I think it might be better to say that's not beyond our current level of scientific understanding (it doesn't require any exotic materials or new physics). Howe

I think it might be better to say that's not beyond our current level of scientific understanding (it doesn't require any exotic materials or new physics). However it's likely beyond our practical engineering capabilities. Early spaceflight up through Apollo was done with an army of humans with slide rules and rivet guns. We did it because there was a political imperative to do so. A lot of new technology was developed as a result. But it ended up being unsustainable because it was too expensive. I think the key to making O'Neill cylinders feasible will be large scale outer space resource mining and in-space manufacturing. Both will require advanced robotics and AI. We could throw a bunch of money into developing that ... or we could just wait for the technology to develop here on Earth and then apply it to space. Likewise, as much as I'd like to see a Mars outpost in my lifetime, it would be a lot cheaper and safer if we didn't rush it. Maybe by 2075 we'll have cured cancer and radiation won't be an issue, and we'll have self-repairing life support systems and one of those automated medical pods from the Alien movies.