# Covariant derivative of stress-energy tensor

1. Aug 9, 2011

### solveforX

hi, I understand that Tab,b=0 because the change in density equals the negative divergence, but why do the christoffel symbols vanish for Tab;b=0?

2. Aug 9, 2011

### Ben Niehoff

They don't. Why would you think that?

I think your first equation came from flat space, because it is not true in curved space.

3. Aug 9, 2011

### WannabeNewton

$\triangledown _{\mu }T^{\mu \nu } = 0$ can be gotten from $\triangledown_{\mu }G^{\mu \nu } = 0$ which is a consequence of the second bianchi identity. You also know that $T^{\mu \nu }, _{\mu } = 0$ but what you can do is say that locally this is the same thing as $T^{\mu \nu }; _{\mu } = 0$ and if this is true for some coordinate system it will be true for all coordinate systems.

4. Aug 9, 2011

thank you