[awb]

> If they can build Dyson spheres, wouldn't they already be all over the place?

The number I’ve seen quoted is 50M years to colonize the galaxy: http://www.sentientdevelopments.com/2012/01/new-mathematical...

> they calculated that any galactic empire would have spread outwards from its home planet at about 0.25% of the speed of light. The result is that after 50m years it would extend over 130,000 light years, with zealous colonisers moving in a relatively uniform cloud and more reticent ones protruding from a central blob. Since the Milky Way is estimated to be 100,000-120,000 light years across, outposts would be sprinkled throughout the galaxy, even if the home planet were, like Earth, located on the periphery.

It’s actually so fast that advanced civilizations would have had time to colonize the galaxy and go extinct many times over without us noticing.

Or maybe their spheres were built so long ago they’ve already collapsed and been consumed by the stars again.

It’s a great mystery and interesting thought experiment though.

[denton-scratch]

> 50M years to colonize the galaxy

Our ancestors from 50M years ago are all extinct. So a civilisation spreading through the galaxy would have evolved in all kinds of different ways. It's not obvious to me that these diverse evolutionary strands would all still be interested in Dyson spheres, or space travel, or even astronomy.

There's a belief that's hard to shake off, that the properties humans have that we think most important represent some kind of evolutionary pinnacle. Typically, those properties are language, and a large brain for processing language. But if language and a large brain are really such great evolutionary advantages, why are humans the only creatures on Earth that have evolved those properties? Possibly language and a large brain are an evolutionary backwater.

At 0.25% of the speed of light, it would take us 1,600 years to reach Proxima Centauri; but it might take a lot longer to reach a star with habitable planets. We'd definitely need generation ships. After (say) a million years, we'd presumably have evolved to adapt to life on generation ships. It's not obvious to me that such adaptations would leave us fit to inhabit a planet. And perhaps adaptation to life on a generation ship means adapting to eating your fellow passengers.

Given the history of humanity, I find it hard to believe that the population of a generation ship could survive as long as 100 years without war breaking out on-board. We've had large brains and language for about 50,000 years, as far as I can tell; we've been warring the whole time. Maybe large brains and language pre-dispose us to war? If that's right, then it seems unlikely that intelligent life would ever spread far from it's planet of origin.

I'm very sceptical of the idea that any "civilisation" could ever spread far from its home planet. There are two things that we refeer to as a civilisation: a culture, and a species. Culture changes very quickly - over a single lifetime. But on a scale of millions of years, speciation is also pretty quick. So I can't see how any kind of homogenous civilisation or species could spread through a galaxy. They would have diversified before the train even reached its first stop.

So I don't have any insurance against being kidnapped by aliens.

[awb]

Much of what you say is reasonable.

Only two minor nits:

> why are humans the only creatures on Earth that have evolved those properties? Possibly language and a large brain are an evolutionary backwater.

Nearly every animal has language, for cooperation, competition, raising their young or to find a mate. Bees, whales, primates, birds, etc. all communicate strategically with some type of language.

> Given the history of humanity, I find it hard to believe that the population of a generation ship could survive as long as 100 years without war breaking out on-board. We've had large brains and language for about 50,000 years, as far as I can tell; we've been warring the whole time.

There’s been war somewhere the whole time, but not everywhere. Costa Rica, Iceland, Panama and several minor countries have no military: https://en.m.wikipedia.org/wiki/List_of_countries_without_ar...

It’s possible to find many countries who have not fought wars in the past 100 years. If you look more narrowly it’s possible to find communities unaffected by local war in hundreds or maybe even thousands of years (in other words, they fought a war but did so by traveling great distances, not fighting each other).

A ship would be much more like a tiny, isolated island colony rather than the geopolitical tensions that dominate the news.

Finally, wars rarely result in the extinction of both sides. Even if there was a large scale conflict it would likely be resolved prior to social collapse. There would be little incentive to kill so many of your fellow crew to the point where you’re putting every survivor at risk.

[mannykannot]

>> why are humans the only creatures on Earth that have evolved those properties? Possibly language and a large brain are an evolutionary backwater.

> Nearly every animal has language, for cooperation, competition, raising their young or to find a mate. Bees, whales, primates, birds, etc. all communicate strategically with some type of language.

There is currently only one species on Earth capable of even imagining the things being discussed here. On the other hand, it seems there may well have been more, but the others are now extinct.

One of these issues only we are discussing is the evolutionary viability of those abilities, and the fact we can do so suggests that it is, to some extent, up to us whether or not they will be.

[denton-scratch]

It's just struck me that a civilisation propagating through space can be envisaged a bit like light-cones, as in GR, except that the velocity factor isn't c, it's b (the maximum speed at which beings can move), which kinda depends on the beings.

But I think the cones thing is still interesting; it means that for any given maximum travel speed, there must be other civilisations that can never know anything about you, nor you about them.

[denton-scratch]

> Nearly every animal has language

Yes, I elided that bit, in the aim of brevity. I think there is a difference between human language and animal languages, in that human language is "creative" - we can freely create new utterances with new meanings, and expect to be understood. We know this of human language, but I would be very interested to learn of evidence of it in other species (chimps come close).

[webmaven]

> Our ancestors from 50M years ago are all extinct. So a civilisation spreading through the galaxy would have evolved in all kinds of different ways. It's not obvious to me that these diverse evolutionary strands would all still be interested in Dyson spheres, or space travel, or even astronomy.

There is no need for all, or even most, of the civilizational variants to retain those characteristics. As long as some variants do, civilization would continue its inexorable interstellar spread, it would just end up being lumpy rather than a relatively even wavefront.

Indeed, survivorship bias would seemingly tend to conserve the required characteristics and possibly start constraining the ways in which civilizational offshoots can vary. And it isn't hard to imagine stay-at-home variants succumbing to the influence of later arriving spreader variants to become spreaders once more.

[ncmncm]

Indeed, any culture that actually lasted long enough to get to the next star would have become, in the interval, comfortable with not being near any star, and could roam freely, doing nothing that could attract our attention.

[awb]

True, unless their lifespans are high enough that a trip to a nearby star is only a fraction of their existence (or they travel cryogenically and aren’t conscious during transit).