Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: The Halo Drive: Fuel-Free Relativistic Propulsion of Large Masses via
  Recycled Boomerang Photons   Gravitational slingshots around a neutron star in a compact binary have been
proposed as a means of accelerating large masses to potentially relativistic
speeds. Such a slingshot is attractive since fuel is not expended for the
acceleration, however it does entail a spacecraft diving into close proximity
of the binary, which could be hazardous. It is proposed here that such a
slingshot can be performed remotely using a beam of light which follows a
boomerang null geodesic. Using a moving black hole as a gravitational mirror,
kinetic energy from the black hole is transferred to the beam of light as a
blueshift and upon return the recycled photons not only accelerate, but also
add energy to, the spacecraft. It is shown here that this gained energy can be
later expended to reach a terminal velocity of approximately 133% the velocity
of the black hole. A civilization could exploit black holes as galactic way
points but would be difficult to detect remotely, except for an elevated binary
merger rate and excess binary eccentricity.

10
The categories of this paper are: q-bio
1
The categories of this paper are: cs
22
The categories of this paper are: chao-dyn
37
The categories of this paper are: supr-con
21
The categories of this paper are: adap-org
30
The categories of this paper are: q-alg
35
The categories of this paper are: mtrl-th
28
The categories of this paper are: alg-geom
34
The categories of this paper are: chem-ph
3
The categories of this paper are: physics
18
The categories of this paper are: econ
26
The categories of this paper are: bayes-an
9
The categories of this paper are: nlin
32
The categories of this paper are: atom-ph
16
The categories of this paper are: stat
33
The categories of this paper are: plasm-ph