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Coordinate-time acceleration of weak-field geodesics: the Newtonian leading term with uniform error

Statement

Let (Ω,K,L,C,ε0,φ,r) be stationary weak-field data with spatial base U=π(Ω), compact K⊆U and spatial potential Φ=εc2φ (Stationary weak-field data: the diagonal ansatz, its bounds, stationarity and the Einstein-family condition), let c>0, let ε∗ and C2 be the threshold and constant of Uniform Christoffel-symbol bounds for stationary weak-field data, let 0<ε≤ε∗, and let Γabc be the Christoffel symbols of g(ε) in the inertial chart.

(a) Reparametrization identity. Let γ:S→Ω be a geodesic of g(ε) in an affine parameter s on an interval S (Geodesic of an affine connection), suppose that t(s):=x0(γ(s))/c is C2 with dt/ds≠0 on S and has image an interval I, and write x(t) for the spatial coordinates of γ and z(t):=(ct,x(t)) for its full chart-coordinate curve. Set ua:=dza/dt=(c,x˙); the geodesic equation holds in the chart (Coordinate geodesic equation). Then for every t∈I,

d2xidt2=−Γibcubuc+uic Γ0bcubuc.

(b) Newtonian leading term. Suppose in addition that γ(t)=(ct,x(t))∈Ω, x(t)∈K and ∣x˙(t)∣≤Cε c for every t∈I. Then there is C3<∞, depending only on C, L and the dimension, such that for every t∈I and i=1,2,3,

∣d2xidt2+∂iΦ(x(t))∣≤C3 c2ε2L.

Facts & Assumptions

Given: Stationary weak-field data with spatial base U=π(Ω), 0<ε≤ε∗, a geodesic γ reparametrized by t=x0/c, with γ(t)=(ct,x(t))∈Ω, spatial path x(t)∈K⊆U and ∣x˙∣≤Cεc.

[L1]

At every spacetime point with spatial coordinate in K and for the given ε, the Christoffel symbols of g(ε) satisfy ∣Γi00−∂iΦ/c2∣≤C2ε2/L, ∣Γi0j∣≤C2ε3/2/L, ∣Γijk∣≤C2ε/L, ∣Γ000∣≤C2ε5/2/L, ∣Γ00j∣≤C2ε/L and ∣Γ0jk∣≤C2ε3/2/L (Uniform Christoffel-symbol bounds for stationary weak-field data, with supplies Stationary weak-field data: the diagonal ansatz, its bounds, stationarity and the Einstein-family condition, Stationarity, timelikeness and uniform inverse-metric bounds for stationary weak-field data, Metric compatibility, torsion freedom and the metric-selected connection class, Existence and uniqueness of the metric-selected connection of a nondegenerate metric, and its extension to tensor fields and Christoffel symbols of an affine connection).

Proof

technique · direct; reparametrize the chart geodesic equation by the coordinate time, then insert the uniform Christoffel bounds
1.1L2givenalgebra

(Reparametrization identity.) Write va:=dza/ds for the affine velocity of the geodesic and ua:=dza/dt for its coordinate-time velocity, so that v0=c dt/ds≠0 and ua=cva/v0; in particular u0=c, so u=(c,x˙). The geodesic equation of [L2] reads dva/ds=−Γabcvbvc. Since va=(v0/c)ua, substituting vbvc=(v0/c)2ubuc and using the chain rule ddt=cv0dds gives d2xidt2=cv0dds(cviv0)=c2(v0)2dvids−c2vi(v0)3dv0ds=−Γibcubuc+uicΓ0bcubuc, which is (a).

1.2givenalgebra

(Uniform smallness of the velocity data.) The hypothesis ∣x˙∣≤Cεc and u0=c give ∣ua∣≤max⁡(c,Cεc)≤C4c for a constant C4 depending only on C, since ε≤ε∗≤1 by the definition of the threshold.

2.1step 1.1step 1.2L1algebra

(Insertion of the bounds.) Insert the components of the identity of step 1.1 and the estimates of [L1], using step 1.2 to bound the velocity factors: −c2Γi00=−∂iΦ+O(c2ε2/L),∣2c Γi0jx˙j∣≤2c⋅C2ε3/2L⋅Cεc=C′c2ε2L, ∣Γijkx˙jx˙k∣≤C2εL⋅C2εc2=C′′c2ε2L, and each of the three terms containing the factor x˙i/c is bounded by C′′′c2ε2/L: for the Γ000 term, ∣x˙i∣c ∣Γ000∣≤Cεc⋅c⋅C2ε5/2/L=C′′′c2ε3/L≤C′′′c2ε2/L; for the Γ00j term, ∣2x˙iΓ00jx˙j∣≤2Cεc⋅C2ε/L⋅Cεc; and for the Γ0jk term, ∣x˙iΓ0jkx˙jx˙k∣/c≤Cε⋅C2ε3/2/L⋅C2εc2. Collecting all terms and renaming the constants gives (b).

3.1step 1.1step 2.1given∎

(Conclusion.) Step 1.1 proves (a) and step 2.1 proves (b), with constants depending only on C, L and the dimension.

Remarks

  • Coordinate time, not proper time. The parameter t=x0/c is the chart time; the conversion from proper time to t along the worldline is part of the reparametrization of step 1.1 and uses only dt/ds≠0. No synchronization of distant clocks and no global time function is asserted.
  • Uniform in the family. The error term is uniform on K and for the whole family, with constant depending only on (C,L); this uniformity is what the trajectory-limit theorem on this page uses.
  • Scope. The estimate concerns the model's geodesics only: no claim is made that any physical body follows these geodesics, and the slow-motion and small-field conditions are hypotheses, not consequences.

Depends on

Dependency tree · 2 levels

Sources