I was reading Isham, Chris J. Modern differential geometry for physicists. Vol. 61. World Scientific, 1999. p.52
In the first chapter, he gives mathematical preliminaries that'll be useful for the rest of the chapters. There I came across this interesting text about open sets and their role in physics.
An important question in any topological space $X$ is the extent to which points can be distinguished from each other by listing the collection of open sets to which each belongs.
From the viewpoint of conventional physics, this is related to the idea that if $X$ represents physical space, then any real ‘object’ exists inside an open set. More precisely, it cannot exist as a subset of a closed subset unless this has a non-trivial interior. It thus seems plausible to argue that it is physically meaningless to distinguish between two points in $X$ if the collections of open sets to which they belong are identical. In the context of quantum field theory, this remark is related to the analysis by Bohr and Rosenfeld of the need to smear quantum fields with test functions that are non-vanishing on an open set.
One could say that all open sets are ‘fat’ whereas closed sets come in both thin and fat varieties. For example, a segment of a line in the plane is thin whereas a closed disc is fat.
Then the author defines what is a $T_0, T_1, T_2$ space, then he comments
Any topological space that represents spacetime must be at least $T_0$ at least if all its points are to have ‘physical meaning’ in the sense of being distinguishable by objects located in open sets.
I know one way to define topological space, continuous maps are in terms of open sets, but there also exist equivalent formulations in terms of closed sets for topology and continuous maps.
- So why open sets are important in physics?
- And what does it mean to put real objects in open sets?
- How does one understand 'fat' and 'thin' sets?