Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: On a nonstatic Painleve-Gullstrand spacetime   A time dependent geometry outside a spherically symmetric mass is proposed.
The source has zero energy density but nonzero radial and tangential pressures.
The time variable is interpreted as the duration of measurement performed upon
the physical system. For very short time intervals, the effect of the mass
source is much reduced, going to zero when $t \rightarrow 0$. All physical
quantities are finite when $t \rightarrow 0$ and $r \rightarrow 0$ and also at
infinity. The total energy flux measured on a hypersurface of constant $r$ is
vanishing.

18
The categories of this paper are: econ
17
The categories of this paper are: q-fin
8
The categories of this paper are: hep-ex
34
The categories of this paper are: chem-ph
15
The categories of this paper are: nucl-ex
5
The categories of this paper are: gr-qc
27
The categories of this paper are: comp-gas
2
The categories of this paper are: math
13
The categories of this paper are: quant-ph
10
The categories of this paper are: q-bio
22
The categories of this paper are: chao-dyn
16
The categories of this paper are: stat
30
The categories of this paper are: q-alg
36
The categories of this paper are: cmp-lg
19
The categories of this paper are: eess
21
The categories of this paper are: adap-org