Whenever we talk about alien life, we should always, absolutely always limit our discussion to the Milky Way. Any alien life outside of the Milky Way will remain completely irrelevant for us for any timespan that has any meaning for us as humans. Just hearing "You had me at Hello" back from the Andromeda galaxy would take 5 million years. 5 million years ago humans and chimps were still not distinct species.

I get what you're saying as far as "practical" purposes go. It's also arguable that (in the spirit of your argument) the outer reaches of the Milky Way are also irrelvant.

However, when people sit around and think about the possibilities of alien life, or rather - the probability that alien life exists outside of Earth, it's *100%* relevant to consider galaxies outside our own.

However, I absolutely hate it when people say "We are perhaps/probably/certainly alone in the universe" and then go on to explain why (they think) we are perhaps/probably/certainly alone in the galaxy. These two things are not even slightly the same! Even if there's only on average one technological civilization per galaxy (say), we might well be alone in the milky way but share the universe with a wondrous number of other civilizations.

But what is the relevance of knowing that? Why would anyone care? It's a bit like the question of how many angels can sit on the tip of a needle. Existence of aliens in a different galaxy has absolutely zero consequences for us now and forever. We may "share the universe" in a theoretical way, but we don't share anything in the real sense of the term.

I find it much more fun to speculate about and imagine things that potentially really exist. Aliens in other galaxies exist with 99.99% plus probability. Angels don't exist and are of no interest to me.

no necessarily, assuming a galaxy wide expanding type II civilization that converts every star it encounters to a Dyson swarm as the tracked intelligent civilization. they will easily take a few 10s of Million years to colonize their galaxy. in this process we should be able to see such transformation 'in real-time' via changing radiation signature of the region.

Fermi-paradox is paradox for a reason!

> Fermi-paradox is paradox for a reason!

The Fermi paradox is just a way to have fun at parties (or over the internet, especially on HN). There are so many possible solutions to the Fermi paradox. But each solution has questions, and answers, and more questions and more answers. That's why it's so fun.

But it's not that deep.

To the point, the Dyson swarm is virtually guaranteed to not be used by any advanced civilization. Once a civilization tames nuclear fusion (and we are on the brink of doing so), then harnessing the energy produced by the local star via a bunch of solar panels becomes just a clunky way of getting energy. Controlled nuclear fusion can be many, many orders of magnitude more productive and convenient compared to solar energy. A star is indeed a massive fusion reactor, but it's a very slow one. Our Sun in particular produces about as much energy per pounds as the heat produced by the fermentation of manure. ITER is projected to produce hundreds of millions of time more. And ITER is just the beginning. Sure, the Sun contains most of the matter in the solar system, but plenty exists in the planets. We could use hydrogen stripped from Jupiter to produce trillions of times more power than a Dyson swarm could produce, and we can have that in a portable form that we could take with us in interstellar travels.

Bottom line, whoever is looking to find Dyson spheres out there in the sky, is simply wasting time.

I think nuclear fusion may not be a very efficient method of converting mass to energy for very advanced civilizations. In absence of discovery of large amounts of antimatter, a super-advanced civilization is likely better off taming small black holes that can convert matter into heat. It doubles as a waste disposal mechanism as well.

its been a few days so doubt you'll read this but, even if you have really advanced fusion tech what people often overlook is that ultimately the civilizations are just heat engines. They would still have a heat signature & as you described that would be orders of magnitude more that their respective stars. you could literally track the growth of a type-II civilization by the radiation signature as long as they are emitting anything above CMB.

Also, yes you dont have to look for Dyson spheres but you'll agree that Sun has 99.9% of the mass of solar system so it would make sense to turn to it after you have 'used up' Jupiter. And yes you can extract mass out of a Star with currently know physics, its just big project. THAT should be visible from far away.

ultimately I doubt there is any stealth for cloaking a type-II civ so whats the best bet? you go the other way & expand as fast as possible. But either way we would have noticed .. at least close to our galactic cluster.

Since controlled fusion isn't mutually exclusive to using Dyson Spheres, why not use both?

It seems that constructing Dyson Spheres can only add to the amount of energy that an advanced civilisation is capable of harnessing even if they're also using controlled fusion, so it wouldn't be surprising if they used those unless their energy needs were totally satiated by controlled fusion (or some as yet undiscovered alternative source)

It's highly relevant from a perspective of philosophical and scientific curiosity, and mostly (seemingly) irrelevant from a perspective of practical impact on our lives.

I'm also cautious to jump to such conclusions given how nascent we are technologically, even though the light speed barrier does seem absolute. Our recent ancestors probably would've thought a similar thing about life orbiting a nearby star, and it's only with the hindsight of recent advances that we're able to conclude that that's not as far out of our reach as once believed. Who knows what one million years of additional technological advances will make us capable of?

Our understanding of the universe could be arbitrarily rudimentary which means actually advanced technology, say that which improves somewhat like ours for, say, a million years would just look like magic to us.

Unless they're some super advanced species that either intentionally or unintentionally destroys or sterilizes the universe.

Don’t forget that we can put a lower bound on the size of the actual (ie, not just the observable) universe and it’s many times greater in size. It could even be infinite, but at the very least it’s larger than we can see.

Two trillion galaxies (2e12) according to:

https://phys.org/news/2017-01-universe-trillion-galaxies.htm...

That would be on average 1 out of every 10 stars - hard to believe.

Our solar system has Earth and Mars. Venus is on the edge of the habitable zone. Had the asteroid belt formed a planet, it would be on the edge as well.

Right, but, what does it all mean?

There might be only one way to find out ;

https://en.wikipedia.org/wiki/Magic_mushrooms

It means we won the cosmic lottery.

If qualifying as a technical civilization can be defined as being able to launch a self-replicating interstellar probe (a/k/a a Bracewell-von Neumann probe), we can be confident that our galaxy has not yet had one. Once those probes are out there, they'll saturate the galaxy permanently; and once a technical civilization can launch one, it'll happen eventually. A modern billionaire could probably fund such a project now.

> We can be confident that our galaxy has not yet had one

How can we be confident of that? For all we know the Cambrian explosion was precisely that.

Or - there's tons of lottery winners and we're bad at finding them.

Maybe nothing, but just the fact that we're talking about it is something, IMHO.