I like that. A thing to keep in mind, too, is that if you were to observe the sun from 4 billion lightyears away, there would hardly be an earth. You'd have to get with in a few hundred million lightyears to even find any signs of life here, and we've only been here for 2,000 lightyears' worth of time-distance. We're seeing all those planets from hundreds of millions to billions of lightyears away. That's a lot of time for a civilization to grow, fall, grow again, fall again, and so on until it escapes the star or the star explodes.
Hundred of billions of times throughout the distance of the galaxy.
The problem is though, that's a
huge amount of time. In a mere few thousand years, we went from building mud huts to building spacecraft. In a mere few decades, we went from mechanical computers to devices so capable and widespread that we now have
raw computing power exceeding that of a human brain. A civilization like ours, left unchecked for a mere millenia? It would likely be solar-system spanning.
But what about 100 times that? A mere 100,000 years; if our civilization were to continue for such a length, we would have complete control over our solar system through sheer economics, even if intellectual progress halted at exactly where it is today. If technological progress were to continue as it is today? I would expect nothing less than widespread colonization of our corner of the galaxy.
But what about 100 times that? A mere 10 million years. Even if we presume FTL travel is utterly impossible (which it seems it could be from where we now stand), that's plenty of time for a spacefaring civilization to cross from one side of the galaxy to the other. By that point, colonization could well be commonplace enough to continue onwards at a significant portion of the speed of light, seeding worlds with the technology and lifeforms required in an automated process of expansion, driven by small autonomous craft optimized to perfection over thousands of generations.
And yet, that's a mere 10 million years. There have been around 1380 such time periods since the beginning of our universe. Many of those were largely uninhabitable towards the beginning, as the prerequisite materials for life (as we know it) weren't available. But even if we narrow it down to the formation of our solar system, that still leaves 460 such time periods, after which we can be absolutely sure the prerequisite materials for life were around. This means that, even given a mere 0.25% head start on us, one would expect not just to see life on another planet, but signs of life on
effectively every planet. This, of course, is only one possible mode of civilization; there are many other potentials, but there are enough possibilities similar to this that it would be really frickin weird if we don't exist in a veritable sea of interstellar life.
Of course, it may still be that we do. By the end of such a 10 million year period for such a civilization, they would look radically different from what we would call life; or what we would look for when looking for life. It may look like this:
http://news.discovery.com/space/galaxies/mysterious-cold-spot-fingerprint-of-largest-structure-in-the-universe-150420.htmIt may look like engineered lifeforms which survive in an environment without altering it in any way visible from space, or anywhere in between. The only thing we can be fairly certain of is that it won't be any form of life that looks anything like humanity's present.
As for roy's point 3, that becomes more and more like grasping at straws given recent information. Nearly everything we learn about the universe and life is drastically improving the odds of most of the steps. Life is apparently relatively easy to build, all things considered, and there isn't anything that particularly sticks out as being hard to go from life to intelligent life aside from the transition from cellular to fully multicellular, which apparently took a while on Earth. After intelligence, civilization seems quite likely, as you are likely to have a succession of intelligent species capable of it due both to evolutionary relationships and the apparent rise of intelligence in close proximity to other intelligent life forms. All in all, there isn't really anything that stands out as particularly challenging given trillions of planets and 5-10 billion years. By all estimates, from astronomy, chemistry, biology, etc, there isn't any particularly good reason why intelligent civilizations would be rare.
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