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4.7 First post-Newtonian equations of motion

Next, at 1 PN order, we need h ττ 6 and hμν 4. The n = 1 term in the retardation expansion series of ττ h, Equation (76View Equation), gives no contribution at the 1 PN order by the constancy of the mass mA, i.e. 5hττ = 0.

Now we obtain 4hτi from the Newtonian momentum-velocity relation:

τi ∑ mAviA- 4h = 4 rA . (117 ) A=1,2

We evaluate the surface integrals in the evolution equation for τ PA at 1 PN order. The result for star 1 is

dP τ d ( 1 3m ) ---1 = ε2m1 --- -v21 + ---2 + 𝒪 (ε3), (118 ) dτ dτ 2 r12
where we used the Newtonian equations of motion. From this equation we have the mass-energy relation at 1 PN order,
[ ( )] P τ= m 1 + ε2 1v2 + 3m2- + 𝒪 (ε3). (119 ) 1 1 2 1 r12

Then we have to calculate the 1 PN order QiA. The result is QiA = ε2m2AviA∕(6εRA ). As QiA depends on RA, we ignore it (see Section 4.5). As a result we obtain the momentum-velocity relation at 1 PN order, P i= Pτvi + 𝒪 (ε3) A A A from Equation (86View Equation).

Now as for ij 4h, we first calculate the surface integrals ij Q A, ij R A, and kij R A from Equations (91View Equation) and (92View Equation). We then find that they depend on RA, hence we ignore them and obtain

i j ij 4 ∑ mAv-AvA-- 5 h B = 4ε rA + 𝒪(ε ). (120 ) A=1,2

To derive h ττ 6 and hij 4, we have to evaluate non-compact support integrals for hττ 6 N ∕B and ij 4hN ∕B, and the n = 2 term in Equation (76View Equation) for h ττ:

∫ ττ 4 ∑ PAτ 6 d3y ττ 6 ∂ ∑ τ h = 4ε --- + 4ε -------6[(− g )tLL] + 2ε--2- PArA, (121 ) A=1,2rA N ∕B |⃗x − ⃗y| ∂τ A=1,2 ∑ m vi vj ∫ d3y hij = 4ε4 --A-A--A-+ 4ε4 ------4[(− g)tijLL], (122 ) A=1,2 rA N∕B |⃗x − ⃗y |
with
[(− 16πg )tττ ] = − 7-h ττ,k hττ , (123 ) 6 LL 8( 4 4 ,k ) ij 1 i i 1 ij ττ,k ττ,l 4[(− 16πg )tLL ] =-- δ kδk − --δ δkl 4h 4h . (124 ) 4 2
The evaluation of the twice retardation expansion term (the last term in Equation (121View Equation)) is straightforward. For the non-compact support integrals, it is sufficient to consider the following integral:
∫ 3 ∑ ∫ 3 i j ∫ 3 (i j) ---d-y-----hττ,i hττ,j = 1- P τ2 -d-y--rAr-A + 2-P τPτ --d-y--r1 r2-. (125 ) N ∕B 16π|⃗x − ⃗y|4 4 π A N ∕B |⃗x − ⃗y| r6A π 1 2 N∕B |⃗x − ⃗y| r31r32 A=1,2
To evaluate the above Poisson integrals, we use Equation (101View Equation), thus we need to find super-potentials for the integrands. For this purpose, it is convenient to transform the tensorial integrands into scalar integrands with differentiation operators,
( ) riArjA 1-( i j ij ) -∂2--- 1-- r6 = 8 δ kδ l + δ δkl ∂ykyl r2 , (126 ) Aj ( ) A ri1r2 -∂---∂-- -1-- r3r3 = ∂zi ∂zi r1r2 . (127 ) 1 2 1 2
Then it is relatively easy to find the super-potentials for these scalars. The results are Δ ln r1 = 1∕r21 and Δ ln S = 1∕ (r1r2) where S = r1 + r2 + r12 [77]. Equation (101View Equation) for each integrand then becomes
( ) ∫ d3y rirj δij rirj 4π δij --------A6A-= − 4π --2-− -A-A4- + -------+ 𝒪 (εRA ), (128 ) N∕B |⃗x − ⃗y | rA 4rA 4r A 3εR1r1 ∫ 3 i j ( ij i j i j i j i j i j ) --d-y--r1r2 = − 4π − -δ---− --r1r12--+ r12r12-+ r12r12 − -r1r12-+ -r12r2--- N ∕B |⃗x − ⃗y|r31r32 r12S r1r12S2 r212S2 r312S r1r2S2 r12r2S2 +𝒪 (εRA ). (129 )
The second last term of Equation (128View Equation) and the terms abbreviated as 𝒪 (εRA ) in the above two equations arise from the surface integrals in Equation (101View Equation). Since they depend on εRA, we ignore it (see Section 4.8). Substituting Equations (128View Equation) and (129View Equation) back into Equation (125View Equation), we can compute the non-compact support integrals.

Using the above results and the Newtonian equations of motion for the twice retardation expansion term, we finally obtain

∑ P τ hττ = 4ε4 --A A=1,2 rA [ + ε6 − 2 ∑ mA-{ (⃗n ⋅⃗v )2 − v2} + 2m1m2--⃗n ⋅ (⃗n − ⃗n ) rA A A A r212 12 1 2 A=1,2 ] ∑ m2 m1m2 m1m2 ∑ 1 + 7 -2A-+ 14 ------− 14 ------ --- + 𝒪 (ε7), (130 ) A=1,2 rA r1r2 r12 A=1,2rA ∑ i j hij = 4ε4 mAv--AvA- A=1,2 rA [ 4 ∑ m2A i j 8m1m2 i j + ε r2-nAn A − --r-S-- n12n12 A=1,2 A 12 ( 1 ) m1m2 ] − 8 δikδjl −--δijδkl ---2--(⃗n12 − ⃗n1)(k(⃗n12 + ⃗n2)l) + 𝒪 (ε5), (131 ) 2 S
where ⃗nA ≡ ⃗rA ∕rA.

Evaluating the surface integrals in Equation (111View Equation) as in the Newtonian case, we obtain the 1 PN equations of motion,

dvi1 m1m2 i m1 ----= − --2---n12 dτ r12 [ ( ) 2m1m2-- i 2 2 3- 2 5m1- 4m2- + ε r212 n12 − v1 − 2v2 + 2(⃗n12 ⋅⃗v2) + 4(⃗v1 ⋅⃗v2) + r12 + r12 ] +V i(4(⃗n12 ⋅⃗v1) − 3(⃗n12 ⋅⃗v2)) , (132 )
where we defined the relative velocity as ⃗V ≡ ⃗v1 − ⃗v2 and we used the Newtonian equations of motion as well as Equation (119View Equation).

Finally let us give a summary of our procedure (see Figure 4View Image). With the n PN order equations of motion and μν h in hand, we first derive the n + 1 PN evolution equation for τ P A. Then we solve it functionally and obtain the mass-energy relation at n + 1 PN order. Next we calculate i QA at n + 1 PN order and derive the momentum-velocity relation at n + 1 PN order. Then we calculate QKAli and RKlAij. With the n + 1 PN mass-energy relation, the n + 1 PN momentum-velocity relation, QKli A, and RKlij A, we next derive the n + 1 PN deviation field μν h. Finally we evaluate the surface integrals which appear in the right hand side of Equation (111View Equation) and obtain the n + 1 PN equations of motion. In the above calculations we use the n PN order equations of motion to reduce the order of the equations of motion whenever an acceleration appears in the right hand of the resulting equations of motion. For instance, when we meet ε2dvi∕d τ 1 in the right hand side of the equations motion and we have to evaluate this up to 2 ε, then using the Newtonian equations of motion, we replace it by 2 i 3 − ε m2r 12∕r 12. Basically we shall derive the 3 PN equations of motion with the procedure as described above.


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