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Question on Weak Energy Condition

  1. Oct 10, 2005 #1

    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


    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???

  2. jcsd
  3. Oct 10, 2005 #2


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    Staff Emeritus
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    u is the 4-velocity of a hypothetical observer. An observer can't have a space-like 4-vector, an observer must be moving slower than light. The above equation is thus telling you that the energy density must be positive for all observers, where an observer can have any 4-velocity as long as it's timelike.
  4. Oct 10, 2005 #3
    aaah, ok thanks a lot.

    That makes sense.
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