The Newman–Penrose (NP) formalism
[ The original paper by Newman and Penrose, which introduces the formalism, and uses it to derive example results.][Ezra T Newman, Roger Penrose. ''Errata: An Approach to Gravitational Radiation by a Method of Spin Coefficients''. Journal of Mathematical Physics, 1963, 4(7): 998.] is a set of notation developed by
Ezra T. Newman
Ezra Theodore Newman (October 17, 1929 – March 24, 2021) was an American physicist, known for his many contributions to general relativity theory. He was Professor Emeritus at the University of Pittsburgh. Newman was awarded the 2011 Einstein P ...
and
Roger Penrose
Sir Roger Penrose (born 8 August 1931) is an English mathematician, mathematical physicist, philosopher of science and Nobel Laureate in Physics. He is Emeritus Rouse Ball Professor of Mathematics in the University of Oxford, an emeritus fello ...
for
general relativity
General relativity, also known as the general theory of relativity and Einstein's theory of gravity, is the geometric theory of gravitation published by Albert Einstein in 1915 and is the current description of gravitation in modern physics ...
(GR). Their notation is an effort to treat general relativity in terms of
spinor
In geometry and physics, spinors are elements of a complex vector space that can be associated with Euclidean space. Like geometric vectors and more general tensors, spinors transform linearly when the Euclidean space is subjected to a sligh ...
notation, which introduces
complex
Complex commonly refers to:
* Complexity, the behaviour of a system whose components interact in multiple ways so possible interactions are difficult to describe
** Complex system, a system composed of many components which may interact with each ...
forms of the usual variables used in GR. The NP formalism is itself a special case of the
tetrad formalism
The tetrad formalism is an approach to general relativity that generalizes the choice of basis for the tangent bundle from a coordinate basis to the less restrictive choice of a local basis, i.e. a locally defined set of four linearly independe ...
,
where the tensors of the theory are projected onto a complete vector basis at each point in spacetime. Usually this vector basis is chosen to reflect some symmetry of the spacetime, leading to simplified expressions for physical observables. In the case of the NP formalism, the vector basis chosen is a
null tetrad: a set of four null vectors—two real, and a complex-conjugate pair. The two real members asymptotically point radially inward and radially outward, and the formalism is well adapted to treatment of the propagation of radiation in curved spacetime. The
Weyl scalars, derived from the
Weyl tensor
In differential geometry, the Weyl curvature tensor, named after Hermann Weyl, is a measure of the curvature of spacetime or, more generally, a pseudo-Riemannian manifold. Like the Riemann curvature tensor, the Weyl tensor expresses the tidal f ...
, are often used. In particular, it can be shown that one of these scalars—
in the appropriate frame—encodes the outgoing
gravitational radiation
Gravitational waves are waves of the intensity of gravity generated by the accelerated masses of an orbital binary system that propagate as waves outward from their source at the speed of light. They were first proposed by Oliver Heaviside in 1 ...
of an asymptotically flat system.
Newman and Penrose introduced the following functions as primary quantities using this tetrad:
* Twelve complex spin coefficients (in three groups) which describe the change in the tetrad from point to point:
.
* Five complex functions encoding Weyl tensors in the tetrad basis:
.
* Ten functions encoding
Ricci tensor
In differential geometry, the Ricci curvature tensor, named after Gregorio Ricci-Curbastro, is a geometric object which is determined by a choice of Riemannian or pseudo-Riemannian metric on a manifold. It can be considered, broadly, as a measur ...
s in the tetrad basis:
(real);
(complex).
In many situations—especially algebraically special spacetimes or vacuum spacetimes—the Newman–Penrose formalism simplifies dramatically, as many of the functions go to zero. This simplification allows for various theorems to be proven more easily than using the standard form of Einstein's equations.
In this article, we will only employ the
tensor
In mathematics, a tensor is an algebraic object that describes a multilinear relationship between sets of algebraic objects related to a vector space. Tensors may map between different objects such as vectors, scalars, and even other tenso ...
ial rather than
spinor
In geometry and physics, spinors are elements of a complex vector space that can be associated with Euclidean space. Like geometric vectors and more general tensors, spinors transform linearly when the Euclidean space is subjected to a sligh ...
ial version of NP formalism, because the former is easier to understand and more popular in relevant papers. One can refer to ref.
[Peter O'Donnell. ''Introduction to 2-Spinors in General Relativity''. Singapore: World Scientific, 2003.] for a unified formulation of these two versions.
Null tetrad and sign convention
The formalism is developed for four-dimensional spacetime, with a Lorentzian-signature metric. At each point, a
tetrad
Tetrad ('group of 4') or tetrade may refer to:
* Tetrad (area), an area 2 km x 2 km square
* Tetrad (astronomy), four total lunar eclipses within two years
* Tetrad (chromosomal formation)
* Tetrad (general relativity), or frame field
** Tetra ...
(set of four vectors) is introduced. The first two vectors,
and
are just a pair of standard (real)
null vectors such that
. For example, we can think in terms of spherical coordinates, and take
to be the outgoing null vector, and
to be the ingoing null vector. A complex null vector is then constructed by combining a pair of real, orthogonal unit space-like vectors. In the case of spherical coordinates, the standard choice is
:
The complex conjugate of this vector then forms the fourth element of the tetrad.
Two sets of signature and normalization conventions are in use for NP formalism:
and
. The former is the original one that was adopted when NP formalism was developed
and has been widely used
[Subrahmanyan Chandrasekhar. ''The Mathematical Theory of Black Holes''. Chicago: University of Chikago Press, 1983.][J B Griffiths. ''Colliding Plane Waves in General Relativity''. Oxford: Oxford University Press, 1991.] in black-hole physics, gravitational waves and various other areas in general relativity. However, it is the latter convention that is usually employed in contemporary study of black holes from quasilocal perspectives (such as isolated horizons
[Abhay Ashtekar, Christopher Beetle, Jerzy Lewandowski. ''Geometry of generic isolated horizons''. Classical and Quantum Gravity, 2002, 19(6): 1195-1225]
arXiv:gr-qc/0111067v2
/ref> and dynamical horizons). In this article, we will utilize for a systematic review of the NP formalism (see also refs.[Jeremy Bransom Griffiths, Jiri Podolsky. ''Exact Space-Times in Einstein's General Relativity''. Cambridge: Cambridge University Press, 2009. Chapter 2.][Valeri P Frolov, Igor D Novikov. ''Black Hole Physics: Basic Concepts and New Developments''. Berlin: Springer, 1998. Appendix E.][Abhay Ashtekar, Stephen Fairhurst, Badri Krishnan. ''Isolated horizons: Hamiltonian evolution and the first law''. Physical Review D, 2000, 62(10): 104025. Appendix B]
gr-qc/0005083
/ref>).
It's important to note that, when switching from to , definitions of the spin coefficients, Weyl-NP scalars and Ricci-NP scalars need to change their signs; this way, the Einstein-Maxwell equations can be left unchanged.
In NP formalism, the complex null tetrad contains two real null (co)vectors and two complex null (co)vectors . Being ''null'' (co)vectors, ''self''-normalization of naturally vanishes,
,
so the following two pairs of ''cross''-normalization are adopted
while contractions between the two pairs are also vanishing,
.
Here the indices can be raised and lowered by the global metric
Metric or metrical may refer to:
* Metric system, an internationally adopted decimal system of measurement
* An adjective indicating relation to measurement in general, or a noun describing a specific type of measurement
Mathematics
In mathem ...
which in turn can be obtained via
NP quantities and tetrad equations
Four covariant derivative operators
In keeping with the formalism's practice of using distinct unindexed symbols for each component of an object, the covariant derivative operator is expressed using four separate symbols () which name a directional covariant derivative operator for each tetrad direction. Given a linear combination of tetrad vectors, , the covariant derivative operator in the direction is .
The operators are defined as
which reduce to when acting on ''scalar'' functions.
Twelve spin coefficients
In NP formalism, instead of using index notations as in orthogonal tetrad
In mathematics, orthogonality is the generalization of the geometric notion of ''perpendicularity''.
By extension, orthogonality is also used to refer to the separation of specific features of a system. The term also has specialized meanings in ...
s, each Ricci rotation coefficient in the null tetrad is assigned a lower-case Greek letter, which constitute the 12 complex ''spin coefficients'' (in three groups),
Spin coefficients are the primary quantities in NP formalism, with which all other NP quantities (as defined below) could be calculated indirectly using the NP field equations. Thus, NP formalism is sometimes referred to as ''spin-coefficient formalism'' as well.
Transportation equations: covariant derivatives of tetrad vectors
The sixteen directional covariant derivatives of tetrad vectors are sometimes called the ''transportation/propagation equations,'' perhaps because the derivatives are zero when the tetrad vector is parallel propagated or transported in the direction of the derivative operator.
These results in this exact notation are given by ODonnell:
Interpretation of from and
The two equations for the covariant derivative of a real null tetrad vector in its own direction indicate whether or not the vector is tangent to a geodesic and if so, whether the geodesic has an affine parameter.
A null tangent vector is tangent to an affinely parameterized null geodesic if , which is to say if the vector is unchanged by parallel propagation or transportation in its own direction.
shows that is tangent to a geodesic if and only if , and is tangent to an affinely parameterized geodesic if in addition . Similarly, shows that is geodesic if and only if , and has affine parameterization when .
(The complex null tetrad vectors and would have to be separated into the spacelike basis vectors and before asking if either or both of those are tangent to spacelike geodesics.)
Commutators
The metric-compatibility or torsion-freeness of the covariant derivative is recast into the ''commutator
In mathematics, the commutator gives an indication of the extent to which a certain binary operation fails to be commutative. There are different definitions used in group theory and ring theory.
Group theory
The commutator of two elements, a ...
s of the directional derivatives'',
which imply that
Note: (i) The above equations can be regarded either as implications of the commutators or combinations of the transportation equations; (ii) In these implied equations, the vectors can be replaced by the covectors and the equations still hold.
Weyl–NP and Ricci–NP scalars
The 10 independent components of the Weyl tensor
In differential geometry, the Weyl curvature tensor, named after Hermann Weyl, is a measure of the curvature of spacetime or, more generally, a pseudo-Riemannian manifold. Like the Riemann curvature tensor, the Weyl tensor expresses the tidal f ...
can be encoded into 5 complex Weyl-NP scalars,
The 10 independent components of the Ricci tensor
In differential geometry, the Ricci curvature tensor, named after Gregorio Ricci-Curbastro, is a geometric object which is determined by a choice of Riemannian or pseudo-Riemannian metric on a manifold. It can be considered, broadly, as a measur ...
are encoded into 4 ''real'' scalars and 3 ''complex'' scalars (with their complex conjugates),
In these definitions, could be replaced by its trace-free
In linear algebra, the trace of a square matrix , denoted , is defined to be the sum of elements on the main diagonal (from the upper left to the lower right) of . The trace is only defined for a square matrix ().
It can be proved that the trace o ...
part or by the Einstein tensor
In differential geometry, the Einstein tensor (named after Albert Einstein; also known as the trace-reversed Ricci tensor) is used to express the curvature of a pseudo-Riemannian manifold. In general relativity, it occurs in the Einstein field ...
because of the normalization relations. Also, is reduced to for electrovacuum ().
Einstein–Maxwell–NP equations
NP field equations
In a complex null tetrad, Ricci identities give rise to the following NP field equations connecting spin coefficients, Weyl-NP and Ricci-NP scalars (recall that in an orthogonal tetrad, Ricci rotation coefficients would respect Cartan's first and second structure equations),
These equations in various notations can be found in several texts. The notation in Frolov and Novikov
is identical.
Also, the Weyl-NP scalars and the Ricci-NP scalars can be calculated indirectly from the above NP field equations after obtaining the spin coefficients rather than directly using their definitions.
Maxwell–NP scalars, Maxwell equations in NP formalism
The six independent components of the Faraday-Maxwell 2-form (i.e. the electromagnetic field strength tensor) can be encoded into three complex Maxwell-NP scalars
and therefore the eight real Maxwell equations
Maxwell's equations, or Maxwell–Heaviside equations, are a set of coupled partial differential equations that, together with the Lorentz force law, form the foundation of classical electromagnetism, classical optics, and electric circuits.
T ...
and (as ) can be transformed into four complex equations,
with the Ricci-NP scalars related to Maxwell scalars by
It is worthwhile to point out that, the supplementary equation is only valid for electromagnetic fields; for example, in the case of Yang-Mills fields there will be where are Yang-Mills-NP scalars.
To sum up, the aforementioned transportation equations, NP field equations and Maxwell-NP equations together constitute the Einstein-Maxwell equations in Newman–Penrose formalism.
Applications of NP formalism to gravitational radiation field
The Weyl scalar was defined by Newman & Penrose as
:
(note, however, that the overall sign is arbitrary
Arbitrariness is the quality of being "determined by chance, whim, or impulse, and not by necessity, reason, or principle". It is also used to refer to a choice made without any specific criterion or restraint.
Arbitrary decisions are not necess ...
, and that Newman & Penrose worked with a "timelike" metric signature of ).
In empty space, the Einstein Field Equations
In the general theory of relativity, the Einstein field equations (EFE; also known as Einstein's equations) relate the geometry of spacetime to the distribution of matter within it.
The equations were published by Einstein in 1915 in the form ...
reduce to . From the definition of the Weyl tensor, we see that this means that it equals the Riemann tensor
In the mathematical field of differential geometry, the Riemann curvature tensor or Riemann–Christoffel tensor (after Bernhard Riemann and Elwin Bruno Christoffel) is the most common way used to express the curvature of Riemannian manifolds. ...
, . We can make the standard choice for the tetrad at infinity:
:
:
:
In transverse-traceless gauge, a simple calculation shows that linearized gravitational waves
Gravitational waves are waves of the intensity of gravity generated by the accelerated masses of an orbital binary system that Wave propagation, propagate as waves outward from their source at the speed of light. They were first proposed by Oliv ...
are related to components of the Riemann tensor as
:
:
assuming propagation in the direction. Combining these, and using the definition of above, we can write
:
Far from a source, in nearly flat space, the fields and encode everything about gravitational radiation propagating in a given direction. Thus, we see that encodes in a single complex field everything about (outgoing) gravitational waves.
Radiation from a finite source
Using the wave-generation formalism summarised by Thorne,[ A broad summary of the mathematical formalism used in the literature on gravitational radiation.] we can write the radiation field quite compactly in terms of the mass multipole, current multipole, and spin-weighted spherical harmonics
In special functions, a topic in mathematics, spin-weighted spherical harmonics are generalizations of the standard spherical harmonics and—like the usual spherical harmonics—are functions on the sphere. Unlike ordinary spherical harmonics, t ...
:
:
Here, prefixed superscripts indicate time derivatives. That is, we define
:
The components and are the mass and current multipoles, respectively. is the spin-weight -2 spherical harmonic.
See also
* Light cone coordinates
* GHP formalism
* Tetrad formalism
The tetrad formalism is an approach to general relativity that generalizes the choice of basis for the tangent bundle from a coordinate basis to the less restrictive choice of a local basis, i.e. a locally defined set of four linearly independe ...
* Goldberg–Sachs theorem
The Goldberg–Sachs theorem is a result in Einstein's theory of general relativity about vacuum solutions of the Einstein field equations relating the existence of a certain type of congruence with algebraic properties of the Weyl tensor.
More ...
Notes
References
* Wald treats the more succinct version of the Newman–Penrose formalism in terms of more modern spinor notation.
* Hawking and Ellis use the formalism in their discussion of the final state of a collapsing star.
External links
Newman–Penrose formalism on Scholarpedia
{{DEFAULTSORT:Newman-Penrose Formalism
Theory of relativity
Mathematical notation