I have next question, we have in the next printcreen the equation:(adsbygoogle = window.adsbygoogle || []).push({});

[tex]\frac{dU^b}{d\tau}=U^a \nabla_a U^b[/tex]

why is this right?

(the printscreen is from the book of Woodhouse in GR, page 34)

Thanks.

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# Clarification on mass-energy tensor and 4-velocity.

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