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ISO an elegant derivation of the Geodesic Equation from the Einstein Equation

Physics Asked by John of Tellus on April 19, 2021

Assuming a particle filled with perfect pressure-less fluid can the Geodesic Equation be derived from the Einstein Equations?

One Answer

The usual way is to assume a non-interacting "dust" energy momentum $T^{munu}= rho u^mu u^nu$ with $nabla_mu( rho u^mu)=0$. Then $$ nabla_mu T^{munu} =0quad Rightarrow quad u^mu nabla_mu u^nu=0 $$ so the integral curves $x(tau)$ (worldlines) $$ frac {dx^mu}{dtau}= u^mu $$ of the particles are autoparallels and therefore geodesics.

Answered by mike stone on April 19, 2021

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