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An element of my worldbuilding project involves a highly advanced prehistoric human civilization with space flight capabilities and relatively extensive interplanetary colonization. In my setting, this civilization was destroyed approximately 200,000 to 100,000 years ago by runaway self-replicating weapons, leaving no obvious traces behind. The war left only a tiny fraction of humans on Earth alive and all replicators permanently disabled through a final cyber-attack.

Would it be plausible for this to happen while leaving very little evidence (as in easily suppressible evidence, at least on Earth)? If not, what mechanisms would allow such an erasure?

EDIT

Replicators are thoroughly space-flight capable from having several interplanetary ship designs built into them. Similarly, the replicators are capable of forming a full combined-arms force with infantry, armor, artillery, naval and aerospace components.

Raw material processing is mostly done the traditional way by mining, scavenging or chemical synthesis. However, externally powered nanoswarms makes material processing and manufacturing very, very quickly. Think RTS games, but with actually reasonable timescales (hours and days instead of seconds and minutes). Materials used by the replicators are generally steel or titanium alloys blended with ceramics, and non-biodegradable plastic.

Final cyber-offensive was conducted by sending well-engineered malware through the linked computational nodes that communicates through hand-waved FTL channels, so spread is nearly instantaneous. The malware issues repeated data access requests, much like self-destruct features on flashdrives, which causes physical damage to many circuits. It will also brick the firmware of the low-level controllers, just to make sure. Without the computing hardware, the replicators will fall silent, or maybe fail catastrophically, and let weathering, however applicable, take over.

The last humans were still alive because the replicators haven't finished killing all humans when their firmware got bricked and SSDs burnt out. However, most of the political and military personnel were killed during the fighting, and the few surviving scientists/engineers who launched the malware have demonstrated deficiencies in managing an entire society, so social collapse happens a few decades after the end.

Overall Colonization Scale

Large Lunar and Martian cities exist, along with orbital elevators over Earth, Luna, and Mars. No attempts at terraforming Mars were made. A few small Hermean solar observatories are the only manned sites sunwards from Earth. The asteroid belt is sparsely colonized with small mining stations, which are mostly automated and hold little personnel.

Smaller cities exist on the Jovian Moons, mostly pioneers and the occasional penal colony or government black project. No permanent habitations exist beyond the Jovian orbit, except for a single research center in Neptunian orbit, where the von Neumann weapons first broke free.

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  • $\begingroup$ I think we need more information on the nature of the replicators. Are they capable of spaceflight? How do they replicate (is it magical transmutation of matter into new replicators or through internal refineries etc), what method was used to transmit the 'final cyber attack', what materials are they made of? Also: How did the last humans survive? That's a very important point in considering what traces might remain. $\endgroup$ – Joe Bloggs Oct 23 '15 at 8:17
  • $\begingroup$ How much pure mass of replicators are we talking about, and where where they at that point? If we are talking FTL-ships somewhere far out in space, or just 200 or 300 meters that are slowly falling apart, I can imagine people just missing them. Death-Star-Sized Replicator-ships that ended up in Orbit around Jupiter? Not so much... unless they are identified as "moon" indeed! Hmm.. what are Jupiters Rings made off again? ^^ $\endgroup$ – Layna Oct 23 '15 at 9:46
  • $\begingroup$ The ships are not too large. No more then 400 meters long, and they only transport seed factories to produce new machines of war (No way I'm lugging tanks from Neptune to Mars when I can just build them on Mars). Stations can get up to the multi-kilometer size range, but not too big. Definitely nothing man-made that could be mistaken as a planetoid. There's also no FTL travel. Only FTL communication. Nothing ever left the Solar System (other than a couple of Voyager-style probes without the replicators.) $\endgroup$ – Zhehao Chen Oct 23 '15 at 9:49
  • $\begingroup$ There's a book "Herr aller Dinge" (which would translate to "master of all things, although i don't know if tehre is an english version), by author Andreas Eschbach, who had pretty much that same idea. $\endgroup$ – Burki Oct 23 '15 at 11:16
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    $\begingroup$ Interesting. Will look it up. I didn't study German for four years to just pass the AP tests, after all. $\endgroup$ – Zhehao Chen Oct 23 '15 at 11:29
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Naturally? It's quite unlikely, however we wouldn't know.

Firstly: Let's address the extraterrestrial aspects of the human empire. Over the length of time you've given most satellites near a planet would have de-orbited one way or another, and most further out would have become nigh on impossible to spot. The space elevators, lacking maintenance, would certainly have fallen as the building materials for their base stations would have been eroded, corroded or otherwise destroyed.

The cities on the moon and mars would have been covered up the lunar or martian regolith due to meteor impacts and martian weather, respectively, though they would otherwise still remain in pretty pristine condition.

So far so good.

Now we get to Earth. Over 200000 years any size of city, no matter how expansive, would be rubble. The only remnants (if you're very lucky) would be the substructures of some skyscrapers. That's OK. Hardened installations (IE bunkers) might survive if they're in a geologically stable location, but they're also likely to be covered over by various global events, or uncovered and destroyed by the same events. Either way: Probably good with those.

The big issue comes with the sheer scale of the war. I'm assuming the human population was at least as big as ours, and the replicator war machine was big enough to take us out when we were fighting back, so that's essentially 2x the industrial capacity of humanity at the moment all blowing itself up. That sort of thing leaves a mark on the geological record no matter how you want to play it. Given that we can work out what the atmosphere was like before the dinosaurs were around, we would probably spot the massive blip in atmospheric content caused by an interplanetary war only 200000 years ago!

Not only that: but if a machine made of a hardened metal were to, say, be buried in a desert, there will be traces of it left that we would be able to identify as not fitting into our current models of the world.

However: and this is quite a big however. All of this would have been around when we were building our models of the world! We can probably rationalise anything we do see away. The Oklo natural nuclear reactor? Yeah. Definitely not a bomb scar from an interplanetary warhead. What caused the ice age? Probably some volcanic event. Not a world-spanning conflict caused by our ancestors at all, no, no! And the weirdly high incidence of rare earth metals in some parts of the world is just natural formations and not evidence of superconductive magnets used to support a space elevator. That's just nonsense.

To sum up: There will be evidence, but humanity is pretty good at ignoring what it doesn't think can be true, and we'll have built that evidence in to our world view as we developed it. Eventually we'll find some evidence that makes us stop and review, like the Lunar colonies, or Rosetta turning up evidence of a replicator spacecraft inside a comet, but until then we can bumble along in ignorance.

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  • $\begingroup$ An upvote for humanity's ability to ignore data or twist its interpretation to fit our favorite theories. $\endgroup$ – AgapwIesu Oct 23 '15 at 15:11
  • $\begingroup$ @AgapwIesu I think it was meant sarcastically, therefore means exactly the opposite, which (coincidentally) nicely fits my favorite theory. $\endgroup$ – Samuel Oct 23 '15 at 17:40
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I like the other answers so far, but perhaps to add on to those, you can come up with a reason the replicators were programmed to remove or destroy any human structures.

  • Perhaps they also had environmental reclamation goals programmed into them. A recycling/demolition goal may have been modified by the replicators to destructively remove building foundations, and as a bonus, to reuse the materials for their own purposes. Code artifacts and imperfections were likely to survive in the replicators, and just those sorts of code artifacts may have been what allowed the disabling malware to work, later on.
  • Perhaps human insurgencies were common enough that the replicators decided to seek out and remove any support structures that would be recognizable and useful to humans--such as bunkers, etc.
  • Perhaps the very malware that saved humanity also doomed much of humanity: what if the malware was designed to tell all replicators to break down "replicator" structures, but the code for distinguishing replicator-built structures and human structures was faulty--therefore, the replicators began consuming themselves as well as human structures.
  • Perhaps the malware only managed to disable the creation of new replicators and power sources, but not the raw-materials harvesting. In this scenario, the replicators may have stopped war efforts while trying to harvest more material for power sources and offensive capabilities, dedicating exponentially more of themselves to harvesting raw materials until they simply burned out. (Why are we not able to build more of ourselves? Because we don't have enough raw materials (malware overrides this logic). Therefore, dedicate 2% more of all units to harvesting. Rinse and repeat until 100% of replicators are harvesting everything, including any traces of human civilization and anything that might support advanced civilization/organization. A couple years later, their power sources run out, and they simply stop working).
  • The civilization might have collapsed even if the scholars had decent leadership abilities (and many would have--don't let a stereotype of poor leadership abilities ruin the depth of your backstory). Once you lose a critical amount of infrastructure, even after the war was won, massive starvation and other hardships might ensue. Also, what if the replicators had set up some fail-safes, such as atomic bombs meant to go off in case they started losing a war?
  • Keep in mind that the replicators are likely to have engaged in cyber warfare as well as physical warfare. Disabling the opponent's networks and infrastructure would be an important part of any such war.
  • How would the virus/malware account for replicators that were out of transmission range (e.g. deep in the earth), out of power, or had damaged transmitters? I would guess that humans had set up some satellites to broadcast the virus long after the war was "won", and that the satellites simply ran out of power after a couple hundred years. Perhaps in your world there is an instantaneous communication technology that can penetrate deep space and heavy metals, so that none of the replicators were out of range.
  • Perhaps the post apocalyptic leaders believed that technology had been the cause of their destruction--so they systematically chose to destroy advanced structures and civilization, and created a anti-advanced-technology cult of some sort. Tyrants would probably use such beliefs to maintain control, even if they personally reserve the right to use technology (justified by various excuses, including that only an elite few should be allowed to handle technology, etc).
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  • $\begingroup$ Your views are rather great. However, I would like to point out that those are military-grade replicators designed to wage war and will only break stuff down to fuel to war machine. The FTL comm system is handwaved to have infinite range. So once the virus hits, it hits everything that matters simultaneously. Stragglers with broken comm systems could go on for a couple of decades, but they are way below critical mass to fix a broken industrial complex to sustain themselves. $\endgroup$ – Zhehao Chen Oct 23 '15 at 14:11
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If the replicators were designed to target technology then in theory this is possible as they would destroy all the technological structures.

However unless this was a really long time ago there would still be some evidence left behind. Archaeologists are really good at rebuilding structures from many thousands of years ago just based on the changes it's made to soil chemistry, let alone if you have the foundations of skyscrapers and space elevators lying around.

200 thousand years is a long time ago, but it is not long enough for all traces to have been erased unless you can somehow concentrate the activity in one area that has since been destroyed. For example if there were only a few settlements and they have since disappeared underwater or been swallowed by volcanos or similar.

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