The question-modifying discussion that we're having in a now lengthy comment chain. After reviewing your edits, I agree with @sphennings that I no longer know what you're asking for. We're working on how to deal with review my idea questions, but please realize that they don't fit in Stack Exchange's model, which is expecting objective problems with the hope of a best answer. – JBH Oct 30 '23 at 21:34
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    From a review standpoint, @Tom's point about everything moving is important. The two stars and their planets are moving (I'm assuming the ends connect with inhabited planets...). That means your cable is "whipping" around like an egg beater and constantly needing to have additional length added to the cable. I'm also assuming that the orbital planes of the two systems are facing each other. If they're edge-on rather than plane-to-plane, then the cable will be pulled through one or both stars at least once each year. To use this idea, you need to build solar systems that can handle it. – JBH Oct 30 '23 at 21:39
  • I do like the basic nature of this question. I hope it gets edited for reopen. Author: questions can be reopened if you edit them. – SRM Oct 31 '23 at 04:31
  • Could this be reopened? It doesn't seem to need changes right now. I edited it two days ago with the intent of getting ready to reopen, but nothing has changed. – Jakav Nov 07 '23 at 23:05

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