# Galilei-covariant tensor formulation

The Galilei-covariant tensor formulation is a method for treating non-relativistic physics using the extended Galilei group as the representation group of the theory. It is constructed in the light cone of a five dimensional manifold.
Takahashi et al., in 1988, began a study of Galilean symmetry, where an explicitly covariant non-relativistic field theory could be developed. The theory is constructed in the light cone of a (4,1) Minkowski space. Previously, in 1985, Duval et al. constructed a similar tensor formulation in the context of Newton–Cartan theory. Some other authors also have developed a similar Galilean tensor formalism.

## Galilean manifold

The Galilei transformations are
    ${\begin{aligned}\mathbf {x} '&=R\mathbf {x} -\mathbf {v} t+\mathbf {a} \\t'&=t+\mathbf {b} .\end{aligned}}$
where $R$ stands for the three-dimensional Euclidean rotations, $\mathbf {v}$ is the relative velocity determining Galilean boosts, a stands for spatial translations and b, for time translations. Consider a free mass particle $m$; the mass shell relation is given by $p^{2}-2mE=0$.
We can then define a 5-vector,
    $p^{\mu }=(p_{x},p_{y},p_{z},m,E)=(p_{i},m,E)$,
with $i=1,2,3$.
Thus, we can define a scalar product of the type
    $p_{\mu }p_{\nu }g^{\mu \nu }=p_{i}p_{i}-p_{5}p_{4}-p_{4}p_{5}=p^{2}-2mE=k,$
where
    $g^{\mu \nu }=\pm {\begin{pmatrix}1&0&0&0&0\\0&1&0&0&0\\0&0&1&0&0\\0&0&0&0&-1\\0&0&0&-1&0\end{pmatrix}},$
is the metric of the space-time, and $p_{\nu }g^{\mu \nu }=p^{\mu }$.

### Extended Galilei algebra

A five dimensional Poincaré algebra leaves the metric $g^{\mu \nu }$ invariant,
    ${\begin{aligned}[P_{\mu },P_{\nu }]&=0,\\{\frac {1}{i}}~[M_{\mu \nu },P_{\rho }]&=g_{\mu \rho }P_{\nu }-g_{\nu \rho }P_{\mu },\\{\frac {1}{i}}~[M_{\mu \nu },M_{\rho \sigma }]&=g_{\mu \rho }M_{\nu \sigma }-g_{\mu \sigma }M_{\nu \rho }-g_{\nu \rho }M_{\mu \sigma }+\eta _{\nu \sigma }M_{\mu \rho },\end{aligned}}$
We can write the generators as
    ${\begin{aligned}J_{i}&={\frac {1}{2}}\epsilon _{ijk}M_{jk},\\K_{i}&=M_{5i},\\C_{i}&=M_{4i},\\D&=M_{54}.\end{aligned}}$
The non-vanishing commutation relations will then be rewritten as
    ${\begin{aligned}\left[J_{i},J_{j}\right]&=i\epsilon _{ijk}J_{k},\\\left[J_{i},C_{j}\right]&=i\epsilon _{ijk}C_{k},\\\left[D,K_{i}\right]&=iK_{i},\\\left[P_{4},D\right]&=iP_{4},\\\left[P_{i},K_{j}\right]&=i\delta _{ij}P_{5},\\\left[P_{4},K_{i}\right]&=iP_{i},\\\left[P_{5},D\right]&=-iP_{5},\\[4pt]\left[J_{i},K_{j}\right]&=i\epsilon _{ijk}K_{k},\\\left[K_{i},C_{j}\right]&=i\delta _{ij}D+i\epsilon _{ijk}J_{k},\\\left[C_{i},D\right]&=iC_{i},\\\left[J_{i},P_{j}\right]&=i\epsilon _{ijk}P_{k},\\\left[P_{i},C_{j}\right]&=i\delta _{ij}P_{4},\\\left[P_{5},C_{i}\right]&=iP_{i}.\end{aligned}}$
An important Lie subalgebra is
    ${\begin{aligned}[P_{4},P_{i}]&=0\\[P_{i},P_{j}]&=0\\[J_{i},P_{4}]&=0\\[K_{i},K_{j}]&=0\\\left[J_{i},J_{j}\right]&=i\epsilon _{ijk}J_{k},\\\left[J_{i},P_{j}\right]&=i\epsilon _{ijk}P_{k},\\\left[J_{i},K_{j}\right]&=i\epsilon _{ijk}K_{k},\\\left[P_{4},K_{i}\right]&=iP_{i},\\\left[P_{i},K_{j}\right]&=i\delta _{ij}P_{5},\end{aligned}}$
$P_{4}$ is the generator of time translations (Hamiltonian), P$_i$ is the generator of spatial translations (momentum operator), $K_{i}$ is the generator of Galilean boosts, and $J_{i}$ stands for a generator of rotations (angular momentum operator). The generator $P_{5}$ is a Casimir invariant and $P^{2}-2P_{4}P_{5}$ is an additional Casimir invariant. This algebra is isomorphic to the extended Galilean Algebra in (3+1) dimensions with $P_{5}=-M$, The central charge, interpreted as mass, and $P_{4}=-H$.
The third Casimir invariant is given by $W_{\mu \,5}W^{\mu }_{5}$, where $W_{\mu \nu }=\epsilon _{\mu \alpha \beta \rho \nu }P^{\alpha }M^{\beta \rho }$ is a 5-dimensional analog of the Pauli–Lubanski pseudovector.

## Bargmann structures

In 1985 Duval, Burdet and Kunzle showed that four-dimensional Newton–Cartan theory of gravitation can be reformulated as Kaluza–Klein reduction of five-dimensional Einstein gravity along a null-like direction. The metric used is the same as the Galilean metric but with all positive entries
    $g^{\mu \nu }={\begin{pmatrix}1&0&0&0&0\\0&1&0&0&0\\0&0&1&0&0\\0&0&0&0&1\\0&0&0&1&0\end{pmatrix}}.$
This lifting is considered to be useful for non-relativistic holographic models. Gravitational models in this framework have been shown to precisely calculate the Mercury precession.