We present two kinematical Lie algebras contraction processes to improve the Bacry and Lévy-Leblond contractions (H Bacry, et al, 1968 J. Math. Phys., 9, 1605-1614 :(speed-time, speed-space and space-time contraction). For the first one, we introduce kinematical parameters, namely the radius r of the Universe, the period τ of the Universe and the speed of light c r 1 t = -. Next we present them as static, Newtonian and flat limits through the use of the dynamical parameters, namely the mass, m, the energy, E 0 and the compliance C, all depending on mass as well as length and time. We consider that the second one as the best. To give a little physical taste for each kinematical Lie algebra, we set up the equations of change with respect each group parameter through the use of the Poisson brackets defined by the Kirillov form.
IntroductionGroup (algebra) contraction is a method which allows to construct a new group (algebra) from an old one. Contraction of Lie groups and Lie algebras started sixty six years ago with nonu and Wigner [1] in 1953, when they were trying to connect Galilean relativity and special relativity. Eight years later, in 1961, Saletan [2] provided a mathematical foundation for the Inonu-Wigner method. Since then, various papers have been produced and the method of contraction has been applied to various Lie groups and Lie algebras [3][4][5][6][7][8][9][10].The method has also been used by Bacry and Lévy-Leblond [11] to connect the de Sitter Lie algebras to all other kinematical Lie algebras through three kinds of contractions: speed-space contractions, speed-time contractions and space-time contractions. The terminology is related to the fact that Bacry and Lévy-Leblond have, first of all, scaled the velocity-space generators, the velocity-time translation generators and the space-time translation generators by a parameter ò to obtain, in the limit 0 , the respective contractions that we prefer to call velocity-space contractions, velocity-time contractions and space-time contractions. The Lévy-Leblond contraction approach has been also extended to supersymmetry [12] and kinematical superalgebras [13][14][15].Within the corresponding eleven Lie groups, four of them, namely the Galilei group G governing the Newtonian physics (Galilean relativity), the Poincaré group P governing the Einstein physics (special relativity), the Newton-Hooke groups NH ± describing Galilean relativity in the presence of a cosmological constant and the de Sitter Lie groups dS ± governing the de Sitter relativity of a space-time in expansion or oscillating universe, are well known in physics literature.Within the remaining five ones, the Para-Poincaré groups P ± and the Static S are still unknown in physics, but the Para-Galilei group G ± and the Carroll group C are gaining more interest in recent times.The Para-Galilei group has been identified as governing a light spring [16].The Carroll group has been associated to tachyon dynamics [17][18][19], to Carrollian electromagnetism [20] versus Galilean electromagnetism...