He has just read an early draft of a book by his friend Max Tegmark, a physicist at the Massachusetts Institute of Technology, arguing that the only reason nuclear bombs can't be made from instructions downloaded from the Internet is that the laws of physics luckily make it hard to do. "There's no guarantee that wouldn't be possible," he says, referring to homemade nuclear bombs.

Yea that's gonna be a problem once 3D printers and home biolabs get sufficiently powerful and wide-spread, it'll be the analogue to a world in which you could use a microwave and sand to make fissile material. Great Filter anyone?