The other topic got sufficiently derailed that I thought I'd start another. Here goes.
Suppose you were to speak to someone living in the pre-industrial world about the way people live today by jumping right into it by talking about hairdressers and computer programmers and delivery truck drivers. They would be confused by how all this works. How can the world feed itself without everyone farming? Well there's tractors and fertilizers and irrigation and such. But where do all those things come from? Well there are factories and supply lines and retail stores and agricultural colleges. But how is all of this afforded? Just three centuries ago, nearly everyone was dependant on farming to make ends meet and couldn't dream of affording these things that are such a minor expense to us. How could the world make something as complicated as a laptop? It's all very confusing and everything is co-dependant, making a complicated story because you are approaching it backwards, starting at the end.
The way to approach things is to start your story by going back to the early 19th century and start discussing textiles. Textiles of course are the first thing that were profitably industrialized. Some people figured out how to use machines to make cloth much more easily, getting them a lot of money and freeing up a lot of labor. That money and labor was used to do other things efficiently like steel and concrete and fertilizer. And very quickly the process snowballed until everything is being done with machines and you have the industrial revolution. Then things that used to be very expensive, like food and clothing, are now much much cheaper then they used to be.
When thinking about space colonization, we need to approach things the same way. It wouldn't make sense to make a bunch of colonies on mars if everyone of them is going to have a huge price on earth. It needs to be a sustaining cycle. And that cycle needs to start somewhere. That place is solar power.
Space has a lot of solar power. Well, duh, you might think, the sun is everywhere. But it really does. There is no day or night in space, so a solar panel in space is already twice as efficient as on earth. There is no atmosphere, increasing efficiency again. There are no fluctuations due to clouds. There isn't a need to kept it pointed at the sun. No seasonal fluctuations. All in all, space based solar panels should be about 4 times as efficient as earth based ones.
Now, you might be thinking that we'd need a long extension cord to get that electricity back to earth. But there's an easier way, microwave power transmission. It's safe, reliable and the loss in transmission would be fairly small. Plus that power could be beamed anywhere on half the earth. That means all the redundancy that we have on earth to deal with peak demands in different places and all the electricity lost in transmision would be reduced enormously.
So why don't we do this? Because launching satellites from earth is really, really expensive, $3000 a pound or more. But there is another way. What if we made those satellites in space? The moon has all the raw materials we need, silicon, iron, carbon. And launching something from the moon is easy, even a catepault could do it. The only problem is that there aren't any people living on the moon. But what if there were? This moon colony would be very, very profitable as it would have an export that the earth has a nearly limitless appetite for: electricity. This would fuel further colonization. And as the colony got bigger, manufacturing would get more efficient. Eventually you reach a snowball effect and you start having a self sustaining move to the moon.
And once you have really colonized the moon, not just dusted your feet and placed a flag, the universe is your oyster. Because we don't need to stay on the moon. It's easy to leave the moon, remember? We can make entirely artificial habitats in space with materials from the moon and the asteroids. It sounds crazy but it's quite simple, just make a two mile across steel ring, spin it for artificial gravity and fill it with air. Then all you need are a few mirrors and solar panels and you've created yourself a giant greenhouse to live in. But this greenhouse would be far better then any greenhouse on earth. No polution, a zero environment environment to do you manufacturing and construction in and a frictionless environment to handle all your transportation needs. Plus expanding this environment would be easy. You will never run out of space and there are enough materials in the moon and asteroids alone to sustain trillions and trillions of people.
It's difficult to take that first step. The people who started the industrial revolution, by first tinkering around with textile machines like the spinning jenny, had no clue what they were starting. But we can start the space revolution, the move into a low gravity environment. And once we do, the process will eventually become self sustaining. But the challenge is, can we take that first step?