How long does it take a Space Opera setting to recover after losing more than 80% of its collective civilization

Question

In a space opera soft sci-fi setting, the typical War in Heaven happens as two superpowers go to war and drag everyone into it. Over the course of about 30-ish years, planets were trashed, empires shattered and whole civilizations wiped out.

Refugees ran and hid wherever possible. Most were eventually found and killed. The ones that survived were the ones who landed on un-populated worlds and shut down any technology that could be used to find them.

Then at some point it stops. Anyone alive has no idea when it ended or how it ended. The War in Heaven just seems to have stopped. No winner and the super-powers are just as shattered as anyone else.

Question: How long does it take to rebuild the collective galactic civilization after most of the people are dead and most infrastructure is gone?

Clarifications:

Planets were damaged but usually not to the point of permanent uninhabitablity. Roughly on scale with destruction of dinosaurs. Limited evacuations of planets could be achieved, usually early on in the war when there were still military fleets to buy civilian ships time.

In traditional space opera logic, inhabitable planets are common, and most alien species can live happy in a least one Earth like biome environment.

FTL Faster then Light tech can be developed by any civilization. It does not require any unobtainium or element zero. Just a lot of R&D.

People Most survivors are aware that high tech civilization is possible. There is a likely to be people alive in most groups who have middle to advanced levels of technical knowledge. It can be assumed that the survivors typically want to re-build their civilizations because being a hunter gather sucks.

Edit the 1st. The survivors do have tech. They would have the ships they landed in. The ships may have been grounded, submerged( standard landing for some races) or half crashed, but much of the ships remain. They would have shut down every system and completely power off for decades, but some level of functionally can be restored.

Space ports and developed worlds were hit early and hard. Until advanced infrastructure can be re-build, the existing ships are irreplaceable.

Answer 1

Scenario 1: Not everything is lost
Going for a general answer, since there is a lot left up in the air, I think it will happen at different paces in different places. Like, there will be civilizations who's home planets were hit less hard than others, and so will retain more of their scientific and manufacturing infrastructure.
When the war ends and they are able to get their escape ships functioning enough to return home, then these will rise first, and will probably work at building new empires. Expanding from their home planets they'll begin to expand outward, looking for resources, subjugating or helping other groups as they find them.
Time frame to get most of the surviving civilizations back up to prewar levels? 100 - 300 years.

Edit:

This is kind of a general answer. Removing 80% of the galactic population is going to be really bad, but some civilizations will be hit harder than others, and the ones that lose the least population will be the ones with the most infrastructure left. Depending on who gets lucky, the time frame could be cut way down. A civilization that is organized and far thinking enough that they are able to minimize their losses is going to be well suited to bringing civilization back to the rest of the galaxy. There will still be civilizations that lag behind through bad luck, bad organization, or bad feelings causing new wars, but 100 years really isn't that long when it comes to rebuilding an entire planets infrastructure from rubble, let alone dozens/hundreds of planets.

So can it be sped up? Sure. Space opera soft science gives you a LOT of leeway here. Self replicating machines feeding on the scrap could be dropped from orbit and rebuild cities in years instead of decades, with an exponential growth curve.
A similar solution could be achieved from a biological point, with insects being used to create giant hives that survivors could live in and get a basic starting point to rebuild, combined with strategic drops of mini factories that can be used to manufacture larger factories.

Or since there is now a lot less population, you could even just pull all of the survivors from all of the undeveloped worlds and settle them on one lucky world, so that they can all work together and pool resources and talents. Once that world is built back up then all that manpower can be directed toward building up another world.

There are a number of ways where the rate of repair can be sped up very quickly.
At that point it's just up to you what kind of "technology akin to magic" you want to allow, and how quickly you want things to happen.

Scenario 2: We have these sticks and rocks
So every single civilization is reduced to a hunter gatherer level, and the highest form of technology in the galaxy is the pointy stick. There is no medical care, so life is hard and short. Anyone that knows about advanced technology dies out in a fairly short amount of time, and because everyone is learning how to survive without technology no one has time to actually build anything or do more than basic education. The very lucky find a way to write things down in a way that wont get destroyed in a decade, hoping that their descendants will know how to read.

Time passes and it's now several generations later. All knowledge of the past is oral tradition, with stories about how their ancestors used to navigate between the stars. These fade over time too.

Time frame to get a couple civilizations back up to a prewar level? 10,000 - 20,000 years.

Answer 2

These civilizations usually have advanced custodian A.I. systems doing all the "hard work" behind the scenes, if not outright running the entire show (like the Culture) so it would really depend on how trashed the A.I. "infrastructure" is. A single 3D fabbing plant with an A.I. that has all of the technical blueprints in memory and access to enough resources ought to be able to bootstrap the whole civilization within a few years, barring some sort of macguffin resource scarcity (like anti-matter mining) that limits expansion.

Otherwise if recovery is limited to what humans and aliens can do themselves, they are hosed for generations at best. Even if a number of super-engineers/scientists did survive, without access to all the techno-goodies they rely on their ability to rebuild is minimal at best. This is especially true if enough advanced hardware is lying around to enable scavengers and pirates to raid scientific enclaves, since the protective authority system is presumably gone.

A larger problem is if the advanced culture is vitally dependednt on a resource that lower tech can either not make (the aforementioned anti-matter) or is simply used up. Imagine if all the surface metals from all the planets were stripped away and sent into orbit. Anyone left on the surface is now stuck a below bronze age living since they can't even retrace their primitive steps, much less jump past it (unless there are enough wrecks and ruins around to allow for recycling materials).

Another consideration is will they want to rebuild? This is a common trope in sci-fi, since we modern humans can't imagine regressing to a more low tech society. But if advanced technology is what resulted in planet smashing wars, perhaps the survivors will instead take a more pastoral "low tech" approach with certain retained advancements for comfort. So you may have a broad movement to avoid high technology, which would hinder those who chose to pursue it.

Folks in ships in space have no choice, they either restart the advanced tech that keeps their ships going or they make planetfall and start tanning hides. If they at least have 3D printers and good libraries, it could take several decades for a relatively intact refugee society, with the appropriate specialists and technological resources, to get enough advanced technology manufacturing going to preserve at least a semblance of their previous culture. If individuals are particularly long lived and can personally usher in this new era, then very little may change as planets are repopulated and rebuilt. But if it is the descendants of the survivors doing this task, their priorities will be very different and there is no guarantee that recreating the old civilization will occur rather than something entirely new (and much more splintered) that just uses some of the same tech.

Answer 3

Technology Dependency

Depending at what point this people where reliant in technology the time tables change removing or adding years.

Giving an example with us right now, if a EMP bomb goes out and fries all our infraestructure and 80% of the population is eliminated. This means that as race we could go on. but as civilization there is going to be a looong way to recover (a few hundreds being luckly).

Remember that right now at our tech level most jobs and tools depend in that we have energy. Most people can't survive on his own to get food, shelter or cure themselfs, because as specie this traits and knowledges aren't a necesity for each member of our specie to thrive.

Now a few thousands years in the future, the dependency in tech would had cover alot more if no all our basic and middle needs. At a point that people would had to study in a history book about farming, husbandry and medicine. In the same level that we now study about hunting, gathering and trepanning.

Answer 4

For sufficiently space-operatic settings, how about hours or days from the decision?

Slightly less than 20% of the "civilization" are left. They come from multiple multi-planet polities, and presumably they all have experience and a "manual" for bootstrapping colony worlds to industrial level. Some things to consider:

• Did anybody think to hide away complete industrial startup kits, mothballed and fitted with a big red "On" button? Or will the survivors have to go scavenging to find an encyclopedia galactica in one ruined town, a nano-assembler in another, and a working geothermal power plant in yet a third?
• Or did people actively smash these things to avoid detection? That would put a shoe into my timescales. They may or may not have been right in their belief that a shutdown was necessary, but if they were wrong, some installation would have survived both the deliberate attacks and the EMCON efforts.

But if you do not want that, you can make it as long as you like. Imagine all the microchip production plants and their supporting industries being wiped out along with most microchips. We'd know that something like the Intel i9 is possible, but even if somebody had thought to print the design on paper, we'd spend a long time building transistor breadboards to control the production lines for much less sophisticated chips.

Answer 5

As soon as the hunting of the civilized stops, there are huge social and financial incentives for finding tech and using it. There will be cached knowledge bases in every prepper uncle's backyard. Video instruction works on Earth, today. Tech salvage will be a profession. Every government will prioritize bootstrapping.

Given FTL, there will be populations that just like to go as far out as possible, and when they get back, having skipped the war, they'll have trained personnel and wikis.

Lots of planets, so, plenty of space and resources. They're low on people, but 80% of billions is still a lot of folks.

I imagine the rebuild would look a lot like how Europe/Japan rebuilt after world War Two: 15-20 years, starting from rubble.

Answer 6

Unhelpful answer: I would expand on the answer above by saying anywhere from almost instantly (good thing we have these self replicating nano auto-factory that can build anything out of anything and this super artificial intelligence to tell it what to build now press the button)

to never (It's nice to have plans for a starships. To bad we don't have any fossil fuels to mine metals to make them)

More Helpful answer. You have a bunch of questions: How long did it take to create the Stellar empire in the first place from our technology? 100 years, 1000 years, 10 000 years? then how far did they fall back after the war.

Was someone planning for this? It world really speed things up. If someone was building colonies or colony ships just in case of an empire crushing emergency and putting them in out of the way places that would help (see the Foundation series)

How long will it take the colonies to look out once the war stops? Do the colonies have a faster than light radio receiver to know when the war is over. Or big telescopes that are 50 light years away to detect the end of the ware, Or are going to have nothing and they are just going to wait 1000 years before venturing back out to the stars and hope the war is over.

After they get the Empire going again do they all come they do they have to fight with other people starting their empires or do they all come together. Multiple communities that join together will help.

All that is a none answer which is sort of not the point. I am going to ballpark that if they if they have the will the knowledge and the resources it will take them 25% of the time it took originally to recreate the Empire. So assuming 1000 years to make it in the first place 250 years does not seem unreasonable.

Answer 7

If you have AIs that actually are the workhorses of civilization then recovery will be fast.

Otherwise, I do not expect recovery in any reasonable time frame.

1) By going to ground like that you get basically random samples of the population. Unless the groups that go to ground are very large (and how are they going to sustain themselves without high tech---and thus get stomped on??) you can basically be assured that said groups lack some critical professions necessary to build their tech. As their stuff wears out they can't replace it. They fall way down the tech ladder.

The groups on uninhabited worlds at least have the advantage that there are some easily-accessible minerals but they have no information as to where they are and likely don't have the information on how to refine them. At least they have a slim chance to rebuild.

The ones on inhabited worlds are much worse off--there are no easy to get minerals unless they can salvage them from the stuff already built out of them--but the cities have almost certainly been burned to a crisp at this point (inevitable when there is no longer anyone who puts out the fires that do start, it doesn't need enemy action) and thus there will be little to salvage. Most metals will have oxidized and washed away before anyone is looking.

2) Having samples of their tech doesn't give them the ability to build the tech. I'm writing this on a box with a pretty nice processor in it--but that is useless from a standpoint of building a replacement. Chip fabricators are huge operations that require huge infrastructure support, many millions of people per fabricator. That means small groups (and how did large groups survive??) can't build such tech. Even with resources they're knocked back to the discrete component era and they're going to stay there until the population builds enough.