Gonna kinda jump ahead here, you raise valids points on a lot of these but keep in mind I originally said we should expect aliens to have used coal and oil if they had them, I am merely establishing things can be done without them, so your objections kinda reinforce my original point, still your objections hit mostly out how impractical compared to what we have some of these things are, not impossible or even implausible.

I also had a mid-message power outage so I'm kinda repeating some points in haste I already typed, so excuse me if I get a bit hasty or jumpy in the new version. I am, from my POV, kinda repeating myself.



Engines rarely run anywhere near 50% efficiency, so the answer for both is 'radiator', also keep in mind you don't have to throws logs on or something, you'd use compressed woodchips and such if you were actually building an infrastructure around this.




Superconducting materials let you bypass distance and those materials aren't related to coal or oil, more Yttrium and nitrogen really than hydrogen or carbon, albeit the nitrgoen is for cooling them. And we've known how to build flywheels as power storage for a long time, they predate batteries by and large, we've only recently started screwing around with levitating flywheels in a vacuum over superconductors, that's also not coal/oil related. No particular reason they would not be pursued, the big boost of an oil economy might make us do stuff way faster but we've really only been at this for a century out of thousands of years of effort, some civ taking a millenia or two to go from ox-pulled plows to superconducting magnets instead of a century or two in't all that big a deal in the grand scheme of things.






I only removed fission as a route for a hypothetical where life took a long time to evolve around a slightly dimmer star, and there's really no difference between coal or wood when we start talking about fusion or antimatter. For that matter a big old rail gun could fire a slug with a tether up into space sans coal/oil/rocket fuel and leave you with a space elevator and prototypes could give you the sattelites we have... that is actually how we always we assumed we'd do it. Newton and the lot knew perfectly well how to get a satellite up there just had no damn reason to do it, what with no radio or computers or micro-stuff. Very little of our tech is even vaguely related to fossil fuels, they just provide the cheap power that let's us devote a lot of people to research, hell a culture with better memories or longer lives or a mild obsession with math - say a religious devotion to numbers - might be able to devote an effectively equal amount of resources to research, physical scientists and engineers make up way less than 1% of the pop, a feudal culture where the noble ladies pursued the 'feminine art' of math or chemistry instead of embroidery might end up with semiconductors while everyone was still drag plows behind oxen, don't make the mistake of assuming science is some ladder you climb in one specific route. If I knew latin I could teach an old roman aquaduct engineer how build and power telegraph system in a few days, he might come up with it on his own if he was dwelling on how to send messages fast after seeing a bolt of lightning hit a metal rod. They could easily end up with TVs and a 200 cable channels while still trying to figure out how the hell to make a useful steam engine. The processes are not linear.




There's maybe a 1000 atoms per cubic meter in the space between local stars, the 'local bubble' isn't very dense as these things go, so serious vacuum there, but a ship trucking along a decent fraction of light speed with be hitting about a trillion atoms a second per square meter of cross-section. That gas is moving at speed pretty much dead-stop relative to local stars meaning at 1/3 light speed each little bit of hydrogen you smack is going to give you a braking energy of ~ 10^-11 Joules, pretty tiny but every square meter is hitting 10^11 of those things a second at .33c, or basically 1 joule per square meter [yes I cherry picked that speed for the example]. That's not a lot of drag, some 1 ton spaceship with a meter of cross-section at that speed would decelerate under that braking force over many trillions of year, more since as it slowed the braking energy would decrease with the cube of velocity. Cube because you're breaking comes from how many atoms you hit... which is linear to your speed, and their kinetic energy, square of relative velocity, so cube total... works great at very close to light speed to slow but not well at lower speeds... but close to stars density rises and more importantly you start getting light pushing on you too, here near earth that's around 1000 watts per square meter. If you're using something like Graphene, which a 1 ton sheet literally cover a few billions square meters, a cargo bay full of sail is giving you something the size of Texas to slow down with. Realistically it's the sort of thing you'd want to combine with other propulsion to basically save fuel and thus get a higher top speed, but yeah it can play a role.




Like I said, if you've got fusion then stellar or planetary gravity wells are like mild uphill slopes to a formula 1 racecar. Lacking fusion or something better you might want to on-site construct say a bigger laser platform than ran by burning methane in orbit around the local Neptune or whatever to push you, or a bunch of locally fabricated mirrors, you work with what you got. We have the tech to do a lot of this stuff already it's just the difference between a little retaining wall in your terraced garden behind the house and the Great Wall of China, kinda sucks if you're halfway through with the wall and someone invents landmines to render big walls redundant.




Your mistaking cheap mass production with ability, that's why I keep talking about $5 and $50 hammers. You don't need coal to make steel, charcoal works, so would dried out algae put through the same process if it comes to it. Ceramics and glass can do a lot stuff steel can, so could aluminum. Silicon, the main ingredient of glass really, does not turn into microchips through any petroleum based process I know off. Fundamentally it's important use is that it's cheap, it means we can devote large portions of our pop to doing other things, but I feel pretty confident that if we only had a few hundred scientists alive at once, which even a tiny agrarian society of a million could support without too high a burden, we'd get all those things just take longer and likely in different orders. When we're talking about alien physiology keep in mind some species of sapient whales with long lives and good memories might sit around for a billion years belching calculus back and forth to each other without having ever developed a tool culture or a race of smart raccoons whose love of grubs let's them mega-breed because insects get you very high calories per acre compared to cattle... on top of agriculture, they might only need 1/10th of their pop to produce food way back in their megalith days and jump from Stonehenge to Hubble in a century or two just because they're really curious and don't need coal or oil to let them have 10% of their pop doing research and engineering. Earthcentrism is fairly legit, if probably limiting, but species centrism is a bigger problem when thinking about this stuff. If you're curious tool users with language who can spare a decent percent of your pop for curiosity, you probably can and will develop advanced tech, coal and oil just were handy for us because they let us spare a lot of people from normal toil and incline to toward more development. In my opinion, moveable type printing presses and an obsession with teaching everyone to read so they could read the bible probably had a much huger effect in causing the industrial revolution than coal or oil did.