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Zachary Nasipak committed Dec 19, 2023
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# Summary

In general relativity, the motion of a free falling test particle in a curved spacetime is
described by a geodesic - the generalization of a "straight line" path to a curved space.
described by a geodesic - the generalization of a "straight line" path to a curved space.
[comment: maybe we just say "...by a geodesic - the minimal path between two points in a curved space"]
The geodesics of Kerr spacetime are of particular interest in the field of black
hole perturbation theory because they describe the zeroth order motion of a small object
moving through the background spacetime of a large spinning black hole. For this reason, computing
geodesics is an important step in modelling the gravitational radiation emitted by an
moving through the background spacetime of a much more massive spinning black hole. For this reason, computing
geodesics is an important step in modeling the gravitational radiation emitted by an
extreme mass ratio inspiral (EMRI) - an astrophysical binary in which a stellar mass
compact object, such as a neutron star or a small ($10^1 - 10^2 M_\odot$) black hole,
spirals into a massive ($10^4 - 10^7 M_\odot$) black hole.
compact object, such as a neutron star or black hole (with mass $10^1 - 10^2 M_\odot$),
spirals into a massive black hole (with mass $10^4 - 10^7 M_\odot$).

Kerr spacetime has several nice properties which simplify the problem of computing geodesics. Since
it has both time translation symmetry and rotational symmetry, energy and angular momentum are conserved quantities. It is also
it has both time-translation symmetry and rotational symmetry, energy and angular momentum are conserved quantities. It is also
equipped with a higher order symmetry which gives rise to a third constant of motion called the Carter
constant. These three constants of motion, along with the spin of the black hole, uniquely define a geodesic up to
initial conditions [@schmidt].Alternatively, geodesics can be identified using a suitably generalized
version of the parameters used to define a Keplerian orbit (eccentricity, semi-latus rectum and inclination angle).
Bound geodesics also possess fundamental frequencies since their radial, azimuthal and polar motion is periodic.
initial conditions [@schmidt]. Alternatively, geodesics can be identified using a suitably generalized
version of the parameters used to define a Keplerian orbit (eccentricity, semi-latus rectum, and inclination angle).
Bound geodesics also possess fundamental frequencies since their radial, azimuthal, and polar motions are periodic.

`KerrGeoPy` is a Python implementation of the `KerrGeodesics` [@kerrgeodesics] Mathematica library
which computes both stable and plunging geodesics in Kerr spacetime using the
`KerrGeoPy` is a Python package which computes both stable and plunging geodesics in Kerr spacetime using the
analytic solutions to the geodesic equation derived in [@fujita] and
[@dyson]. These solutions are written in terms of elliptic integrals, which are
[@dyson]. It mirrors and builds upon much of the functionality of the `KerrGeodesics` [@kerrgeodesics] Mathematica library.
Geodesic solutions are written in terms of elliptic integrals, which are
evaluated using `SciPy`. Users can construct a geodesic by providing the initial position and
four-velocity, or by providing either the constants of motion or the Keplerian parameters described above.
`KerrGeoPy` also provides methods for computing the fundamental frequencies
and constants of motion associated with a given geodesic and implements the algorithm described
in [@stein] for finding the location of the last stable orbit orbit, known as the separatrix.
in [@stein] for finding the location of the last stable orbit, known as the separatrix. The package also
includes several methods for visualizing and animating geodesics.

![Example of an equatorial (left), spherical (center) and generic (right) orbit computed by `KerrGeoPy`](orbits.png)

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Although other Python packages [@kerrgeodesicgw] with similar functionality do exist, they mostly rely on numerical
integration to compute geodesics. The analytic solutions used by `KerrGeoPy` have two main advantages
over this approach. First, they are guaranteed to be numerically stable and can be quickly evaluated at
any point in time. Second, they produce several useful intermediate terms which cannot easily be
computed using numerical integration.
over this approach. First, they can be much more numerically stable over long time periods and can be quickly evaluated at
any point in time. Second, they produce several useful intermediate terms which are not calculated by other packages that rely on
numerical integration. Modeling EMRIs typically requires long time-averages over the geodesic motion. Therefore,
`KerrGeoPy`, with its analytic solutions and various orbital parametrizations, is specifically tuned to support
perturbative models of binary black holes and their gravitational waves.

`KerrGeoPy` is a part of the [Black Hole Perturbation Toolkit](https://bhptoolkit.org). The source code
is hosted on [Github](https://github.com/BlackHolePerturbationToolkit/KerrGeoPy) and the package is
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