Spacetime symmetries

Spacetime symmetries refers to aspects of spacetime that can be described as exhibiting some form of symmetry. The role of symmetry in physics is important, for example, in simplifying solutions to many problems. Spacetime symmetries are used to simplify problems and find ample application in the study of exact solutions of Einstein's field equations of general relativity.

Physical motivation

Quite often, physical problems may be investigated and solved by noticing features of the problem which have some form of symmetry. For example, in the Schwarzschild solution, the role of spherical symmetry is important in deriving the Schwarzschild solution and deducing the physical consequences of this symmetry (for example, the non-existence of gravitational radiation in a spherically pulsating star). In cosmological problems, symmetry finds a role to play in the cosmological principle which restricts the type of universes that are consistent with large-scale observations (e.g. the Friedmann-Robertson-Walker (FRW) metric). Symmetries usually require some form of preserving property, the most important of which in general relativity include the following:

*preserving geodesics of the spacetime
*preserving the metric tensor
*preserving the curvature tensor

These and other symmetries will be discussed in more detail later. This preservation feature can be used to motivate a useful definition of symmetries.

Mathematical definition

A rigorous definition of symmetries in general relativity has been given by Hall (2004). In this approach, the idea is to use (smooth) vector fields whose local flow diffeomorphisms preserve some property of the spacetime. This preserving property of the diffeomorphisms is made precise as follows. A smooth vector field $X$ on a spacetime $M$ is said to preserve a smooth tensor $T$ on $M$ (or $T$ is invariant under $X$) if, for each smooth local flow diffeomorphism $phi\_t$ associated with $X$, the tensors $T$ and $phi\_t\{\}^*(T)$ are equal on the domain of $phi\_t$. This statement is equivalent to the more usable condition that the Lie derivative of the tensor under the vector field vanishes:

:$mathcal\{L\}\_X\; T=0$

on $M$. This has the consequence that, given any two points $p$ and $q$ on $M$, the coordinates of $T$ in a coordinate system around $p$ are equal to the coordinates of $T$ in a coordinate system around $q$. A "symmetry on the spacetime" is a smooth vector field whose local flow diffeomorphisms preserve some (usually geometrical) feature of the spacetime. The (geometrical) feature may refer to specific tensors (such as the metric, or the energy-momentum tensor) or to other aspects of the spacetime such as it's geodesic structure. The vector fields are sometimes referred to as "collineations", "symmetry vector fields" or just "symmetries". The set of all symmetry vector fields on $M$ forms a Lie algebra under the Lie bracket operation as can be seen from the identity:

: $mathcal\{L\}\_\{\; [X,Y]\; \}\; T\; =\; mathcal\{L\}\_X\; (mathcal\{L\}\_Y\; T)\; -\; mathcal\{L\}\_Y\; (mathcal\{L\}\_X\; T)$

the term on the right usually being written, with an abuse of notation, as $[mathcal\{L\}\_X,\; mathcal\{L\}\_Y]\; T$.

Various examples of symmetries are briefly described below.

Killing symmetry

Killing vector fields are one of the most important types of symmetries and are defined to be those smooth vector fields that preserve the metric tensor:

:$mathcal\{L\}\_X\; g\_\{ab\}=0$

This is usually written in the expanded form as:

:$X\_\{a;b\}+X\_\{b;a\}\; ,\; =0$

Killing vector fields find extensive applications (including in classical mechanics) and are related to conservation laws.

Homothetic symmetry

A homothetic vector field is one which satisfies:

:$mathcal\{L\}\_X\; g\_\{ab\}=2c\; g\_\{ab\}$

where c is a real constant. Homothetic vector fields find application in the study of singularities in general relativity.

Affine symmetry

An affine vector field is one that satisfies:

:$(mathcal\{L\}\_X\; g\_\{ab\})\_\{;c\}=0$

An affine vector field preserves geodesics and preserves the affine parameter.

The above three vector field types are special cases of projective vector fields which preserve geodesics without necessarily preserving the affine parameter.

Conformal symmetry

A "conformal vector field" is one which satisfies:

:$mathcal\{L\}\_X\; g\_\{ab\}=phi\; g\_\{ab\}$

where $phi$ is a smooth real-valued function on $M$.

Curvature symmetry

A "curvature collineation" is a vector field which preserves the Riemann tensor:

:$mathcal\{L\}\_X\; R^a\{\}\_\{bcd\}=0$

where $R^a\{\}\_\{bcd\}$ are the components of the Riemann tensor. The set of all smooth curvature collineations forms a Lie algebra under the Lie bracket operation (if the smoothness condition is dropped, the set of all curvature collineations need not form a Lie algebra). The Lie algebra is denoted by $CC(M)$ and may be infinite-dimensional. Every affine vector field is a curvature collineation.

Matter symmetry

A less well-known form of symmetry concerns vector fields that preserve the energy-momentum tensor. These are variously referred to as "matter collineations" or "matter symmetries" and are defined by:

:$mathcal\{L\}\_X\; T\_\{ab\}=0$

where $T\_\{ab\}$ are the energy-momentum tensor components. The intimate relation between geometry and physics may be highlighted here, as the vector field $X$ is regarded as preserving certain physical quantities along the flow lines of $X$, this being true for any two observers. In connection with this, it may be shown that every Killing vector field is a matter collineation (by the Einstein field equations, with or without cosmological constant). Thus, given a solution of the EFE, "a vector field that preserves the metric necessarily preserves the corresponding energy-momentum tensor". When the energy-momentum tensor represents a perfect fluid, every Killing vector field preserves the energy density, pressure and the fluid flow vector field. When the energy-momentum tensor represents an electromagnetic field, a Killing vector field does "not necessarily" preserve the electric and magnetic fields.

Applications of symmetry vector fields

As mentioned at the start of this article, the main application of these symmetries occur in general relativity, where solutions of Einstein's equations may be classified by imposing some certain symmetries on the spacetime.

Classifications of spacetimes

Classifying solutions of the EFE constitutes a large part of general relativity research. Various approaches to classifying spacetimes, including using the Segre classification of the energy-momentum tensor or the Petrov classification of the Weyl tensor have been studied extensively by many researchers, most notably Stephani et al. (2003). They also classify spacetimes using symmetry vector fields (especially Killing and homothetic symmetries). For example, Killing vector fields may be used to classify spacetimes, as there is a limit to the number of global, smooth Killing vector fields that a spacetime may possess (the maximum being 10 for 4-dimensional spacetimes). Generally speaking, the higher the dimension of the algebra of symmetry vector fields on a spacetime, the more symmetry the spacetime admits. For example, the Schwarzschild solution has a Killing algebra of dimension 4 (3 spatial rotational vector fields and a time translation), whereas the Friedmann-Robertson-Walker (FRW) metric (excluding the Einstein static subcase) has a Killing algebra of dimension 6 (3 translations and 3 rotations). The Einstein static metric has a Killing algebra of dimension 7 (the previous 6 plus a time translation).

The assumption of a spacetime admitting a certain symmetry vector field can place severe restrictions on the spacetime.

See also

*Field (physics)
*Killing tensor
*Lie groups
*Noether's theorem
*Ricci decomposition
*Symmetry in physics

References

* See "Section 10.1" for a definition of symmetries.
*
* See "Chapter 3" for properties of the Lie derivative and "Section 3.10" for a definition of invariance.