[puglr]

It routinely bothers me a bit that, IME, most IRL discussions of the Fermi paradox tend to omit this rather simple explanation.

Half the time it's brought up, FTL is offered as the solution. Which as best we can tell is fundamentally impossible.

That squishy or otherwise organic bodies are generally unable to travel interstellar distances has always seemed to me to be the simplest solution.

Assuming intelligent life is out there, surely there are civilizations that have destroyed themselves and so on. But lack of FTL travel would be a common constraint, regardless of all other scenarios.

[jjoonathan]

It routinely bothers me a bit that, IME, most IRL discussions of the Fermi Paradox settle on this rather simple explanation when the whole idea was that this "simple explanation" was extremely unlikely, pushing speculation to more complicated and interesting hypotheses.

Emergence of life on Earth took 13.7 billion years, galactic colonization should only take millions of years. We should not expect to find the galaxy half-colonized, as this would be a staggering feat of synchronization. We should find the galaxy completely full or completely empty. It seems to be completely empty.

[withinboredom]

I doubt a civilization could survive being multi-starred.

If you can expend that kind of energy, a pilot with a bad day can destroy the whole planet. A pissed off colony in the asteroid belt can sling asteroids at the home planet, etc.

I also think you’re making a lot of assumptions but the speed of light is quite limiting in every aspect. If it takes 40 years to send a message, you need to either live a ridiculously long time — in which case your birth rate will be quite low — or figure the colonization as a one-way trip. No one would colonize another star system just for kicks, there would need to be a reason and I can’t think of a reason to colonize an entire galaxy that would make sense for a whole civilization, especially when it takes multi-decades just to send a message one way.

[jjoonathan]

> A pissed off colony in the asteroid belt can sling asteroids at the home planet

If planets become indefensible then people simply won't colonize them. Problem solved. They'll stay in artificial habitats, and I would expect this to accelerate the colonization of the galaxy, if anything, because then they wouldn't need to wait for a planet to fill up before sending out the next generation of colony ships, they would just need to wait for the prime asteroids to be claimed.

> the speed of light is quite limiting in every aspect

Yes, exactly, and the non-interaction limit only needs one ship of space mormons to become exponential growth

Diffusion-limited exponential growth, to add the next layer of modeling sophistication, but the ultimate point is that you still don't need all that many generations before the galaxy is full. Just multiply the number of steps by generation time, and if you are tempted to pick a really long generation time, remember that space mormons get a vote.

> No one would colonize another star system just for kicks, there would need to be a reason and I can’t think of a reason

Come on, the reason is resource competition.

[withinboredom]

> If planets become indefensible

Planets are indefensible, as is most of space if your orbit is predictable. All you need is a body bigger than your margin of computational error, and reaction mass.

> exponential growth

My point is that in order for civilization to be multi-starred, they'd need to communicate. Once a new star is colonized, due to the constraint on light speed, they become two separate civilizations. This means each star basically starts from the beginning, each time. So, if you estimate a couple of million years for a civilization to just to consider expanding... it'd take awhile. A lot more than a few million years to colonize the galaxy because not every star will consider expanding.

> the reason is resource competition.

Resources for what, exactly? Populations self-govern growth. It isn't always pretty, since over-population means a lot of deaths. But hungry people aren't going to say "I'm hungry so I'm going to build a space ship and hunt Space Cows. Who's coming with me?"

Further, there is no evidence that dinosaurs left this planet. They had 165 million years to do so. So, it doesn't 'just take' a few million years and there is no guarantee that a civilization will expand beyond a star. Sure, you could argue that dinosaurs probably weren't intelligent, but there's no evidence for that other than a small cavity size for brains; which doesn't necessarily mean they were stupid. Whales have bigger brains than humans, but I don't see them leaving the earth any time soon either.

My point is, even if you decide to colonize another planet, you are effectively cut off from your home. Technology WILL be lost since you basically have to restart manufacturing from nothing, with a small population at that, and there are a whole different set of filters to pass through. The odds of that civilization growing to the point of wanting to move on, without dying out itself, is remarkably low.

Interestingly, colonization of stars beyond a certain distance might even be impossible without some kind of artificial gravity or cryo. Especially if it requires multiple generations to arrive.

[shireboy]

I really like this thread. Both make good points. I still land on space being really big and really hard. Maybe so much so that multi-stellar civilization is next to impossible and multi-galactic moreso.

If you think resources are scarce on Kepler-452b, wait til you see how bad things on the Space Mormon ship. Especially since it’s 60% harder to exit the gravity well than it would be on our destination at Sol-3. We used all our lift capabilities just to get the basics to orbit. Once we get out there, it’s 1400 light years away, so will take us 5600 years to get to Sol assuming we can get the drives going full speed and still retain enough delta-v to brake in Sol orbit. If not, it may even take longer or worse - we zoom by as a blip at the edge of the system. The good news is our telescopes are pretty sure it’s habitable, but we don’t hear any techno signatures. So with any luck we arrive in 6000 years and have enough gametes in storage for the incubators to build a viable colony from. I just hope the kids behave once they get there.

[chrononaut]

> Emergence of life on Earth took 13.7 billion years

13.7 billion years? Isn't that the age of the universe, and didn't life emerge on Earth only after about a billion years?

[yreg]

Yes, but the parent means we are 13.7B years late to the party. Other civs should have been spawned before Earth was formed.

[jjoonathan]

The formation of Earth wouldn't have been the starting gun for alien life evolving somewhere other than Earth.

[pmontra]

Even with FTL you have to know where to go. Let's say you do 100 solar systems per year (going somewhere close to the star and some local jumps to look around.) There are 100 billion stars in the Milky Way. It takes a billion years to sequentially scan the galaxy. Optimizations in the search algorithm to cut that number down to 100 years are left as an exercise ;-)

[bottled_poe]

Branching factor of two means around 30 hops to visit every star.

[pmontra]

2^30 probes at the final hop. Maybe those FTL ships will turn out to be cheap to build like in Vance's Demon Princes novels.