[pfdietz]

This also means very long lived extraterrestrial civilizations can colonize the galaxy with slow very short range starships. Just wait for stars to come within a fraction of a light year and send over ships at 1% of the speed of light (or even slower). The star will move away, and it also becomes a target for close approaches of stars. The number of colonized stars grows exponentially (if gradually) until most stars have been colonized.

[denton-scratch]

> very long lived extraterrestrial civilizations

We're talking about stellar visitors that arrive every 20 million years or so. We have no evidence of any civilization that lasted much more than 1,000 years.

So you're going to hitch a ride with a stellar visitor, with a view to colonizing the galaxy? Suppose the visitor comes as close as a few thousand AU, and the colonist ship can reach it in a hundred years or so; cool. But how long before the visitor reaches the next star on its tour? That's going to be millions of years.

So the colonists set up camp on a planet orbiting the visitor; the hypothesis is "a very long-lived" civilization, so we assume they have brought their knowledge with them to this camp, from the home planet, and are able to conserve it (strong assumption). But for tens of millions of years?

Civilizations evolve much more quickly than organisms, if Earth history is anything to go by. It seems far-fetched that a civilization might survive the evolution of the underlying organism; there's something Canute-like about trying to preserve a civilization in the teeth of genetic evolution. Evolution is going to happen, even during the initial journey to the visitor.

I'm very sceptical about this notion of "very long lived extraterrestrial civilizations".

[pfdietz]

The civilizations themselves don't have to be long lived, but once a star system is colonized it has to stay inhabited long enough. Civilizations could come and go, but not completely die out too quickly. As long as each colonized star system produces enough child colonies before it dies out the exponential growth will continue, with colonies eventually being reestablished at systems where former colonies may have expired.

Eventually there would be selection for colonies that can produce faster and/or longer lived colony ships, so they can plant new colonies faster. This will eventually shade over into the more conventional galactic colonization scenarios where stars can be treated as near stationary.

[Vecr]

Technically, you can stop evolution from happening. Cloning is annoying, but if you had enough compute and algorithms you should be able to make in-place edits to stop drift. By "in-place" I mean even optimistically just a bit after the blastocyst phase, editing humans that are even slightly grown to that level is just not going to be practical.

[denton-scratch]

> Technically, you can stop evolution from happening.

Yeah, but would anyone do that to an entire civilization? Evolution leads to greater 'fitness' - improved adaptation to the environment. The travellers are jumping from $HOME_PLANET, into a generation ship, onto a new planet presenting novel adaptation challenges. That planet's star-system then eventually sets off on a long journey, until it encounters a new star. Each of these stages takes many times the time humans have existed on Earth. If you consider only modern humans, we've only been around for 50,000 years, maybe.

And "modern humans" isn't a civilization; it's really all of the different, successive human civilizations.

If you could imagine that, in addition to making stone tools and animal-hide clothes, neanderthals had also learned how to freeze evolution, do you think those guys would be well-adapted to a planet-hopping future? I don't.

[V-2]

Once you have the technology to freeze evolution, you also have the technology to accelerate it and steer it, and given the level of incentive (to gain competitive advantage), I don't imagine it would be a balanced sword-and-shield race in the long run. The "conservatives" couldn't keep up with the "modernists", unless it's some sort of a totalitarian system.

There's been this great (relatively) short story, both humorous and profound, by Stanisław Lem; "The Twenty-first Voyage" in "The Star Diaries". It's based on that exact premise.

[justinclift]

At a rough guess, even with an approach between stars that's very close, the difference in velocities between them may make for swapping between the two systems near impossible.

There could be a good measure of good luck needed too. eg the spaceship is lucky enough to get some gravitational assistance from a large body or two in the target system

[pfdietz]

The average relative velocity of stars in the Sun's neighborhood is about 30 km/s. For certain high velocity stars, it can be in the hundreds of km/s (but those are particularly interesting as they leave to enable colonization of new parts of the galaxy much more quickly.)

If the time between encounters is a million years, at 30 km/s the stars will have separated by 100 light years.

[ummonk]

Okay that’s a lot slower than I’d have imagined. Won’t be getting as much out of a flyby / gravitational slingshot as I’d have hoped.

[justinclift]

> 30 km/s

Given the likely time scales, do you reckon that'd be challenging?

To me, that sounds like an awful lot of acceleration would be needed.

[wongarsu]

Getting from earth's surface to low earth orbit is about equivalent to accelerating by 9 km/s, in terms of fuel. Some it lost to drag and gravity, but at the orbit of the ISS you have an orbital sites of about 7.6 km/s.

If you could teleport a fully fueled Saturn V into orbit, it could speed up by about 18 km/s. More if you reduced the payload.

Of course going from 18 to 30 exposes you to the tyranny of the rocket equation: to go faster you need more fuel, which makes your rocket heavier, which means it needs more fuel, which makes your rocket heavier, ... Any small efficiency increase matters a lot here. It might be possible with today's tech if you build the rocket in orbit (which gets rid of aerodynamic constrains), or alternatively with orbital refueling and a couple more decades of progress in more efficient engine types (which you could greatly accelerate by throwing money at it).

[pfdietz]

Just using chemical rockets, speeds at infinity in excess of 100 km/s are possible using the Oberth effect: go into an elliptical orbit that passes close to the Sun, then accelerate at perihelion. Because the change in kinetic energy is thrust times velocity, and velocity is very high, large amounts of energy are added to the vehicle. The source of this energy is that the ejected propellant is left at low gravitational potential relative to where it was initially.

Solar sails released at perihelion can also achieve very high speeds.

Slowing down in the target system could be done with solar sails or electrodynamic sails (working on the stellar wind).

Ion or plasma engines are another possibility, with nuclear power plants.

[pdonis]

There is also the issue of losing speed as you climb up out of the Sun's gravity well. You gain some of it back by falling into the close approach star's gravity well, but from what I can tell, the close approach star is likely to be considerably less massive than the Sun, so you won't gain back all of what you lost. Which means you need even more rocket power.

[EForEndeavour]

Changing speed by 30 km/s would "only" require 51 minutes of acceleration at 1 g (9.8 m/s/s):

https://www.wolframalpha.com/input?i=%2830+km%2Fs%29%2Fg&ass...

[justinclift]

Interesting, thanks. :)

[pfdietz]

The required accelerations would be quite small, given the time the trip would take.

[alserio]

Sorry, I'm not getting this. The star will move away from what? Wouldn't the spaceship move with the star? I apologize if I'm misinterpreting what you said

[mtlmtlmtlmtl]

Once you've colonised the star system during its close passage to your own, it will continue its journey and eventually pass nearby other star systems, which can then be colonised as well. So every colony will spawn more colonies, leading to exponential growth.

At least I think that's what they meant. It's an interesting idea for sure.

[pfdietz]

Yes, that's exactly what I meant.

[0cf8612b2e1e]

I think they mean that if you start in solar system A, when solar system B flies by, you enter its gravity and colonize the system. Eventually solar system B will fly by solar system C, where you can repeat the process.

[dboreham]

B-Arc