Yes, the universe is indeed a big place and getting bigger all the time (not unlike my ex’s waistline). I’ve been told that most of the galaxies in the universe may actually exist beyond the Hubble Sphere. So, how would you feel if you spent months souping up your spaceship to exceed the speed of light, then go chasing after a distant galaxy at full throttle, only to have your shotgun-riding passenger yell, “step on the gas, grandpa, your losing ‘em!”
IOW, we’re probably not going to bump into the vast majority of our universe-mates…
True; we only have one data point. We have some hints as to other behavioral patterns from other animal species on earth. We can imagine a civilization run by cats, or dogs, or Mynah birds, etc. This expands our imaginary modeling of possible alien civilizations.
Or…yeah! What you said!
We know that aggressive territorial expansion is one behavior pattern possible for intelligent species. There might be others.
A docile and sessile species probably won’t be building exploratory spacecraft.
An extremely vicious and hostile species probably (we can hope!) will destroy itself before getting out into space. (The odds are too damn high that this will be our fate.)
The “golden mean” is a curious species, but one that can curb its aggressive instincts. We might manage to become that species.
The science fiction author David Brin wrote a very useful formal paper (under the name Glen David Brin) on the subject of the Fermi Paradox, listing all the solutions he could find at the time of writing.
This remains one of the best overviews of the subject.
I like to collect solutions that Brin didn’t think of or know about; one interesting idea is described in a paper by Milan Circovic - the most persistent species on our planet are not sentient, but instead are well adapted to survive in particular niches - sharks and crocodiles, for example. A sentient species that values survival rather than intellect could adopt such a lifestyle, losing the expensive processing tissue that allows abstract thought.
No need to apologize. We actually prefer that you excerpt, without changes, the part you are responding to.
That was intended as a hint to the people who DON’T excerpt and quote the entire bloody post. :mad:
I’m sorry, you were saying?
Short answer, we still don’t know enough even to reasonably constrain the parameters, much less answer the question. Here’s what we could do to narrow the error bars a bit:
[ul]
[li]confirm liquid water exists in the rest of the Solar system, and see if it contains life. If it does, then we know for sure that abiogenesis is inevitable in the right conditions. If we find liquid water but it’s completely sterile, then we know microbes require special circumstances to form[/li][li]Expand our deep space imaging capacity in all wavelengths by at least two orders of magnitude. Narrow down the possibility that large artificial sources of waste heat are out there, modulated electromagnetic signals, and whether any exoplanets we can spot have biological signatures[/li][li]Figure out not as an informed guess but as a confirmed experiment exactly how you go from simple organic molecules (like amino acids, nucleotides, sugars, lipids and phosphates), and end up with a Von Neumann machine[/li][/ul]
To refine this a bit, we should distinguish between microbes found in extraterrestrial locations which are related to Earth life from microbes which are not. The Solar System may be chock-full of bacteria, all of which are descended from Earth life; this would tell us precisely nothing about how widespread life is in other systems. If, on the other hand, one or more examples of life with a demonstrably different origin can be found, then we might guess that abiogenesis is a commonplace event.
An excellent idea. I would like to think that the first signature of life that we ever detect outside our system will be the thermal emissions from an advanced civilisation.
But I’m not so hopeful about biosignatures. Even decades later we still don’t know if Viking discovered signs of life on Mars, and the Martian meteorite ALH84001 is still a source of controversy, depite close examination. How can we hope to distinguish biosignatures at a distance of many light years?
Another good idea, but also likely to be inconclusive. There may be numerous paths from non-life to life, and without a time machine we might never know if we have found the right one.
I suspect and hope there are many routes to abiogenesis, and if we ever get to examine other distant biospheres, we might with luck find some where abiogenesis is actually in progress. If so we might be able to build up a classification system for such events - there might be so many different types that each one is unique. If so we may never know exactly how our own biosphere originated.
Actually, I’m not even sure we’ve found intelligent life on earth here yet! 
“Are we alone in the Universe?”
from 2006-ish, but the all science and hypothesizing about the universe is still true.
introduced by a UK physicist, Sir Martin Rees. episode 1 of 3 in the series.
Economist Robin Hanson proposed the concept of the ‘great filter’ to explain why we might be alone. The great filter is a thing that stops progress of civilization, or which prevents civilizations from forming in the first place.
If it’s the latter, then we’re the lucky ones - we made it through some kind of very improbable event. Life on Earth was simple until the Cambrian explosion. Maybe that was a one-in-a trillion event. Or we’ve just been rolling the dice for four billion years and coming up sixes over again in terms of avoiding some catastrophe - impact with another body, gamma ray burst, solar flare, whatever.
The worrying part is considering that the great filter is still ahead of us. Maybe lots of civilizations get to be at about our level, and then get wiped out. Maybe it’s mutating viruses and bugs that finally get the best of us, or just that the advance of technology increases the ability to destroy ourselves much faster than the ability to protect against that destruction, and accident or malice wipes us out.
Or maybe there is some future physics or biology or nanotech advancement we inevitably stumble upon which unleashes something that kills us all. Or maybe an alien race that doesn’t like competition and has a habit of wiping out any planet that shows any sign of real space-faring capability.
Of course these aren’t the only options.
I’m still quite drawn to the scenario that we are among the very first intelligent life. Because as old as the universe is, there are a number of requirements, such as needing at least a second- maybe third-generation star, that really push forward how early life could start.
So we’re destined to leave behind mysterious and powerful artefacts for the plethora of sentients that there will be someday, before disappearing equally mysteriously 
Or, alternatively, as Zach Weiner, esteemed cartoonist would point out, maybe societies just discover some very risky behavior that’s just more fun that living. Something like wingsuiting on LSD.
I liked Larry Niven’s fictional explanation: 3.5 billion years ago, all multi-cellular life in the galaxy was wiped out, and spacegoing species are only now re-evolving.
Yes, I agree with this point of view. The Fermi Paradox seems to involve a time bias that says that we are late to the party, and that the galaxy, and the universe should be teeming with intelligent, technological life. I just don’t see it.
A lot of preliminary events had to happen before even the elements that compose our solar system and our life could occur. And then it has taken fully 1/3 of the known age of the universe for intelligent life to evolve and develop on this little blue dot so we can have this conversation. Maybe 5 billion years is just how long it takes once you get the appropriate rock in the goldilocks zone.
If we as a species survive long enough to spread beyond this solar system to a point where localized extinction events will not eliminate us, then we will see if there are others like us.
And rather than being a young race that will be introduced to the greater galactic community, which almost all of our speculative fiction postulates, we will find that we are the Ancient Ones, the early ones to the party.
Firstborn, babysitters of the galaxy.
It wasn’t all multi-cellular life; it was all life with a mind (strictly speaking, all life with a mind subject to telepathy). Stage trees, for instance, are multicellular, but survived that event.
It’s still not an adequate explanation, though, at least not with that timescale. Starting from a species like us, it would only take a few million years for that species to expand across the Galaxy. There are some constraints on the speed of evolution, I’m sure, but over billions of years, some planets are going to end up a heck of a lot more than millions of years ahead of others.
An interesting idea, but is it supported by the facts? Searches for extrasolar planets are finding some surprising data- including planets around quite low-metallicity stars that were formed in a much earlier era of the universe.
Small, rocky worlds like the Earth might even be commonplace in low metallicity systems, which do not have tend to have large gas giants. If rocks in the goldilocks zone is the only requirement for life, then there could have been thriving biospheres many billions of years ago.
But without abstract thought, they would no longer be sentient.
And even if there are sentient creatures out there, they can’t see us.
Maybe they can see the Earth, but they can’t see modern humans. If they are looking at us from 200 light years away, they are seeing us 200 years ago–riding horses and using sailing ships to transport slaves from Africa.
Why would they want to come visit us?
There’s also the matter of time. Our universe is quite young – a mere 13.7 billion years old, with enough matter & potential energy to keep creating new stars and star systems for the next 1 to 100 trillion years (assuming the Ever-Expanding Universe theory is true – if our eventual future’s The Big Rip or The Big Crunch, it’ll all be over much, much sooner!) So while Fermi’s Paradox may hold true, that intelligent life will colonize every corner of the universe eventually, there’s plenty of time remaining for that.
Carl Sagan was real big on that. Personally, I believe our modern civilization was extremely lucky to pass through the Nuclear Era relatively unscathed; no doubt due to the fact that we actually did nuke a couple cities during wartime, so we had a chance to directly observe the aftereffects. Even so, we’ve come pretty damn close a few times since then, and we’re not out of the woods yet.
That’s right. The idea is that sentient species regularly become ‘post-sentient’; rather than striving forever to become smarter and to take control over and responsibility for the natural world, a species might instead find a way of becoming successful at merely surviving without forever relying on their wits.
I doubt that ‘post-sentience’ happens very often, but it might happen sometimes.
This brings us to one of the great problems with almost all solutions to the Fermi Paradox; any particular solution might work to explain why some, or many species do not conquer the galaxy, but very few of them explain why none of them do so.
Even if there is a Great Embargo that prevents civilisations from contacting Earth, for instance, why should all the different races that have ever evolved obey this embargo? If some species self-evolve into post-sentience, why should they all do so? If some civilisations prefer to stay at home, inhabiting increasingly elaborate virtual worlds, why should they all do this?