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I know some people here are concerned with national demographic shifts, but there's a larger and more esoteric question. Epistemic status: Deep Thoughts with Jack Handey.
If we assume that our current state of understanding the physical laws of the universe are mostly correct (especially with the feasibility of FTL space travel), it seems to me that in the medium to long term, Malthusian limits are a foregone conclusion.
At some point the lines of human population, possible food/power/water resources and our technological advancement will intersect. Could be ten billion, could be a hundred billion, could be much more, but without FTL travel, we get there someday. The obvious answer is to have less people, and there's any number of horrific ways to achieve that, and one painless one. Given our assumption, and further assuming that we want to do this the most moral way possible, zero population growth is the way to go. We don't have to kill anyone or stop fighting disease and starvation, we just have to limit everyone to two kids at a global scale. Failing FTL travel, there is a maximum number of humans that are physically, practically, and politically possible to keep alive, and we don't know what those numbers are until we hit them.
Now we're stacking assumptions, which is always a bad idea. If we accept these two basic propositions: No FTL leading to someone someday having to stop people from breeding too much. This brings us to the issue that the demographics of the world change over time, so when this event takes place will have a huge impact on what sort of humans are represented in this hypothetically fixed future population. There will be a lot more africans in ten years than there are now, and fewer europeans, but in a hundred years, or a thousand? Who knows? If the decision is outside the next few decades, it's very hard to say what the population trend lines will be. People tend to slow production as they develop economically, so the whole thing may be solved organically.
This brings us to the questions, if it happens and isn't solved organically: How long do we "hold the door" for more people at the cost of the resources available to each? Does the shifting demographic composition play any role in your decision? Should we aim for maximum diversity? Maximum resources per person? Maximum people?
Bonus question: Do you feel strongly enough about the moral correctness of your current socio-political unit to want the decision? Every day the resources get less and the people more. Someone will make the call at some point. If we defer, someone else makes it. We have our own concerns, but the Chinese have different ones, and the Russians still different ones to that. Or some future superpower nation (or group of nations) not yet in existence. Do we put the decision off as long as possible because people are getting better over time? Or do we act as soon as is possible because we think we're the best possible people to do the moral calculus? And can we trust anyone who thinks that?
Even without FTL travel, there is enough energy and material being produced in our current solar system to support an unimaginable number of humans. The Dyson sphere gets built incrementally. You just keep building new permanent space habitations ( each could support 50,000 to 1 million people) that get sent into close orbit around the sun, with an array of solar panels running at 100X the efficiency you get running solar panels on earth. Each of the habitations would have enough energy to both make its world a utopian human experience and allow for building new habitations for the colony's children. In the far future, human life will be primarily non-terrestrial, and visitors from the habitations will come on vacation to Earth and marvel at how people put up with blizzards and mosquitos and rain and other hazards of planetary life. What happens when the Dyson sphere is complete and the solar system has reached capacity of quadrillions people? We'll cross that bridge when we come to it...
Also, there is not reason you couldn't send out colonies to other solar systems even without FTL travel. The colony ships would just need to be self-sustaining habitations that would house many generations of people.
The issue with this is that without FTL there's a limit to what we can reach due to the expansion of the universe, so we're likely stuck in our local group of galaxies, where gravitational attraction holds things together.
Note that the universe doesn't expand at any fixed speed but at a speed per unit of distance, which we normally measure in kilometers per second per megaparsec (one megaparsec is about 3.26 million light years). If the expansion rate is 70 km/s/Mpc, that means, on average, an object that’s 10 Mpc away should expand away at 700 km/s; one that’s 200 Mpc away should recede at 14,000 km/s; and one that’s 5,000 Mpc away should appear to be moving away at 350,000 km/s.
An analogy that's often offered up to illustrate the expansion of the universe would be the balloon analogy, where coins are placed all over a balloon then the balloon is inflated to show that every coin will be moving away from all other coins at a rate proportional to how far away they are (note that you'll be ignoring the interior of the balloon here, this "balloon" universe is represented by the surface). The balloon analogy isn't a perfect one, since the balloon is a 2D universe and is curved whereas our universe obviously isn't 2D and also is flat, but it helps illustrate the concept. What this means is that there's some distant event horizon of sorts beyond which everything will be receding from us at a rate that means we won't ever be able to reach it.
This basically ensures that only a finite portion of the matter and energy that exists in the universe will be available to us, and eventually we'll be reaching some kind of limit assuming no Great Filter scenarios rear their head before then. Improvements in technology can only really take us so far, because eventually there'll be no more increments of efficiency to squeeze out of what we have (it seems reasonable that there will eventually be some sort of sheer physical limit we'll bump up against).
Ultimately, this pushes the problem very far down the line, but doesn't at all eradicate it.
Nah, the "reachable universe", while not as large as the "observable universe" and slowly shrinking, is bigger than that (it's something along the lines of a billion galaxies IIRC). The Local Group's only the eventual size of the reachable universe, as t -> infinity, not its current size or anywhere close.
Obviously, exponential growth will still hit the "reachable universe" eventually though.
Of course, if FTL is real then many estimates for the size of the universe boil down to "time and/or aliens are the limit, not space". 10^10^10^122 makes exponential growth go cry in a corner.
Yes, the reachable universe at the moment isn't only the Local Group. However the size of our reachable universe is premised on the assumption that we leave today, and at the speed of light. What's currently in our reachable universe is a very generous estimate as to what we can practically reach.
In retrospect the way I phrased it was probably misleading - the statement that we might be restricted to the Local Group was my extrapolation of what in practice might be our limit, incorporating my own quite pessimistic estimates as to the difficulty of achieving anything close to relativistic speeds (let alone speeds nearing that of light) as well as the difficulty of keeping a crew alive and the ship working when going at these speeds.
Given the constraints that relativity imposes, this seems like it might be unlikely absent some revolution in our understanding of physics.
EDIT: added more
Laser sail/Bussard brake. Antimatter ramjet. (The Bussard idea doesn't work as far as we know, because of the scoop's drag and the difficulties getting p-p fusion to happen in a scramjet throat, but it works as a brake - or injecting antimatter into a scramjet throat would certainly get it to burn.) Baryon number nonconservation does also seem allowed, which implies that non-antimatter-based total conversion engines could be possible. There is, indeed, trouble with keeping a ship intact in the intergalactic void given the cosmic rays, relatively-relativistic dust, and lack of new material to replace that blown off - but that's mostly an issue of building a bigger ship.
General relativity does allow for FTL in the broad sense of "get from A to B faster than light conventionally could" - the Alcubierre metric and wormholes being the most obvious. And there are some plausible answers to the time-travel problem (there's a hypothesis that attempting to convert a wormhole into a time machine would collapse it, for instance). Whether FTL's possible is an open question.
Okay now I'm getting into things I'm not too certain on (obviously IANAP), but from what I understand apparent FTL that entails the warping of spacetime is one of these things that we're not 100% sure is impossible but does pose a lot of problems. Apart from the whole "closed timelike curve" problem that these apparent FTL methods seem to create (which, granted, as you noted one can try to resolve through all kinds of difficult-to-verify chronology protection conjectures), there's also the fact that both Alcubierre drives and traversable wormholes alike require unobtainum exotic matter that at best isn't impossible but there's no evidence for its existence and at worst violates an energy condition.
So they're not exactly impossible per se, but there's reasons to believe they probably are.
Clarke's First Law is a decent heuristic, and there's no clear no-go theorem (Earnshaw's theorem is the obvious example of a theorem with a lot of important loopholes). I recall reading about somebody trying to build an Alcubierre metric using the Casimir effect, though I'm not sure how it turned out and that's well beyond my own paygrade.
Overall I'd say it's in the "maybe" category; I'm leery of saying it can be done, but at least as leery of saying the opposite.
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Is it not the case that, once we start moving towards those distant objects (in say a colony ship), the expansion behind us compensates for a growing portion of that total expansion? It's my understanding that there IS an inflection point as you describe, but we haven't reached it yet.
Perhaps "Save the Universe" is the ultimate point of the simulation we built ourselves. Seems fitting.
The case for an inflection point is pretty strong. It’s my understanding that for objects that have already crossed the boundary of the event horizon, no reduction of the distance between us and that object will occur.
Think about it this way: There are objects far enough away from you that they are moving away at a rate that exceeds the speed of light, meaning without FTL travel they will be receding from you faster than you can travel to them. The space between you and any object beyond that horizon will only increase and the further they go, the faster they recede. If you try to reach it in a relativistic colony ship, all that happens is that you’ll be stranded from your original galaxy group and will never reach the new one as your galaxy of origin passes out of your event horizon. Sure, you are closer to the object and further away from your point of origin than you would've counterfactually been, but that does not equate to closing the distance.
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