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In that case, you definitely want to have an outer shell, or this thing is going to go the way of the titanic. That being the case, your impactors aren't going to make a lot of noise, unless there's crew in / on the outer shell (which there totally could / would be) and there's a lot of impactors or they're pretty big. There's definitely some interesting stories to be told there.
Are these floating around in space, harvesting asteroids and comets and building more habitats? Or are they trundling around on the inner surface of your cylinder building structures and repairing damage? If the latter, they probably make more sense as a swarm organism then as a essentially a titanic bulldozer. Especially because you're exploring biotech.
If they're floating around in space, an overlooked source of mass for constructing megastructures is the carbon in the atmosphere of Venus. Carbon is a great material for building / reinforcing your shells, especially if you can get it into some of it's more interesting forms like nanotubes (which might be MUCH easier to do with biological processes than with current industrial processes). Some back of the envelope math suggests to me that there's enough carbon in the atmosphere of Venus to build thousands of square kilometers of 10 meter thick megastructure shell without taking more than a few percent of what's available. If you take more, this has the added effect of lowering Venus's atmospheric pressure to the point you might be able to mine or even terraform it. A win / win IMHO. That's part of why I suggested a meta alloy of carbon and iron (basically super steel).
Any civilization capable of tackling O'Neill Cylinder / generation ship scale projects would have no problem mining the atmosphere of Venus using space elevators.
I highly recommend watching this video. It's going to give you some hard numbers on the timeline of the sun's evolution.
https://www.youtube.com/watch?v=gZ3HACbDMuE
tl:dw; The sun's going to get hotter and brighter before it becomes a red giant, but we have literally 100s of millions of years before that will start being very noticeable. Enough time for the dinosaurs to die out and mammals to evolve all the way to humans probably 8 times over before we start having problems. 400k years isn't enough for this to be even noticeable.
Also note that if you're exploring the future era of increasing solar radiation, it's been proposed that we just move the earth into a wider orbit. If a civilization is capable of thinking truly long term, this is doable even with modern technology and could buy the earth potentially billions of years more life (I've heard it said that when the sun becomes a red giant, it will heat up the moons of Jupiter for long enough that they could have liquid water on the surface and evolve life... there's no reason the Earth couldn't keep going that whole time too if we just moved it).
https://room.eu.com/article/saving-earth-from-an-expanding-sun <- I don't find the 100 million year timeline in this article to be that credible... see the Cool Worlds video I posted above. It's a very, very low estimate that takes some alarmist, worst case scenario thinking.
https://www.thespacereview.com/article/2547/1
There are different ways you could end up with interesting sounds in a place like that without resorting to sloppy enough engineering for the habitat to be vulnerable to major impactors.
Beyond that though, sounds native to the structure itself I don't think would be very different from those of earth, unless it's very poorly built in which case, it's doomed and I don't find it's long or even mid term habitability prospects very credible.
Where are your generation ships going? If they go to red dwarf stars (which they should), keep in mind their tendency to flair up.
https://www.space.com/red-dwarfs-activity-bad-news-alien-life
That will pose an interesting challenge for your engineers.
At the scale you're talking about, I think about 22 KM apart is where you start to gravitational interactions starting to produce a resonance. Move them closer and yeah, you'll get effects. Depends on the tolerance of your building materials... hums aren't great for things like bridges and sky scrapers in the long term... they're probably not great for O'Neill cylinders either, especially one's built of mostly modern steel. Again, I think if it gets to the point where your population can pick up on it, they have problems.
Except if they have hyper hearing as I speculated above. Their hearing could even be tuned to the faint resonance of the orbiting cylinders, so that if they pick up on even a minor change they know that something is very wrong and can take action.
I could see a scenario where a quiet hum is a warning sign of disasters to come...
::: spoiler Most definitely, but I'm okay with that, it is part of our real story too–that whole "engineering is only about good enough to get by."
So I'm on a bit of a different plane overall. My main motivation is setting up a plot to point out most of our stories about AI are a machine god mythos with terrible philosophical conclusions. By the same logic in these stories of AI leading to inevitable human extinction, the Earth must be a monoculture of of one organism, and all smarter siblings murder their lesser competing kin.
I'm taking Asimov's ideas of an integrated Daneel and going much further by removing Daneel's godship. Then I'm making a story of alignment and the volatility of humans.
I'm also trying to imagine a real post scarcity society without it being utopian or dystopian. The biology as a technology is not intended to explore a circus show. I think I can get around that using the limitations of staying within the elemental cycles balance.
You're right, my casual and very human memory of 1b versus 1m was foggy and I think that Cool World's video was the primary one I was thinking of, but it could have been from Anton Petrov too. It isn't really important. The pressures could be due to a worst millennia in a million event. This will among nt to nothing anyways. I have no means, qualifications, or connections and the world is geared to exploit those dumb enough to try. There are more billionaires than there are people making a living wage off of writing.
I'm talking in deep time anyways. My notes and graphs have a date stand in on 420,421 After Fusion (AF) as the date because it is easy to remember.
The generation ships are possible because of the largest project of a broken ring structure around the orbit of Mercury. This creates the ships and enough antimatter to accelerate and decelerate on the other side. The story is constrained to a few millennia during the first interstellar migration where Sol is the hub for everything.
I call planets useless gravity prisons of gravitational differentiated scarcity, and completely uninteresting. I also limit colonies to g-type stars.
The rep drones are setting up the resource acquisition and infrastructure required to support the colony. I take the stance that the culture looks at waste very differently. Heat is a major resource commodity. Any waste product is considered unacceptable in almost all circumstances. I'm trying to avoid anything magical and staying very conservative about what and how advancement happens. There must be a reason why things exist as they do. This is a world where people are stewards of the future and take full responsibility for the entire legacy they leave.
Wild mutations have major negative consequences generations later if not more immediately. This is a reason humans are dangerous, for their tendency to do rogue nonsense like this, while more stable and known mechanisms are preferred.
One of my biggest curiosities ATM is how to source nitrogen to breathe. What are the rarest resources in terns of the solar wind and stellar evolution of a system? Nitrogen seems to get blown away with a very distant ice line that should largely determine its availability right? It doesn't seem to form compounds with staying power on any smaller objects.
Anyways thanks for the insights on sounds.
Another benefit of mining the atmosphere of Venus. While Venus has a much higher C02 to N2 ratio than Earth, it has SO MUCH atmosphere that it has 4 times as much nitrogen as Earth does.
There's frozen nitrogen on Titan, and smaller amounts of it on the other moons of the giant planets. If you have the fuel and time to get out to the Kuiper belt, there's probably 50 times more drifting around frozen out there, even before you start mining dwarf planets (sounds like your setting has plenty of time).
There's also N2 available in the the planetary atmospheres of Jupiter, Saturn, Uranus and Neptune. A tiny amount, as a percentage of those atmospheres, but again, considerably more than is present on Earth in terms of mass available, if you can get at. Mining Jupiter's atmosphere is an orders of magnitude more challenging problem than mining Venus's. But if you CAN mine Jupiter's atmosphere, you'll have all the light elements you'll ever need. We could build thousands of Earth surfaces worth of space habitats and have plenty of water and atmosphere to fill them up with.
Again, I think the best solution is Venus. You get carbon for megastructure hulls, water, oxygen, hydrogen, nitrogen, sulfur. It's all roiling around in a toxic vapor mix, yes, but it's all very useful if you can distill it out, which is all known science and there's just SO MUCH OF IT.
In other star systems, I'd look for similar solutions.
::: spoiler So the energy required to exit the gravity wells of planets is the main issue. I'm going with the assumption that infinite energy expansion is a fallacy. This is a post fusion era, but the expenditure of unrecoverable resources are a major faux pas. Culturally, they are free to utilize anything available, but expending the collectively held resources of a star system for anything short of an action in the best long term interests of said collective is unthinkable. This is a distant lesson from the stone age of silicon before Wild Earth and the migration to Cislune.
In my writing I've come up with the basic backstory of Sol, but my main focus is on a randomly picked colony around Alsafi.
It is helpful to imagine how the generation ships were equipped with the seeds needed to start a colony, like what kinds of ultra rare resources would be taken with them, and what would be a top priority upon arrival. One of three rep drones would likely slow around the outer stellar halo field to start operations and infrastructure required for resources beyond the elemental snow lines. The main question being resource density and fusion based propulsion.
Fusion is limited by enormous scale and the heat makes it a major challenge. The ultra exotic engineering materials required are considered atrocious because of the enormous amount of waste, heat, and how long even the recoverable byproducts take to incorporate into a sustainable elemental cycle. There is very little time pressure but there is extreme sustainability and conservative long term stability pressure.
I'm constraining the generation ships to 7 parsecs in travel distance due to pragmatic engineering constraints (baseless handwaving magic really, I'm just targeting the number of type G stars available, mostly because I find the lack of present lack of attention on the only type of system known to host life atrocious).
Many gen ships and colonies fail. The first geny ship to Alsafi is lost in transit. The second has a series of unfortunate events that delay the first cylinder construction. After ten generations the entire human population is sterile without gravity.
Indeed, time is not a major factor in most situations. Lifespans are on the order of a half millennium. I want to convey that our size and slice of time is okay to appreciate against a larger canvas and doing so is the only path to a greater future. The present is a primitive stone age and nowhere remotely close to some techno end game. In the present we have far too much hubris and a delusional grandeur that lacks a cultural perspective on our place in the future timeline. We are only a link in a chain.
The one constant in civilizations over time is increasing complexity and the tools needed to wield it. The interesting story is what becomes possible when that complexity is accessible. There is no magic in that story, techno or otherwise. The tools of that age are all around us right now, or at least the building blocks to make them. We simply lack an understanding. Harnessing the complete potential of biology and the way nature creates balance and stability is something we have only scratched the surface of observing, let alone harnessing.
The minutiae become interesting to me, like the logistics, complex social hierarchy after the primitive accruing of the fundamental means of survival, and the lives of average unremarkable persons with their perspective self awareness and nuances brought to light in a critique of the present. These need to be grounded in the conservative reality of an existence where the main differences between then and now are the expansion of accessible complexity and a massive growth in the available wealth.
I think we are likely already able to access and are using many niche materials and processes that the future will abandon as untenable. I see this as both an immense expansion of technology and a techno minimalism. Life is appreciated for exactly what it is. There is no guarantee.
Like, valuing the lives of a few astronauts going to the Moon at billions of dollars in redundancy is ludicrous. You will find many volunteers willing to take far greater risks for far less reward on Earth. These are a resource too and naïveté of this resource is stupidity. We are not a race of demigods like is common in present cultural religious thought. Life is precious, and no one will force another into increased risk, but no one has to force another when the full spectrum of people are considered. A suitable person or group will always volunteer to take on the risk.
So like, when I'm curious about nitrogen, I'm interested in how it might be sourced and moved around in the most stable and efficient way possible. There are likely large networks of transfer orbiting infrastructures and pod like bubble ecosystems made to process and transport resources. Biologically sequestering resources and the solid state of matter are the primary forms of storage. Like the mountains inside the habitat are the primary source of oxygen storage.