The sun goes nova

[Batman]

(note for example there is only two companies in the whole world which is capable of producing wide body aircrafts, for a market of billions

Which has nothing whatsoever to do with the population base? In fact, in the past, when the world's population was smaller, there were more high tech airplane manufacturers? The question is what technological infrastructure is left.

... so for a human civilization of a few millions wide body aircrafts are not possible to produce,

Says you. Between them Boeing and Airbus directly employ less than than a quarter million people.
Not that I see why widebodies are necessary, there was intercontinental air travel even before WW2.

and most of the high-tech stuff is similar, only accessible with a reasonably big population)

Because of? Most of the high-tech stuff needs the infrastructure, not the manpower. You only need the manpower if the infrastructure's gone.

so the aliens can practically leave alone all the habitats if they are small and isolated.

Provided they've taken care of our industrial complex, because if they haven't we'll be back up and running in even less time than Junghalli predicted for having to start from scratch.

So unless a yet unknown invention change the nature of manufacturing

Which for the past half century if not longer has seen a continuous reduction in the need for manpower,

a scattered band of survivors pose no threat as they can only produce the most basic stuff by themselves.

They're not by themselves, at least not if pruning the population was all the aliens did. If the industrial infrastructure is still there (like, say, after the aliens relying on bioweapons to get rid of us), so is a noticeable portion of our manufacturing capacity.

This also means that if they can not find a suitable location to restart the things they will forget all advanced knowledge (or at least the practical part of it) within a few generations.

Why? The libraries, the internet, a gazillion of educational DVDs are still there, as are the means to use them.

Advanced robotics might change the fact a bit, but robots have limited lifetime also (and the more sophisticated units tend to be the less damage prone), so without a proper support infrastructure they might degrade pretty soon too.

Why would that infrastructure be gone? So far you have mentioned nothing but them decimating the population.

[bz249]

In case of an alien invasion, well they do not have to kill everyone to totally demolish us. They just have to kill enough to collapse civilization forever

Forever is a long time, and civilization could plausibly spring back in eyeblinks compared to galaxy-colonization timescales (since even 10-20 thousand years isn't that much time next to that, and in that time we went from cave men to a global industrial civilization). Leaving behind survivors on the logic that you've done enough just by knocking them back to the stone age strikes me as complacent.

Okay then just taking them out for a sufficiently long time that they would cause no pain during the establishment phase. The invading forces have enough problem by the colonization itself. So hunting down every survivor is the least they would want.

[bz249]

Which has nothing whatsoever to do with the population base? In fact, in the past, when the world's population was smaller, there were more high tech airplane manufacturers? The question is what technological infrastructure is left.

What has some connenction with the population base is the demand the more population you have the more. And there comes the economic of scales into, the production of one unit would be more manpower intensive.

Because of? Most of the high-tech stuff needs the infrastructure, not the manpower. You only need the manpower if the infrastructure's gone.

What they need besides infrastructure is a bunch of specialized guy, a manufacturing background which allows these guys to work in those field (a general efficiency in the more basic stuff, because that is the source of the free manpower) and a society that uses their product, thus have a nice complex hierarchy of high-tech industry.

Provided they've taken care of our industrial complex, because if they haven't we'll be back up and running in even less time than Junghalli predicted for having to start from scratch.

Most probably they took care of it. But yes, if they leave the warships than a counter strike is even easier.

Which for the past half century if not longer has seen a continuous reduction in the need for manpower

And exactly that is thing we would lose if the market size is reduced. Because this reduction is made available by producing huge quantities, producing enough stuff for a small community requires different, way more inefficient methods.

They're not by themselves, at least not if pruning the population was all the aliens did. If the industrial infrastructure is still there (like, say, after the aliens relying on bioweapons to get rid of us), so is a noticeable portion of our manufacturing capacity.

Disrupting the trade network, taking out some vital factories... just like in a nuclear war. That would lead to a massive shock to the production capabilities.

Why? The libraries, the internet, a gazillion of educational DVDs are still there, as are the means to use them.

Firts books, DVDs whatever does not meant to last too long. Second reading something in a book is not the same as doing it with a real machine.

Why would that infrastructure be gone? So far you have mentioned nothing but them decimating the population.

Okay, they might do more than just killing people. Some of them can blow trucks, power lines and spaceships too.

[Junghalli]

Okay then just taking them out for a sufficiently long time that they would cause no pain during the establishment phase. The invading forces have enough problem by the colonization itself. So hunting down every survivor is the least they would want.

The problem is if you leave survivors and they rebuild you might run into them later, under unknown circumstances. It's one of those things that's asking for Murphy to bite you at some point in the distant future.

[Swindle1984]

A friend of mine manufactures his own firearms and parts for his cars as a hobby. He has a CNC machine and lathe in his garage; he doesn't even need to know how something is made, just download CNC plans off the internet, load them into the machine, and load it with the material he needs. Very little manpower is needed, he does it all by himself and lets the machines do a lot of the work.

In a civilization which has colonized the solar system, and given that each O'Neill cylinder is designed to be more or less self-sufficient as far as gathering resources, manufacturing, and producing food for its population of millions, I don't think it would be overly difficult for them to manufacture whatever they want, especially if they're pissed off and have genocide and revenge on their minds.

Reducing the population without reducing the infrastructure doesn't do a whole lot. And no, reducing demand for products does not mean that capacity for production will be reduced accordingly.

That's why the scout ship tried to drop a comet on us when it did; we were primitive enough to not know it was an attack, and if it had succeeded but still not wiped us out, it'd take long enough for us to recover that the fleet could pass through unmolested and we'd never be the wiser. But we'd still recover.

[sirocco]

Though, If they are really acting like us, they will have a part of their swarm that wants to establish a colony in our solar system. And if they believe that we may become a menace at some point, they will do their best to keep the humans in the dark ages as long as possible.

And if they are a little smarter than your average SciFi Evil Alien, we won't recover.

[Swindle1984]

Or they could be entirely nomadic and not tie themselves to any one system for very long, preferring to simply expand the fleet (or split into more fleets after reaching a certain size) as they travel across the galaxy.

[Uncluttered]

If it's nasty radiation you want then you probably want to look at a star brighter than our sun. The papers I've read though suggest a habitable planet around a red dwarf would be stable (not in danger of atmosphere collapse due to the air freezing out on the tidally locked planet's permanent night side) with ~.1 bar partial pressure of CO2, which is way more than we evolved for; the air probably wouldn't be very healthy for us.

You're correct. I should have specified a "young" red dwarf. They flare up a lot, without interference from a companion.

To be sure, if they lived in habs, the type of star wouldn't matter at all, brown, red, yellow, black etc. Much of the scenery could be a convincing illusion.

If my planet was dual spanked by a comet, then by xenos, i'd never move my ass back to a vulnerable rock again. I'd go all Culture.