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Building my sci-fi setting deciding what's the in-universe answer to the Fermi-paradoxon, the reason why the universe still seems to be empty of K3 civilisations despite its age, has been quite important to me. I've settled on the notion that a species with a "functional setup" on a "suitable world" arises once per billion years per billion stars, but that a series of strong filters keeps everyone else from building galactic empires. Mankind is the only great civilisation in the next dozen superclusters. The enormous distances of space and time make seeing other civilisations in this early age of the universe impossible.

I have most of these filters pinned down, but the last filter should be that FTL is a suicide pact technology. This means that while FTL is theoretical possible in my universe, as soon as you use it to violate causality you are toast. I've encountered this idea elsewhere before, but the suicide part was either way too strong (the entire timeline where causality was violated stops existing) or way too weak (only the researchers and equipment working on the FTL project and all the data implying that there ever was such a project and such people stops existing).

I want a "causality correction" that wipes out a possibly already interstellar civilisation for good, but leaves ruins, artifacts and enigmatic data for my protagonists to find. I'd love it if the apocalypse would be spooky and enigmatic. However it should not wipe out a civilisation which has already spread to several thousand star systems, although it is acceptable if the event destroys hundreds of systems of such a civilisation. What exactly happens?

The setting is supposed to be sci-fi as hard as the narrative can take, but this is my little bit of Lovecraftian horror in the setting. Causality violation shall be a spectre that is enigmatic, unproveable and devastating.

The best answer will survive a meeting with Occam's razor, yet still give me the desired horror aspects.

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  • $\begingroup$ Do you have a specific flavor of FTL in your setting? $\endgroup$ – Frostfyre Dec 3 '19 at 15:37
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    $\begingroup$ At least one SF story has involved FTL being associated with the nearest star going super-nova. Though it was ambiguous whether it was because of something caused by the FTL drive, or if it was some aliens doing some cosmic censorship. $\endgroup$ – puppetsock Dec 3 '19 at 15:46
  • $\begingroup$ The Mass effect syndrome : a trans-dimensional race that comes and destroy upper civilizations once they reach a certain point would work ? $\endgroup$ – MakorDal Dec 3 '19 at 16:12
  • $\begingroup$ @Frostfyre Not really, the setting has several possible pathways to FTL. Slower-than-real-space Wormholes are common, sub-light warp drives seem to exist and some odd kind of probability manipulation might be within the capabilities of some superminds. In the end it doesn't matter how you violate causality, it matters that you did. $\endgroup$ – TheDyingOfLight Dec 3 '19 at 16:15
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    $\begingroup$ You could have a causality correction which requires a massive object to use more and more energy as it accelerates as more of the acceleration is devoted to increasing the mass of the object? That's one way the universe could correct causality. :) $\endgroup$ – Muuski Dec 3 '19 at 18:24

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The final great filter

Any civilisation that attempts FTL causality breaking will very quickly twig that it can be used for all sorts of shenanigans. This means the any race brave (or foolish) enough to make the attempt is destined to become a race that uses these causality bending effects for their own gain. To whit: they’ll be time travellers.

But no race is perfect, especially not when your descendants can come back in time and shout at you for doing it wrong, and so divisions will quickly appear. Factions will rise, extremists will pop into existence, and thanks to the fact that you have a race of time travellers you will suddenly find that every era of your race’s history is filled with war. Endless, unceasing, trans-temporal war. A race to the past will begin with each side trying to effect changes as far back in the history of their territory as they can, diverting almost all of their resources into temporal expansion instead of physical.

Then there will be no safety from the paradoxes. Waves of history will make and unmake whole armies. Resources will be made, lost and never have been all the way through your race’s history, spawning countless enigmatic artefacts, disparate histories and blasted battlegrounds. Any race in contact with these time travellers will either join in the war or, recognising the danger, cut off all contact and seal away the madness.

Of course, all things (even those across time) must come to an end, and your time war will eventually fizzle out as the complexities of keeping track of the endless variations of causal violations prevent any meaningful ideological conflict. The survivors might ban time travellers and try to rebuild, but it’s simply no longer possible.

Your race will eventually be utterly destroyed by alternate versions of itself, as terrible trans-temporal weapons unleashed centuries later land on communities barely getting by after agreeing a ceasefire. Occasional war machines or bands of half mad soldiers pop into existence, flung across spacetime by the vagaries of relativity, and slaughtering everything around them before getting sucked back into whatever tangle of probable futures and pasts spawned them. Any races that joined them will either be similarly annihilated or might have any contact with them erased from history.

Even these last spasms of spacetime will eventually cease, leaving nothing behind but ruins that (as far as any other race can tell) have always been there.

Now, a race sensible enough to not try exploit this (even if they do it once and then back the heck away) won’t ever trigger this apocalypse, and will pass the last great filter, and any race lucky enough to engage in it and survive can only do so by editing a deep fear of causal violations into its own history, thus leaving that race (narratively speaking) no different to any other.

It also has the advantage that you don’t have to really explain the nature of trans-temporal war for this explanation to work, as for the purposes of your story no race can engage in it without risking annihilating itself from history and leaving nothing but ruins behind.

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    $\begingroup$ Great Answer + 1. I would only add temporal shielding to your mix. At various meta-moments in the simultaneous all-time-spanning war, brilliant scientists repeatedly discover ways to shield themselves from paradoxes which now exist in their pasts. The shielding thus created allows artifacts and buildings to survive even after the entire histories which brought them into existence have been washed away. These islands of civilization find themselves cut off from the galaxy-spanning infrastructure that now never existed. They collapse, leaving abandoned ruins from the never-was. $\endgroup$ – Henry Taylor Dec 3 '19 at 17:35
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    $\begingroup$ @HenryTaylor: Amongst other methods of messing with relativity, such as ‘high tech death cult surviving by diving past a black hole’ and ‘this pristine building just bootstrapped itself into existence with all it’s occupants long dead’ $\endgroup$ – Joe Bloggs Dec 3 '19 at 17:41
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You exist in reality, where causality will not be violated

A reality with FTL, can not exist and it have never existed. Actually what happens is controversial pending on interpretation of quantum physics:

  • if multiverse actually exist, then all timelines with FTL simply end or even are retrospectively removed (for practical purposes think this way, regardless whether from technical perspective it's the right one, there is a survivorship bias - you are in timeline in which FTL has not been activated)
  • if there is not multiverse, then reality sort of follows a path when mild random quantum fluctuations are diverging towards a path in which causality would not be violated

But the second explanation does not sound scary enough, right? Right???

Not necessary, if you're near discovering FTL, then otherwise unlikely events would happen against it, so actually it would not run. Effectively reality would follow the most likely path in which FTL would not be launched. So a minor explosion? Depends. Does it stop whole research or merely delays it? Delays, so every week there should be another highly unlikely accident? What's a cumulative chance of such events? Insane low. Collapse of whole civilisation is more likely than that...

So the problem is not obvious. You don't see many exploding FTL drives prototypes. You just see dozens of civilisations that were on straight path to create in decade or century a FTL research project... and somehow diverged to a path when this did not happen. There is no fixed pattern of destruction - war, economic crisis, computer virus, weird culture shift against technology, virtual reality decadence, other unrelated experiment going wrong. You only observe the scenarios, in which FTL was not to be constructed. The only clear and odd pattern for an observer, is the more advanced and technological minded civilisation, the more likely something odd would go terribly wrong.

Bonus: the pattern that would happen should be also noticed by other characters, but could be interpreted as usual decadence and moral decay. For bonus horror points a one should notice that his civilisation is on verge of a total war, so is trying to develop a FTL to bail out himself and family.

Bonus2: as you are going to be need all unlikely scenarios how civilisation missed their chance for ultra high tech, I can drop a few less used ideas (another grey goo, nuclear war and global warming... [yawn]):

  • Read Ted Kaczynski "Industrial Society and Its Future", there are quite a few original thoughts, why people would feel badly in seemingly nice high tech society.
  • We're right now running a serious dysgenic project as high IQ individuals are having less kids plus thanks to safety net and modern medicine usually deleterious mutations are not being eliminated. Potentially such mechanism could make a civilisation stuck in situation, where it's no longer producing enough geniuses to actually advance civilisation (to be able to produce designer babies), while simultaneously it would be good enough to maintain otherwise quite high tech civilisation
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Here's a possibility that doesn't quite meet your criteria, but is somewhat closer to actual, known physics: Invoke the Chronology Protection Conjecture.

In short, this conjecture asserts that whenever general relativity would appear to make time travel possible, quantum mechanics will step in one way or another to thwart it. It's an explanation for why we haven't seen any time travelers yet.

Say you invent a device capable of creating stable wormholes. Cool. It works fine, until you get the two mouths of one wormhole close enough to each other that you can use it to travel backward through time. Suddenly, some virtual particle mumbo-jumbo happens, and both mouths collapse into inescapable black holes. If you build an Alcubierre drive, as soon as you turn it on, the warp bubble will irradiate its interior with Hawking radiation so intense as to incinerate both your starship and the drive itself. And heaven forbid you try to use a jump drive to save on propellent in getting spacecraft into orbit: The black hole that collapses into will be big enough to consume the entire planet over the next century or two.

Maybe it is possible to build an FTL drive that actually works and doesn't destroy itself, but any given civilization is far, far more likely to destroy itself by accidentally feeding their home planet to a black hole than to get it right on the first try. And those that do manage to survive their first FTL failure are unlikely to try it again.

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My approach to punishing violations of causality is a phenomenon that I have dubbed 'Paradox Reverberations'.

Let us suppose that we have a time traveller who goes back in time. Should this time traveller do something that invalidates the course of events that led to the time traveling event, a reverberation begins... time flows onwards, the time traveling event does not take place, and since no time travel event occurred, reality resets at the instant the paradox occurred, restoring the preconditions for the time traveling event... or does it?

The thing is, quantum fluctuations will ensure that the unfolding of the sequence of events between the paradox-causing event and the time traveling event will not play out deterministically... but the differences may not be sufficient to prevent the time traveling event during any particular iteration.

This leads to alternating temporal paradox / normal chronology sequences, though each iteration will play out differently due to quantum uncertainty, until a chance sequence of events occurs that breaks the loop, preventing the paradox from occurring.

From the point of view of the time traveller, they can only be aware of the latest iteration of the paradox reverberation loop, so to them it would appear that some event occurs in the non-paradox part of the loop that prevents them from going back in time, or having gone back in time, prevents them from causing the paradox.

A paradox reverberation, once broken, would appear to have the appearance of an incredibly unlikely sequence of events that just happens to lead to its causative event not occuring in the non-paradox part of the loop. This may be highly localised and specific, resulting in no more than the time traveller simply deciding not to perform the actions that cause the paradox, or it may be entirely more catastrophic, rendering the time-travelling event impossible.

Since the paradox reverberation will continue to cycle until a chance sequence of events invalidates its preconditions, potentially billions of times if necessary, it would have the effect of being an infinite improbability engine: whatever sequence of events that is necessary to prevent the paradox will occur, no matter how improbable it seems.

Should use of technology with the potential to cause paradoxes come into common use before a paradox reverberation is initiated, it becomes quite likely that, should the first reverberation catastrophe not invalidate time travel, the first reverberation catastrophe could trigger further time traveling in an attempt to prevent the initial catastrophe, each additional event either invalidating the initial paradox reverberation's exit conditions, reinstating that reverberation, or it may cause an entirely new paradox reverberation.

In such a case, the only exit point becomes a highly improbable sequence of events that prevents all paradoxes. Something sufficient to prevent any further time-travel paradoxes in a setting with many time travel machines, especially if they pre-exist the earliest time-point of the oldest reverberation loop, is likely to be both highly improbable and calamitous.

What would be left after the exit event of a paradox reverberation involving multiple time-travelling devices? That may be highly variable... but it is highly likely to have the appearance of a sequence of highly unlikely and calamitous events.

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Time Traveling Super-Viruses

I think a mundane answer might be interesting to consider: any interstellar civilization developed to a certain point will have to have an extremely high level of medical know-how. With this know-how, they will have eliminated some of the pathological organisms (eg. viruses, bacteria, etc.) that are less adaptable, with only the most adaptable and deadly viruses surviving until that age.

Using the FTL technology, the civilization will probably attempt some experiments with time travel, but any time traveler, no matter how well disinfected, risks carrying pathological organisms back in time. When these highly advanced organisms are taken back, they will adapt and advance even more quickly than before, given a lack of medical know-how in treating them. The further back the traveler goes, the more time the organisms have to evolve even further, and the less prepared the civilization will be to welcome it. This is, of course, presuming we operate on the principles of creating alternate timelines when we introduce causality errors and paradoxes when violating causality.

With each loop back, it will be like a positive feedback loop for the pathological organisms, as each loop back will make the organisms progressively stronger (we bring the N-strength organism back, causing it to be N+1 in the future, when we go back again, it becomes N+2, etc.). If too many time travels are done, the loop will have made the pathological organisms deadly enough that the living creatures on the entire planet will be wiped out, leaving only the artifacts behind.

This should fit the bill of a gloomy/creepy planet, and a dangerous one too; there would be a conspicuous lack of almost any living creatures left on the planet, with a select few more adaptable ones harbouring incredibly deadly viruses. Any dormant super-viruses left on the surfaces of the artifacts could also pose an extreme risk, threatening to wipe out entire civilizations should they be accidentally brought back with the artifacts.

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Every time a FTL spaceship violates causality and returns the systems it departed this sets up a temporal barrier. Every subsequent FTL vessel that violates causality, these will come newly spawned timelines created by the first and any subsequent causality violations.

Effectively every FTL spaceship will return to the same point in time. They are effectively cut off from returring to the futures from whence they came. Each spaceship will be equipped with technology and weapons vastly superior to those of the system from which they originally departed. Naturally they will want to conquer this system or planet.

The problem is all the FTL spaceships will have to fight all the other FTL spaceships. The most probable outcome will be total annihilation for the civilization that dispatched the FTL spaceships in the first places (that's because multiple converging timelines will be involved). Only rubble will remain.

Should one or two FTL spaceships survive the destruction of their native civilizations, they will be doomed to ply the spaceways like some galactic version of the Flying Dutchman. Preying on lesser civilizations like the Lovecraftian eldritch abominations of old.

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You could perhaps have FTL trigger vacuum decay. The FTL drive could somehow trigger a vacuum decay, which would more or less ensure everything around the source would be destroyed at the speed of light. Nothing of the civilization in question would remain which fits great with the apocalypse part of your question.

The problem with this apocalypse scenario however is that (at least to my understanding) it wouldn't just stop there. It would keep expanding until everything is destroyed. One possible workaround would be that if the expansion rate of the universe is faster than the speed of light, the false vacuum bubble wouldn't reach other galaxies. This only leaves the fact that you want some sort of ruin or artifact for the protagonist to find. The only thing that really pops into mind is that if the person testing FTL traveled far enough and reached another galaxy, he could (in theory) be the only survivor (albeit briefly) of that civilization. If/When they die in the middle of nowhere it could be a potential artifact for the protagonist to find.

I'll admit I don't really know enough about the subject to tell you how exactly an ftl drive could cause vacuum decay, but it seemed apocalyptic enough to post as an answer to this.

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Gradual degradation of spacetime collapsing into oblivion

FTL travel has exactly the expected causality breaking affects, except it just happens incredibly slowly at first and builds exponentially as the technology is continuously used.

Say it initially begins at the site of the first FTL launch, but nobody actually notices aside from a random, nearly undetectable spike of energy, or interference, or what have you. This continues to happen with each new FTL launch only the affect is doubling in strength each time, while simultaneously doing the same at the previous launch sites. This becomes noticed, but not quite as fast the technology takes hold as a staple of the civilization.

Ultimately the gradual collapse becomes powerful enough to actually disintegrate matter and seems to stem from the civilizations central planets where the FTL experiments originally began. Unaware of the cause, individuals attempt to escape the expanding wave of destruction by the fastest means they know, FTL, and end up compounding the problem so that everything the civilization has built simply cascades into dust.

You could almost think of it as a massive pain of glass, that you throw a small rock at that only scratches it. Then you throw another rock that also only scratches it, but it causes the first scratch to grow deeper. Then a third rock deepens the second scratch and the first becomes a crack. This affect continues with each rock, expanding more and more cracks into a larger and larger web of fractured pieces. Eventually the glass can't resists anymore and simply shatters.

As for having ruins, you could come up with some explanation, like remains of what existed before FTL travel existed, or explain how some materials were shielded in some way, or simply had the structure and/or luck to resist the destruction like the Nagasaki Arch surviving the atomic bomb.

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FTL has a slow, cumulative effect on the very fabric of hyperspace

We assume that FTL requires to jump into a different section of space-time (let's call it hyperspace) and back to travel FTL.

When the first civilization were born (probably around 10-12 bilions of years ago), the hyperspace was very easy to access.
Developing and using an FTL drive was relatively cheap and easy, but what nobody could foresee was that every FTL jump slightly damaged the structure of hyperspace (both on a universal scale and - on a bigger amount - on a local scale).
After a cumulative of some tens of thousands of jumps, these damages cumulated up to a threshold level, after which the hyperspace sudden became harder to access, requiring much higher energies than before. This basically brought these civilizations into a cul-de-sac, since at some point the engines of their starships suddenly could no more access the hyperspace and travel FTL. As a consequence, all of their supply routes became unexpectedly impraticable, leaving isolated, non-self-sustaining colonies and decadent single-planet civilizations.
They could no way discover new technologies to access hyperspace again, so they slowly decayed into barbarism on their home planet and got extinct on the other colonized planets, leaving only ruins.

And through the eons, every following civilization had to invest increasingly more energies and resources in developing and operating FTL technologies, every time without realizing the long term effects of their use of hyperspace.

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Ripped.

Spacetime in the area where FTL was used gets ripped and stays ripped. Causality becomes unpredictable. The laws of physics can change, and are mutable. Other adjacent timelines are affected by this deep tear and things can bleed through - or come through the gaps. Strange eddies produced by this disturbance can create new things or allow the emergence of things that once were. Property values plummet.

Solemn angels arrive to darn up the edges of the the tear with reality anchors and prevent its propagation. These anchors stabilize things enough that the vicinity of an anchor can be used as a base from which to explore The Torn.

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