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We always assume that civilizations expand into space without consideration for benefit, but I don't think that will be the case, honestly.

Should we want a colony on another planet? In another star system? Absolutely. It doesn't take much expansion for us to achieve the goal of having a 'redundant backup' for humanity.

Beyond that, there has to be some sort of economic incentive to drive it. Given there is little chance we can economically trade between planets, much less between star systems, I don't think that incentive is there.

One benefit is reduced risk of extinction by the various nasty ways the grand cosmos might annihilate a local-only civilisation.

Let's consider extinction mitigation as a luxury? I posit that sooner or later, as technology improves, the relative cost of such projects will go down to a point where some group of people think it is a good idea.

Again, you don't have to expand to more than a couple of self sustaining planets in a couple of solar systems to establish a reasonable redundant backup for mankind.

I don't think new discoveries suddenly change the physics of escaping gravity wells. I question whether interplanetary trade will ever move beyond intellectual property.

Basically, colonization will have to be altruistically funded without any expectation of economic return. We've proven we have limited capacity for that.

>We've proven we have limited capacity for that.

Currently.

In a system where resources are allocated not to cover the needs of all, but instead to ensure that the victor gets their spoils, this is true.

But IF a civilization makes it to a point where self-sustaining planet settlement is even a real possibility, I think you just have to assume they are post-scarcity and post-currency. Economic return isn't really a thing, if everyone can get what they want whenever they want it.

Therefore, any species with a natural propensity to exploration and curiosity as an individual is going to necessarily seek out new places as a whole. The individual will drive the societal, I guess is what I mean there. Because I want to see something new, and there is no scarcity of resources, society as a whole gets to see something new.

>I question whether interplanetary trade will ever move beyond intellectual property.

Honestly, I don't think so. Because, again, I'm of the firm belief that once a society is able to establish off-world colonies successfully, economics stops meaning anything at all, really.

> Because I want to see something new, and there is no scarcity of resources, society as a whole gets to see something new.

You might _explore_. You're less likely to _colonise_. The only way you get the sort of geometric expansion people love to talk about is if there's a very strong imperative to colonise, and it's just not clear that it exists.

I’ve heard it said that if humanity simply maintains its growth pace, there should be enough human bodies to pile up into the entire solar system just by waiting X number of years.

> if humanity simply maintains its growth pace

But humanity is not doing that. The current trend is world population will peak at just over 9 billion in about 30 years. Trends can change, but that is the current one.

Every growth curve is an S curve. Every time you see one of those studies where someone takes a system that is still in an early growth phase and projects it out to infinity at the current growth rate it is always nonsense.

Absolutely agree. I think that once you've established your civilization over a few stars and planets your survival is pretty much guaranteed against astronomical destruction (if there's anything huge enough to destroy ALL the stars you've colonized, you probably can tell with billions of years in advance), the incentive changes and you now want to basically go on stealth mode to make sure you don't actually attract any nasty neighbour's attention, because that may be the only thing you can NOT see that may be dangerous.

And notice that people from different star systems in the same civilization will very likely only have extremely rare direct encounters due to the brutally long distances involved... it's highly likely the civilization will split up into completely separate "star nations", with their own internal issues to keep them busy and at some point may lose interest even in communicating with other star systems... without any incentive at all to expand further as the species survival is guaranteed... curiosity wouldn't be strong enough, probably, because after getting to know several star systems, you may decide they are look more or less the same, and to find anything really interesting might take multi-million year journeys and no one would be able to embark on something like that just for curiosity.

> if there's anything huge enough to destroy ALL the stars you've colonized, you probably can tell with billions of years in advance

I can't think of any phenomena that could come close to doing such a thing. But if there was one, why couldn't it be like gamma bursts?

I see no way a species that can technologically spread to a couple of star systems would stop expanding.

If you're scared of nasty aliens, the best defence is to conquer the whole galaxy before them.

There is no way to hide from an interstellar civilization.

Nasty or not, they could always produce self replicating probes and keep track of the whole galaxy.

Also, if you were able to colonize a couple of Star Systems, there's no way you are an anxious/scared species,

Exactly this. I was thinking about this the other day and using the web as a metaphor (bear with me).

It's easy to know where to go next when we only have one option to pursue (such as travelling to the next nearest planet or star). This is a lot like the early web when there were only a handful of interesting sites and discovering new cool sites was a goal in and of itself.

Today, I think we intuitively understand that there's no longer any value in trying to visit every website on the internet. Most sites offer zero value or even net negative value. The entire problem space pivots to one of knowing which 0.00001% of websites are valuable to a given situation.

When a civilization gains the ability to visit any planetary system, the problem shifts to one of filtering choices and information to minimize the amount of unnecessary travel or world-settling.

I assume this is a well-known concept. I'd love to read more if anybody knows what it's called.

The big issue here is different understanding of "reasonable time". The galaxy is very large, and the time spans involved are very large, but they are not in the same ballpark of "large", the time gaps involved are so much larger. If one civilization gets their evolution of intelligent life just 1% quicker than us, that's a head start of 50 million years. On that scale, "reasonable speed" required to fill the galaxy (to use round numbers, 10000 "hops" of 10 ly each for 100000ly distance across the whole galaxy) means something like a planet launching a single colony ship to a nearby star once in 5000 years (and the new colony populating the planet and launching another ship in 5000 years, which seems quite reasonable). You don't need FTL, you don't need fast ships, you don't need many ships, just one generation ship in a few millenia moving at 0.01c or even less is amply sufficient to colonize all of the galaxy many times over in that time range. And if someone was 2% or 3% faster than us, then they'd have even more excessive time to swarm the galaxy even if they're doing it at snail's speed.

So to explain the fact that the aliens aren't swarming everywhere because they can't afford it means accepting that either they launching a single generation-ship every 5000 years is a huge expenditure (which IMHO it wouldn't be even with our current economy size, definitely not an issue with the tech of 2100 or 2200 assuming at least some growth because at that point a Musk-type figure with a wish to "make humanity interstellar" would be able to afford it from private funds) or that we're the first (only?) life in galaxy or that all the aliens before us didn't try to expand, like, at all - because getting just one single rich eccentric every 5000 years (like a bunch of billionaires here are) would be sufficient to make the galaxy swarm with juuust a bit higher tech level than ours now.

Well, you're assuming that technological advancement is linear and one way. We simply have not been technologically advanced in any notable way that we know of to be the least bit certain that this is true. A lot can happen within 5000 years.

The problem with generation ships is we assume we can just "zip" all knowledge and technology and ship it via a generation ship and "unzip"/bootstrap at destination.

I'm sure we are absolutely not capable to do that now. Maybe with AI.

But we haven't done that with our Moon and that's not even a few lightseconds away.

We certainly can "zip" enough knowledge and technology to match Earth as it was in year 1000 (and almost certainly year 1900) and as experience shows, it's sufficient to build up in a millenium or two i.e. very tiny amount of time on that scale.

Well, maybe "constant speed" is a bad wording.

An example:

We have 1000 stars within the 30 light year radius neighborhood. Traveling at Voyager 1 speed, it takes half a million years to do a 30 light year trip. [1]

Given another half a million years for bootstrapping at the destination for ten more probes, we can send ten more probes, 1 million years from the launch of the first one. At two million years we get 100 probes.

How many probes do we get in a 11 million years? 10^11 probes. We have 10^11 stars in the galaxy.

Dinosaurs roamed the earth aleady 200 million years ago, and died in the mass extinction 65 million years ago. 20 to 5 million years ago was the Miocene, when life was pretty close to modern already. [2]

It's really the nature of exponential growth that makes things seem instantaneous compared to celestial phenomena or evolution. Colonizing the galaxy, even with abysmally slow speeds, is possible, because of the magic of exponentials.

(Certainly this has a lot of assumption, like that you can bootstrap, or that the probe survives the trip. Anyway, optimal probes would be a lot faster and bigger, and there would be more of them. This was just to tie in with something concrete that we know we can do, as Voyager 1 really is out there traveling at 15 km/s, as are Voyager 2, Pioneers and New Horizons probes.)

1: https://www.wolframalpha.com/input/?i=+30+light+years+%2F+vo...

2: https://en.wikipedia.org/wiki/Miocene

> And at that point, where is the incentive to colonize other star systems?

Just to put this out there, unless every advanced civilization becomes a hive mind with one unified will, some individual or splinter group or just hodgepodge of determined individuals would probably do it even if there were no incentive, and that's not to point out that incentives are somewhat subjective and differ between individuals and groups. If there are enough individuals on a civilization and the capability exists, some crazy person will do it.

Even without FTL travel, just having constant acceleration at 1g is enough to travel the width of the galaxy in 12 years ship-time: https://en.wikipedia.org/wiki/Space_travel_using_constant_ac....

That is a _huge_ 'just', tho. That's a lot of energy.

Welcome to the delta V problem....

Try plugging 12 years of 1G acceleration into the Rocket Equation.

If reactionless drives remain science fiction this mode of travel is flat impossible. There literally isn't enough mass in the solar system.

> if you can produce energy that cheaply you can sustain a large population in comfort at home

Don't see the reasoning here.

For example, we currently can feed the whole Earth population "cheaply". But the wars didn't end. Same with cheap energy.

E.g. we can try to install vast amounts of mirrors to direct Sun's light towards an accelerating ship. But that won't bring us here more comfort.