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**"gravitational field strength"**

Hi all,

I've been reading some lecture notes by G. t'Hooft, available from

http://www.staff.science.uu.nl/~hooft101/lectures/genrel_2010.pdf

On page 12, t'Hooft is discussing the Rindler space of an observer using co-ordinates [itex]\xi^{\mu}[/itex] with constant acceleration g in the [itex] \xi^3[/itex] direction when he says says: "The gravitational field strength is given by [itex]\rho^{-2} \vec{g} (\xi)[/itex]",

where [itex]\rho = 1+g\xi^3[/itex] is the "local clock speed" (which, with a bit of help from MTW, I take to mean [itex]\frac{d \tau}{d \xi^0}[/itex]).

I can't see that he defines "gravitational field strength", or [itex]\vec{g}(\xi)[/itex] anywhere (just a constant vector [itex]\vec{g}=(0,0,g)[/itex].)

Could someone help me understand what precisely is meant by the "gravitational field strength" in GR, as well as where the above formula comes from?

Thanks in advance.