Spinning Test Particles in Spacetime with Torsion Physics Department
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1239 Progress of Theoretical Physics, Vol. 86, No.6, December 1991 Spinning Test Particles in Spacetime with Torsion Koichi NOMURA, Takeshi SHIRAFUJI and Kenji HAYASHI* Physics Department, Saitama University, Urawa 338 * Institute of Physics, Kitasato University, Sagamihara 228 Downloaded from https://academic.oup.com/ptp/article/86/6/1239/1918293 by guest on 27 September 2021 (Received July 18, 1991) We use the Fock-Papapetrou method to derive equations of motion for a spinning test particle moving both in the background gravitational field and a torsion field. The resultant two equations, the equations of world line and spin can be written in a covariant manner (as is the same for Papapetrou's result in general relativity). The equation of spin reproduces all the known equations of spin precession due to torsion. § 1. Introduction In general relativity, the problem of motion of extended bodies has first been 1 studied by Fock ) and developed by Papapetrou,2) who derived the equations of world line and spin for a spinning test particle moving in a background gravitational field. By a test particle we mean that the field produced by this particle can be neglected. Consequently, the back-reaction due to emission of gravitational radiation is ignored. The test particle is assumed to be neutral and of sufficiently small size so that effects due to electromagnetic field and the coupling of its multipole moments to in 3 homogeneities of the gravitational field can be neglected. ) Accordingto the -Fock-Papapetrou method, all test particles move like a spinless mass point in the single-pole approximation. Namely, the world line is the familiar geodesics of the metric. The effects due to spin appear in the pole-dipole approxima tion. In effect, the world line of a spinning test particle deviates from the geodesics of the metric. In this paper we study the motion of an extended body such as a gyroscope or a wave packet moving both in the background gravitational field and a torsion field. At present we have two theoretical frameworks of gravitation constructed on the spacetime with torsion: Poincare gauge theory4)-7) and new general relativity.S) The former theory is based on the Riemann-Cartan spacetime characterized by curvature and torsion. The latter theory, on the other hand, is built on the Weizenbock spacetime which possesses absolute parallelism: The gravitation is then ascribed to the torsion of the spacetime, since the curvature is identically vanishing. These two theories give the same predictions as those of general relativity to all the experiments so far performed. The torsion is coupled to the intrinsic spin of fundamental particles (quarks and leptons), and it will give, if it exists, nontrivial effects on the motion of spin of these particles. Thus, observation of spin precession may possibly give us a clue to detect the torsion of the spacetime. In a previous paper,9) we applied the WKB method to the Dirac equation and the Proca equation, and derived the equations of world line and spin precession for massive particles with spin 1/2 and 1 in units of fz moving in 1240 K. Nomura, T. Shirajuji and K. Hayashi the background gravitational and torsion fields. In particular, we showed that the world line for these particles in free fall is the familiar geodesics of the metric, but that the equation of spin precession is not represented in a geometric manner. Namely, the spin vector. of these particles is neither Fermi-Walker transported relative to the asymmetric connection with torsion, nor parallel propagated with respect to the ordinary symmetric connection. 10 It has been suggested by Adamovicz and Trautmann ) using a spin-fluid model/I) on the other hand, that the spin vector of a spinning test particle is Fermi-Walker transported along the world line relative to the asymmetric connection. Our result Downloaded from https://academic.oup.com/ptp/article/86/6/1239/1918293 by guest on 27 September 2021 obtained by the WKB method does not agree with their suggestion. It seems highly desirable to apply the Fock-Papapetrou method to spinning test particles moving in the Riemann-Cartan spacetime. We shall arrange the content as follows: In § 2 a brief review is given of the spinor and tensor analyses in the spacetime with torsion in order to fix our notation. In § 3 the Fock-Papapetrou method is summarized, and the basic response equations are extended to those for a spinning body in the Riemann-Cartan spacetime. In §§ 4 and 5 the equations of motion are derived in the single-pole and pole-dipole approxima tions, respectively. In § 6 a possible way to get the equation of spin precession is suggested. The equation of world line is discussed in § 7. The final section is devoted to conclusion. Detailed proofs of basic relations are given in three appen dices. § 2. A brief review of the Riemann-Cartan spacetime Spacetime has a locally Lorentzian metric field gpv(x) and a global tetrad field ekV(x) with gpvekPe/= 7Jkl. Here !}kl is the Minkowski metric, which we choose as 7Jkl =diag( -1, + 1, + 1, + 1).*) The Latin indices and the Greek ones refer to the Lorentz basis ek and the coordinate basis E p , respectively. The fundamental particles such as quarks and leptons are described by spinor fields if, on which the covariant derivative acts like (2'1) where (Jmn is the generator of the local Lorentz group, and Amnv is the Lorentz (or the spin) connection. In this paper we shall assume that matter fields are minimally coupled to the gravitational field and a torsion field: Namely, the interaction of matter fields with torsion is introduced through the covariant derivative, which satisfies the tetrad condition and the metric condition, (2·2) (2·3) with r;v being the affine connection coefficients. Here the r;v is not assumed to be *) We shall mainly follow the notation and convention of Ref. 7). Spinning Test Particles in Spacetime with Torsion 1241 symmetric with respect to fl and v, and accordingly the torsion tensor is defined by TA pv = rtv - rtp. The affine connection can then be expressed by (2·4) where {~v} is the Christoffel symbol and KApv denotes the contorsion tensor, (2·5) Downloaded from https://academic.oup.com/ptp/article/86/6/1239/1918293 by guest on 27 September 2021 which is anti symmetric with respect to It and fl. Putting (2·4) in (2·2), we see that the Lorentz connection is represented as Amnv = L1mnv + Kmnv " (2·6) where L1mnv is the Ricci rotation coefficients for the spin connection of general relativ ity. We shall write by l7 p the covariant derivative with respect to gv} and L1mnv: It also satisfies the tetrad condition and the metric condition. The comml,ltator of the covariant derivative reads (2·7) . with the curvature tensor given by (2·8) As for the covariant derivative l7 v, its commutator is (2·9) where Rmnpv is the Riemann-Christoffel curvature tensor which is obtained from Fmnpv of (2·8) by replacing Amnp and rtv with L1mnp and gv}, respectively. The curvature tensor Fp(JPv is thus related to the Rp(JPv by (2·10) The torsion tensor has three irreducible components; the tensor part tAPV, *l the vector part Vp and the axial-vector part ap, in terms of which the contorsion tensor is expressed by (2·11) In general relativity, the starting point of the Fock-Papapetrou method is the response equation of the energy-momentum tensor density TPV of matter fields in the background gravitational field. The response equation takes the familiar form: 17vTPv =O. (2·12) *l'The tAPV is symmetric with respect to A and p. and traceless. Moreover it satisfies the cyclic identity. tAPV+ f pvA + tVAP=O. See Refs. 7) and 8) for details. 1242 K. Nomura, T. Shirafuji and K. Hayashi Here TI-'v is symmetric with respect to fL and v, and the covariant derivative acts on the tensor density Tl-'v=J=[iTl-'v like [7ATl-'v=j-g[7ATl-'v. In the theory of gravitation constructed on the spacetime with torsion, on the other hand, the starting point is the response equations of the energy-momentum tensor density and the spin tensor density of matter fieds in the background gravita tional and torsion fields. These equations can be derived from Noether's 14 theorem. ),15) Since the action of matter fields is invariant under general coordinate transformations and local Lorentz ones, there are two sets of Noether's identity which 5 Downloaded from https://academic.oup.com/ptp/article/86/6/1239/1918293 by guest on 27 September 2021 are obtained by the well-known procedure. ) Using the field equations of matter fields in those identities which follow, we can get the following response equations:*) (2·13) (2'14) where the energy-momentum and spin tensor densities of matter fields are defined by smnv=_2 oLM (2·15) oAmnv' Here LM=j-gLM is the Lagrangian density of matter fields. Derivation of (2'13) and (2·14) is given in Appendix A: It should be stressed that these response equations are valid irrespectively of whether matter fields are coupled to the gravitational field and the torsion field minimally or not. We note that TI-'v is not symmetric with respect to fL and v: It is reduced to the canonical energy-momentum tensor in special relativistic limit, when the interaction is minimal coupling. § 3. Papapetrou's method 2 3.1. A brief review of Papapetrou's method ) Let us consider an extended body, whose motion sweeps a world tube in spacetime.