Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: No observational proof of the black-hole event-horizon   Recently, several ways of obtaining observational proof of the existence of
black-hole horizons have been proposed. We argue here that such proof is
fundamentally impossible: observations can provide arguments, sometimes very
strong ones, in favour of the existence of the event horizon, but they cannot
prove it. This applies also to future observations, which will trace very
accurately the details of the spacetime metric of a body suspected of being a
black hole.

18
The categories of this paper are: econ
28
The categories of this paper are: alg-geom
24
The categories of this paper are: dg-ga
5
The categories of this paper are: gr-qc
32
The categories of this paper are: atom-ph
22
The categories of this paper are: chao-dyn
21
The categories of this paper are: adap-org
19
The categories of this paper are: eess
37
The categories of this paper are: supr-con
9
The categories of this paper are: nlin
26
The categories of this paper are: bayes-an
16
The categories of this paper are: stat
20
The categories of this paper are: acc-phys
14
The categories of this paper are: math-ph
0
The categories of this paper are: hep-ph
35
The categories of this paper are: mtrl-th