> I imagine making plastic out of air would take a lot of energy.

Not much more than you are otherwise loosing by using the oil instead of burning it. The hard part is getting carbon - not a lot of CO2 in the air.

> I'm going to pre-emptively reply "Exponential growth".

But we are not having exponential population growth right now. And even if we were, the universe is big REALLY big, stupendously unimaginably big. Even just the Earth is enormous, and we are nowhere close to filling it up.

Yes I know the universe is REALLY BIG but infinity is INFINITELY bigger. I am saying population cannot grow, infinitely. There will be a limit.

How do you know? No one knows if the universe is infinite or not. And even exponential growth takes time - if it takes longer than the heat death of the universe (assuming a finite universe in both time and space) to fill it up, then it doesn't matter if it's exponential.

And to bring things back to this earth, this isn't going to be a problem for such a long time that predicting anything whatsoever about it is futile.

>> And to bring things back to this earth Let's end this with a Asimov short story. http://www.multivax.com/last_question.html

> Earth's energy income will always be limited by the Sun's energy output.

This seems to discount tidal, wind, and geothermal energy sources. Wearable devices that capture kinetic movements are a possibility too. As technology progresses, efficiency increases.

While what you're saying seems theoretically possible, it also seems a long way off (perhaps several millennia).

Wind energy is solar energy, indirectly.