In my last 'blog post I may have come across as a little over-pessimistic. My theme was the fundamental impossibility of telling "realistic" future stories. In particular, I mentioned that by the time humans venture to other star systems, they will no longer be human by our standards.
Is that true?
Well, consider. It's going to take a while for human civilization to expand into the Solar System. Probably longer than I would wish — I've pretty much given up on being able to afford a vacation in space before I'm too frail to go. But I do think expansion beyond Earth is inevitable. In time there will come a point when the bulk of human economic production is off Earth, and sometime later the bulk of human population will no longer be on Earth, either. The only question is when that will happen.
So how long will it take?
Depending on who you listen to, the space industry will be somewhere between one and three trillion dollars by 2040. Now the space industry is not the same as industry in space, but that's where most of the future growth has to come from. The more you can make in space, the less you have to launch on expensive rockets (and yes, even a reusable SpaceX BFR with costs to orbit approximating FedEx international overnight rates is still expensive).
Anyway, let's take the low end figure of 1 trillion dollars as our starting point, and the United Launch Alliance's estimate of 1000 people in space by roughly the same year of 2040. Even if the space economy grows at an anemic 1 percent annual average, that means it would nearly double by the end of the century. By 2200, it would be 5 trillion in constant dollars. If we assume the population scales with the value, that means 5000 people living in space a century and a half from now.
That's pathetic. Where's my interplanetary civilization?
Ah, but behold the magic of compound interest. A thousand years later, in A.D. 3200, those 5000 people have become 100 million. And a thousand years after that, the number of people in space is 2 trillion! So even with a low-ball estimate for growth, the number of people off Earth will surpass this planet some time in the middle of the Fourth Millennium. That may seem like a long time, but by the standards of everything except our mayfly lifespans, that's nothing. If the human species lasts only a million years — and the oldest known Homo Sapiens remains are about 300,000 years old — then for two-thirds of human history, we will be an interplanetary species, and probably an interstellar species for a significant fraction of that time.
Arthur C. Clarke put it better. I can't find the exact quotation, but to paraphrase, "If Mankind survives as long as the least successful of those creatures we deride as Nature's failures, the dinosaurs, for all but a vanishingly brief instant in the childhood of the race, the word 'ship' will mean 'spaceship.'"
However, there are a couple of things which do advance faster than compound-interest economic and population growth: scientific knowledge and technology. Consider the famous Moore's Law: that computer processing power roughly doubles every 18 months. If computing power continues to double every couple of years for the next millennium, we're looking at a world with machines something like ten to the 150th power smarter than what we've got today. That's ten followed by 150 zeroes. A googol of Googles.
That is why I said the first humans to visit other star systems won't be human by our standards. Individuals will command millions of times more information and processing power than our entire civilization possesses right now. Even if we can capture only a millionth of the Sun's energy output, individuals will have megawatts of power at their disposal.
Those aren't humans. Those are gods.
There's a common trope in science fiction: the Elder Space Gods. Ancient alien civilizations with powers far beyond our comprehension, who've been around since before intelligent life arose on Earth. Well, here's a data point: so far all our searching hasn't uncovered any signs of technological civilizations elsewhere in the Galaxy. We know that planets are common, we think that life should be common, so where is everybody? Well, someone's got to be first, and it looks like we're elected. Congratulations, humanity: you get to be the Elder Space Gods. Let's try to be the benevolent white-robes-and-glowy-crystals kind, okay?
One more thing to blow your mind: this picture of godlike humans as the Elder Race of the Galaxy is the pessimistic scenario.