An interesting theme unmentioned so far here appears in Fire.., Children… & (somewhat) in Deepness…: technology progress and it’s possible acceleration depending on the information you have. Level of civilization tech has fallen (or never existed). People a) recall that some tech worked OR b) Know specific choices are right ones OR c) Have a computer (or at least a book)with detailed instructions. Now: is it possible to rebuild tech? If so, which paths and what’s quickest way? Often one needs to build a chain of techs first. For example, as a thought experiment, a time machine can send back a VLSI design textbook (which makes clear that planar CMOS wins). If that’s to 1947, diverts from building big vacuum tube computers into pushing to VLSI as fast as possible. But if sent to 1900? Likewise, complete design for iPhone sent to 1990 doesn’t help much.