Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: A solution of the non-uniqueness problem of the Dirac Hamiltonian and
  energy operators   In a general spacetime, the possible choices for the field of orthonormal
tetrads lead (in standard conditions) to equivalent Dirac equations. However,
the Hamiltonian operator is got from rewriting the Dirac equation in a form
adapted to a particular reference frame, or class of coordinate systems. That
rewriting does not commute with changing the tetrad field $(u_\alpha)$. The
data of a reference frame F fixes a four-velocity field $v$, and also fixes a
rotation-rate field $\Mat{\Omega}$. It is natural to impose that $u_0=v$. We
show that then the spatial triad $(u_p)$ can only be rotating w.r.t. F, and
that the title problem is solved if one imposes that the corresponding rotation
rate $\Mat{\Xi}$ be equal to $\Mat{\Omega}$ - or also, if one imposes that
$\Mat{\Xi}=\Mat{0}$. We also analyze other proposals which aimed at solving the
title problem.

18
The categories of this paper are: econ
3
The categories of this paper are: physics
32
The categories of this paper are: atom-ph
37
The categories of this paper are: supr-con
16
The categories of this paper are: stat
24
The categories of this paper are: dg-ga
10
The categories of this paper are: q-bio
9
The categories of this paper are: nlin
34
The categories of this paper are: chem-ph
1
The categories of this paper are: cs
20
The categories of this paper are: acc-phys
11
The categories of this paper are: hep-lat
25
The categories of this paper are: patt-sol
19
The categories of this paper are: eess
0
The categories of this paper are: hep-ph
6
The categories of this paper are: astro-ph