trivialities as quantum mechanics and the Planck scale.  Relativity and quantum mechanics finally meet in Section \[QG\], on the computational complexity of quantum gravity theories, but the whole point of the section is to explain why this is a premature subject.  Sections \[TIME\] and \[ANTHROPIC\] finally set aside the more sober ideas (like solving the halting problem using naked singularities), and give zaniness free reign.  Section \[TIME\] studies the computational complexity of time travel, while Section \[ANTHROPIC\] studies anthropic computing, which means killing yourself whenever a computer fails to produce a certain output.  It turns out that even about these topics, there are nontrivial things to be said!  Finally, Section \[DISC\] makes the case for taking the hardness of $\mathsf{NP}$-complete problems to be a basic fact about the physical world; and weighs three possible objections against doing so.

I regret that, because of both space and cognitive limitations, I was unable to discuss *every* paper related to the solvability of $\mathsf{NP}$-complete problems in the physical world.  Two examples of omissions are the gear-based computers of Vergis, Steiglitz, and Dickinson [@vsd], and the proposed adiabatic algorithm for the halting problem due to Kieu [@kieu].  Also, I generally ignored papers about hypercomputationthat 