Pramana – J. Phys. (2022) 96:121 © Indian Academy of Sciences https://doi.org/10.1007/s12043-022-02361-8 Conservation laws and solution of the geodesic system of Gödel’s metric via Lie and Noether symmetries F ALKINDI1, A H KARA2 ,∗ and M ZIAD3 1General Directorate of Education of A’ddakhiliyah Province, Ministry of Education, Sultanate of Oman 2School of Mathematics, University of the Witwatersrand, Johannesburg, South Africa 3Department of Mathematics, College of Science, Sultan Qaboos University, Muscat, Sultanate of Oman *Corresponding author. E-mail: Abdul.Kara@wits.ac.za MS received 19 August 2021; revised 19 January 2022; accepted 7 February 2022 Abstract. We consider the geodesic system for Gödel’s metric as a toy model and solve it analytically using its Lie point symmetries. It is shown that the differential invariants of these symmetries reduce the second-order non-linear system to a single second-order ordinary differential equation (ODE). Invariance of the latter under a one-dimensional Lie point symmetry group reduces it to an integrable first-order ODE. A complete solution of the system is then achieved. The sub-algebras of Noether symmetries and isometries are then found with their corresponding first integrals. Keywords. Geodesic equations; Lie symmetries; Noether symmetries; isometries. PACS Nos 04.25.-g; 02.30.Hq; 02.30.Jr; 02.40.-k 1. Introduction Lie point symmetries provide operative tools to find solutions of differential equations [1–9]. Various tech- niques are applied using Lie point symmetries to inte- grate ordinary differential equations (ODEs), which under some considerations, can be applied for systems of ODEs [1,7,8]. If a system admits a transitive solvable Lie algebra of the Lie point symmetries of enough dimen- sions, then the solution can be found by line integrals. If there are several symmetries but they do not satisfy the condition of solvability, method of successive reduction of order using canonical forms of the symmetry gener- ators may be purposive. Furthermore, invariants of Lie point symmetries can, in some cases, provide invari- ant solutions or first integrals obtained from differential invariants. However, if the Lagrangian of the system is known, then the variational symmetries (if exist) pro- vide first integrals via Noether theorem. Similarly, if the ambient space–time of the system admits enough isome- tries, then according to Cartan theory [1] these readily provide the reduction of order of the system. We show the efficiency of using Lie point sym- metries to find a complete analytic solution of the geodesic system of Gödel’s metric. Gödel’s metric belongs to the family of the Einstein and the de Sitter space–times which are well known homogeneous Petrov type-D static metric [10–14]. It admits five space–time isometries and is stationary compared to the other two members of its family. It gained popularity and attracted much attention of many investigators [15–23] since its emergence because of its unusual features. The stress– energy tensor of this metric represents a rotating matter without having a singularity. Another notable charac- teristic of this space–time metric is that a portion of the trajectories of the free particles in it appears as time-like closed curve. Here we exhibit this feature of Gödel’s metric explicitly. We provide a complete solution of the geodesic sys- tem of Gödel’s metric in a cylindrical coordinate system (t, r, φ, z), where t < ∞, 0 ≤ r ≤ ∞, 0 ≤ φ ≤ 2π , −∞ < z < ∞, given by [15,18] ds2 = a2([dt + √ 2 sinh2 r dφ]2 − dr2 − dz2 − sinh2 r cosh2 r dφ2), (1) with the aid of Lie point symmetries. Here a = 2/ω, where ω is a measure of the constant rotation of the matter flow. Assuming ω = 2, the geodesic equations for this metric turn out to be ẗ + 4 sinh r cosh r ṫṙ + 2 √ 2 sinh3 r cosh r ṙ φ̇ = 0, (2a) r̈ + 2 √ 2 sinh r cosh r ṫ φ̇ 0123456789().: V,-vol http://crossmark.crossref.org/dialog/?doi=10.1007/s12043-022-02361-8&domain=pdf http://orcid.org/0000-0002-0231-0198 121 Page 2 of 7 Pramana – J. Phys. (2022) 96:121 − sinh r cosh r(1 − 2 sinh2 r)φ̇2 = 0, (2b) φ̈ − 2 √ 2 sinh r cosh r ṫṙ + 2 sinh r cosh r ṙ φ̇ = 0, (2c) z̈ = 0, (2d) where dot over the variables t , r , φ and z denote the derivatives with respect to the arc-length parameter s. In the next section, Lie point symmetry generators of geodesic system (2) are found which form a ten- dimensional Lie algebra and a brief analysis of the resulting Lie algebra is provided in terms of its various sub-algebras and their applicability to solve the system. In §3, the system is reduced, with the aid of the admit- ted Lie point symmetries, to an integrable autonomous first-order ODE which is then analytically solved in details. In the following section, Noether symmetries of the geodesic system besides the isometries of the space as well as their corresponding first integrals are found. 2. Lie point symmetries of the geodesic system Finding Lie point symmetries (using Einstein summa- tion convention) X = ξ(s, xa) ∂ ∂s + ηa(s, xa) ∂ ∂xa (3) of a system of k second-order ordinary differential equa- tions Ei (s, x a, ẋa, ẍa) = 0, i, a = 1, . . . , k, (4) means finding the general solution ξ(s, xa) andηa(s, xa) of the determining equations obtained from the symme- try condition X̂(Ei ) = 0, (5) where X̂ is the extension of the symmetry operator X written as X̂ = ξ(s, xa) ∂ ∂s + ηa(s, xa) ∂ ∂xa + (ηa)′(s, xa, ẋa) ∂ ∂ ẋa +(ηa)′′(s, xa, ẋa) ∂ ∂ ẍa and (ηa)(n) = d(ηa)(n−1) ds − (xa)(n) dξ ds . The solution of symmetry condition (5) applied onto system (2) gives a ten-dimensional Lie algebra (L10) whose symmetry generators are X1 = s ∂ ∂s , X2 = z ∂ ∂s , X3 = ∂ ∂s , X4 = −√ 2 tanh r sin φ ∂ ∂t + cos φ ∂ ∂r − 2 cosh2 r − 1 sinh r cosh r sin φ ∂ ∂φ , X5 = √ 2 tanh r cos φ ∂ ∂t + sin φ ∂ ∂r + 2 cosh2 r − 1 sinh r cosh r cos φ ∂ ∂φ , X6 = ∂ ∂φ , X7 = ∂ ∂t , X8 = s ∂ ∂z , X9 = z ∂ ∂z , X10 = ∂ ∂z , (6) with non-vanishing commutators given by [X1, X2] = −X2, [X1, X3] = −X3, [X1, X8] = X8, [X2, X8] = X9 − X1, [X2, X9] = −X2, [X2, X10] = −X3, [X3, X8] = X10, [X4, X5] = 2 √ 2X7 + 4X6, [X4, X6] = X5, [X5, X6] = X4, [X8, X9] = X8, [X9, X10] = −X10. The derived algebra for L10 is L10 itself showing that it is neither solvable nor nilpotent. The sub-algebra H7 = 〈X1, X2, X3, X6, X7, X9, X10〉 (7) is the maximal solvable sub-algebra of L10. With a change of basis, L10 can be written as L10 = 〈 X1 + X9, X1 − X9, X2, X3, X4, X5, X6 + √ 2 2 X7, X7, X8, X10 〉 . The symmetries involving the arc length s would nec- essarily be gauge-dependent and those that correspond to zero gauge form a sub-algebra as, viz., [20] L7 = 〈 X3, X4, X5, X6 + √ 2 2 X7, X7,X8, X10 〉 . (8) Equation (2d) is exclusive from the rest of the ODEs of the system and can be integrated readily to give z as a linear function of s. Thus, we consider eqs (2a)–(2c) which form a coupled system admitting H6 = 〈X1, X3, X4, X5, X6, X7〉. (9) In the next section, we show the employment of H6 on solving geodesic systems (2a)–(2c). Pramana – J. Phys. (2022) 96:121 Page 3 of 7 121 3. Solving the geodesic system using H6 The generators X6 and X7 show that the variables φ and t do not appear explicitly in the system. Thus, these generators will be no more useful for applying further reduction of order. So is the case with the generators X4 and X5 as their coefficients are dependent on the variable φ. Thus, we have only two generators X1 and X3 to be taken in the reduction process. The commutator [X1, X3] = −X3, sets the order X3, then X1 is to be taken into considera- tion. Using X3, which is in the normal form with respect to arc-length parameter s, the system can be reduced to first order with respect to all dependent variables by change of variables τ = r, v1 = t, v2 = s, v3 = φ. (10) Then, transforming the system into the space of the canonical variables (τ, u1, u2, u3) with respect to gen- erator X1 using the transformation: τ = τ, u1 = ln v̇2, u2 = v̇1, u3 = v̇3, (11) where v̇i = (dvi/dτ) reduces the system into the form u̇1 = 2 √ 2 sinh τ cosh τ u2u3 − sinh τ cosh τ(1 − 2 sinh2 τ)u2 3, (12a) u̇2 = −4 sinh τ u2 + 2 √ 2 sinh3 τ u3 cosh τ + 2 √ 2 sinh τ cosh τ u2 2u3 − sinh τ cosh τ(1 − 2 sinh2 τ)u2u 2 3, (12b) u̇3 = 2 √ 2 u2 − 2 u3 cosh τ + 2 √ 2 sinh τ cosh τ u2u 2 3 − sinh τ cosh τ(1 − 2 sinh2 τ)u3 3, (12c) where u̇i = dui/dτ . The last two equations of sys- tem (12) form a system of two first-order ODEs in the two variables u2 and u3. In principle, solving this lat- ter system for u2 and u3, substituting into eq. (12a) and transforming back into the original variables give the complete solution of geodesic system (2). Practi- cally, this is a non-autonomous system of two nonlinear first-order ODEs which are in a non-integrable form. Moreover, it does not admit a Lie point symmetry. Thus, we turn to use the differential invariants which appear to be practically operative [7,8]. The differential invariants of the transitive Lie sub- group H6, which form the solution i (x) of the system X̂ N i (x) = 0, N = 2, . . . , 6, where x = (s, t, r, φ, ṫ, ṙ , φ̇) and X̂ N is the prolonged form of XN up to the first derivatives of the dependent variables, are 1 = ṫ + √ 2 sinh2 r φ̇, (13a) 2 = ṙ2 + sinh2 r cosh2 r φ̇2. (13b) Invariants (13) satisfy geodesic system (2). Thus, they are first integrals, which are analogous to the first inte- grals found in ref. [15] according to his form of Gödel’s metric. To reduce the order of the system, one requires a full set of first integrals which needs to be three in this case. However, one can use them to reduce the number of equations in the system. Substituting invariant solu- tions (13) into the geodesic system reduces the system of three equations to a single ordinary differential equation given as r̈ + 2 √ 2 1 √ 2 − ṙ2 − 2( 2 − ṙ2) tanh 2r = 0, (14) where i are constants. The above equation admits the only symmetry generator ∂/∂s by which it is reduced, via the transformation v = ṙ , x = r , to the first-order ODE vv̇ + 2 √ 2 1 √ 2 − v2 2 ( 2 − v2 ) = 2 tanh 2x , (15) where v̇ = (∂v/∂x). The solution of eq. (15) is given implicitly as √ 2 1 cosh 2x − √ 2 − v2 sinh 2x + 3 = 0, (16) where 3 is a constant. Substituting back into the space of the original variables s, r, ṙ gives ṙ2 = 2 − (√ 2 1 cosh 2r + 3 )2 sinh2 2r . (17) Equation (17) offers the third functionally independent first integral of system (2) which, with eqs (13), provide a complete solution of system (2). This is given in detail as follows. Equation (17) can be written as ṙ2 = 4 2 sinh2 r cosh2 r − ( √ 2 1(sinh2 r + cosh2 r) + 3) 2 4 sinh2 r cosh2 r (18) or (2ṙ sinh r cosh r)2 = 4 2 sinh2 r cosh2 r −[√2 1(sinh2 r + cosh2 r) + 3]2. 121 Page 4 of 7 Pramana – J. Phys. (2022) 96:121 The left-hand side of this equation is nothing but( d ds sinh2 r )2 . Thus, we can write it as( d ds y )2 = 4 2y(1 + y) −[√2 1(y + y + 1) + 3]2, where y = sinh2 r . Hence ẏ2 = 4 2(y 2 + y) − [√2 1(2y + 1) + 3]2. (19) Expanding and regrouping the terms, this can be written as ẏ2 = −4(2 2 1 − 2) × [ y2 − 2 − 2 2 1 − √ 2 1 3 2 2 1 − 2 y +( √ 2 1 + 3) 2 4(2 2 1 − 2) ] . (20) From eqs (13), it is clear that 2 ≥ 0, and 2 2 1 − 2 = 2(ṫ + √ 2φ̇2 sinh2 r)2 − ṙ2 −φ̇2 sinh2 r cosh2 r. (21) Dividing eq. (1) by ds2 1 = (ṫ + √ 2φ̇2 sinh2 r)2 − ṙ2 −φ̇2 sinh2 r cosh2 r − ż2 (22) and subtracting from eq. (21) gives 2 2 1 − 2 = (ṫ + √ 2φ̇2 sinh2 r)2 + 1 + ż2 > 1. (23) This proves that 2 2 1 − 2 is a positive quantity. More- over, from eq. (20) 2 − 2 2 1 − √ 2 1 3 = ẏ2 + 4(2 2 1 − 2)y2 + ( √ 2 1 + 3) 2 4y > 0. (24) Thus, one can write eq. (20) as ẏ2 = −4A2 [ y2 − B2y + C2] , (25) where A2 = 2 2 1 − 2, B2 = 2 − 2 2 1 − √ 2 1 3 2 2 1 − 2 , C2 = ( √ 2 1 + 3) 2 4(2 2 1 − 2) . (26) An alternative form of eq. (25) is ẏ2 = 4A2 [ B4 − 4C2 4 − ( y − B2 2 )2 ] , (27) from which we can deduce that B4 − 4C2 > 0. (28) Hence ẏ = 2A √ B4 − 4C2 4 − ( y − B2 2 )2 , (29) whose integral y = B2 2 + √ B4 − 4C2 2 cos (2As + s◦) = sinh2 r. (30) In terms of s, y(s) = 2 − 2 2 1 − √ 2 1 3 2 ( 2 2 1 − 2 ) + √ 2 ( 2 − 2 2 1 + 2 3 ) 2 ( 2 2 1 − 2 ) × cos ( 2 √ 2 2 1 − 2 s + s0 ) . (31) To find ṫ and φ̇ we use eqs (17) and (13b) from which we can find that φ̇2 = [√ 2 1(sinh2 r + cosh2 r) + 3 ]2 4 sinh4 r cosh4 r . (32) Taking the square root of both sides and regrouping the terms we can write it as φ̇ = √ 2 1 − 3 2 cosh2 r + √ 2 1 + 3 2 sinh2 r . (33) Substituting eq. (30) gives φ(s) = √ 2 1 − 3 2 I + √ 2 1 − 3 2 J, (34) where I = ds 1 + B2 2 + √ B4−4C2 2 cos(2As + s0) and J = ds B2 2 + √ B4−4C2 2 cos(2As + s0) . This gives φ(s) = tan−1 ⎛ ⎝ √ 2+B2−√ B4−4C2 2+B2+√ B4−4C2 tan ( As+s0 2 )⎞ ⎠ + tan−1 ⎛ ⎝ √ B2−√ B4−4C2 B2+√ B4−4C2 tan ( As+s0 2 )⎞ ⎠ +φ0. (35) Pramana – J. Phys. (2022) 96:121 Page 5 of 7 121 Now substituting eq. (33) into eq. (13a) we get t (s) = − √ 2 ( 3 − √ 2 1 ) √ B4 − 4C2 I − 1s, (36) yielding t (s) = √ 2 tan−1 ⎛ ⎝ √ 2+B2−√ B4−4C2 2+B2+√ B4−4C2 × tan ( As+s0 2 )⎞ ⎠ − 1s + t0. (37) In terms of the constants s, the time-like trajectories in the Gödel Universe in (t, r, φ)-hypersurface are given as (t, r, φ) = ⎛ ⎝√ 2 tan−1 ⎛ ⎝ √ 2+B2−√ B4−4C2 2+B2+√ B4 − 4C2 ×tan ( As+ s0 2 )⎞ ⎠ − 1s + t0, sinh−1 ⎛ ⎝ √ B2 2 + √ B4 − 4C2 2 cos (2As + s0) ⎞ ⎠ , tan−1 ⎛ ⎝ √ 2 + B2 − √ B4 − 4C2 2 + B2 + √ B4 − 4C2 tan ( As + s0 2 )⎞ ⎠ +tan−1 ⎛ ⎝ √ B2−√ B4−4C2 B2+√ B4−4C2 tan ( As+ s0 2 )⎞ ⎠+φ0, (38) where A, B and C are as defined in eq. (26). 4. Noether/gauge symmetries, conservation laws and isometries It is well known that for a variational system, the Lie pint symmetry that leaves the action integral invariant leads to a conservation law via Noether’s theorem. In this case, they are called variational or Noether symmetries [1,3, 4,12]. Since the geodesic equations are derived from a metric, we have a ‘natural’ Lagrangian, with a = 1, arising from (1), viz., L = ṫ2 − ṙ2 − ż2 + (2 sinh4 r − sinh2 r cosh2 r)φ̇2 +(2 √ 2 sinh2 r)ṫ φ̇. (39) L is substituted into a Killing-type equation given by XL + LDsξ = Ds f, (40) where Ds is the total derivative and f (s, t, r, z, φ) is a gauge term. The conserved quantity or first integral is then given by T = Lξ + (ηα − ẋαξ) ∂L ∂ ẋα − f, (41) where, by definition, DsT = 0 (42) along the solutions of geodesic equations (2). Alternative to determining X by eq. (40), one may utilise the ‘multiplier approach’ [4] which determines the multipliers that render a system of equations to be closed/conserved. Here, each multiplier is of the form (ηα − ẋαξ) and is a ‘symmetry’ since the system is variational. The multiplier approach leads to table 1. The method does not explicitly generate the gauge functions. The ‘energy’, from table 1 isT = −( ṫ+ √ 2φ̇ sinh2 r). The T s in table 1 are constants along the solutions of the geodesic equations. Moreover, according to Cartan’s theory, there exists a first integral, Xa ẋa for each Lie point symmetry gener- ator X = ξa∂a satisfying the Killing equations [1]. Xa;b + Xb;a = 0, (43) where solution X of eq. (43) is an isometry of the space– time metric and is called a Killing vector. It was shown that for a natural Lagrangian, the Killing vectors of the metric form a sub-algebra of the Noether symmetries [23]. If the metric admits enough isometries, they are used to instantly reduce the order of the geodesic system by one, leading to a first-order system. Gödel’s metric possesses five isometries which are the symmetry gener- ators Xi where i = 4, . . . , 7, 10 in eq. (6), providing five corresponding first integrals. This reduces the geodesic system to ṫ = c2 [ 1 − 2 sinh2 r cosh2 r ] + √ 2c1 cosh2 r , (44a) φ̇ = √ 2c2 cosh2 r − c1 sinh2 r cosh2 r , (44b) ṙ2 = c2 2 − c3 − (√ 2c2 sinh r cosh r − c1 sinh r cosh r )2 , (44c) ṙ = − (a cos φ + b sin φ) , (44d) where c1, c2, c3, a and b are constants. It is notable that 121 Page 6 of 7 Pramana – J. Phys. (2022) 96:121 Table 1. First integrals corresponding to Noether/gauge symmetries of system (2). X T X3 ṫ2 − ṙ2 − ż2 + φ̇2 sinh2 r(sinh2 −1) + 2 √ 2ṫ φ̇ sinh2 r X4 ṙ cos φ + 2 √ 2ṫ sin φ sinh r cosh r + φ̇ sin φ sinh r cosh r(2 sinh2 r − 1) X5 ṙ sin φ − 2 √ 2ṫ cos φ sinh r cosh r − φ̇ cos φ sinh r cosh r(2 sinh2 r − 1) X6 − sinh2 r((sinh2 r − 1)φ̇ + √ 2ṫ) X7 −( ṫ + √ 2φ̇ sinh2 r) X8 sż X10 sż − z eq. (44c) is identical to eq. (17) with 1 = c2, 2 = c2 2 − c3, 3 = −(2c1 + √ 2c2). Thus, solving this equation with eqs (44a) and (44b) gives the complete solution for the geodesic system. 5. Conclusion The geodesic system for Gödel’s metric is shown to admit a 10-dimensional Lie algebra which con- tains a solvable sub-algebra of dimension seven. We focussed our attention on solving this system in the hypersurface z = constant by making use of the admit- ted Lie point symmetries. Applying Lie’s integration theorem requires the admitted transitive solvable Lie algebra to be of at least six dimensions, which is not the case here and thus the solution cannot be found through line integrals. Method of successive reduction reduces the problem to a system of two nonlinear first-order ODEs. Differential invariants pro- vide two first integrals which reduce the problem to an autonomous second-order ODE. The invariance of this latter ODE under translation along the indepen- dent variable reduces it to an integrable first-rder ODE, which form an additional functionally-independent first integral for the geodesic system. Accordingly, these first integrals provide a complete analytic solution for the time-like geodesics. Besides, variational symme- tries via Noether theorem are found which constitute a sub-algebra of Lie point symmetries. These provide conserved quantities (first integrals) of the geodesic sys- tem. Furthermore, isometries of the space–time, which form a sub-algebra of Noether symmetries, are singled out of the Lie point symmetries and used to find the first integrals, thus reducing the order of the geodesic system by one. 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