Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: On the position operator for massless particles   It is always stated that the position operator for massless particles has
non-comutting components. It is shown that the reason is that the commutation
relations between coordinates and momenta differs for massive and massless
particles. The correct one for massless particles and a position operator with
commuting components are derived.

22
The categories of this paper are: chao-dyn
4
The categories of this paper are: cond-mat
37
The categories of this paper are: supr-con
34
The categories of this paper are: chem-ph
20
The categories of this paper are: acc-phys
23
The categories of this paper are: solv-int
31
The categories of this paper are: ao-sci
32
The categories of this paper are: atom-ph
2
The categories of this paper are: math
8
The categories of this paper are: hep-ex
0
The categories of this paper are: hep-ph
27
The categories of this paper are: comp-gas
6
The categories of this paper are: astro-ph
17
The categories of this paper are: q-fin
3
The categories of this paper are: physics
1
The categories of this paper are: cs