Galilean transformation

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In physics, a Galilean transformation is used to transform between the coordinates of two reference frames which differ only by constant relative motion within the constructs of Newtonian physics. These transformations together with spatial rotations and translations in space and time form the inhomogeneous Galilean group (assumed throughout below). Without the translations in space and time the group is the homogeneous Galilean group. The Galilean group is the group of motions of Galilean relativity acting on the four dimensions of space and time, forming the Galilean geometry. This is the passive transformation point of view. In special relativity the homogenous and inhomogenous Galilean transformations are, respectively, replaced by the Lorentz transformations and Poincaré transformations; conversely, the group contraction in the classical limit c → ∞ of Poincaré transformations yields Galilean transformations.

The equations below are only physically valid in a Newtonian framework, and not applicable to coordinate systems moving relative to each other at speeds approaching the speed of light.

Galileo formulated these concepts in his description of uniform motion. The topic was motivated by his description of the motion of a ball rolling down a ramp, by which he measured the numerical value for the acceleration of gravity near the surface of the Earth.

Translation

Standard configuration of coordinate systems for Galilean transformations.

Although the transformations are named for Galileo, it is the absolute time and space as conceived by Isaac Newton that provides their domain of definition. In essence, the Galilean transformations embody the intuitive notion of addition and subtraction of velocities as vectors.

The notation below describes the relationship under the Galilean transformation between the coordinates (x, y, z, t) and (x′, y′, z′, t′) of a single arbitrary event, as measured in two coordinate systems S and S′, in uniform relative motion (velocity v) in their common x and x′ directions, with their spatial origins coinciding at time t = t′ = 0:

${\displaystyle x'=x-vt}$

${\displaystyle y'=y}$

${\displaystyle z'=z}$

${\displaystyle t'=t.}$

Note that the last equation holds for all Galilean transformations up to addition of a constant, and expresses the assumption of a universal time independent of the relative motion of different observers.

In the language of linear algebra, this transformation is considered a shear mapping, and is described with a matrix acting on a vector. With motion parallel to the x-axis, the transformation acts on only two components:

${\displaystyle {\begin{pmatrix}x'\\t'\end{pmatrix}}={\begin{pmatrix}1&-v\\0&1\end{pmatrix}}{\begin{pmatrix}x\\t\end{pmatrix}}}$

Though matrix representations are not strictly necessary for Galilean transformation, they provide the means for direct comparison to transformation methods in special relativity.

Galilean transformations

The Galilean symmetries can be uniquely written as the composition of a rotation, a translation and a uniform motion of spacetime. Let x represent a point in three-dimensional space, and t a point in one-dimensional time. A general point in spacetime is given by an ordered pair (x, t).

A uniform motion, with velocity v, is given by

${\displaystyle (\mathbf {x} ,t)\mapsto (\mathbf {x} +t\mathbf {v} ,t),}$

where v ∈ R³. A translation is given by

${\displaystyle (\mathbf {x} ,t)\mapsto (\mathbf {x} +\mathbf {a} ,t+s),}$

where a ∈ R³ and s ∈ R. A rotation is given by

${\displaystyle (\mathbf {x} ,t)\mapsto (R\mathbf {x} ,t),}$

where R : R³ → R³ is an orthogonal transformation.

As a Lie group, the group of Galilean transformations has dimension 10.

Galilean group

Two Galilean transformations G(R, v, a, s) and G(R' , v′, a′, s′) compose to form a third Galilean transformation,

G(R′, v′, a′, s′) ⋅ G(R, v, a, s) = G(R′ R, R′ v + v′, R′ a + a′ + v′ s, s′ + s).

The set of all Galilean transformations Gal(3) forms a group with composition as the group operation.

The group is sometimes represented as a matrix group with spacetime events (x, t, 1) as vectors where t is real and x ∈ R³ is a position in space. The action is given by

${\displaystyle {\begin{pmatrix}R&v&a\\0&1&s\\0&0&1\end{pmatrix}}{\begin{pmatrix}x\\t\\1\end{pmatrix}}={\begin{pmatrix}Rx+vt+a\\t+s\\1\end{pmatrix}},}$

where s is real and v, x, a ∈ R³ and R is a rotation matrix. The composition of transformations is then accomplished through matrix multiplication. Care must be taken in the discussion whether one restricts oneself to the connected component group of the orthogonal transformations.

Gal(3) has named subgroups. The identity component is denoted SGal(3).

Let m represent the transformation matrix with parameters v, R, s, a:

• ${\displaystyle \{m:R=I_{3}\},}$ anisotropic transformations.
• ${\displaystyle \{m:s=0\},}$ isochronous transformations.
• ${\displaystyle \{m:s=0,v=0\},}$ spatial Euclidean transformations.
• ${\displaystyle G_{1}=\{m:s=0,a=0\},}$ uniformly special transformations / homogenous transformations, isomorphic to Euclidean transformations.
• ${\displaystyle G_{2}=\{m:v=0,R=I_{3}\}\cong \left(\mathbf {R} ^{4},+\right),}$ shifts of origin / translation in Newtonian spacetime.
• ${\displaystyle G_{3}=\{m:s=0,a=0,v=0\}\cong \mathrm {SO} (3),}$ rotations (of reference frame) (see SO(3)), a compact group.
• ${\displaystyle G_{4}=\{m:s=0,a=0,R=I_{3}\}\cong \left(\mathbf {R} ^{3},+\right),}$ uniform frame motions / boosts.

The parameters s, v, R, a span ten dimensions. Since the transformations depend continuously on s, v, R, a, Gal(3) is a continuous group, also called a topological group.

The structure of Gal(3) can be understood by reconstruction from subgroups. The semidirect product combination (${\displaystyle A\rtimes B}$) of groups is required.

1. ${\displaystyle G_{2}\triangleleft \mathrm {SGal} (3)}$ (G₂ is a normal subgroup)
2. ${\displaystyle \mathrm {SGal} (3)\cong G_{2}\rtimes G_{1}}$
3. ${\displaystyle G_{4}\trianglelefteq G_{1}}$
4. ${\displaystyle G_{1}\cong G_{4}\rtimes G_{3}}$
5. ${\displaystyle \mathrm {SGal} (3)\cong \mathbf {R} ^{4}\rtimes (\mathbf {R} ^{3}\rtimes \mathrm {SO} (3)).}$

Origin in group contraction

The Lie algebra of the Galilean group is spanned by H, P_i, C_i and L_ij (an antisymmetric tensor), subject to commutation relations, where

${\displaystyle [H,P_{i}]=0}$

${\displaystyle [P_{i},P_{j}]=0}$

${\displaystyle [L_{ij},H]=0}$

${\displaystyle [C_{i},C_{j}]=0}$

${\displaystyle [L_{ij},L_{kl}]=i[\delta _{ik}L_{jl}-\delta _{il}L_{jk}-\delta _{jk}L_{il}+\delta _{jl}L_{ik}]}$

${\displaystyle [L_{ij},P_{k}]=i[\delta _{ik}P_{j}-\delta _{jk}P_{i}]}$

${\displaystyle [L_{ij},C_{k}]=i[\delta _{ik}C_{j}-\delta _{jk}C_{i}]}$

${\displaystyle [C_{i},H]=iP_{i}\,\!}$

${\displaystyle [C_{i},P_{j}]=0~.}$

H is the generator of time translations (Hamiltonian), P_i is the generator of translations (momentum operator), C_i is the generator of rotationless Galilean transformations (Galileian boosts), and L_ij stands for a generator of rotations (angular momentum operator).

This Lie Algebra is seen to be a special classical limit of the algebra of the Poincaré group, in the limit c → ∞. Technically, the Galilean group is a celebrated group contraction of the Poincaré group (which, in turn, is a group contraction of the de Sitter group SO(1,4)). Formally, renaming the generators of momentum and boost of the latter as in

P₀ ↦ H / c

K_i ↦ c ⋅ C_i,

where c is the speed of light (or any unbounded function thereof), the commutation relations (structure constants) in the limit c → ∞ take on the relations of the former. Generators of time translations and rotations are identified. Also note the group invariants L_mn L^mn and P_i Pⁱ.

In matrix form, for d = 3, one may consider the regular representation (embedded in GL(5; R), from which it could be derived by a single group contraction, bypassing the Poincaré group),

${\displaystyle iH=\left({\begin{array}{ccccc}0&0&0&0&0\\0&0&0&0&0\\0&0&0&0&0\\0&0&0&0&1\\0&0&0&0&0\\\end{array}}\right),\qquad }$ ${\displaystyle i{\vec {a}}\cdot {\vec {P}}=\left({\begin{array}{ccccc}0&0&0&0&a_{1}\\0&0&0&0&a_{2}\\0&0&0&0&a_{3}\\0&0&0&0&0\\0&0&0&0&0\\\end{array}}\right),\qquad }$ ${\displaystyle i{\vec {v}}\cdot {\vec {C}}=\left({\begin{array}{ccccc}0&0&0&v_{1}&0\\0&0&0&v_{2}&0\\0&0&0&v_{3}&0\\0&0&0&0&0\\0&0&0&0&0\\\end{array}}\right),\qquad }$ ${\displaystyle i\theta _{i}\epsilon ^{ijk}L_{jk}=\left({\begin{array}{ccccc}0&\theta _{3}&-\theta _{2}&0&0\\-\theta _{3}&0&\theta _{1}&0&0\\\theta _{2}&-\theta _{1}&0&0&0\\0&0&0&0&0\\0&0&0&0&0\\\end{array}}\right)~.}$

The infinitesimal group element is then

${\displaystyle G(R,{\vec {v}},{\vec {a}},s)=1\!\!1_{5}+\left({\begin{array}{ccccc}0&\theta _{3}&-\theta _{2}&v_{1}&a_{1}\\-\theta _{3}&0&\theta _{1}&v_{2}&a_{2}\\\theta _{2}&-\theta _{1}&0&v_{3}&a_{3}\\0&0&0&0&s\\0&0&0&0&0\\\end{array}}\right)+\ ...~.}$

Central extension of the Galilean group

One may consider a central extension of the Lie algebra of the Galilean group, spanned by H′, P′_i, C′_i, L′_ij and an operator M: The so-called Bargmann algebra is obtained by imposing ${\displaystyle [C'_{i},P'_{j}]=iM\delta _{ij}}$, such that M lies in the center, i.e. commutes with all other operators.

In full, this algebra is given as

${\displaystyle [H',P'_{i}]=0\,\!}$

${\displaystyle [P'_{i},P'_{j}]=0\,\!}$

${\displaystyle [L'_{ij},H']=0\,\!}$

${\displaystyle [C'_{i},C'_{j}]=0\,\!}$

${\displaystyle [L'_{ij},L'_{kl}]=i[\delta _{ik}L'_{jl}-\delta _{il}L'_{jk}-\delta _{jk}L'_{il}+\delta _{jl}L'_{ik}]\,\!}$

${\displaystyle [L'_{ij},P'_{k}]=i[\delta _{ik}P'_{j}-\delta _{jk}P'_{i}]\,\!}$

${\displaystyle [L'_{ij},C'_{k}]=i[\delta _{ik}C'_{j}-\delta _{jk}C'_{i}]\,\!}$

${\displaystyle [C'_{i},H']=iP'_{i}\,\!}$

and finally

${\displaystyle [C'_{i},P'_{j}]=iM\delta _{ij}~.}$

where the new parameter ${\displaystyle M}$ shows up. This extension and projective representations that this enables is determined by its group cohomology.