# Null energy condition

#### 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.

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#### 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

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