This is how you get Dark Forest strikes

[canpakes]

Aliens coming here to impregnate our women and then take over? There has to be some psy-fi movie/book somewhere with this as a plot line.

I believe that Tucker Carlson rants about this very thing at least once a week.

[Physics Guy]

I'm skeptical of any attempt to put numbers on the likely density of intelligent life in the universe, because you get a product of so many uncertainties that the result isn't really any better than a guess. But I have a hard time believing we could really be alone in the universe.

Yes, there are some things about Earth that are likely weird. The composition of our sun doesn't seem all that unusual; most stars we see now are third-generation stars with heavy elements from previous-generation supernovas. The standard nebular theory of solar system formation makes terrestrial inner planets with outer gas giants seem generic, too. Beyond that there doesn't seem to be anything terribly special about our distribution of planets. So I have a hard time seeing how these kind of factors could make us as rare as one system in a million. Conceivably the size of our moon is a bigger fluke. That seems pretty odd, and maybe it's important for giving us tides.

The fact that life only seems to have evolved on Earth once doesn't necessarily indicate that it's a one-in-many-billions-of-years fluke. It may just be like having a fire only start once, in a forest. Once life evolves, it proliferates to the point where it chokes competition before any rivals emerge.

But it could indeed very well be that intelligent life is rare enough that our nearest neighbours are really far away. And it could be that there are no good ways for traveling that far. In that case nobody is going to launch an interstellar mission that would bankrupt their whole civilisation just to kill off a distant rival. Maybe all we'll ever be able to do is wave at each other now and then, with centuries of time lag.

[Analytics]

The fact that life only seems to have evolved on Earth once doesn't necessarily indicate that it's a one-in-many-billions-of-years fluke. It may just be like having a fire only start once, in a forest. Once life evolves, it proliferates to the point where it chokes competition before any rivals emerge.

That's certainly possible, and scientists have noted that life seems to have gotten a toe-hold on this planet at the very earliest time it could have. They then argue that is evidence that once the conditions are right, life is inevitable. Maybe, but as you said, we are dealing with a sample of 1 and really don't know.

What I can't get past is how unlikely it was for us to come out of the process. I note that SETI thinking began in the 1960's, which is the same decade that Pierre Boulle wrote Planet of the Apes. That book depicts a world where chimpanzees, orangutans, and gorillas all evolve over the course of a not-too-many centuries into a multi-species civilization with technology and culture that is pretty-much like ours. The whole thing is based on the terrible idea that evolution is all directed towards human perfection.

The reality is that over the last 3 billion years, a string of highly unlikely evolutionary and earth-history events led to us. Prokaryotic life evolving into eukaryotic life. Eukaryotic life evolving into animals. Just the right species surviving a long series of extinction-level events. If any other needle would have been threaded, we wouldn't be here. And if we weren't here, would another animal have risen to the challenge of mastering electromagnetic technology? It doesn't seem likely.

But it could indeed very well be that intelligent life is rare enough that our nearest neighbours are really far away. And it could be that there are no good ways for traveling that far. In that case nobody is going to launch an interstellar mission that would bankrupt their whole civilisation just to kill off a distant rival. Maybe all we'll ever be able to do is wave at each other now and then, with centuries of time lag.

Looking at it from another angle, what are the chances that within the next several hundred years, we'll develop the technology to send a probe into space that has the capabilities of collecting energy and matter, repairing itself, and reproducing itself? That seems pretty high. And what are the chances there will be some eccentric trillionaire who will make it happen? That seems pretty high, too.

Say these probes will be able to travel at the speed of Voyager 1, which is about 10 miles per second, and that each probe multiplies itself exponentially every time it gets to a new solar system. If that is the case, then within about 2 billion years, there will be fleets of these probes in every solar system of the galaxy, and the entire galaxy will be pulsating with radio waves from these probes.

When we look up into the sky, we don't see fleets of probes from other planets. We see nothing. This implies that as-of 2 billion years ago, this galaxy had precisely zero civilizations that had the means and inclination to launch such probes. So even if there is other intelligent life out there (intelligent like modern humans, not intelligent like dolphins, nor intelligent like humans 100,000 years ago), they must be relatively young.

Or maybe they aren't there.

[Moksha]

Could some weird alien influence, seeking to devolve us to the Dark Ages times, have caused modern Republicanism?

[Analytics]

Could some weird alien influence, seeking to devolve us to the Dark Ages times, have caused modern Republicanism?

That actually seems quite likely. You can't really expect me to believe Donald Trump is an earthling.

[Physics Guy]

Hmm, I hadn't thought about filling the galaxy with self-replicating probes. The idea is clearly not that it would be easy, just that it might be feasible enough within a few centuries to be nearly inevitable over a million years.

At speeds on the order of 10 miles per second you take on the order of ten thousand years to cover a light year, so it should indeed take only about a billion years to flood the galaxy. So it should already have happened.

Would we really notice this if it were happening, though? I don't really want to do the calculation now, but I'd bet a beer that it would take a lot more energy to send a signal that could be picked up many light years away than just to replicate a self-replicating probe. So maybe eccentric trillionaire probe swarms have indeed flooded the galaxy many times, and we've just been oblivious. Maybe galactic civilisation treats self-replicating probe swarms as a form of littering and slaps trillionaires with misdemeanours for it all the time.

My track record on physics beverage bets is poor, though. Perhaps this eccentric trillionaire self-replicating probe idea is a sharpened form of Fermi's "Where is everybody?" Paradox.

[Manetho]

You can't really expect me to believe Donald Trump is an earthling.

He is far too human, in all the worst ways, to be an alien. But perhaps aliens constructed him as a sort of racist caricature of a human being.

[dantana]

So, does the fact that there should be be aliens out and about but aren't bode better or worse for the - God did it - crowd?

[Moksha]

Trump possibly views life from the perspective of Sauron's Eye.

[Dr. Shades]

Also, what the Hell is a "Dark Forest strike??"

It’s from the second book of the trilogy that begins with the Three Body Problem. In the book, advanced alien races that discover the existence of other planets with intelligent life simply obliterate the planet as a survival strategy.

Thanks, Res. Am I the only person on this message board who hasn't read the book and thus didn't get the reference?