I have tried to conceive the most realistic hard sci-fi human/alien conflict scenario without invoking FTL magic. By realistic, I mean “War is the wrong answer but we do it anyway”. Diplomacy has failed and “here we are.”

Two interstellar civilizations have had a long-standing trade relationship, exchanging goods with centuries of lead time using 2 billion metric ton cargo barges (like the Nostromo), and exchanging information in real time through a quantum entanglement communication link. (The fictional world observes the PT Symmetry theory of quantum mechanics which means the “no-signaling principle” is not needed or valid). As such the quantum device must be exactly in the center of the two receivers at each system. See diagram below:

Interstellar arrangement

The civilizations are 12 light years separated, which means the communications relay is 6 light years from either civilization. This took almost a century to set up (It's not easy). This is basically a follow-on to Rocheworld.

The original trade agreement was insufficient, things have changed for both sides, but both stubbornly refuse to give up their leverage. One side began with suppressing critical medical exchange information claiming “your shipments are not sufficient.” Thousands were permanently disabled or died as a result. The other responded with coercive tactics of their own. Your goal is to increase shipments from the other side due to a pending long-term climate crisis; they are refusing.

How can these trading societies have information warfare leverage over each-other?

Physical resources of value are centuries away from making any impact. Information warfare is the only tool available. An answer identifies ways to weaponize information assumed to be valuable to the other side. Please see full answer checklist below.



  • Fastest space flight capability is 0.15c (1 billion mph) for either civilization

  • Cargo ships travel 0.1c on autopilot like Nostromo from Alien. Living pilots carry the outbound barges to the edge of their system, set the barge loose on course, then transfer to an inbound ship to pilot it home. (Barges are closely synchronized so pilots are not flying home empty)

  • The long lead time makes these resource transactions impractical for a free market. No one could take possession of the purchased goods, so the resources are traded via this pipeline which requires government oversight as a commodity with the product portioned out at the end (much like the US National Pipeline).

  • Stasis (cryogenic sleep) is not possible. One civilization is human, the other is alien with a 120% lifespan, but 75% of human constitution.

  • The climates are completely incompatible, which is why the resources are so valuable. We have things they can't (like liquid water), and vice versa.

  • Like our Internet, the link is vital both for civilian exchange as well as propaganda and conflict. Just as Isis would never destroy the Internet (how would they recruit or stream beheadings??), neither civilization wants to damage the link. Everyone loses - even the activists - if the link is broken.

  • The communications data rate is roughly 100Gbps - enough for 20 simultaneous 3D HD multi-angle video conferences (holograms). This link also exchanges valuable culture, scientific, educational, and entertainment resources unique to each world.

  • The relay is automated. It has a nuclear reactor, some stabilizing ion propulsion drives, and quantum entanglement laser emitters. Maintenance signals go back and forth twice per year but take 6 years to be received, then another 6 to be responded to.

  • ALL other communication happens at the speed of light. It takes years to communicate with anything between the planet’s, such as a barge or the relay.

  • Corpses can not fight. There can be no ground war or AI invasion, as nothing more than a microbe can survive the trip and sentient AI has not been developed.

  • Radical factions lack the resources to launch an attack on the relay (which would take decades with the best technology), but local receiving equipment could be sabotaged by wealthy factions.

Good answers have:

  • A cyberwarfare weapon which can coerce your position.

  • A plot device with Whatever information resource you choose to be weaponized - it will magically become valuable (but try to be reasonable here). E.g., "Planet A has an aquatic species which doesn't exist on Planet B. This species responds to an illness just like a problem on Planet B; A contributes medical research." "A demands more shipments, and 'delays' research findings until they get compliance"

  • Any support agency needed to make the weapon effective. For example, a religion (jihad), political (activist group tampering with an election), science (global warming council), diplomatic (United Nations), civil (labor unions) can exist, to give the weapon a punch.

  • An assumption of real-world (not utopian / Star Trek) ideologies. Do not try to "avert the war" or "solve the crisis" in the answer. All wars are fundamentally stupid, but a "let's all get along" mentality is fantasy. Don't challenge the premise of the war in an answer, just win the battle... ACCEPTING that it risks losing the war.

  • No suicide devices. Killing everyone is not a win.

Example from Star Trek: Voyager:

In Prime Factors the Sikarians highly valued “stories” as a commodity. Withholding or tainting these could be used as leverage, sending low quality or quantity “stories” until the other side bends.

(Note: I don’t like many Star Trek plot devices, I find them campy)

I will read comments if this needs expanding, please let me know before you answer.

  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – Monty Wild
    Nov 1, 2019 at 1:40
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    $\begingroup$ Read that article 200 years from now, many physicists are getting paychecks to prove your "Quantum impossibility" comment wrong. Physics usually builds on past theories rather than simply disproving them--general relativity still reduces to Newtonian gravity in certain well-defined limits, for example. It's not impossible future physics will allow for FTL communication, but I don't think physicists would see this any more likely than future physics allowing for physical FTL travel (and unless relativity is totally wrong, both would imply the possibility of the future affecting the past). $\endgroup$
    – Hypnosifl
    Nov 1, 2019 at 2:46
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    $\begingroup$ ^ This. If you want to have FTL communication and not FTL travel, and don't want people with knowledge of quantum mechanics to roll their eyes, I recommend just calling your communication method an ansible. $\endgroup$ Nov 1, 2019 at 12:39

5 Answers 5


Cyber warfare does not work with a single access point.

Hacking relies on redteam having more possible attack vectors, than blueteam can control. Many individual systems don't have a single hole in their design, but are still vulnerable because they are just one part of a much, much bigger system. Attacking the other civilization across a single communications access point is like attacking a specific server with a single access point and the combined IT security forces of the World making sure nothing malicious gets through. Every piece of data that passes through that system will be so meticulously catalogued and sanitized that there would be no point.

Genocide Has some merit

The only thing you can really send another civilization that will still have value in 120 years is going to natural resources. Manufactured goods and cultural commodities will all be obsolete by the time they get where they are going; so, it seems safe to assume that we are talking natural resources here. Even though neither race is designed to colonize the other one's worlds, we can both build robots rugged enough to mine the commodities we need. And unlike trade partners, we don't need to send a robot slave army any of our valuable goods for them to send us what we need in return.

At 2 billion metric tons, these freighters can carry enough ordnance to depopulate and robotically colonize a planet many times over. Basically you send a freighter as usual, but this time you equip it to exterminate the other species with an orbital bombardment, and deploy its own mining infrastructure.

While this may sound like it will cause a significant disruption in supplies, the long lag times don't make a difference. Civilization-A could prepare a new freighter (or more accurately a Q-ship) specifically for this task while all other shipments continue to come and go. It may take many years to build but orders are still being fulfilled during this time. They then inform Civilization-B that they are adding a new freighter to the trade route and inform them of when to expect it. This way, nothing will cause Civilization-B to be alarmed by its approach. This means that by the time it arrives at its target, Civ-B is still sending supplies back to Civ-A.

The hand-off is also theoretically not a big deal. Cargo scanning is highly ineffective at these volumes because particle absorption very quickly becomes weaker than background radiation when going through any significant thickness of materials. As long as all your payload is behind enough legitimate cargo, not even gamma rays would penetrate deep enough to see through it; so, unless they do some serious hands on inspection, all they will see is a ship full of the same stuff they've been processing without variation for past several hundred years.

When your world killer destroys Civ-B, the clock for how long you need to go without supplies still has not started yet because this was always an "extra" freighter. None of your shipments actually risk being late until your NEXT freighter after it arrives. In preparation for this, you don't need to spend a lot of time on infrastructure as long as your mining robots come fully ready for deployment. You also don't need to waste much time on surveying since your previous freight missions could gather more than enough surveillance footage to be able to know where the aliens are getting their stuff from before you even disembark. This means you can start mining as soon as the aliens are all dead, and in theory, not miss a single shipment.

While this works in theory... it has a few strong caveats. One is that the resources needs to be something that would not be destroyed by the bombardment. If you need an organic resource or unstable isotope, there may be nothing left when you are done. The second problem is that this civilization needs to be mostly homeworld bound. If they have developed enough of colonies there may be a MAD response planed for such an attack or they may have enough remaining military assets to destroy your new mining operation.

Trade sanctions are more realistic.

The past few years have shown us that warfare is a bit of "dying art" in a world where MAD is becoming increasingly hard to avoid. Instead, international hostilities are becoming increasingly expressed in the form of tariffs, trade sanctions, trade regulations, and boycotts. Since trade is the primary interaction these people have, cutting off trade is the best way to harm one another.

The best way to beat the other would be to become the 1st race to come up with an effective substitute for this life blood resource. Think of American oil imports vs European oil imports. The US is farther from the middle east than Europe and has a much bigger demand for petrol, but pays a lot less for it because the US CAN meet its own oil needs or replace that need with other fuels such as natural gas much more easily than Europe could. Once you have an alternative resource (even if it is inferior), you could use this ability to survive without their thing to strong arm them into an asymmetric trade deal where they have to give you "fair" rates on their thing, but you can charge exploitative rates for yours.

  • $\begingroup$ Well, logistically, we have A: Load up army and weapons (5 years) B: Ship invasion force (t +125 years) C: Locate resources, develop mines/ manufacturing facilities, establish power infrastructure (you nuked what they had), and overtake their management of the alien quantum link (??)(t + 150 years ish) D: Load up freighter with hand-mined cargo and send it home (while not interrupting any shipments in progress) (t + 270 years). Your DIY shipment arrives to a dead planet. Margin for error: Zero. (But the robots live on forever!!!) $\endgroup$
    – Vogon Poet
    Oct 31, 2019 at 17:47
  • $\begingroup$ Also the barges are sent and received by pilots who fly out to the edge of the system, hook up, and fly them home. They don't magically fall into a geosynchronous orbit. These are realistic tug operators - not Hollywood. They scan the inventory before taking it home. Possibly they had trust issues / customs like real nations do? $\endgroup$
    – Vogon Poet
    Oct 31, 2019 at 17:58
  • $\begingroup$ @VogonPoet The logistics are not as bad as you think. You can load up and send out forces while shipments are still on going; so you can cut out all of your concerns there. You can use footage from previous freight missions to geosurvey the planet for where the resources are coming from, and you can send your freighter fully loaded with modular mining equipment so you don't need to spend much time on infrastructure to get started mining. Your only interruption would be however long it takes to mine your first shipment, and you can be absorbed with a little bit of rationing. $\endgroup$
    – Nosajimiki
    Oct 31, 2019 at 18:27
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    $\begingroup$ This is actually amazingly complicated. You are talking about overthrowing an alien civilization you have never visited in an uninhabitable environment while maintaining full infrastructure capabilities (which you know nothing at all about). These "spy missions" we sneak in on cargo ships - that take over a century to get there - then report back their reconnaissance. This is well beyond what I could imagine. How did the aliens let our probes just snoop around on their planet? We barely know where to locate crude oil on our own planet and somehow we're going to gamble on finding theirs? $\endgroup$
    – Vogon Poet
    Oct 31, 2019 at 18:53
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    $\begingroup$ There is no "invasion". You just nuke them out of existence, it only takes a few moments with propulsion systems that can hit .15C; so, there is no war, no occupation. Robots don't care if you irradiate the planet, or if it's hot or cold or has a poison atmosphere. As for planetary formations, there are only so many ways a planet can be formed, this story by necessity has to be is so far in the future that we'd already have had centuries to analyze data on thousands if not millions of exoplanets. $\endgroup$
    – Nosajimiki
    Oct 31, 2019 at 19:37

First mistake: your quantum entanglement whatever doesn't work. Other than the fact that quantum entanglement has nothing to do with what you are describing, there are no fixed points in space - everything is in constant motion compared to everything else. Whatever you tried to place in the middle of two "points" will immediately no longer be in the middle. You can theoretically program it to remain on a course that keeps it in the general vicinity of equidistance, but due to the unpredictability of even simple orbital systems work, this is simply not possible with any level of precision, and even a small rounding error will quickly propagate along the various gravitational influences.

But even if you somehow handwaved that, you have a second, equally major problem. What if something moves in the path between sender/receiver? How are you going to ensure information flow does not simply cease? Or account for the influence of gravitational/magnetic fields of stellar events such as supernovas or gamma ray bursts near the path of your data stream? Honestly this "explanation" is impossible. Just say they somehow found a method of instant communication and be done with it. After all, if you say that "Neither civilization is willing to damage the link, which was built under condition of “no ownership”", you probably aren't intending of making it a plot point to be used in the war, so why even have it? Furthermore, since your ships are travelling at speeds far lower than c, you are already ensuring information transfer hundreds of times faster than material transfer.

Second mistake: if your civilizations can build ships with computers capable of guiding them across interstellar distances with efficient precision break-and-stop as well as return mechanisms... they can and they should be programming them to do a hell of a lot more than that, including react to instructions sent after their departure (which, due to their "slow" travel speed, will reach them before the ships themselves reach their destination). So if civ A sends a shipment to civ B, and when the ship is halfway, civ A decides they don't want to trade any more, they should still be able to send a signal to their ship to stop and turn around or do any number of things, and unless the ship is already fairly close to civ B's systems, it will arrive in time.

Third mistake: "each society is mutually dependent on the unique resources from the other"... no. They have reached interstellar travel. They are not "dependent" on any other society because if they were, they would have died out long before that point. The other civilization might have stuff that is nice to have (although this premise is really pushing the story far away from any scientific hardness, since civilizations separated by lightyears are unlikely to have anything whatsoever in common in any context that makes sense). The whole premise of "reliance on interstellar trade" is weak to say the least.

Fourth mistake: you are assuming (physical) war is difficult for two such societies. It is not. It is literally as simple as using the ships they are already shooting at relativistic speeds at each other as weapons. It is made especially easier since the target is actually expecting these ships - they won't realize the attack is coming until it is way too late to stop it (if even possible at all). Under the conditions you describe, one civilization causing extinction-level events across the other's main planets / star systems will be easy. Then they can swoop in and coerce the survivors to do what they want. It literally becomes a question of who shoots first, and because of that, there will be very powerful incentive to shoot first.

The premise is so flawed that there is no reasonable answer to this question. Sure I could handwave the above points and describe two ultra-pacifictic civilizations, that somehow managed to integrate to such an extent to another civilization across the stellar void that not only are they basically treating them as equal (a pipe dream for people on the same Earth, it seems...) but they have become materially dependent on each other to survive. But if your civilizations are that reasonable to each other, then there is no plot in the first place. Simply put, their reaction to any crisis would be to sit down and resolve it together, because after all they have established a symbiotic relationship that is much tighter than any international treaty, and I'd argue stronger than most national units, on our present Earth. Such states would not go through with any hostile action against the other, physical or otherwise, unless they are threatened with extinction-level events - at which point they will skip the information war and go straight to relativistic rocks.

Disclaimer: yes, I know the above is not an answer. I am challenging the premise of the question because I feel any answer that fits into the stated premise will lead to a particularly weak plot (at least in terms of suspension of disbelief). Evidence I've seen in other questions indicate that frame challenge can be a valid answer. If I am mistaken, let me know.

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    $\begingroup$ Frame challenges are acceptable, but you could do with some better formatting to make your points clearer. It's a bit of a wall of text right now. $\endgroup$
    – Separatrix
    Oct 31, 2019 at 15:45
  • $\begingroup$ Interesting. Mistake 1: Why Are you putting my receivers in an orbit? Mistake 1.5: "Cyclic Redundancy Check." Mistake 2: Please explain in your answer how a pilotless barge turns around? Mistake 3: Civil rights are nice to have. Tea is nice to have. Even some might argue that oil is nice to have. And yet somehow we are still alive through periods of denial? AND we even war over them occasionally. Mistake 4: "Britain should have sent a bomb into their tea shipment." OK. Not sure that would get the desired results? $\endgroup$
    – Vogon Poet
    Oct 31, 2019 at 15:53
  • $\begingroup$ Have you tried your hand in the Palestine/Israel affair? "Simply put, their reaction to any crisis would be to sit down and resolve it together, because after all they have established a symbiotic relationship that is much tighter than any international treaty" I really wish reality worked like Star Trek, but this is not fantasy. $\endgroup$
    – Vogon Poet
    Oct 31, 2019 at 15:59
  • $\begingroup$ "Why Are you putting my receivers in an orbit?" -> I'm not putting anything in an orbit (although technically everything in space is orbiting something). I am just saying that everything is constantly moving, which is true. $\endgroup$
    – Nightmayre
    Oct 31, 2019 at 16:05
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    $\begingroup$ I just feel like you've written yourself in a corner by introducing a some flawed elements into your setting, which in turn lead to the issues that you are trying to solve with this question - which I believe is more useful than trying to formulate a cohesive answer based on a flawed premise. If you do not see it this way, I respect your view and will not try to convince you - however I believe that demanding I delete my answer is not a fair response. If you do not agree with my answer, such as it is, you can downvote or not accept it. $\endgroup$
    – Nightmayre
    Oct 31, 2019 at 16:38

Presumably any trade ship can be contacted by its homeworld via your quantum link.

If that’s the case you’re looking at a trade war. Side A can tell their ship’s automated systems to not sell/return home with/destroy the valuable resources if their demands are not met. They can do the same, but demand on both sides for some of the resources will lead to a struggle of wills to see who actually triumphs. Tariffs aren’t applicable here (you don’t have any local competition for the resources), but you can certainly threaten to withhold your goods and ‘haggle’ your opposition down.

On any given trade mission you’ll want to send a decent chunk of resources but only ever ‘sell’ what your government has agreed to in return for resources from the other side. You also can’t stop the trade missions just in case you suddenly need to deliver lots of resources in order to get lots of resources. There will be a constant tide of resources moving back and forth, even if they don’t all get sold immediately. You could even build a trade station to securely store your resources until the opposition agree to give you what you want.

Any physical attempt to take the resources will likely fail if the resources can be destroyed or spoiled at short notice, and even if they can’t you can heavily fortify your ships and space stations to give you time to permanently remove the resources from contention. If your militaries can breach and take over a ship or a station you get your typical space war, though actual conflict might be considered a bit MAD. If you attack their ships, they attack yours. It’s more likely you’ll get a tense political hostage situation for both sides as the politicians and traders fight with words and brinkmanship. But that seems a bit too much like diplomacy, so:

If your communications to/from your trade vessels are compromised somehow (perhaps passing through an opposing communication node) they you start looking at a crypto/cyber war. The communication system itself can’t be destroyed, but if your opponent controls part of the communications pathway they could edit your messages or flat out prevent them from reaching your ships.

The war is no longer about the actual resources, it’s about the processes, security, cryptography and even good old cyphers that your government uses to communicate intent to the ships. Your opposition could (in theory) break your encryption somehow, then stop the ‘Do not sell cargo for agreed price’ message, or even change it to ‘sell the cargo at half price’. You’ll need layer upon layer of protocol to protect your interests (all of which would need establishing before the ships left your ‘cyber-territory’). What should your ship systems do if they don’t receive regular updates? Does every update need encryption? If so where is the information on your encryption scheme stored? Do you want code words so your computers can spot deception? Is there a trade equivalent of alert levels?

At which point you’re in a two layer cold/trade war. Both dependent on each other economically and politically, and both battling furiously for control of the information and automated systems which, in turn, control the flow of resources. Spies (especially in the companies responsible for your ship control systems) and double agents abound. At all times the threat of an actual shooting war over the ships currently in transit or waiting to offload looms heavily over both sides.

That sounds a bit more like a war.

  • $\begingroup$ I’ll think about this but the ships are automated and unmanned. There are emergency override systems which can divert or abort for whatever reason, but the communication happens at the speed of light (years for a message) $\endgroup$
    – Vogon Poet
    Oct 31, 2019 at 11:54
  • $\begingroup$ @VogonPoet: Then the cyber warfare aspect is even more critical, your ships need to be automated to hell and back, and the trade aspect will just take a lot longer for disputes to be settled/occur (which works out great, since the news of a trade dispute/resolution will reach the ships at the same time as the people on the other side). All other points still apply (even the military ones. Drones can be so lethal nowadays). $\endgroup$
    – Joe Bloggs
    Oct 31, 2019 at 12:12
  • $\begingroup$ @Vogonpoet Edited our the references to captains. Exactly the same logic applies though. $\endgroup$
    – Joe Bloggs
    Oct 31, 2019 at 12:16
  • $\begingroup$ ..Ok Thank you! I put more details about the cargo trade but the cyberwar is the focus of the question. Supply chain disruption is not an effective weapon. $\endgroup$
    – Vogon Poet
    Oct 31, 2019 at 12:24
  • $\begingroup$ @Vogonpoet: The supply chain is never disrupted. What may fluctuate based on who can exert the most political pressure (or who can control the automated systems of their enemy best) is what proportion of the supplies held by Party A are actually delivered all the way to party B rather than sitting in the interstellar equivalent of the parking lot. As that is literally the only point of contact these two nations need to have its also the viable battleground. Everything else (cyber warfare included) is about exerting control on that battleground. $\endgroup$
    – Joe Bloggs
    Oct 31, 2019 at 20:07

War and coercion really don't seem effective in this situation. Or really the best answer that the human race as a whole could come up with.

A much more likely scenario is that the humans in your setting would offer help to avert whatever crisis is happening on the alien home world. Then in exchange for this help the aliens would pay the humans back with increased shipments in the future. Basically the humans would loan their help to the aliens, for future increases in shipment quantity (as the debt accrued).

This might be done via information transfer, think-tanks using the quantum communication to debate a solution, or long term relief ships.

A war on the other hand, in no way helps. If you pressure the other side to deliver shipments in the middle of a planetary crisis and the crisis isn't averted, there will be no more shipments. The world may disappear, or the global government may become firmly anti-human. So the shipment that you strongarm, may become the last shipment that you ever get. And then what?

In that case, those shipments better not have been important because they will be gone.

Extended Edit: War because we want war:

I think information warfare is really the only power that you can leverage against this distant ally. You would need to somehow infiltrate their infrastructure and hold it ransom. I think this would require some kind of sentient virus, because once your quantum communicator is turned off you stop being able to interact with their world altogether.

So you would need some kind of ransomware that turns off their entire worldwide infrastructure unless they comply with your demands. This might work in the short term, but for sure you will turn a friend into an enemy.

  • $\begingroup$ This utopian answer assumes civilizations make sensible decisions. I’m trying to be realistic. $\endgroup$
    – Vogon Poet
    Oct 31, 2019 at 11:55
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    $\begingroup$ Also worth noting that race A depend on race B for some goods. If race B is allowed to perish those goods disappear altogether. Better to keep them just hanging on but constantly requiring more of your goods in order to survive. $\endgroup$
    – Joe Bloggs
    Oct 31, 2019 at 12:20
  • $\begingroup$ “What do you mean the atmospheric scrubbers wear out after fifteen years? Oh well, you’d better keep buying them from us then!” $\endgroup$
    – Joe Bloggs
    Oct 31, 2019 at 12:20
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    $\begingroup$ @VogonPoet given the massive resource expenditures needed to even ship anything 12 light years away, continuing the trusting cooperation which started shipments in the first place is vastly more realistic than somehow starting a "war" with whom the only meaningful physical interactions has centuries of lag. You have no meaningful "war" interactions possible - the only interaction is basically just stopping the shipments, which can hardly be described as a "war". $\endgroup$ Oct 31, 2019 at 13:34
  • $\begingroup$ @Vogon Poet, I have heard some debate that war among the stars may never become a thing, because any species that becomes space fairing will have too much to risk and too little to gain from armed conflict of that scale. If they did give in to war at that technology level, they would gain nothing and quickly go extinct. I would have to find you the link, but I am also basing my answer on that knowledge. $\endgroup$ Oct 31, 2019 at 13:36

I don't really think that war is a plausible scenario in this case.

  1. It takes unbelievably vast amounts of energy to travel interstellar distances. According to my calculations, to get an aircraft carrier (100,000 ton) sized spaceship up to 0.1c, you need 12577118.204822823 terawatt hours of energy, not counting efficiency (probably below 50%, not a physicist), which is over 600 times more than the combined annual electricity usage of all of modern humanity. Also, you would then need that amount of energy again to decelerate. While this could be done using fusion/antimatter engines or giant solar powered lasers pushing on sailships, it would be extremely expensive and impractical. Solar systems are full of tons of resources, with metal available in asteroids, planets, and moons, water in comets, hydrogen from water or in gas giants, so transporting inorganics at those distances is sort of pointless. Organics stuff can be grown in greenhouses at a fraction of the cost (remember, over a thousand times the entire modern world's energy budget just to move an aircraft carrier sized ship). Anything you don't have here can be made by starlifting hydrogen from the star and synthesizing it into whatever you need via fusion (might take energy to fuse together, but will get you what you need). Also, at 0.1c, it would take over 120 years (ships would be traveling slower at beginning when accelerating and at end when decelerating) for a ship to make the trip. Unless one or both sides has some sort of magical unobtainium the other side lacks, interstellar trade is pointless, because everything could be sourced much more easily from one's own solar system at a tiny fraction of the cost.

  2. Any civilization capable of interstellar travel would be incomprehensible and perhaps even Godlike to our eyes. If you are comfortably capable of moving between the stars, your society probably has an economy billions of times larger than our modern one, at the very least. With much less technology than needed for interstellar travel, you would be able to fill your solar system with O'Neill cylinders and colonize every planet and moon available, so your planet of origin is no longer that important. You will be able to stop almost every threat that presents itself, and to evacuate from most other things. You talk about global warming as a possible threat, but to an interstellar civilization, it would be nothing. They could easily stop it either by placing numerous mirrors around their planet to block sunlight or by using fusion powered plants to such CO2 from the air directly. Such measures would be expensive and impractical for us, but if they have the energy to travel at relativistic speeds, these things would be trivial.

  3. Invasions are sort of impractical. Firstly, it would take decades for your ships to arrive, and when they did, they would be outdated (might be able to upgrade them en route based on new information, but not to latest level, probably) and outnumbered. The enemies would have had decades to build up defenses and your ships would be alone. With the resources available to them, they would probably be able to fill their Oort Clouds, Kuiper Belts, and planetary orbits with billions or trillions of self replicating autonomous defense platforms, while building reinforced bunkers on all their planets and placing guns on their planets larger than your ships. A better strategy would be to launch RKKVs or to burn your enemies with a Nicoll-Dyson beam, but while those might be very effective against a planetbound civilization, an interstellar one will have a presence across the system and would survive to retaliate.

  4. Cyberwarfare is probably impractical. You might be able to hack the relay, but what would that give you? Turning it off would hurt you as well. The relay will probably be routed through a data center or something on the other side. If you try to hack them, they could just pull the plug on the thing, given that all your information has to go through that one route.

  • $\begingroup$ It sounds like two civilizations which establish a trade route are incapable of war? Not even on ideological or political grounds, when resources become desperate? $\endgroup$
    – Vogon Poet
    Nov 6, 2019 at 22:13
  • $\begingroup$ Unless you have some sort of magical unobtanium or a extremely energy cheap FTL method, then no. If you rely on STL travel or one of the more plausible FTLs (Alcubierre style or wormhole, which are both energy intensive), both war and trade are pretty impractical. $\endgroup$ Nov 7, 2019 at 2:32
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    $\begingroup$ Trade, except perhaps of information, just doesn't make sense when there are just so many resources in a solar system. Even after you've completely mined out every planet, moon, and asteroid (which will be very hard, given that you can make a Dyson swarm at Mercury's orbit just with the materials from strip mining Mercury), you can still starlift hydrogen from the star and fuse it into whatever you want. It would be incredibly expensive, but still probably much cheaper and faster than trading. $\endgroup$ Nov 7, 2019 at 2:35
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    $\begingroup$ But you did give me a good idea. Political Activists years ago wanted to end the trade deal, and included a culture or spore of the organism in a shipment, which just arrived. Now B can become obsolete, the balance of power can shift. But the government has kept this secret from the public, saving it as a wild card for emergencies - like now. $\endgroup$
    – Vogon Poet
    Nov 7, 2019 at 13:37
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    $\begingroup$ Just one note: you don't touch the star in starlifting, but either shoot parts of it with giant lasers to make an eruption of hydrogen at that point or use large electromagnets or whatever to suck matter off or something like that. And you don't need super advanced technology to transmute elements, just particle accelerators, which we have now. It is extremely energy intensive to do so, but if you have interstellar craft, you have that energy. $\endgroup$ Nov 7, 2019 at 14:21

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