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Comment by cgore | Hacker News Reader

I have trouble believing there's more than a handful of spacefaring species per galaxy. Even without FTL travel, one species could fill up all of the habitable planets in a galaxy the size of ours in a few million years or less, thanks to the fact that we breed exponentially. And there's been Sun-like stars for a while now in our galaxy, presumably with Earth-like planets orbiting them. So it's a matter of getting there first.

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I wonder if there are intelligent species that are incapable of developing spaceflight due to environmental reasons. For example, an intelligent water-breathing species on a planet that is quite a bit more massive than Earth.

A ship filled with water has a lot more mass than a ship filled with air. That extra mass compared to what we face, combined with starting from a high gravity world, would mean they would need significantly better propulsion systems than we need to get to orbit.

Could this mean that chemical based rockets would not be sufficient for them to do manned spaceflight? If so, they may never establish a manned orbital presence, and a manned orbital presence may be necessary as a stepping stone to interstellar travel.

If there are a "handful," then there are probably 0 (besides us). You have to believe that there's a very narrow range of values to get precisely to a handful.

Like, the range quoted above is from "10,000 to a few million," so a factor of 200ish from high to low range.

If you think that there are 10, per galaxy, why don't you believe instead that there are 0.3 per galaxy (only off by a factor of 30?)? How much confidence can you have in your highly speculative variables that your output can be constrained down to just a single order of magnitude or so?

By handful I mean < 5. Probably usually just one. Because no others would get a chance to evolve, with all the habitable planets quickly being occupied by whichever species figures out interstellar travel first.

I think you have a very Golden-Era-of-Sci-Fi space-opera conquering view of spacefaring.

Why do we need to expand? Our population growth is already levelling off.

Why do we need to send _our bodies_ rather than with AI, or with our consciouses uploaded into computers? (a la Charles Stross' Accelerando or Greg Egan's Diaspora)

If we're sending AI probes around, why do we need to take all resources to the extent that we prevent other species from developing?

Most of the fundamental science that we want to do can be done right here in our own solar system. There's just not a need to expand.

Once you have sufficiently advanced technology, unless you also have some kind of totalitarianism, it only takes a few groups of people who disagree about the allocation of resources (or, really, anything else) in this star system and want to leave. And then the same thing happens recursively in whatever star systems they go to, and you have exponential growth until it saturates to cubic or whatever.

Yes, of course, they will be AIs or uploads or body printers. Even for an advanced civilization, shipping bags of meat to other star systems would be wasteful. That doesn't mean they won't want to use resources, or that there would be no evidence of them if they were already here.

edit: There's certainly scenarios in which the first species to explore a galaxy doesn't literally prevent other species from evolving. But that doesn't mean they wouldn't be obviously detectable. The likely number of starfaring species in the galaxy given our observations is zero.

> There's just not a need to expand.

It doesn't take a need, just a desire. Someone wanting to be able to say, "I went there. I saw it. I got the T-Shirt."

They don't need to be spacefaring, just broadcasting. Of course if there are 10,000+ aliens advanced enough to broadcast radio and laser, we do have to wonder why this hasn't led to the galaxy being filled up over the past 5 billion years. Drake's answer to this is that space travel is too expensive, cramped and lengthy to be worth it.

A couple of assumptions here. The first is that our solar system has habital planets from their perspective, I wouldn't completely rule out that there chemistry could be very different to ours. The second is that they need habitable planets. The idea of living in under ground tunnels isn't palatable to us because we evolved in wide open savanahs, an underground, underwater or nocturnal species might not feel the same.

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