Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: On the Meaning of General Covariance and the Relevance of Observers in
  General Relativity   Since the appearance of General Relativity, its intrinsec general covariance
has been very often misinterpreted as implying that physically meaningful
quantitities (and conclusions extracted from the theory) have to be absolutely
independent on observers. This incorrect point of view is sometimes expressed
by discarding the very concept of observer in the structure and applications of
the theory. As we shall stress in this essay, through some examples, the
concept of observer is as essential to General Relativity as it is to any
physical theory.

22
The categories of this paper are: chao-dyn
2
The categories of this paper are: math
13
The categories of this paper are: quant-ph
21
The categories of this paper are: adap-org
18
The categories of this paper are: econ
9
The categories of this paper are: nlin
24
The categories of this paper are: dg-ga
12
The categories of this paper are: nucl-th
25
The categories of this paper are: patt-sol
4
The categories of this paper are: cond-mat
28
The categories of this paper are: alg-geom
16
The categories of this paper are: stat
15
The categories of this paper are: nucl-ex
6
The categories of this paper are: astro-ph
34
The categories of this paper are: chem-ph
35
The categories of this paper are: mtrl-th