General covariant transformations
# 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 $FX$.

