Joel Garreau’s provocative new book, Radical Evolution, is divided into different scenarios. One that he calls “Heaven” is largely the vision of Ray Kurzweil, one of the founders of modern assistive technology.

Kurzweil imagines a future where the positive aspects of the new technology are available freely to everyone, allowing each of us to customize our own selves to the point where immortality — or complete spiritual freedom from the body, if that’s what you want — is more than a promise or a legend or a fable.

Countering Kurzweil’s vision are the prophets of doom, led by Silicon Valley pioneer Bill Joy, who worry that unrestricted experimentation with self-replicating nanobots could result in the entire planet — you, me, and everything around us, right down to the core — turned into food for invisible, ravenous robots. This “grey goo” nightmare is cataloged by Garreau in his “Hell” scenario, along with other dystopias of the Brave New World variety.