I'm not arguing for either position right now, but, if I were to make an argument about "we're the only civilization in the galaxy", I think I'd be basing it on the idea that "civilizations don't last all that long, all things considered." Obviously, it depends on what you call "civilization". If we say that "civilization" started with the Neolithic revolution, then we're saying that civilization started 10,000 years ago, more or less. Let's say that we're still babies -- let's say that we're still only 10% through the way of human civilization.
That would mean that human civilization would last 100,000 years. If civilization on a planet lasts 100,000 years, then that's just a tiny, tiny blip in the history of a planet.
Of coursed, that could be terribly pessimistic. Maybe, once you have civilization, it just goes on and on forever, which would completely change things.
Or maybe humans will last for another 30 million years (let's say that "humans" starts with homo habilis, about 3 million years ago, and, again, we're only now 10% through). Still, is 30 million years all that long?
Or maybe, after humans die off, another creature will evolve and have another civilization. So that would make the statistics a lot better. Maybe there will always be some sort of civilization on Earth from now on, even if it's not always humans. Maybe there are LOTS of civilizations out there, because there are lots of stars about the same age as ours, and there are lots of planets where something similar to on our planet happened, and so there are tons of places where things started to become people three million years ago, and started to have civilizations ten thousand years ago, or a hundred thousand years ago.
In any case, there are lots of options. One guess is that there are maybe 20 billion planets in our galaxy that can support life like us, and who knows how many planets that could support slightly different kinds of life. Maybe there are a hundred billion planets out there that could have life.
If there's a one in a million chance of life showing up, and a one in a million chance of intelligent life showing up if there is life, then there would STILL be twenty civilizations out there.
My gut feeling, though, is that life will show up almost everywhere that it possibly can, and that life will almost always get more and more complex, and that one strategy that complex life can use to get more success is to be able to interact with its environment in more flexible ways, which is almost always going to lead to intelligence, which is almost always going to lead to civilizations.
So I personally think that there are billions of planets with civilizations. If there are a billion planets which could ever have civilizations, and a planet lasts a billion years, and a civilization lasts 100,000 years, then there would still be ten thousand civilizations in the galaxy.
It's all guesswork, though. It's all based on your gut feeling of how common these things are. Really, my actual guess is that there are somewhere between 1 and 100 billion civilizations in the galaxy.
The problem with talking about civilizations is we don't know what a completed one would look like. Is ours old or middleaged or juvenile? Not having anything to compare it with, we've really no idea. I suspect a truly advanced civilization would be so far beyond our understanding we might not even know it was there.
We've seen that life can flourish in all sorts of contexts that would kill us- underwater volcanic vents for example. This suggests there could be civilizations everywhere- including in some (to us) highly unlikely places.
If we define "life" as "any repeating pattern which uses resources from the environment to make occasionally-imperfect copies of itself" -- and I do -- then we can imagine a LOT of places that life can exist other than where we live.
(Occasionally-imperfect copies allows change, which allows evolution, which I think is necessarily a part of life. "Fire" isn't "alive", because it only ever makes more fire, and never anything that is different fire.)
I don't have a definition of life. I'm not sure about reproduction being one of its necessary functions. I'll have to think about it.
His premise was (and I did watch the programme) that simple life like bacteria is probably fairly common, but advanced multicellular life may depend on a very flukey encounter between two types of cells which I think he called the "fateful encounter" i.e. a primitive cell with what became the mitochondrion. The chances of one type of cell swallowing up another without it being killed and digested does seem small, but such a symbiotic event did occur, and it's the basis of all eukaryotic cells. Also he said that if there were other civilisations on other planets they may actually not last long enough to overlap much. So here we are , again on our own, as Whitesnake so memorably put it.
Yes, well, but who knows what's probable and what's not? We have so very little to go on.
Evolutionary biologist and theoretical xenobiology hobbyist (he likes working out possible alien races for science fiction stories) Dr. Jack Cohen suggests that things which only developed once on Earth are "parochials", unique to here, while things which developed multiple times on Earth are "universals" which are likely to show up other places.
Among his universals are flight, fur, transformation of light into usable energy, and the transfer of genetic information among members of a species. He refers to them as "the four 'F's -- flight, fur, photosynthesis, and mating."
That seems like an absurd premise. Because organisms running into each other and becoming new organisms happens lots and lots of times. You've got RNA combining into other forms, organelles combining into cells, cells combining into multicellular structures -- if a thing happens over and over on Earth, then it's likely to be a thing that life will come up with other places, too. Smaller independent structures combining into complex structures is one of the basic mechanisms of evolution. It can't be that unlikely, given how often it happens.
In other words, by the time that you get to mitochondria, you've already gone through, like, three or four levels of "fateful encounters". They cannot possibly be all that rare if they happen that often.
i started looking for brains shows via the internet
we get just a smidgen of things from your end of the world via tv.
we're 3 dr who's behind.
mike asked me what do you think about god
and i thought you might's well ask me to define art.
what is art... what is god....
i could not give him an answer he wanted to hear
people expecting an omnipotent being
in charge of the universe to be focused on them as a specific point?
think they're being unrealistic..
its like the people here assuming the terrorists
are going to knock on their door just for them.
none here is that important other than out figurehead/president.
now groups of people that is a more enticing target
or taking over a country.....
the earth is marked quarantined till we stop poisoning and killing
just not a safe place for a civilized alien...
I don't suppose the aliens are that bothered about us. If they've got interstellar flight they're also likely to have whatever they need to defend themselves against out violence.