General covariant transformations

In physics, general covariant transformations are symmetries of gravitation theory on a world manifold $X$. They are gauge transformations whose parameter functions are vector fields on $X$. From the physical viewpoint, general covariant transformations are treated as particular (holonomic) reference frame transformations in general relativity. In mathematics, general covariant transformations are defined as particular automorphisms of so-called natural fiber bundles.

Mathematical definition

Let $\pi:Y\to X$ be a fibered manifold with local fibered coordinates $(x^\lambda, y^i)\,$. Every automorphism of $Y$ is projected onto a diffeomorphism of its base $X$. However, the converse is not true. A diffeomorphism of $X$ need not give rise to an automorphism of $Y$.

In particular, an infinitesimal generator of a one-parameter Lie group of automorphisms of $Y$ is a projectable vector field

$u=u^\lambda(x^\mu)\partial_\lambda + u^i(x^\mu,y^j)\partial_i$

on $Y$. This vector field is projected onto a vector field $\tau=u^\lambda\partial_\lambda$ on $X$, whose flow is a one-parameter group of diffeomorphisms of $X$. Conversely, let $\tau=\tau^\lambda\partial_\lambda$ be a vector field on $X$. There is a problem of constructing its lift to a projectable vector field on $Y$ projected onto $\tau$. Such a lift always exists, but it need not be canonical. Given a connection $\Gamma$ on $Y$, every vector field $\tau$ on $X$ gives rise to the horizontal vector field

$\Gamma\tau =\tau^\lambda(\partial_\lambda +\Gamma_\lambda^i\partial_i)$

on $Y$. This horizontal lift $\tau\to\Gamma\tau$ yields a monomorphism of the $C^\infty(X)$-module of vector fields on $X$ to the $C^\infty(Y)$-module of vector fields on $Y$, but this monomorphisms is not a Lie algebra morphism, unless $\Gamma$ is flat.

However, there is a category of above mentioned natural bundles $T\to X$ which admit the functorial lift $\widetilde\tau$ onto $T$ of any vector field $\tau$ on $X$ such that $\tau\to\widetilde\tau$ is a Lie algebra monomorphism

$[\widetilde \tau,\widetilde \tau']=\widetilde {[\tau,\tau']}.$

This functorial lift $\widetilde\tau$ is an infinitesimal general covariant transformation of $T$.

In a general setting, one considers a monomorphism $f\to\widetilde f$ of a group of diffeomorphisms of $X$ to a group of bundle automorphisms of a natural bundle $T\to X$. Automorphisms $\widetilde f$ are called the general covariant transformations of $T$. For instance, no vertical automorphism of $T$ is a general covariant transformation.

Natural bundles are exemplified by tensor bundles. For instance, the tangent bundle $TX$ of $X$ is a natural bundle. Every diffeomorphism $f$ of $X$ gives rise to the tangent automorphism $\widetilde f=Tf$ of $TX$ which is a general covariant transformation of $TX$. With respect to the holonomic coordinates $(x^\lambda, \dot x^\lambda)$ on $TX$, this transformation reads

$\dot x'^\mu=\frac{\partial x'^\mu}{\partial x^\nu}\dot x^\nu.$

A frame bundle $FX$ of linear tangent frames in $TX$ also is a natural bundle. General covariant transformations constitute a subgroup of holonomic automorphisms of $FX$. All bundles associated with a frame bundle are natural. However, there are natural bundles which are not associated with $FX$.

See also

• General covariance
• Gauge gravitation theory
• Fibered manifold

References

• Kolář, I., Michor, P., Slovák, J., Natural operations in differential geometry. Springer-Verlag: Berlin Heidelberg, 1993. ISBN 3-540-56235-4, ISBN 0-387-56235-4.
• Sardanashvily, G., Advanced Differential Geometry for Theoreticians. Fiber bundles, jet manifolds and Lagrangian theory, Lambert Academic Publishing: Saarbrücken, 2013. ISBN 978-3-659-37815-7; arXiv: 0908.1886