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I have just read that one of the restrictions on [tex]T^{\mu\nu}[/tex] is that:

[tex]T^{\mu\nu}u^au^b > 0

[/tex]

where u is a timelike vector.

Can someone please tell me WHAT u actually is (other than saying it is a timelike vector).

What sort of vector is it, what is the context???

i.e Once I have obtained the stress energy tensor and I want to test the weak energy condition then what u's to I multiply it by to see if it is greater than zero???

Thanks!