It's a sail, you RETRACT IT when you don't need it. Classic solar sale design before we really knew atomic thick materials like Graphene could exist, basically called for you to line your spaceship up on a straight line between your star and the one you were headed to, then open the sail, slowly accelerating up, then out past the edge of the solar system when your push started to diminish relative to your drag (drag from space dust on one of these goes with the cube of your velocity) you haul it in and coast until you get to the edge of the target solar system and the put it back out, tweaking your course just a little along the way to fall into an orbit where you want to be, then you haul the sail in and recycle it. Something like Graphene, which is thin enough to actually make space dust useful for slowing down and not just starlight and solar wind, you'd probably scrap down for the hull of a rotating habitat a few miles in diameter to give your people-sicles some leg room. We usually assume ten light years as a good round number for trips, even with antimatter you're still talking decades to get anywhere, optimal fusion drive is like .1c IIRC, or a century, something like a sail can do anything from .00...01c to .99c, though probably .5c as a practical top, it's all about how much juice you can send at it. Keep in mind that as huge as the energy we're talking about here is, you're usually talk about around a year to distribute it over, 1 year at 1g or 'normal gravity' means even some gigaton ship the size of New York City would only need 1 minute of the sun's power output to get near light speed, some Texas-sized millimeter thick solar panels converting into lasers would be able to do that, with available tech, but we're talking Great Wall of China or Pyramids in terms of effort scale where we tend to currently do small house fences or sheds.



Well you've got to be careful talking about 'fusion' - and H-bomb is fusion, you can run a spaceship off H-Bombs, just farting them out behind you and riding the wave, you build a cone or hemisphere or dish on the back end of your ship to act as absorber and radiation shield then fire h-bombs out. If you're willing to go really big scale, like ships with megatons of cargo and mass, you can use H-bombs that are mostly fusionable material by mass instead of mostly conventional explosives and plutonium, and get a higher top speed, a few percent of light... you can also use that wonderful rear dish to smack with a laser beam too for more speed. Then you go ahead when your H-bombs are depleted and the laser is too far out and drag your dish in and recycle it into a much bigger thinner sail for braking. You don't need fossil fuels for nukes, nor to develop them, development is pretty much an automatic as soon as you realize mass=energy, and while we think of them as being industrial age, they aren't, they are simply the byproduct of having a generation or two of scientists who knew about relativity and basic quantum mechanics, which really had no involvement or dependence on fossil fuels. It's likely that without a manhattan project it might have taken an extra generation or two but probably not. You just don't get a basic knowledge of radiation and not realize that's how the sun works, every physicists knows how to build a nuke same as they know how to build a car or howitzer or rocket, they couldn't draw you a working schematic but they'd know it would work and the basics to start of and yield you a functional half-ass proto-type, it's inevitable that if you uncover radioactive material and recognize it's gaining it's energy from losing mass you will realize the sun works off the same concept and that you can build reactors or bombs form it.




Right, we have to recognize that initial conditions and cultures can vary the end results a lot, I subscribe to the camp that say life is probably absurdly common but that smart life is absurdly rare, that horses would not evolve into tool users even if left for a billion years, because there have been plenty of stars and planets that could and should have life for billions of years longer than us and even if we have to half ass colonization with millenia long trips to stars off current tech and never a drop better one should have an entire galaxy colonized within tens of millions of years, meaning anyone in the galaxy who beats us to the punch by a billion years should be everywhere by now. To me, the 'oh maybe they don't spread and overbreed' response is absurdity, since a desire to grow one's pop is practically axiomatic for anything that hauled itself to the top of the food chain for an entire planet. It also ignores scale, to go from one planet of a billion people to a billion planets of a billion people over a billion years, the extremely slow growth approach, would require that one double your pop only once every 30 million years, as opposed to 30 years. Even a racial obsession with not taking other possible people's worlds that might evolve wouldn't work, because you don't need to take their worlds, you could just grab a couple useless asteroids and turn them into big ring habs causing no harm and hidden from the natives till they were already at the point that they'd be thinking about how big and potentially inhabited the universe was. It's just a no odds kinda case.



Or the flip side, sentient glaciers who think on that time scale, Alistair Reynold's did a nice story on that, very hard SF guy too, his House of Suns is the only book I ever read that gave a galaxy wide civilization restricted to light speed a sane and probable construct.

As for packing up and moving somewhere else, I really don't know if AGW is as serious as many believe, though I don't think so. Regardless I just can't see it killing us off. We could always, lacking space travel, move a few thousand people to serve as breeder pop along with a lot of frozen sperms and eggs to the tops of tall mountains in pressurized, fission or solar powered habitats, and just wait for the CO2 to settle back down, it does get absorbed, if it goes all runaway and we think Venus might be our future, we could detonate a shit load of gigaton nukes and blow a big chunk of the atmosphere away, the oceans would spill out oxygen to replenish what was lost, the whole human race might be a few tens of thousands of people for a couple centuries or maybe a few millenia [I'm assuming the worst case scenario for everything then doubling it] then come back down and setup shop again running off say a target max pop of a billion all on solar and ethanol. Those, say, 20,000 people could come back down and reseed everything from their zoos and cryogenic lockers and be back up to a billion with normal population growth in about twenty generations, 4-500 years, tops. So even if we kill damn near everyone and we find ourselves having to abandon ship for say 500 years while the planet sucks the carbon back down, you're talking about a thousand year interregnum, we've already done those, not a big deal. I really can't imaigne a scenario... inside our control... that could plausibly wipe humans out, killing 99.9% of the pop is only a short term setback, since it would take ten doublings, about ten generations of normal growth or a couple centuries, to restore yourself.