Universe puts on a happy gravitational lens.

[Sean]

That's one reply to the Fermi paradox.

For the record, Carl Sagan's wackier one was that the von Neumann probes were busy at war destroying each other.

[North Star]

I'm pretty sure there aren't too many intelligent lifeforms in the universe. Something like our bacteria, sure.

[Jo498]

There are too many unknowns. About 20 years ago it was still believed that habitable planets were exceedingly rare. Back then this was one of the main possible explanations for the Fermi paradox: just too few earth-like planets. Now we know that they are not rare at all.
But there may be other factors where our estimations are way off. E.g. the actual difficulties of interstellar travel even for robots or replicants. Maybe also the actual difficulties of AI and replicants (I think most of the wet dreams/nightmares of "transhumanists" are way off, at least in timescale, very often in feasibility.)
Maybe also the actual difficulties of sustaining high tech civilizations at all, even on their home planets! (Remember that Einstein quote about World War IV being fought with sticks and stones) Maybe some biological factor during the evolution of (intelligent) life.

[North Star]

There are too many unknowns. About 20 years ago it was still believed that habitable planets were exceedingly rare. Back then this was one of the main possible explanations for the Fermi paradox: just too few earth-like planets. Now we know that they are not rare at all.
But there may be other factors where our estimations are way off. E.g. the actual difficulties of interstellar travel even for robots or replicants. Maybe also the actual difficulties of AI and replicants (I think most of the wet dreams/nightmares of "transhumanists" are way off, at least in timescale, very often in feasibility.)
Maybe also the actual difficulties of sustaining high tech civilizations at all, even on their home planets! (Remember that Einstein quote about World War IV being fought with sticks and stones) Maybe some biological factor during the evolution of (intelligent) life.

For a life form to have a chance to develop into something as complicated as mammals, a lot of time is needed, and the planet has to have a suitable atmosphere, temperature, and water content during all of that time, not to mention the energy resources, and avoid collisions with large asteroids. And they would also need to avoid developing the TV.  0:)

[Jo498]

Sure, but these factors were also known 20 years ago. The big difference is that the frequency of earth-like planets was estimated to be several orders of magnitude lower than it seems to be from our discoveries of many such planets. But all those factors have such an uncertainty about them that an error of 10^6 or so may not matter much. But in any case, because of the discovery of habitable planets the Fermi Paradox is now about a million times more puzzling than it used to be

[Rinaldo]

I still don't find it puzzling at all. Count out the lifeforms that don't evolve into self-conscious beings, count out the ones who don't survive such evolution (as you mentioned, we might very well exterminate ourselves / set ourselves back by war, pollution, climate change or a combination of many different factors), count out the ones who will get wiped by astronomical events they can't control, count out the ones that already achieved a state that is beyond our grasp (and we'd be ants to them, not worthy of any recognition).

Even when starting with an immense number of Earth-like* planets (and we still don't know if an Earth-like planet automatically gives life), you'll be left with only a handful (I'd say thousands, but that's just a wild guess) of civilizations that might attempt to explore their cosmic surroundings. And not necessarily succeed on stumbling upon us.

* which also doesn't mean simply a planet in the Goldilocks zone.. there are other variables that helped life on Earth to strive - Jupiter positioned so conveniently that it sweeps most of the stuff that would obliterate life on Earth again and again, for example

[Sean]

It may seem like there must be other life out there because there may be 10^22 planets, but the origins of life and the likelihood of it evolving on any one are unknown. And if that turns out to be 10^30 to one then it'll be 10^8 or 100 million to one against there being any other life anywhere else in the universe. I can buy a thousand lottery tickets, which is a big number to me, but it doesn't mean I'm going to win because the real odds are far higher.

Barrow and Tipler in their Cosmological Anthropic Principle book argued from quantum physics that the universe only produces intelligent life once.

[drogulus]

     On the other hand, in a vast ancient Universe everything that can happen does, many times.

[North Star]

     On the other hand, in a vast ancient Universe everything that can happen does, many times.

But not necessarily so that they overlap.

[Jo498]

@Rinaldo: I completely agree. I just wanted to point out that the fact that we have revised one term (# of habitable planets) in the equation for the probability of extraterrestial life/possible contact by 6 orders of magnitude or so within only 20 years shows how uncertain most of those factors are.

@drogulus: vast and old does not mean actually infinite in spatial and temporal directions. And like FTL travel there might be more things that are not only unlikely but physically impossible, so they will not happen even in an infinite universe.
And as North Star points out, for us it is enough that light cones do not overlap so we will never encounter the stuff that is too far away.

[Sean]

Of course we could already have been visited, as Ufologists will insist. In recent years I've seen all the documentaries and presentations on the Rendlesham Forest incident of 1980, great stuff.

I'm also a passing enthusiast for the ancient lunar and Martian ruins hypotheses.