The question is whether there exists a cut X on which the
ingong expansion θ
l
vanishes as well. This is accomplished
in the following sequence of steps.
The minimar assumption dictates that on σ, θ
l
< 0 and
k
a
∇
a
θ
l
< 0. One can choose initial conditions on N
−
k
such
that along every null generator of N
−
k
, k
a
∇
a
θ
l
is constant
and equal to its value on σ: by the cross-focusing equation,
k
a
∇
a
θ
l
¼ −
1
2
R − θ
k
θ
l
þ χ
2
þ ∇ · χ þ 8πGT
kl
; ð8Þ
this can be accomplished by choosing all terms on the right-
hand side to be constant on N
−
k
. This is already ensured for
the intrinsic curvature scalar R and for the (vanishing) θ
k
θ
l
term by stationarity of N
−
k
. The twist, or normal one form,
is defined by
χ
a
¼ h
c
a
l
d
∇
c
k
d
; ð9Þ
where h
ab
¼ g
ab
þ 2l
ða
k
bÞ
is the induced metric on a cut.
The twist evolves according to
k
a
∇
a
χ
i
¼ 8πT
ik
ðþterms that vanish when θ
k
¼ ς ¼ 0Þ:
ð10Þ
To summarize, one can accomplish k
a
∇
a
θ
l
¼ k
a
∇
a
θ
l
j
σ
on N
−
k
by choosing Eqs. (5) and (7) and in addition, along
each null generator of N
−
k
,
T
kl
¼ T
kl
j
σ
and T
ik
¼ 0 on N
−
k
: ð11Þ
Again, EW argue that these choices are always possible.
Let v be the affine parameter associated to k
a
, and let y
be the transverse coordinates (angular coordinates) on σ.
The location of a stationary surface X, v ¼ fðyÞ is
determined by the differential equation
L
a
½f¼−θ
l
j
σ
; ð12Þ
where L
a
is the stability operator (see Ref. [13] for details).
This can be shown to have a solution with −∞ <f<0,so
the HRT surface exists and lies on N
−
k
.
EW then glue the geometry exterior to X (i.e., N
−
k
and the
outer wedge) to its CPT image across X. This constructs a
“two-sided” geometry in which X functions as a kind of
bifurcation surface of a two-sided black hole/white hole
pair. (However, the stationary auxiliary portion N
−
k
does not
in general correspond to the horizon of a Kerr-Newman
black hole, as its intrinsic metric can differ.)
In a final step, EW show that X is not just stationary but
is an HRT surface, i.e., that X is the smallest-area stationary
surface homologous to σ. This step uses the NEC as well as
the second part of the minimar property of σ.
This concludes our summary of the EW coarse-graining
prescription. Again, we refer the interested reader to
Ref. [13] for more detailed definitions and arguments.
III. SEMICLASSICAL COARSE GRAINING
OF BLACK HOLE STATES
In this section, we formulate a semiclassical extension of
the Engelhardt-Wall construction, starting from a quantum
marginally trapped surface σ. We conjecture that the
semiclassical state invoked in our construction exists in
the full quantum gravity theory, and that in this theory this
state has a von Neumann entropy given by the generalized
entropy of σ.
In Sec. III A, we introduce relevant concepts such as
generalized entropy, quantum expansion, quantum margin-
ally trapped surfaces, and quantum HRT surfaces.
In Sec. III B, we state our quantum extension of the EW
coarse-graining proposal.
In Sec. III C, we refine our conjecture by describing key
properties that the coarse-grained state is expected to satisfy
at the level of semiclassical gravity. (These properties will
be shown to have an interesting nongravitational limit in
Sec. IV. In Sec. V, we will show that a recent construction
by Ceyhan and Faulkner [4] generates quantum field theory
states which achieve these properties in a certain limit.)
A. Quantum marginal, minimar,
and stationary surfaces
Before we turn to the question of why and how the EW
construction should be extended to the semiclassical
regime, we introduce here the relevant concepts: general-
ized entropy, quantum expansion, quantum (marginally)
trapped surfaces, and quantum extremal surfaces. More
details can be found, e.g., in Refs. [2,10,15,16].
The notion of generalized entropy was originally intro-
duced by Bekenstein [8] as an extension of ordinary
entropy that includes the contribution from black holes,
S
out
→ S
out
þ
A
4Gℏ
. But in an expansion in Gℏ, it is the
exterior entropy that should be regarded as a quantum
correction
S
gen
¼
A
4Gℏ
þ S
out
þ …: ð13Þ
Equivalently, 4GℏS
gen
represents a quantum-corrected
area.
In Bekenstein’s original proposal, A represented the area
of a cut of a black hole event horizon; and S
out
represented
the entropy in the black hole’s exterior. However, the
generalized entropy can be defined for any Cauchy-
splitting surface σ, with S
out
the von Neumann entropy
of the quantum fields restricted to one side of σ. A=4Gℏ
should be regarded as the leading counterterm that cancels
divergences in the entropy; we suppress subleading terms
here. Given its wide applicability, the notion of generalized
RAPHAEL BOUSSO et al. PHYS. REV. D 101, 046001 (2020)
046001-4