### 7.2 The original construction for curved spacetimes

#### 7.2.1 2-surface twistors and the kinematical twistor

In general spacetimes the twistor equations have only the trivial solution. Thus to be able to associate a kinematical twistor to a closed orientable spacelike 2-surface in general, the conditions on the spinor field had to be relaxed. Penrose’s suggestion [307308] is to consider in Equation (43) to be the symmetrized product of spinor fields that are solutions of the ‘tangential projection to ’ of the valence 1 twistor equation, the so-called 2-surface twistor equation. (The equation obtained as the ‘tangential projection to ’ of the valence 2 twistor equation (46) would be under-determined [308].) Thus the quasi-local quantities are searched for in the form of a charge integral of the curvature:

where the second expression is given in the GHP formalism with respect to some GHP spin frame adapted to the 2-surface . Since the indices and on the right of the first expression are tangential to , this is just the charge integral of in the spinor identity (24) of Section 4.1.5.

The 2-surface twistor equation that the spinor fields should satisfy is just the covariant spinor equation . By Equation (25) its GHP form is , which is a first order elliptic system, and its index is , where is the genus of  [43]. Thus there are at least four (and in the generic case precisely four) linearly independent solutions to on topological 2-spheres. However, there are ‘exceptional’ 2-spheres for which there exist at least five linearly independent solutions [221]. For such ‘exceptional’ 2-spheres (and for higher genus 2-surfaces for which the twistor equation has only the trivial solution in general) the subsequent construction does not work. (The concept of quasi-local charges in Yang-Mills theory can also be introduced in an analogous way [370]). The space of the solutions to is called the 2-surface twistor space. In fact, in the generic case this space is 4-complex-dimensional, and under conformal rescaling the pair transforms like a valence 1 contravariant twistor. is called a 2-surface twistor determined by . If is another generic 2-surface with the corresponding 2-surface twistor space , then although and are isomorphic as vector spaces, there is no canonical isomorphism between them. The kinematical twistor is defined to be the symmetric twistor determined by for any and from . Note that is constructed only from the 2-surface data on .

#### 7.2.2 The Hamiltonian interpretation of the kinematical twistor

For the solutions and of the 2-surface twistor equation, the spinor identity (24) reduces to Tod’s expression [307313377] for the kinematical twistor, making it possible to re-express by the integral of the Nester-Witten 2-form [356]. Indeed, if

then with the choice this gives Penrose’s charge integral by Equation (24): . Then, extending the spinor fields and from to a spacelike hypersurface with boundary in an arbitrary way, by the Sparling equation it is straightforward to rewrite in the form of the integral of the energy-momentum tensor of the matter fields and the Sparling form on . Since such an integral of the Sparling form can be interpreted as the Hamiltonian of general relativity, this is a quick re-derivation of Mason’s [265266] Hamiltonian interpretation of Penrose’s kinematical twistor: is just the boundary term in the total Hamiltonian of the matter + gravity system, and the spinor fields and (together with their ‘projection parts’ and ) on are interpreted as the spinor constituents of the special lapse and shift, the so-called ‘quasi-translations’ and ‘quasi-rotations’ of the 2-surface, on the 2-surface itself.

#### 7.2.3 The Hermitian scalar product and the infinity twistor

In general the natural pointwise Hermitian scalar product, defined by , is not constant on , thus it does not define a Hermitian scalar product on the 2-surface twistor space. As is shown in [220223375], is constant on for any two 2-surface twistors if and only if can be embedded, at least locally, into some conformal Minkowski spacetime with its intrinsic metric and extrinsic curvatures. Such 2-surfaces are called non-contorted, while those that cannot be embedded are called contorted. One natural candidate for the Hermitian metric could be the average of on  [307]: , which reduces to on non-contorted 2-surfaces. Interestingly enough, can also be re-expressed by the integral (55) of the Nester-Witten 2-form [356]. Unfortunately, however, neither this metric nor the other suggestions appearing in the literature are conformally invariant. Thus, for contorted 2-surfaces, the definition of the quasi-local mass as the norm of the kinematical twistor (cf. Equation (52)) is ambiguous unless a natural is found.

If is non-contorted, then the scalar product defines the totally anti-symmetric twistor , and for the four independent 2-surface twistors , …, the contraction , and hence by Equation (49) the determinant , is constant on . Nevertheless, can be constant even for contorted 2-surfaces for which is not. Thus, the totally anti-symmetric twistor can exist even for certain contorted 2-surfaces. Therefore, an alternative definition of the quasi-local mass might be based on Equation (53[371]. However, although the two mass definitions are equivalent in the linearized theory, they are different invariants of the kinematical twistor even in de Sitter or anti-de-Sitter spacetimes. Thus, if needed, the former notion of mass will be called the norm-mass, the latter the determinant-mass (denoted by ).

If we want to have not only the notion of the mass but its reality is also expected, then we should ensure the Hermiticity of the kinematical twistor. But to formulate the Hermiticity condition (51), one also needs the infinity twistor. However, is not constant on even if it is non-contorted, thus in general it does not define any twistor on . One might take its average on (which can also be re-expressed by the integral of the Nester-Witten 2-form [356]), but the resulting twistor would not be simple. In fact, even on 2-surfaces in de Sitter and anti-de Sitter spacetimes with cosmological constant the natural definition for is  [313311371], while on round spheres in spherically symmetric spacetimes it is  [363]. Thus no natural simple infinity twistor has been found in curved spacetime. Indeed, Helfer claims that no such infinity twistor can exist [197]: Even if the spacetime is conformally flat (whenever the Hermitian metric exists) the Hermiticity condition would be fifteen algebraic equations for the (at most) twelve real components of the ‘would be’ infinity twistor. Then, since the possible kinematical twistors form an open set in the space of symmetric twistors, the Hermiticity condition cannot be satisfied even for non-simple s. However, in contrast to the linearized gravity case, the infinity twistor should not be given once and for all on some ‘universal’ twistor space, that may depend on the actual gravitational field. In fact, the 2-surface twistor space itself depends on the geometry of , and hence all the structures thereon also.

Since in the Hermiticity condition (51) only the special combination of the infinity and metric twistors (the so-called ‘bar-hook’ combination) appears, it might still be hoped that an appropriate could be found for a class of 2-surfaces in a natural way [377]. However, as far as the present author is aware of, no real progress has been achieved in this way.

#### 7.2.4 The various limits

Obviously, the kinematical twistor vanishes in flat spacetime and, since the basic idea came from the linearized gravity, the construction gives the correct results in the weak field approximation. The nonrelativistic weak field approximation, i.e. the Newtonian limit, was clarified by Jeffryes [222]. He considers a 1-parameter family of spacetimes with perfect fluid source such that in the limit of the parameter one gets a Newtonian spacetime, and, in the same limit, the 2-surface lies in a hypersurface of the Newtonian time . In this limit the pointwise Hermitian scalar product is constant, and the norm-mass can be calculated. As could be expected, for the leading order term in the expansion of as a series of he obtained the conserved Newtonian mass. The Newtonian energy, including the kinetic and the Newtonian potential energy, appears as a order correction.

The Penrose definition for the energy-momentum and angular momentum can be applied to the cuts of the future null infinity of an asymptotically flat spacetime [307313]. Then every element of the construction is built from conformally rescaled quantities of the non-physical spacetime. Since is shear-free, the 2-surface twistor equations on decouple, and hence the solution space admits a natural infinity twistor . It singles out precisely those solutions whose primary spinor parts span the asymptotic spin space of Bramson (see Section 4.2.4), and they will be the generators of the energy-momentum. Although is contorted, and hence there is no natural Hermitian scalar product, there is a twistor with respect to which is Hermitian. Furthermore, the determinant is constant on , and hence it defines a volume 4-form on the 2-surface twistor space [377]. The energy-momentum coming from is just that of Bondi and Sachs. The angular momentum defined by is, however, new. It has a number of attractive properties. First, in contrast to definitions based on the Komar expression, it does not have the ‘factor-of-two anomaly’ between the angular momentum and the energy-momentum. Since its definition is based on the solutions of the 2-surface twistor equations (which can be interpreted as the spinor constituents of certain BMS vector fields generating boost-rotations) instead of the BMS vector fields themselves, it is free of supertranslation ambiguities. In fact, the 2-surface twistor space on reduces the BMS Lie algebra to one of its Poincaré subalgebras. Thus the concept of the ‘translation of the origin’ is moved from null infinity to the twistor space (appearing in the form of a 4-parameter family of ambiguities in the potential for the shear ), and the angular momentum transforms just in the expected way under such a ‘translation of the origin’. As was shown in [129], Penrose’s angular momentum can be considered as a supertranslation of previous definitions. The corresponding angular momentum flux through a portion of the null infinity between two cuts was calculated in [129196] and it was shown that this is precisely that given by Ashtekar and Streubel [29] (see also [336337128]).

The other way of determining the null infinity limit of the energy-momentum and angular momentum is to calculate them for the large spheres from the physical data, instead of the spheres at null infinity from the conformally rescaled data. These calculations were done by Shaw [338340]. At this point it should be noted that the limit of vanishes, and it is that yields the energy-momentum and angular momentum at infinity (see the remarks following Equation (15)). The specific radiative solution for which the Penrose mass has been calculated is that of Robinson and Trautman [371]. The 2-surfaces for which the mass was calculated are the cuts of the geometrically distinguished outgoing null hypersurfaces . Tod found that, for given , the mass is independent of , as could be expected because of the lack of the incoming radiation.

The large sphere limit of the 2-surface twistor space and the Penrose construction were investigated by Shaw in the Sommers [344], the Ashtekar-Hansen [23], and the Beig-Schmidt [48] models of spatial infinity in [334335337]. Since no gravitational radiation is present near the spatial infinity, the large spheres are (asymptotically) non-contorted, and both the Hermitian scalar product and the infinity twistor are well-defined. Thus the energy-momentum and angular momentum (and, in particular, the mass) can be calculated. In vacuum he recovered the Ashtekar-Hansen expression for the energy-momentum and angular momentum, and proved their conservation if the Weyl curvature is asymptotically purely electric. In the presence of matter the conservation of the angular momentum was investigated in [339].

The Penrose mass in asymptotically anti-de-Sitter spacetimes was studied by Kelly [234]. He calculated the kinematical twistor for spacelike cuts of the infinity , which is now a timelike 3-manifold in the non-physical spacetime. Since admits global 3-surface twistors (see the next Section 7.2.5), is non-contorted. In addition to the Hermitian scalar product there is a natural infinity twistor, and the kinematical twistor satisfies the corresponding Hermiticity condition. The energy-momentum 4-vector coming from the Penrose definition is shown to coincide with that of Ashtekar and Magnon [27]. Therefore, the energy-momentum 4-vector is future pointing and timelike if there is a spacelike hypersurface extending to on which the dominant energy condition is satisfied. Consequently, . Kelly showed that is also non-negative and in vacuum it coincides with . In fact [377], holds.

#### 7.2.5 The quasi-local mass of specific 2-surfaces

The Penrose mass has been calculated in a large number of specific situations. Round spheres are always non-contorted [375], thus the norm-mass can be calculated. (In fact, axi-symmetric 2-surfaces in spacetimes with twist-free rotational Killing vector are non-contorted [223].) The Penrose mass for round spheres reduces to the standard energy expression discussed in Section 4.2.1 [371]. Thus every statement given in Section 4.2.1 for round spheres is valid for the Penrose mass, and we do not repeat them. In particular, for round spheres in a slice of the Kantowski-Sachs spacetime this mass is independent of the 2-surfaces [368]. Interestingly enough, although these spheres cannot be shrunk to a point (thus the mass cannot be interpreted as ‘the 3-volume integral of some mass density’), the time derivative of the Penrose mass looks like the mass conservation equation: It is minus the pressure times the rate of change of the 3-volume of a sphere in flat space with the same area as  [376]. In conformally flat spacetimes [371] the 2-surface twistors are just the global twistors restricted to , and the Hermitian scalar product is constant on . Thus the norm-mass is well-defined.

The construction works nicely even if global twistors exist only on a (say) spacelike hypersurface containing . These twistors are the so-called 3-surface twistors [371373], which are solutions of certain (overdetermined) elliptic partial differential equations, the so-called 3-surface twistor equations, on . These equations are completely integrable (i.e. they admit the maximal number of linearly independent solutions, namely four) if and only if with its intrinsic metric and extrinsic curvature can be embedded, at least locally, into some conformally flat spacetime [373]. Such hypersurfaces are called non-contorted. It might be interesting to note that the non-contorted hypersurfaces can also be characterized as the critical points of the Chern-Simons functional built from the real Sen connection on the Lorentzian vector bundle or from the 3-surface twistor connection on the twistor bundle over  [49361]. Returning to the quasi-local mass calculations, Tod showed that in vacuum the kinematical twistor for a 2-surface in a non-contorted depends only on the homology class of . In particular, if can be shrunk to a point then the corresponding kinematical twistor is vanishing. Since is non-contorted, is also non-contorted, and hence the norm-mass is well-defined. This implies that the Penrose mass in the Schwarzschild solution is the Schwarzschild mass for any non-contorted 2-surface that can be deformed into a round sphere, and it is zero for those that do not link the black hole [375]. Thus, in particular, the Penrose mass can be zero even in curved spacetimes.

A particularly interesting class of non-contorted hypersurfaces is that of the conformally flat time-symmetric initial data sets. Tod considered Wheeler’s solution of the time-symmetric vacuum constraints describing ‘points at infinity’ (or, in other words, black holes) and 2-surfaces in such a hypersurface [371]. He found that the mass is zero if does not link any black hole, it is the mass of the -th black hole if links precisely the -th hole, it is if links precisely the -th and the -th holes, where is some appropriate measure of the distance of the holes, …, etc. Thus, the mass of the -th and -th holes as a single object is less than the sum of the individual masses, in complete agreement with our physical intuition that the potential energy of the composite system should contribute to the total energy with negative sign.

Beig studied the general conformally flat time-symmetric initial data sets describing ‘points at infinity’ [45]. He found a symmetric trace-free and divergence-free tensor field and, for any conformal Killing vector of the data set, defined the 2-surface flux integral of on . He showed that is conformally invariant, depends only on the homology class of , and, apart from numerical coefficients, for the ten (locally existing) conformal Killing vectors these are just the components of the kinematical twistor derived by Tod in [371] (and discussed in the previous paragraph). In particular, Penrose’s mass in Beig’s approach is proportional to the length of the ’s with respect to the Cartan-Killing metric of the conformal group of the hypersurface.

Tod calculated the quasi-local mass for a large class of axi-symmetric 2-surfaces (cylinders) in various LRS Bianchi and Kantowski-Sachs cosmological models [376] and more general cylindrically symmetric spacetimes [378]. In all these cases the 2-surfaces are non-contorted, and the construction works. A technically interesting feature of these calculations is that the 2-surfaces have edges, i.e. they are not smooth submanifolds. The twistor equation is solved on the three smooth pieces of the cylinder separately, and the resulting spinor fields are required to be continuous at the edges. This matching reduces the number of linearly independent solutions to four. The projection parts of the resulting twistors, the s, are not continuous at the edges. It turns out that the cylinders can be classified invariantly to be hyperbolic, parabolic, or elliptic. Then the structure of the quasi-local mass expressions is not simply ‘density’ ‘volume’, but they are proportional to a ‘type factor’ as well, where is the coordinate length of the cylinder. In the hyperbolic, parabolic, and elliptic cases this factor is , , and , respectively, where is an invariant of the cylinder. The various types are interpreted as the presence of a positive, zero, or negative potential energy. In the elliptic case the mass may be zero for finite cylinders. On the other hand, for static perfect fluid spacetimes (hyperbolic case) the quasi-local mass is positive. A particularly interesting spacetime is that describing cylindrical gravitational waves, whose presence is detected by the Penrose mass. In all these cases the determinant-mass has also been calculated and found to coincide with the norm-mass. A numerical investigation of the axi-symmetric Brill waves on the Schwarzschild background was presented in [69]. It was found that the quasi-local mass is positive, and it is very sensitive to the presence of the gravitational waves.

Another interesting issue is the Penrose inequality for black holes (see Section 13.2.1). Tod showed [374375] that for static black holes the Penrose inequality holds if the mass of the hole is defined to be the Penrose quasi-local mass of the spacelike cross section of the event horizon. The trick here is that is totally geodesic and conformal to the unit sphere, and hence it is non-contorted and the Penrose mass is well-defined. Then the Penrose inequality will be a Sobolev-type inequality for a non-negative function on the unit sphere. This inequality was tested numerically in [69].

Apart from the cuts of in radiative spacetimes, all the 2-surfaces discussed so far were non-contorted. The spacelike cross section of the event horizon of the Kerr black hole provides a contorted 2-surface [377]. Thus although the kinematical twistor can be calculated for this, the construction in its original form cannot yield any mass expression. The original construction has to be modified.

#### 7.2.6 Small surfaces

The properties of the Penrose construction that we have discussed are very remarkable and promising. However, the small surface calculations showed clearly some unwanted feature of the original construction [372235398], and forced its modification. First, although the small spheres are contorted in general, the leading term of the pointwise Hermitian scalar product is constant: for any 2-surface twistors and  [372235]. Since in non-vacuum spacetimes the kinematical twistor has only the ‘4-momentum part’ in the leading order with , the Penrose mass, calculated with the norm above, is just the expected mass in the leading order. Thus it is positive if the dominant energy condition is satisfied. On the other hand, in vacuum the structure of the kinematical twistor is

where and with . In particular, in terms of the familiar conformal electric and magnetic parts of the curvature the leading term in the time component of the 4-momentum is . Then the corresponding norm-mass, in the leading order, can even be complex! For an in the hypersurface of the Schwarzschild spacetime this is zero (as it must be in the light of the results of the previous Section 7.2.5, because this is a non-contorted spacelike hypersurface), but for a general small 2-sphere not lying in such a hypersurface is real and spacelike, and hence . In the Kerr spacetime itself is complex [372235].