- #1
Xander314
- 4
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Homework Statement
Write down the Newtonian approximation to the geodesic deviation equation for a family of geodesics. ##V^\mu## is the particle 4-velocity and ##Y^\mu## is the deviation vector.
Homework Equations
[tex]
D = V^\mu\nabla_\mu \\
V^\mu\approx(1,0,0,0) \\
g_{\mu\nu}=\eta_{\mu\nu}+h_{\mu\nu}
[/tex][/B]
The geodesic deviation equation and the desired linearisation are
[tex]
D^2 Y^\mu = R^\mu{}_{\rho\lambda\nu} \\
\partial_t^2 Y^i = -\partial_i\partial_j\phi Y^j
[/tex]
The Attempt at a Solution
Apologies if this is not the correct way to post in this subforum, but I'm not going to write up all of what I have so far as I don't think it's relevant to my question. My issue is with the left hand side, which with the approximation for the 4-velocity becomes
[itex]
D^2 Y^\mu = V^\mu\nabla_\mu V^\nu\nabla_\nu Y^\mu = \nabla_t^2 Y^\mu
[/itex]
Everywhere I have looked, the next step is to replace this by ##\partial_t^2 Y^\mu##, but I don't follow this logic. As far as I can see, the covariant and partial derivatives differ by Christoffel symbols containing terms O(h), which should enter into the linearisation. What am I missing? Is Y also O(h)?