Eur. Phys. J. C (2015) 75:541
DOI 10.1140/epjc/s10052-015-3787-9
Regular Article - Theoretical Physics
Dragging of inertial frames in the composed black-hole–ring
system
Shahar Hod
1,2,a
1
The Ruppin Academic Center, 40250 Emeq Hefer, Israel
2
The Hadassah Institute, 91010 Jerusalem, Israel
Received: 25 June 2015 / Accepted: 6 November 2015 / Published online: 19 November 2015
© The Author(s) 2015. This article is published with open access at Springerlink.com
Abstract A well-established phenomenon in general rela-
tivity is the dragging of inertial frames by a spinning object.
In particular, due to the dragging of inertial frames by a ring
orbiting a central black hole, the angular velocity
BH-ring
H
of
the black-hole horizon in the composed black-hole–ring sys-
tem is no longer related to the black-hole angular momentum
J
H
by the simple Kerr-like (vacuum) relation
Kerr
H
(J
H
) =
J
H
/2M
2
R
H
(here M and R
H
are the mass and horizon-radius
of the black hole, respectively). Will has performed a per-
turbative treatment of the composed black-hole–ring system
in the regime of slowly rotating black holes and found the
explicit relation
BH-ring
H
(J
H
= 0, J
R
, R) = 2J
R
/R
3
for the
angular velocity of a central black hole with zero angular
momentum, where J
R
and R are respectively the angular
momentum of the orbiting ring and its proper circumferen-
tial radius. Analyzing a sequence of black-hole–ring config-
urations with adiabatically varying (decreasing) circumfer-
ential radii, we show that the expression found by Will for
BH-ring
H
(J
H
= 0, J
R
, R) implies a smooth transition of the
central black-hole angular velocity from its asymptotic near-
horizon value
BH-ring
H
(J
H
= 0, J
R
, R → R
+
H
) → 2 J
R
/R
3
H
(that is, just before the assimilation of the ring by the central
black hole), to its final Kerr (vacuum) value
Kerr
H
(J
new
H
) =
J
new
H
/2M
new
2
R
new
H
[that is, after the adiabatic assimilation
of the ring by the central black hole. Here J
new
H
= J
R
, M
new
,
and R
new
H
are the new parameters of the resulting Kerr (vac-
uum) black hole after it assimilated the orbiting ring]. We use
this important observation in order to generalize the result of
Will to the regime of black-hole–ring configurations in which
the central black holes possess non-zero angular momenta. In
particular, it is shown that the continuity argument (namely,
the characteristic smooth evolution of the black-hole angu-
lar velocity during an adiabatic assimilation process of the
a
e-mail: shaharhod@gmail.com
ring into the central black hole) yields a concrete predic-
tion for the angular-velocity/angular-momentum asymptotic
functional relation
BH-ring
H
=
BH-ring
H
(J
H
, J
R
, R → R
+
H
)
of generic (that is, with J
H
= 0) black-hole–ring config-
urations. Remarkably, we find the simple universal rela-
tion
H
≡
BH-ring
H
(J
H
, J
R
, R → R
+
H
) −
Kerr
H
(J
H
) =
J
R
/4M
3
for the asymptotic deviation of the black-hole angu-
lar velocity in the composed black-hole–ring system from the
corresponding angular velocity of the unperturbed (vacuum)
Kerr black hole with the same angular momentum.
1 Introduction
The gravitational two-body problem has attracted much
attention over the years from both physicists and mathe-
maticians. In particular, it is highly important to explore the
physics of a central black hole surrounded by an orbiting
ring: it is expected that this composed two-body system may
be formed as an intermediate stage in the gravitational col-
lapse of a compact spinning star to form a black hole [1–3].
Likewise, the coalescence of two compact objects may pro-
duce a composed black-hole–ring system [1–3]. In addition
to these astrophysical motivations, it is highly interesting to
explore the composed black-hole–ring system in order to
understand how an exterior matter configuration affects the
physical properties of central black holes [1–5].
The general-relativistic problem of a slowly spinning
black hole surrounded by a thin orbiting ring was studied
perturbatively by Will [4,5](seealso[6–8]). It was shown
in [4] that, due to the well-known phenomenon of dragging
of inertial frames by the orbiting ring, the angular velocity
BH-ring
H
1
of the central black hole in the composed black-
1
We use the symbol
BH-ring
H
to denote the angular velocity of the
black-hole horizon in the composed black-hole–ring system.
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