What can kill an interstellar civilisation? [closed]

Question

It is a common trope that interstellar civilisations rise and fall, only to leave ruins behind. The origin of this idea seems to be a mismatch between empires and civilisations. Yet while (West)Rome fell, human civilisation hadn't disappeared from the Italian peninsula. And even when Homo Neanderthalis was replaced by Homo Sapiens, there still was a civilisation on Earth. Even an uninterrupted one.

We, as a species, are currently at a point where we could conceivably wipe ourselves out in a global war between superpowers. However this becomes ever more unlikely as a species spreads across the stars. Even though external hunters or internal wars could deal massive amounts of damage, nothing but an extremely systemetic approach to genocide will get everyone. I assume that there are groups of posthumans from this civilisation living as digital beings of nuclear reactors on icy rocks in the interstellar medium. This would only ever be a small group, only numbering in the billions, but if groups like them survive whatever kills 99.9999% of the civilisation, they will reestablish the civilisation in no time.

Even if they suffer from a maschine rebellion and every member of the original species is turned into biofuel, I don't care. I'm looking at this from the perspective of the Fermi-paradoxon, so it doesn't matter if a maschine-civilisation replaces a biological or diverse one. I only care about utter and total annihilation of anything that could be called advanced.

So what, apart from bathing the entire region of space a member of the civilisation could have reached with star powered gamma ray lasers for years, until every possible hiding hole has become uninhabitable, can kill an interstellar civilisation of a million worlds?

PS: This assume a no FTL hard-scifi universe.

Answer 1

Let's talk about your goal

Historically, a "civilization" was defined in a one-world-centric and highly political way: if the ruling government, its supporters and military, were completely demolished — that civilization was considered destroyed. The Myans, the Easter Island peoples, Cahokia, the Olmec... Oh, for the most part their genetic code can be found lurking around today, but our history is littered with civilizations that once thrived and influenced our world — and yet are gone, leaving only ruins.

This is actually a good starting point for an answer. You want an entire species wiped out — but I posit that there's little to no difference between an entire species and an entire ethnic group. I watched during the 1990s as television brought horrific news about the Bosnian genocide. We're not talking about one empire destroying another — we're talking about peoples in Yugoslavia who had intermarried, done business with one another for decades (and generations), and yet (unbeknownst to the world) were harboring a hate that makes whatever nonsense is going on in the U.S. today look like a schoolroom spat. The result is known today as ethnic cleansing, which is simply a smaller scale of what you're looking for, "special cleansing* (See Merriam-Webster definition 3b).

Whether the ethnicity is wiped off the face of the Earth or the species is wiped off the face of the galaxy — it's the same thing. What's left behind are the memories of a once vibrant people.

Having got that out of the way... what can do this on a Grand Scale?

1. Disease

We're living through that right now and the experience is directly relatable. All you need is a disease that's impossible/ultra-hard to eradicate, that has a very long incubation period (to meet the needs of a non-FTL universe), and is 80%-90% mortality rate and you have a believable condition to wipe out a civilization — especially if your disease does something gratefully rare among human-encountered diseases: make the carrier impotent. Whomever survives the disease can't propagate the species. The civilization is dead in one generation upon every planet it touches.

However, in a non-FTL universe where light can always deliver a message faster than the disease can travel, it becomes less believable that this method would work to completely wipe out a civilization. Not impossible, but unlikely. You could use the excuse that the disease killed quickly to keep the knowledge from spreading ... but then comms would go dark. It wouldn't take long for someone in those million worlds to figure out that something was spreading (would make for a really cool story, though).

2. War

This one is so obvious it's a trope all to itself — but having just discussed the Bosnian Genocide it's a very real and very viable solution to your problem. L. Ron Hubbard used a basic concept to set the stage for Psychlo ownership of Earth (and every other planet they were awarded): they gassed the planet.¹ If you have access to interplanetary resources, it's beyond conceivable that you can release enough poison into any atmosphere to bring down a civilization. Yes, there's always the possibility that someone is quick enough to get into their bunker and survive — but your universe has no FTL, which means its inhabitants are conditioned to thinking in terms of decades and centuries. Send a fleet of drones to gas a planet and keep it gassed for 100 years. Then send your colonists to take over the planet.²

To be fair, "war" could be described as the most viable delivery mechanism for whatever destroys the civilization (in which case it kinda doesn't belong on this list) because it could deliver the disease as easily as it could a poison gas. However, war is likely the only mechanism that can bring about complete destruction of a species: an intentional, focused pogrom that would survive the test of time to complete its task.

3. The collision of galaxies

A million planets without FTL travel means millions of years were used to achieve this marvelous pinnacle of civilization! But millions of years actually count in terms of astronomical events. That's enough time for galaxies to collide. While this could lead to a suspension-of-disbelief solution (you didn't restrict how short the destructive period had to be), it does run up against a fundamental axiom of good science fiction:

Space is big. Really big. You just won’t believe how vastly hugely mindbogglingly big it is. I mean you may think it’s a long way down the road to the chemist’s, but that’s just peanuts to space. (Douglas Adams, The Hitchhiker's Guide to the Galaxy)

Space is so big that it's conceivable for to galaxies to collide and not actually interrupt much of anything. Imagine the millions or billions of years required for the galaxies to actually collide and (let's say) pass through each other. "Whew!" some idiot might say, "that was a near miss!"

4. Something's wrong with the black hole...

If you need a natural phenomena that's not as slow as colliding galaxies but might not be as fast as war, consider something going wrong with the galaxy's central black hole. The easiest solution is an explosion. However, if you want ruins left behind for someone to find, this might not be the best solution.

Conclusion

As a conclusion, I'd like to return to the Bosnian Genocide. Yugoslavia had successfully repressed the hate for a while, but once it was allowed to resurface, it came back with a breath-taking mind-alteringly scary vengeance (if you, the reader, were not alive to experience the 90s... you really have no idea just how deep hate can run). And this is important.

I find a million-planet utopia unbelievable. Completely unbelievable. Unless, like Yugoslavia, it's suppressing the hate. It's nice to believe that humans can outgrow hate (and maybe someday we can), but I simply don't see it.³ But for the sake of your story, let's assume that for the time it took to get to a million planets your civilization was able to suppress all the hate...

5. Civil War

Nobody gives enough credit to the civilization-destroying capacity of civil war. Ancient civilizations were destroyed by it. Just because the U.S. survived its civil war doesn't mean it's improbable that others would be destroyed by it. Let the hate loose and and the doomsday machine tech that a million-world-spanning civilization will certainly have (we already have it here...) and you have the seeds to destroy any civilization no matter how large.

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_{¹ Battlefield Earth.}

_{² Kaiju are uber-cool, but a very inefficient way to take over a planet. I'm just sayin'.}

_{³ This won't be a popular comment, but it's relevant to my statement. The "judgement free" generation is the most judgmental generation that's ever existed. Rather than judging ethnicity, race, sexual orientation, etc., it's judging ideas and philosophies - and the only right answer is the "judgement free" generation's answer - and anyone who disagrees is hated.}

Answer 2

Killing civilizations vs killing species.

When Ancient (Western) Rome fell, the Dark Age begin. It's mean that many things (art, technologies, knowledge) was lost and quality of life became low. That's why we call it civilization fall and that's the difference between another empire fall: when British empire fall, people still enjoy same level of tech and live in general.

The end of interstellar civilization doesn't mean the end of civilization at all

In many apocalyptic scenarios people after global nuclear war still survive but population drops down and technologies degrade. When people lose access to rocket science, when flight become banned because of religion reasons - we say that human civilization is not space civilization anymore. Even if that civilization is great in biotech and achieved personal immortality.

Transformation to something more

It's well known trick in many novels: The Predecessors become somewhat more than space travelling civilization. Some say they transform to energy form, some that they go away to higher dimensions, another argue that they recognized the Universe then they bored from life and then just fall asleep.
In fact, they never was destroyed but in fact they are not in our Universe now. And yes, when we say 'they' we suppose there are many civilizations before.

Answer 3

Boredom, ennui, existential fatigue. A sudden breakthrough in logic overcoming its animal origins. A civilisation which ceases to care effectively ceases to exist as far as Fermi is concerned.

Put it another way: once a civilisation finally achieves the maturity to sit in a room and do nothing, it will never bother to make contact with any interstellar neighbours, having finally appreciated that there is no reason to do so.

The species involved would eventually die out, having no reason to breed.

Alternatively, you could invent video games, virtual reality or a religion promising an afterlife (or scientific proof of an afterlife) which would make IRL life not worth living.

Answer 4

1. If the civilisation hasn’t spread to that many star systems it might be sufficiently unlucky to encounter a nearby Supernova. That could destroy civilisations for many light years in the vicinity. But OK this might not be appropriate for a million worlds.

2. There might exist a very technologically advanced civilisation that has some reason to destroy other civilisations. Perhaps the new spreading civilisation is seen as an exponentially expanding pest to the more technologically advanced one and they might systematically root them out and destroy them utterly.

3. Perhaps at a certain point in the development of most civilisations they make a discovery of some sort or have an encounter of some sort that changes them so utterly that they are transformed to such an extent that they no longer count as a civilisation as we would recognise it.

Case in point Arthur C Clarke's A Childhoods end where Mankind face a very unexpected “end”, although this only applied to Earth there is no reason why something similar shouldn’t happen to a much larger civilisation. A variation on this might be a breakthrough to another dimension of space or of mind offering so much more that they willingly abandon our universe / dimension entirely.

Answer 5

Your number one goal is actually connectivity.

Think of it this way. If you wanted to destroy most of humanity without nuking the hole place, what would you do? An airborne highly contagious virus who is engineered to infect and kill humans. Also make it slow enough that it infects all of humanity before it starts killing people, I actually have a zombie idea similar to that, so before people can even go into bunkers they are hit.

So because air connects all humanity right now, and people were not warned. This virus would destroy humanity. And your added engineering would make it impossible for the immune system to fight it.

Now imagine this virus was only spread through intercourse, or even like the corona virus. It's still deadly. But if you stay at home or use complete protection, it just won't be able to infect you no matter how deadly it is.

So I'm leading to how in Mass Effect, and maybe a spoiler here so beware, how the evil robotic race of murderers actually designed and maintained something called mass relays.

Those relays enable faster than light speed travel and thus they connect the entire galaxy together. Now this had the added benefit of making sure that every single civilization that ever appears would relay on those mass relays making sure that their advancement path is in line with the Reapers. So later when the Reaper emerge from deep space, they just follow the dotted lines set by the mass relays and wipe out all advanced life forms from the face of the galaxy. Easy peasy

Now I think you just need to find a way to connect the entire civilization of that race together with a technology or a even magic, if you want. Bonus points if the same tech is used in spaceships and even on far outposts.

And this would make the premise, imo at least, far more interesting. As it would make the fall much much worse by the same tool they used for them advancement. Technology. I know it's not exactly unique, but it works.

Once you connected them then unleash the doom virus or killer robots or angry demons or whatever you want.

I would offer other more "magical" concept but I don't think they will fit. So I will just casually mention that they might be connected by "souls" and how dappling in this field caused their doom. But that's more of a science fantasy.

Anyway unless they are completely connected, you won't destroy them.

Answer 6

Dual purpose Von Neumann probes.

Imagine early in the space exploration, Earthlings launch a few Von Neumann probes. After some troubles, either technical or political on Earth, contact with the probes is lost and they are forgotten. Over the millennia it takes humanity to spread over the nearby star systems, there is no sign of the first pioneers. But they are around, they keep colonising world unsuitable for humans, extracting resources and growing both faster than humans and with a significant head start.

Then when the humans and the machines meet again, an unpleasant truth comes out. The probes weren't just for exploration and colonisation. They had a military purpose as well. Maybe they were supposed to be a buffer against other species. Maybe they were supposed to limit spread of other nations or people with the other skin colour. Or maybe they were just trained to "recognise" criminals on a racist data set. Whatever is the case, machines consider the new human civilisation their designated enemy. And there is no pleading with them, no talking it out, the machines won't stop till all the undesirables are exterminated.