[sevensevennine]

I don't follow the last point in your argument.

Isn't 4 equivalent to, 'no civilization will invent casual interstellar travel/spread to the stars'?

And isn't that hypothesis extremely likely, given what we now know about the costs of _interplanetary_ travel and its relative ease compared to interstellar travel?

That's another assumption, that it's _possible_ to colonize a galaxy from a planetary base. We don't even know that it's possible to colonize a nearby planet. If we were confident that we could, we'd be filling up Siberia, Ellesmere Island, and Greenland first.

There's also another assumption missing from your list- that technological civilizations can last long enough to colonize the galaxy. I'm also surprised that there isn't any discussion on the site or here of Great Filters. If the average technological civilization wipes itself out within a few hundred years of developing technologies that enable space travel or even radio, then all discussion of "filling the galaxy" is castles in the air.

[pfdietz]

> And isn't that hypothesis extremely likely, given what we now know about the costs of _interplanetary_ travel and its relative ease compared to interstellar travel?

Costs are a function of manufacturing productivity. What is the upper bound on manufacturing productivity? With automation and AI, I don't see any hard upper bound.

The raw resources are certainly available to build starships. I mean, your share of per capita energy consumption over your life would be enough to accelerate your body to maybe 700 km/s, and that's with us just using a small fraction of the energy available on a planet; energy in space would be many orders of magnitude more abundant.

[eschaton]

It also could be that interstellar travel is possible but never inexpensive enough to be casual or useful for ever-expanding colonization. Or it could be that civilizations stabilize before the point where ever-expanding colonization becomes attractive.

As an example of the latter, look at birth rates in different societies on Earth: Almost universally, they decline to replacement level once they hit a certain level of per-capita wealth.

It’s very likely that a society that achieves interstellar travel will do so after it achieves the ability to provide the highest standard of living for all of its members indefinitely using just the resources of its local system. This already describes Earth; the reasons we don’t do this are ideological, not based on any inherent constraints, while interstellar travel isn’t in our grasp yet and is likely to be extraordinarily costly.

Such a society wouldn’t face any pressure to grow, so any colonization would itself likely be ideological—“We don’t want to do things Surak’s way, let’s pull up stakes and find a world where we can live the way we want!”—or as a contingency/hedge against large-scale existential risk. Neither demand colonizing even a small fraction of a galaxy, assuming habitable worlds are even remotely plentiful near and reachable from the origin world.

[Veserv]

You have missed the point. The goal is humanity becoming Star Trek and enclosing every star in a Dyson Sphere; the goal is determining whether 4 is true or false. The question is what is stopping it. The entire point of these thought exercises is setting up proof by contradictions/falsifiable experiments to narrow that down.

Maybe (4 is false) is inevitable now that humanity has reached its current point. Maybe literally every human-like species will become Star Trek and enclose every star in a Dyson Sphere; that would be awesome since it means our goal is now a foregone conclusion. But how would we know? Well, we can do a thought experiment assuming it and extrapolate toward characteristics that we might be able to detect to falsify our hypothesis.

Well if literally every human-like species will enclose every star in the future, then we should not be able to see any stars at some point after one comes into existence. We can still see stars, therefore either it has not happened yet (1), there are no other human-like species (2), or the supposition is false (3). If the supposition is false, then there are risks ahead of us. If the supposition is true, then the risks are behind us, but then (1) or (2) must be true. For (1) to be true, we must either be early or it takes a long time. For (2) to be true, we must either be early or rare. If we can rule out (1) and (2), then we must conclude (3) which means we can be confident that there must be a risk ahead of us preventing us from reaching our goal that we as a species need to be wary of even if we do not know what it is specifically. We just know it has to exist otherwise we would see no stars.

If (1) or (2) is true, then (3) no longer needs to be true. It might still be the case that the goal is impossible, but at least we have a chance. The point of this analysis is trying to theorize where we need to look that gives humanity the most information about if and how to become Star Trek. It is not about coming up with the "correct" answer; we do not have enough information for that. We are trying to create theories that take facts that we can find (or can in theory find) as inputs and generate predictions that can be tested and falsified.