Ian, modern QFT and TQFT is entirely formulated in category theory and higher category theory.

]]>People don't have enough time to write, specially topics which they are not thinking right now.

]]>Incidentally, from the open systems stuff I will eventually link to a new page on closed time-like curves (CTCs) since they are (or can be) related and I think category theory might serve to help shed some light on how they function. This brings up the question: why isn't there a relativity section on nLab? I thought John Baez had done some work applying categories to quantum gravity? Maybe no one ever got to it? ]]>