Could anyone explain me what is the interpretation of t^a filed in(adsbygoogle = window.adsbygoogle || []).push({});

Wald's GR book (p.255). It's defined as any (?) field which

fulfills condition [tex] t^a \nabla_a t [/tex], where t is "time function".

What is the difference between [tex]g^{ab}\nabla_b t [/tex]

and t^a. Thanks for answer.

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# Laps and shift function

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