Hi!(adsbygoogle = window.adsbygoogle || []).push({});

I'm reading General Relativity by Wald. In chpater 4.4a about Newtonian limit of linearized gravity, it says:

"When gravity is weak, the linear approximation to GR should be valid. The assumptions about the sources (relative motion << c and material stresses << mass-energy density) then can be formulated more precisely as follows: there exists a global inertial coordinate system of [itex]\eta_{ab}[/itex] such that:

[tex]

T_{ab} \approx \rho t_a t_b

[/tex]

where [itex]t_a=(\frac{\partial}{\partial x^0})_a[/itex] is the "time direction" of this coordinate system."

I've always thought that

[tex]

T_{ab} \approx \rho v_a v_b

[/tex]

where v is the velocity of the observer (or in other words, the relative velocity between the source and the observer). So, how can I say that

[tex]

v_a=(\frac{\partial}{\partial x^0})_a

[/tex]

?

**Physics Forums | Science Articles, Homework Help, Discussion**

Join Physics Forums Today!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

# Question on General Relativity

**Physics Forums | Science Articles, Homework Help, Discussion**