Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Covariant approach to the thermodynamic structure of a generic null
  surface   We readdress the thermodynamic structure of geometrical relations on a
generic null surface. Among three potential candidates, originated from
different components of $R_{ab}$ along the null vectors for the surface (i.e.
$R_{ab}q^a_cl^b$, $R_{ab}l^al^b$ and $R_{ab}l^ak^b$ where $q_{ab}$ is the
projector on the null surface and $l^a$, $k^a$ are null normal and
corresponding auxiliary vector of it, respectively), the first one leads to
Navier-Stokes like equation. Here we devote our investigation on the other two
members. We find that $R_{ab}l^al^b$, which yields the evolution equation for
expansion parameter corresponding to $l^a$ along itself, can be interpreted as
a thermodynamic relation when integrated on the two dimensional transverse
subspace of the null hypersurface along with a virtual displacement in the
direction of $l^a$. Moreover for a stationary background the integrated version
of it yields the general form of Smarr formula. Although this is more or less
known in literature, but a similar argument for the evolution equation of the
expansion parameter corresponding to $k^a$ along $l^a$, provided by
$R_{ab}l^ak^b$, leads to a more convenient form of thermodynamic relation. In
this analysis, contrary to earlier approaches, the identified thermodynamic
entities come out to be in covariant forms and also are foliation independent.
Hence these can be applied to any coordinate system adapted to the null
hypersurface. Moreover, these results are not restricted to any specific
parametrisation of $k^a$ and also $k^a$ need not be hypersurface orthogonal. In
addition, here any particular dynamical equation for metric is not being
explicitly used and therefore we feel that our results are solely based on the
properties of the null surface.

22
The categories of this paper are: chao-dyn
36
The categories of this paper are: cmp-lg
2
The categories of this paper are: math
1
The categories of this paper are: cs
14
The categories of this paper are: math-ph
30
The categories of this paper are: q-alg
17
The categories of this paper are: q-fin
15
The categories of this paper are: nucl-ex
37
The categories of this paper are: supr-con
33
The categories of this paper are: plasm-ph
0
The categories of this paper are: hep-ph
28
The categories of this paper are: alg-geom
32
The categories of this paper are: atom-ph
10
The categories of this paper are: q-bio
9
The categories of this paper are: nlin
35
The categories of this paper are: mtrl-th