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Then the survivors restarted from almost nothing (building most of the crappy - but suspiciosly similar to very modern architecture - stone based stuff we keep finding), remember, not every person in an advacent civilization knows how to produce advanced technology. i.e. we have TSMC at hand, but I suspect there are other similar examples.
Some of that tech could easily create a global imbalance in society leading the civilization to a collapse eventually.
The tech could already be around us, just evolving slowly being adopted by the global society, just as slow as most of the population needs to be it, to not collapse in a sudden explosion of changes subverting or erasing rules and status quo built through centuries. As we know from previous collapses, the rule of law and the need of societal stability to develop advanced tecnology is not easily achieved, it could take another 500-1000 years to recover from a neo-collapse ala Roman empire or Han dinasty.
Scify gives plenty of examples of revolutionary tech that if becames suddenly broadly adopted, it could tear apart most of the global societies structures.
It could be nothing like interstellar travel or wormholes, nor time travel, or else. I could be something more subtle, not inmediately possible to be perceived by homo-sapiens as a "danger Will Robinson" circumstance.
It could start simply like an - AI powered - app giving us 90% of certainty about if someone is lying in an audio sent in Whatsapp. Then you could start reviewing all recorded messages from politians since the 1930's to check if they were telling the truth or just plainly lying to their populations.
And then we could start just double-checking for lies everything said aloud everywhere.
Chaos
It could not be inmediately clear that a succesful peaceful first contact could lead to disastrous consequences for us.
We should pay attention to every human instint in existence, but controlling the natural agressive response to the unknown.
But the survival instintc should not be disregarded inmediately and automatically replaced with "moderation". The aftermath of an error could be beyond a point of no return for us as species.
And if those nuke produced some currently undiscovered very far (light years), detectable energy / event? Yeah, most close neighbors could have already spotted us in 1950.
somehow, west democracies consider anatema to admit that totalitarin regimes can effectively manage entire countries.
And China has enough success that it cannot be dismissed as luck, or anything else than (totalitarian) government plans which achieved the same good outcomes as western democracies have
just saying
Some group (of humans), have been throwing millions subtlely trying to direct the population to believe in aliens secretly visiting Earth, but only thoroughly surveying the US military?
but there is no way in hell a chance to get a single good reel of few seconds of a clearly visible UFO,
this of course adds a lot for the "is not aliens" team
Many are discussing the alien and the secret tech points of view.
I'll talk about the "desinformation campaign", very popular in "non believers" in UFOs, nor aliens, nor secret tech.
I actually don't see the current need of keeping the 50s UFO mith alive, but it still could be a giant desinformation campaign, targetting some entities in the world, it could be a reaaaally long game of sorts, which many won't get to see how unfolds some time in the future.
It is really interesting not to have a straight answer for this.
Maybe it will be (or it was already), some innate sapiens-sapiens neurological and/or psychological blind spot, like overlooking inminent danger in exponential events.
Because high levels of intelligence and auto-drive can fully operate complexly enough to seem "conscious".
I would start a research stating exactly what would constitute a "conscious human", maybe we could find not every human on the planet is actually conscious.
Maybe most people are just looping into some psychological / environmental / sociological context, just like scripts running happily in loop, but not stepping outside its own code, at least not intentionally.
The hypoteshis of humans being complex algorithms, beside the tabu chit-chat, implies the possibility of hacking minds just we can hack programs, looking for their bugs, triggering exploits to fully manage humans into a different setup, in order to make them work towards a different goal from what they were originally intended (having being pre-programmed by the society and/or parents).