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I have a very basic question about future-directed vectors. Are they defined as those vectors whose temporal component is positive or strictly positive?

I need to check wether a certain system satisfies the null energy condition or not and I was wondering if I am allowed to take a vector [itex]k^M[/itex] such that [itex]k^2=0[/itex] and [itex]k^t=0[/itex].

Thanks a lot!

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# Simple question on future-directed vectors

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