Constant of motion in the Kerr-Newman spacetime
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The Carter constant is a conserved quantity for motion around black holes in the general relativistic formulation of gravity. Its SI base units are kg2⋅m4⋅s−2. Carter's constant was derived for a spinning, charged black hole by Australian theoretical physicist Brandon Carter in 1968. Carter's constant along with the energy
, axial angular momentum
, and particle rest mass
provide the four conserved quantities necessary to uniquely determine all orbits in the Kerr–Newman spacetime (even those of charged particles).
Carter noticed that the Hamiltonian for motion in Kerr spacetime was separable in Boyer–Lindquist coordinates, allowing the constants of such motion to be easily identified using Hamilton–Jacobi theory.[1] The Carter constant can be written as follows:
,
where
is the latitudinal component of the particle's angular momentum,
is the conserved energy of the particle,
is the particle's conserved axial angular momentum,
is the rest mass of the particle, and
is the spin parameter of the black hole.[2] Note that here
denotes the covariant components of the four-momentum in Boyer-Lindquist coordinates which may be calculated from the particle's position
parameterized by the particle's proper time
using its four-velocity
as
where
is the four-momentum and
is the Kerr metric. Thus, the conserved energy constant and angular momentum constant are not to be confused with the energy
measured by an observer and the angular momentum
. The angular momentum component along
is
which coincides with
.
Because functions of conserved quantities are also conserved, any function of
and the three other constants of the motion can be used as a fourth constant in place of
. This results in some confusion as to the form of Carter's constant. For example it is sometimes more convenient to use:
![{\displaystyle K=C+(L_{z}-aE)^{2}}](https://wikimedia.org/api/rest_v1/media/math/render/svg/8ea040f63f1cc9867ec73528f58aa23defe64273)
in place of
. The quantity
is useful because it is always non-negative. In general any fourth conserved quantity for motion in the Kerr family of spacetimes may be referred to as "Carter's constant". In the
limit,
and
, where
is the norm of the angular momentum vector, see Schwarzschild limit below.
As generated by a Killing tensor
Noether's theorem states that each conserved quantity of a system generates a continuous symmetry of that system. Carter's constant is related to a higher order symmetry of the Kerr metric generated by a second order Killing tensor field
(different
than used above). In component form:
,
where
is the four-velocity of the particle in motion. The components of the Killing tensor in Boyer–Lindquist coordinates are:
,
where
are the components of the metric tensor and
and
are the components of the principal null vectors:
![{\displaystyle l^{\mu }=\left({\frac {r^{2}+a^{2}}{\Delta }},1,0,{\frac {a}{\Delta }}\right)}](https://wikimedia.org/api/rest_v1/media/math/render/svg/5e42a3c8ff9032d09d5cb971db235f2b9852dc5e)
![{\displaystyle n^{\nu }=\left({\frac {r^{2}+a^{2}}{2\Sigma }},-{\frac {\Delta }{2\Sigma }},0,{\frac {a}{2\Sigma }}\right)}](https://wikimedia.org/api/rest_v1/media/math/render/svg/c6cc84717b487278aa6021e1ce7ea7dcf2dc01b9)
with
.
The parentheses in
are notation for symmetrization:
![{\displaystyle l^{(\mu }n^{\nu )}={\frac {1}{2}}(l^{\mu }n^{\nu }+l^{\nu }n^{\mu })}](https://wikimedia.org/api/rest_v1/media/math/render/svg/7e16764e42944a067f32c2d8adf0dc86d619cf19)
Schwarzschild limit
The spherical symmetry of the Schwarzschild metric for non-spinning black holes allows one to reduce the problem of finding the trajectories of particles to three dimensions. In this case one only needs
,
, and
to determine the motion; however, the symmetry leading to Carter's constant still exists. Carter's constant for Schwarzschild space is:
.
To see how this is related to the angular momentum two-form
in spherical coordinates where
and
, where
and
and where
and similarly for
, we have
.
Since
and
represent an orthonormal basis, the Hodge dual of
in an orthonormal basis is
![{\displaystyle {\boldsymbol {L^{*}}}=mr^{2}{\dot {\theta }}{\boldsymbol {\hat {\theta }}}+mr^{2}\sin \theta \,{\dot {\phi }}\,{\boldsymbol {\hat {\phi }}}}](https://wikimedia.org/api/rest_v1/media/math/render/svg/e87dc93a41917cbe4143357c917ff27c40e3ace4)
consistent with
although here
and
are with respect to proper time. Its norm is
.
Further since
and
, upon substitution we get
.
In the Schwarzschild case, all components of the angular momentum vector are conserved, so both
and
are conserved, hence
is clearly conserved. For Kerr,
is conserved but
and
are not, nevertheless
is conserved.
The other form of Carter's constant is
![{\displaystyle K=C+(L_{z}-aE)^{2}=(L^{2}-L_{z}^{2})+(L_{z}-aE)^{2}=L^{2}}](https://wikimedia.org/api/rest_v1/media/math/render/svg/a55ee4fef574696e56978a5b3ee05670925651ac)
since here
. This is also clearly conserved. In the Schwarzschild case both
and
, where
are radial orbits and
with
corresponds to orbits confined to the equatorial plane of the coordinate system, i.e.
for all times.
See also
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References