[tekne]

I'll wear the dunce hat if I have to, but I don't want to be an animal. I am one, now, but I don't have to be one. I refuse to be limited by what is "naturally" possible -- my nature is to be a being with a mind, and so the only limit I must respect is the raw laws of physics, whatever they are.

And physics lets me get a lot further than "8 billion Americans."

[Peanuts99]

But that's just what OP is saying, you can 'refuse' to be limited to nature, but nature will have other plans for you.

[tekne]

I suppose there's nature -- i.e. physics -- and Nature, i.e. the false god of the status quo. These are not, in fact, the same thing.

Small-n nature says (as best as I can tell) that I must obey conservation of mass and can't go faster than light -- and hence that I probably cannot, in fact, sustain a population of a 100 billion on Earth alone without significantly cutting down individual resource access.

Until very recently big-N Nature said we'd never have heavier-than-air-flight, an automatic translation program (see: plot point in A Fire upon the Deep), or, before the Green Revolution, a population above a few billion -- but just as there's no fundamental physical law forbidding these things (and hence they came to pass) so there really is no fundamental physical law stopping all of current humanity from living a decent life (happy to be proved otherwise).

It may be impossible with current technology, sure, but there's no new physics required to make it possible -- the physics we have now permit it, with sufficient technology.

In fact -- interestingly enough -- before nuclear energy you'd have a far stronger point (though solar energy now means even without nuclear we can probably scrape by enough energy -- but coal does in fact have a Malthusian problem)