Galilean transformation
In
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.[1] 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
Although the transformations are named for Galileo, it is the
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:[2][3][4][5]
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:
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.[6] 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
where v ∈ R3. A translation is given by
where a ∈ R3 and s ∈ R. A rotation is given by
where R : R3 → R3 is an orthogonal transformation.[6]
As a Lie group, the group of Galilean transformations has dimension 10.[6]
Galilean group
Two Galilean transformations G(R, v, a, s) and G(R' , v′, a′, s′)
- 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
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:
- anisotropic transformations.
- isochronous transformations.
- spatial Euclidean transformations.
- uniformly special transformations / homogenous transformations, isomorphic to Euclidean transformations.
- shifts of origin / translation in Newtonian spacetime.
- rotations (of reference frame) (see SO(3)), a compact group.
- 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
The structure of Gal(3) can be understood by reconstruction from subgroups. The semidirect product combination () of groups is required.
- (G2 is a normal subgroup)
Origin in group contraction
The Lie algebra of the Galilean group is spanned by H, Pi, Ci and Lij (an antisymmetric tensor), subject to commutation relations, where
H is the generator of time translations (Hamiltonian), Pi is the generator of translations (momentum operator), Ci is the generator of rotationless Galilean transformations (Galileian boosts),[8] and Lij 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)).[9] Formally, renaming the generators of momentum and boost of the latter as in
- P0 ↦ H / c
- Ki ↦ c ⋅ Ci,
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 Lmn Lmn and Pi 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),
The infinitesimal group element is then
Central extension of the Galilean group
One may consider[10] 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 , such that M lies in the
In full, this algebra is given as
and finally
where the new parameter shows up. This extension and projective representations that this enables is determined by its group cohomology.
See also
- Galilean invariance
- Representation theory of the Galilean group
- Galilei-covariant tensor formulation
- Poincaré group
- Lorentz group
- Lagrangian and Eulerian coordinates
Notes
- ^ Galilei 1638i, 191–196 (in Italian)
Galilei 1638e, (in English)
Copernicus et al. 2002, pp. 515–520 - ^ Mould 2002, Chapter 2 §2.6, p. 42
- ^ Lerner 1996, Chapter 38 §38.2, p. 1046,1047
- ^ Serway & Jewett 2006, Chapter 9 §9.1, p. 261
- ^ Hoffmann 1983, Chapter 5, p. 83
- ^ a b c Arnold 1989, p. 6
- ^ [1]Nadjafikhah & Forough 2009
- ^ Gilmore 2006
- ^ Bargmann 1954
References
- ISBN 0-387-96890-3.
- JSTOR 1969831.
- ISBN 0-7624-1348-4.
- Galilei, Galileo (1638i). Discorsi e Dimostrazioni Matematiche, intorno á due nuoue scienze (in Italian). Leiden: Elsevier. pp. 191–196.
- Galilei, Galileo (1638e). Discourses and Mathematical Demonstrations Relating to Two New Sciences [Discorsi e Dimostrazioni Matematiche Intorno a Due Nuove Scienze]. Translated to English 1914 by Henry Crewand Alfonso de Salvio.
- Gilmore, Robert (2006). Lie Groups, Lie Algebras, and Some of Their Applications. Dover Books on Mathematics. ISBN 0486445291.
- Hoffmann, Banesh (1983), Relativity and Its Roots, Scientific American Books,
- Lerner, Lawrence S. (1996), Physics for Scientists and Engineers, vol. 2, Jones and Bertlett Publishers, Inc,
- Mould, Richard A. (2002), Basic relativity, Springer-Verlag,
- Nadjafikhah, Mehdi; Forough, Ahmad-Reza (2009). "Galilean Geometry of Motions" (PDF). Applied Sciences. 11: 91–105.
- Serway, Raymond A.; Jewett, John W. (2006), Principles of Physics: A Calculus-based Text (4th ed.), Brooks/Cole - Thomson Learning,