Null Energy Condition: What Is It?

[touqra]

What is null energy condition?
I'm not sure whether this is the right forum to post but I come across this word when reading article about wormholes.

 

[hellfire]

The null energy condition means that:

T_{\mu \nu} A^{\mu} A^{\nu} \geq 0

With A a null vector. For a perfect fluid, it can be proven that this implies:

\rho + p \geq 0

I think that for \inline{\rho > 0}, the violation of the NEC is equivalent to a violation of the DEC (dominant energy condition). A perfect fluid that violates the DEC has faster than light effects, see https://www.physicsforums.com/showthread.php?t=78361