Greetings,
I just discovered this topic earlier today, but feel I have to weigh in now that I've read through the comments. I have found the discussion between Eckard and Lawrence interesting, also earlier comments by Georgina, so I'll try to weave that into my remarks about the initial topic. I find the work of Isham and Döring quite promising - from what little I know of it - though I would have been incredulous a year ago. In my essay for the current FQXi contest, I speak of the need to evolve a broader framework (to reconcile Quantum Mechanics and Relativity) and I am coming more and more to believe that category theory is an essential part of that prescription. I wish that all of you on this thread could have attended the wonderful lecture by Marc Lachieze-Rey on applications of Category Theory to Mathematical Physics, which I attended at FFP10. I will post a link or paper, when I have one to share.
In any case; Lachieze-Rey made it clear that many of the recent advances in Math for Physics have a category-theoretic basis, and that large areas of Math could be seen as special cases of stuff from CT. For example a group is a category with a single object and invertible morphisms. If my notes serve me well, he described Topos as the categorification of the functors. Earlier on this thread, LC commented "Topos theory or topoi describes sets of sheaves which have some categorical equivalence by functors. So what Isham and Doring are setting up is a system where one observer will detect things under one algebraic variety, while in general observations can occur under a whole set of such varieties." That pretty well sums up the value of using Topos theory in the manner described by Isham and Döring, because what they are doing is using different topoi to model various ways in which an observer/participant might be situated, as a sort of lens to view different regimes through.
This could indeed be a good way to reconcile the different mathematical frameworks required for Relativity and Quantum Physics, which would show each to be a subset or topos within a broader and more encompassing mathematical framework. I disagree with Eckard's earlier comment about the Math in Physics already being too general, as I am more in favor of LC's "as physics advances it will most likely require ever more advanced (or better put abstract) mathematics. So a good grounding in higher mathematics makes for a better tool box."
Understand; I am not saying we should mistake Math for Physics, but we must use higher Math to advance our understanding in Physics. I feel we must discontinue the erroneous use of Math - where it is mistaken for the behavior of a physical system rather than a model thereof - while earnestly attempting to find the right Math for the job of modeling physical systems. And category theory gives us some great new insights and tools to work with, for developing better models. As another FFP10 presenter, Marni Shepheard, pointed out in her lecture - this includes a broader definition of Logic.
In respect to the current thread; I have a bunch of comments on the concept of a point, and numbers on a line or the subdivision thereof, combining constructivism with topos theory - plus other ideas recently mentioned here - so if Eckard, Lawrence, or Georgina are following this thread, I will have something more to say.
All the Best,
Jonathan