Two Roads to the Null Energy Condition

Understand

The null energy condition has sweeping consequences in general relativity.

• I argue here that it has been misunderstood as a property exclusively of matter, when in fact it arises only in a theory of both matter and gravity.
• I then derive an equivalent geometric formulation of the null energy condition from worldsheet string theory, where it arises beautifully as simply Einstein's equations in two dimensions.
• But further, I show that this condition also has a thermodynamic origin, following from a local version of the second law of thermodynamics, applied to gravitational entropy.